Torres Strait Treaty - - ResearchOnline@JCUbetween Australia and Papua New Guinea that protects the...

This introductory chapter describes the rationale for my research in the context of the threatened status of

dugongs, with particular emphasis on Torres Strait. I then review some of the main issues inherent in

ensuring the sustainability of the Torres Strait traditional dugong fishery, a fishery endorsed by the Torres

Strait Treaty between Australia and Papua New Guinea.

Given the cultural and socio-economic values of dugongs to Torres Strait Islanders successful

management is reliant on the development of a process for community-based management as well as

empirical data. This chapter provides a background for this approach.

The 'cutting' of dugongs is an important of part of community life on Mabuiag Island and is mainly conducted on the beach in front of the community, an arrangement that facilitated this study in 1997-99.

Chapter 1 Rationale and Context 2

1.1 INTRODUCTION

The ancestors of the dugong (Dugong dugon) were a successful, diverse and widely distributed group that

existed some 55 million years ago (Reynolds and Odell1991; Dornning 1999a, 2001). Today the dugong

is the only extant species in the family Dugongidae and has considerable biodiversity value as the only

herbivorous mammal that is strictly marine. With the emergence of the genus Homo about 4 million years

ago, these large, docile animals were hunted for food and other products by many early coastal societies

(Reynolds and Odell 1991). The only other modem dugongid, the Stellets sea cow (Hydrodamalis gigas)

was hunted to extinction in the eighteenth century (Marsh and Lefebvre 1994).

Historically, dugongs have been subjected to human exploitation throughout their range (Reynolds and

Odell1991; Rogan eta!, in press). Anecdotal reports suggest that dugongs were once a very important

subsistence resource in many countries in the Indian subcontinent and islands, South East Asia, East

Africa, Western Pacific and the South Pacific (Marsh and Lefebvre 1994; Marsh ef a/. 2002). With the

growth of human populations, the subsistence hunting of dugongs has probably contributed to the

extirpation or severe depletion of local populations in many parts of their former range (Marsh and

Lefebvre 1994; Marsh et al. 2002).

Australia is currently one of few countries that have large populations of dugongs and is considered the

dugong's stronghold (see Marsh ef al. 2002). It is believed that the global survival of the dugong will be

largely dependent upon Australian efforts (Bertram 1980; Perrin et al. 1996; Marsh et al. 1999; Marsh et

al. 2002). As such, there is increasing pressure for Australia to fulfil its international and national

obligations to conserve its internationally significant populationsof dugongs (Marsh ef a/, 2002)

The large relatively stable populations of dugongs that occur in Australia combined with relatively low

hunting pressure has allowed dugongs to remain of great cultural, nutritional and socio-economic value to

coastal Aboriginal and Torres Strait Islander peoples in tropical Australia (Smith and Marsh 1990;

Johannes and MacFarlane 1991; Bradley 1997). The largest population of dugongs is in Torres Strait

(Marsh et al. 1997, 2002), where the long-standing importance of dugongs for subsistence by Torres Strait

lslanders has been traced in archaeological deposits dating back at least 2000 years (Vandetwal 1973).

The dugong hunting rights of Torres Strait lslanders is protected by the Tores Sfraif Treaty, an agreement

between Australia and Papua New Guinea that protects the traditional way of life of Torres Strait Islanders

(Kwan et a/. 2001).

Following the lead of lndigenous peoples in Canada (Usher 1991; Treseder 1999), north-westem United

States (The International Harpoon 1995) and Alaska (Freeman 1989), lndigenous peoples in Australia,


including Torres Strait Islanders, are actively seeking to exercise their rights over both the commercial and

subsistence use and management of their natural resources (Mulrennan and Scott 2000; Kwan eta/.

2001; Mulrennan and Scott 2001; Smyth 2001; Sutherland and Muir2001). For Indigenous peoples in

these countries, continued access to and use of natural resources is fundamental for their aspirations and

assertions for self-determination. These lndigenous peoples seek to exercise their right to subsistence

hunting, which includes the right to trade or sell harvested wildlife products, in order to support their family

or group (see Nuffal1998).

The challenge for Australia is to meet the multiple goals of: (1) protecting Native Title rights over the sea

including hunting rights; (2) allowing Torres Strait Islanders to actively pursue self determination including

management of hunting; and, (3) fulfilling its international obligations for biodiversity conservation including

the conservation of dugongs. Thus, the management frameworks to achieve these goals have to be socio-

politically, socio-economically and culturally appropriate for Torres Strait Islander peoples as well as being

ecologically relevant (Marsh 1996; Marsh ef a/. 1997; Kwan et a/. 2001).

Information that provides insights into the interface and interdependence of human societies and

ecological systems is increasingly recognised as being crucial for the effective management of natural

resources (Berkes and Folke 2000). This study provided me with a rare opportunity to work at the interface

of natural and human systems (see Berkes and Folke 2000). This opportunity was provided by: (1) my

funding source, the Australian Fisheries Management Authority which has a statutory responsibility for the

Torres Strait dugong fishery, (2) communities in Torres Strait who were willing to host and work with me

and (3) my ability to draw on my experience and skills from my training as a scientist. I was privileged to

be able to take advantage of this opportunity to obtain empirical information on the biological variables that

define the resource (i.e., reproductive and other life history parameters of dugongs) and the social context

of the linkages between the cultural, social, economic and environmental variables that determine hunting

pressure. Knowledge of such linkages is critical to management efforts to ensure sustainability of the

traditional fishery for dugongs in Torres Strait.

The conservation and management of dugongs is a complex challenge particularly as our knowledge of

the factors that determine sustainable harvest rates is limited. The cultural and socio-economic factors that

affect hunting activity and catch rates have not been studied in any depth. Such information, which

provides the social context of dugong hunting in Torres Strait, is vital if management is to be effective.

Similarly we have limited empirical information with which to calculate reliable sustainable catch levels as

these depend (among other data) on information on the intrinsic rate of increase of the dugong population.

Given the cultural and socio-economic values of dugongs to Torres Strait Islanders successful

management is reliant on the development of processes for community-based management as well as on


empirical data. To achieve this it is also vital to build trust between the Indigenous community and the

scientist generating the data. The subsequent sections of this chapter provide a background for this

approach, which is the one I have taken in this thesis.

1.2 STATUS OF AND THREATS TO THE DUGONG

The Sirenia, the Proboscidea (elephants), the extinct orders Dermostylia and Embrithopoda and possibly

the Hydrocoidea (hyraxes) are members of the Tethytheria, which apparently evolved in the Tethys

Seaway in Eurasia during the early Eocene some 55 million years ago (Domning 1999a). The order

Sirenia comprises the only extant herbivorous aquatic mammals. The order includes two families, the

Dugongidae (the dugong) and Trichechidae (manatees) (Domning 1999a, 2001). Extant sirenians are

represented by the dugong (Dugong dugon) and three species of manatees: the Amazonian manatee

(Trichechus inunguis), the West African manatee (T. senegalensis) and the West Indian manatee which

includes two subspecies, the Antillean manatee (T. manatus manatus) and the Florida manatee (T.

manatus lafirostris). The only other recent dugongidid, the Stellets sea cow was hunted to extinction

within 27 years of its discovery in 1741 (Stejneger 1887).

Throughout their ranges all sirenians, including dugongs, are impacted by threatening processes which

include direct human exploitation, habitat destruction or modification, pollution, mortality or injury from

boats and interactions with fisheries (Reynolds and Odell 1991; Marsh and Lefebvre 1994; Preen 1998;

Marsh et al. 1999, 2002; Rogan eta/. in press). All extant members of the Order Sirenia are now listed in

The World Conservation Union Red Data Book of Threatened Species as 'Vulnerable to Extinction' (IUCN

2000). The Amazonian, Antillean and Florida manatees and the dugong are listed in Appendix I and the

West African manatee is listed in Appendix II of the Convention on International Trade of Endangered

Species (CITES), which regulates trade in listed species. In Australia, the dugong is included as a 'Listed

Migratory Species' and 'Listed Marine Species' under the Commonwealth Environmental Protection and

Biodiversity Conservation Act 1999. Throughout its Australian range, the dugong is also protected under

other statelterritory legislation. In Queensland, the dugong is listed as 'Vulnerable' under the Nature

Conservation Act 1992.

The dugong's range extends through the waters of about 37 countries in the tropical and subtropical

coastal and island waters of the IndoPacific from East Africa to the Solomon Islands and Vanuatu, and

between about 26-27" north and south of the equator (Nishiwaki and Marsh 1985; Marsh et a/. 2002). In

most of this range, only relict populations of dugongs now exist. Large areas where dugongs are believed

to be close to extinction separate these populations (Nishiwaki and Marsh 1985; Preen 1998; Marsh et al.


2002). Over much of this range, dugongs are known only from incidental sightings, accidental drownings

and anecdotal reports of fishers (Marsh et a/. 2002).

In Australia, dugongs occur in tropical and subtropical waters from Moreton Bay in the east to Shark Bay

in the west (Marsh and Lefebvre 1994; Marsh ef a/. 1999,2002). Torres Strait is the most important area

with the largest population of dugongs in Australia and the world (Marsh et a/. 1997,2002). Queensland

waters support other populations of international significance. Within the Great Barrier Reef World Heritage

Area, the Cape York to Cooktown region supports important dugong habitat (Marsh and Lefebvre 1994;

Marsh ef a/. 2002) and this is one of the reasons for its world heritage listing (GBRMPA 1981). The

dugong population in Shark Bay, Western Australia is of global significance. Other significant populations

occur in the north-west coast of Western Australia, Northern Territory, the Gulf of Carpentaria and South

East Queensland (Preen 1998; Marsh et a/. 2002).

Dedicated aerial surveys of dugong populations estimate the Australian population to be around 85 000

individuals (Marsh etal. 1999). However, this is considered to be an underestimate for two reasons: (1)

not all of the dugong's range has been surveyed and (2) the aerial survey methodology used to estimate

population size provides estimates of relative, rather than absolute, abundance.

While most Australian dugong populations, including that in Torres Strait, are believed to be stable or are

of unknown status, there is evidence that numbers on the urban coast of Queensland have declined

significantly over the last forty years (Marsh 2000; Marsh ef a/. 2001,2002). The observed decline is

believed to result from a variety of causes (Marsh etal. 1999, 2002). These include habitat loss or

modification, incidental drowning in nets, acoustic pollution, environmental pollutants, vessel strikes,

disease and hunting.

A significant threat to the Torres Strait dugong population is the loss of seagrass systems upon which

dugongs depend for food. Seagrass beds are very susceptible to both natural andlor anthropogenic

impacts. Seagrass have been shown to be vulnerable to extreme weather events such as cyclone and

floods (Poiner and Peterkin 1996). Seagrass habitats may also be susceptible to impacts caused by

increased sediment and nutrient loads from changes in land-use practices such as mining or forestry

developments, which affect catchment areas. However, such anthropogenic impacts upon the condition of

seagrass habitats are extremely difficult to to determine. Loss of seagrass results in a decrease in dugong

reproductive rates (Marsh 1995a; Boyd et a/. 1999; see Chapters 7 and 9) while other threatening

processes result in their increased mortality (see Marsh ef a/. 2002; see Chapters 7 and 9). Dugong

populations can tolerate only a low level of mortality because of their life history (Marsh 1995a; Boyd et a/.

1999; see Section 1.3.3 below). Similarly, a sustained reduction in their reproductive rates will affect the

population's persistence and resilience to anthropogenic impacts.


While there have been previous concems that catch rates of dugongs in Torres Strait has at times been

unsustainable, this study will demonstrate that catch rates are highly variable because of a number of

biological, environmental, social, cultural and economic factors. Obtaining information on these variables

is relevant to ensuring the sustainability of the dugong fishery and is one of the main foci of my study.

The globally threatened status of dugongs and Australia's obligations to: (1) recognise the hunting rights of

Torres Strait Islanders and (2) conserve biodiversity places considerable responsibility on Australia for

dugong conservation.

1.3 CHALLENGES FOR CONSERVATION AND MANAGEMENT

1.3.1 The Need for Conservation

Conservation has been broadly defined as 'the sum total of actions taken to preserve and maintain items

to which we attribute a positive value' (Webb 1995). The wider Australian community accords high priority

to the conservation of wildlife but the issue is complicated by value judgements of comparative usefulness

or importance (Caughley et a/. 1996). Furthermore, the need for conservation of certain species is defined

by their 'usefulness', which provides the incentive to conserve (Caughley et a/. 1996; Webb 1996). The

conservation of marine mammals is given high priority because of their importance in the ecosystem

(Bowen 1997), the decline of populations and wide public appeal (Entwistle and Dunstone 2000).

As conspicuous components of marine ecosystems, marine mammals, including sirenians, typify the

'charismatic megafauna', which are highly valued by the general public for their 'intrinsic value' (see

Freeman and Kreuter 1994). The prominent position of marine mammals on the conservation agenda

means that they are very important as 'flagship' and 'umbrella' species. Thus species such as the dugong

and manatee are considered effective vehicles by conservation biologists, conservationists and

government managers for conveying the broader issues of conservation to the public (see Entwistle and

Dunstone 2000). As such, sirenians are often a focus of intense public and political interest, For example,

the attention given to the dugong in Australia demonstrates the special concern the general public has for

a 'native' species that is considered threatened (Marsh et a/. 1999). In the USA, another sirenian, the

Florida manatee attracts even more public attention (Lavigne et a/. 1999; Reynolds 1999).

High public interest in the conservation of marine mammals is generated by the threatened status of many

marine mammals, including dugongs and manatees. Such concems have resulted from the extinction of

species such the Caribbean monk seal (Monachus tropicalis), Stellets sea cow and populations of several

other species over the past 250 years. In addition, many species of marine mammals are now considered


depleted, threatened or endangered (Reynolds et a/. 1999). Several anthropogenic threatening processes

(noted above) including the consumptive use of marine mammals have been linked to their decline.

Marine mammals have long been of importance to humans as resources used for food, clothing and other

products (Lavigne et a/. 1999; Rogan et a/. in press). A few marine mammal species continue to be

commercially exploited while others are the basis of the subsistence economies of some coastal

lndigenous peoples (Lavigne et a/. 1999). For these peoples, marine m ammals also have considerable

ecological, spiritual and cultural values beyond subsistence. Dugongs are significant in this broad sense to

Aboriginal peoples and Torres Strait Islanders (Smith 1987; Marsh 1996; Bradley 1997).

However, as discussed above the 'general public' (Non-Indigenous peoples) now also value marine

mammals for ethical, aesthetic and ecological reasons. Thus, the consumptive use of wildlife such as

dugongs by lndigenous peoples tends to raise conservation concerns because of the tensions between

the conservation values of the Non-Indigenous community and the cultural and consumptive values of

lndigenous peoples (see Webb 1996). These diverse values create situations which have considerable

potential for conflict between competing stakeholder interests (Meffe et a/. 1999).

The hunting of wildlife by lndigenous peoples in Australia and the concerns that wildlife may be over-

harvested raises such conflict. Ponte et a/. (1994) reported that a major concern in the wider community of

Australia is that harvest levels (from subsistence hunting) may be increased as a result of modem

technology increasing the efficiency of hunting and gathering and the geographic range of subsistence

activity. The Australian Law Reform Commission Report (1986) defines traditional hunting on the basis of

the purpose of the hunt rather than the methods used. As this view is now reflected in most Australian

conservation legislation (e.g. Environment Protection and Biodiversify Conservation Act 1999 (Cwlth) and

Nature Conservation Act 1992 (Qld) there are few restriction on hunting technology. In Torres Strait,

hunters are required to be traditional inhabitants of the region, the hunting of dugong is only permissible

with the use of the traditional harpoon (the wap, see Chapter 5) and the sale of dugong is not allowed.

However, there are currently no other limits (see Kwan eta/. 2001; Marsh et a/. 2002). Therefore, hunters

are allowed to use modern technology such as outboard-powered dinghies (see Chapter 5).

In spite of the concerns about over-harvesting of wildlife, there have been few studies that have examined

the impact of lndigenous hunting on wildlife populations and wildlife conservation objectives. Most of the

research on hunting in Australia has focused on the contribution that hunting makes to the economy and

diet of Aboriginal people living on Aboriginal owned land (Altman 1987; Wilson et a/. 1992). Hunting does

not necessarily lead to over-exploitation. However, for most species, the paucity of data available on

population size and dynamics prevents accurate estimates of the size of a sustainable take of wildlife

(Johnson 1994).


To complicate the issue, understanding of the concept of sustainability has been shown to differ

considerably between lndigenous and non-Indigenous peoples (Caughley et a/. 1996). This point is well

illustrated by the ambiguous use of the term 'sustainable' in International Whaling Commission (IWC)

documents pertaining to Aboriginal subsistence whaling (Nuttal 1998). For non-Indigenous peoples, the

concept broadly implies the maintenance of maximum economic productivity of land and seas. Most

lndigenous peoples view sustainability as the continuation of use of wildlife resources for subsistence.

However, many lndigenous peoples are also increasingly interested in pursuing hunting of traditional

wildlife species as a means of economic development. I use the term 'sustainability' as defined by the

World Comniission on Environment and Development (WCED1987) to be 'development that meets the

needs of the present without compromising the ability of future generations to meet their own needs'. In

the context of this study, this definition includes ecological, social and local economic dimensions (see

Berkes and Folke 2000).

Given the national and international legislative framework relevant to Torres Strait, closure of the dugong

fishery by banning of dugong hunting in Torres Strait is not an option for government management

agencies (see Section 10.3.1). Such an approach would violate the Islander's Native Title rights and

contravene the provisions of the Tones Strait Treaty. This Treaty explicitly protects the traditional way of

Torres Strait Islanders, which includes their right to hunt dugongs (unless the population is considered

endangered) (see below). While there have been long-standing concerns of over-harvesting, there is no

compelling scientific evidence that banning of dugong hunting is warranted in Torres Strait. Aerial surveys

used to estimate the size of the dugong population have indicated that the population is large and not in

imminent danger of collapse (Marsh et a/. 1997; Marsh et a/. 2002), although it must be conceded that the

power of surveys to detect a decline in the population is low (Marsh 1995b).

1.3.2 Co-management Initiatives: the Community-Based Management Approach

Increasingly, the potential conflict between international and national conservation goals and the use of

wildlife by lndigenous peoples is being resolved through participatory decision-making processes that lead

to the development of consensus arrangements for the co-management of the natural resources (Webb

1996). The fundamental principle of co-management is that all stakeholders have a say in the

management of a resource on which they depend (White eta/. 1994; Berkes 1989, 1994, 1999). Social,

cultural and economic objectives are an integral part of the management framework (White ef a/. 1994;

Berkes 1994, 1999). The sharing of management responsibility varies according to specific conditions but

generally involves a level of government. Such an approach has its strength in better assuring the

commitment and participation of all stakeholders and allows the incorporation of lndigenous and Western

knowledge, experiences and aspirations (White et a/. 1994).


The main benefits of co-management approaches for governments is that user participation in

management is likely to lead to a stronger commitment to sustainable use, a higher degree of acceptability

and compliance, and lower enforcement costs (Berkes 1994; Webb 1996). For lndigenous peoples, co-

management provides an avenue to articulate community concerns and protect traditional economies. It

also enables them to assert their aspirations for self-determination (Berkes 1994). Furthermore, co-

management can bring economic benefits through employment and offer training to lndigenous peoples in

westem resource management techniques (Webb 1996). The employment and training of community

rangers in Aboriginal (Davies et a/. 1999; Robinson and Munungguritj 2001) and Torres Strait Islander

communities (Marsh 1996) is a potential benefit of community-based management of wildlife such as

dugongs and sea turtles.

Innovative programs based on participatory approaches to wildlife management initiated in Africa in the

1980s have provided models for a wide range of similar projects in many developing countries around the

world. Over the last 10-15 years, a range of parallel approaches involving lndigenous communities has

also been initiated in countries such as Canada, Africa, Australia and the South Pacific (IIHED 1994;

White et a/. 1994; Davies et a/. 1999; Treseder ef a/, 1999). Many of these initiatives have been based on

the shared management of the environment and natural resources, particularly wildlife (IIHED 1994) and

fisheries (Ruddle and Akimichi 1984; Cordell 1989; Ruddle and Johannes 1990; White ef a/. 1994),

between government and lndigenous people.

Participatory approaches that acknowledge the potential benefits of traditional resource management

systems or other local-level systems in planning for sustainability are only now gaining wide recognition

(WCED 1987; Suzuki and Knudtson 1992; Stevens 1997; Berkes and Folke 2000). These approaches

recognise that the 'non-scientific' knowledge of local experts can be substantive and essential for

management (see Johannes 1981; Johannes et a/. 2000). This 'traditional ecological' or 'local' knowledge

is a key component of the approaches of lndigenous peoples to management.

Aboriginal peoples and Torres Strait Islanders have made valuable contributions to research and

management programs, particularly in central Australia (Burbridge ef a/. 1988; Reid et al. 1993; Nesbitt ef

a/. 2001). lndigenous people are actively working with scientists to promote understanding and synergies

between their respective knowledge systems, lndigenous and non-Indigenous Australians are becoming

engaged in 'two way learning' about how to manage their country in contemporary contexts (Baker et a/.

2001). 1 use the term 'country' as it relates to lndigenous peoples in Australia to mean a 'place that gives

and receives life' (Rose 1996). This 'country' encompasses both the land and the sea and their resources

(see Rose 1996). Webb (1996) points out that the combination of lndigenous peoples' knowledge and

western science provides a more comprehensive database (than either one does alone) from which to

construct appropriate and relevant management.


In Australia, innovative approaches based on collaborative approaches are being applied in contemporary

management of 'country' (Bomford and Caughley 1996; Davies et al. 1999; Baker et a/. 2001).

Collaborative or participatory management projects involving Torres Strait Islanders and Aboriginal

peoples have tended to focus on management of protected areas rather than wildlife species perse (see

Davies et a/. 1999). However, some co-management arrangements to monitor and manage the impact of

hunting on wildlife have been developed for dugongs (Marsh 1996; Kwan et al. 2001) and sea turtles (see

Kennett ef a/. 1997; Robinson and Mununggritj 2001) in northern Australia. Aboriginal peoples are also

centrally involved in conservation programs for endangered marsupials in northern and central Australia

(Burbridge et al. 1988; Reid et al. 1993; Nesbitt ef a/. 2001).

As summarised by Berkes (1994), co-management is an increasingly attractive alternative in a

contemporary world in which local level traditional restrictions are breaking down and are therefore

insufficient, and state level controls and monitoring are inadequate. For example, in geographically remote

regions like Torres Strait, the capacity of government authorities to undertake surveillance and

enforcement of regulations is very limited. In such areas, management strategies, that recognise the

sociocultural significance of target species, are likely to be most effective because they operate at a

consensus (local level) base (Kwan ef al. 2001).

Throughout the world, lndigenous peoples are now gaining political influence over management of natural

resources, including wildlife (Klein 1989). In Canada, lndigenous groups now have the legal right to

participate in resource management decisions based on two key principles: (1) that subsistence wildlife

harvesting by lndigenous peoples may be limited only for valid conservation reasons, and (2) that

lndigenous peoples can harvest wildlife anywhere in their land claim settlement including national parks

(MacLachlan 1995; Nuttal1998).

In Australia, the Nafive N le Act 1993, which recognised lndigenous common law rights to land also has

implications for lndigenous rights to wildlife resources. Occupation andlor use of land by Torres Strait

Islander and Aboriginal peoples and the subsistence use of wildlife are inextricably linked. Native Title

provides an avenue through which Torres Strait Islanders and Aboriginal peoples can exercise their right

to harvest wildlife and their preferred management strategies (Bomford and Caughley 1996; Collins et a/.

1996; Sutherland and Muir 2001).

The Torres Strait Treaty also provides some legal basis for marine conservation initiatives proposed by

Torres Strait Islanders through its clauses protecting the way of life and livelihoods of lndigenous

inhabitants in the Torres Strait Protected Zone (TSPZ), and the marine environment (Ross ef a/. 1994;

Kwan ef a/. 2001). Consistent with the earlier decision of the Croker Island case (Commonwealth of

Ausfralia v. Yamirr (1999) 168 ARL 426) (Sutherland and Muir 2001), in October 2001 a full bench

Chapter I Rationale and Context 11

decision of the High Court of Australia determined that Native Title exists in areas beyond the low-water,

albeit as a non-exclusive and non-commercial right. This decision clears the way for some 120 cases in

pursuit of sea claims by Aboriginal and Torres Strait Islander peoples and has major implications for the

management of marine resources. This will require the recognition of the rights of Aboriginal and Torres

Strait Islander peoples to use and manage their marine resources, including dugongs (Smyth 2001;

Sutherfand and Muir 2001).

In Australia, a fundamental question concerning the contemporary use of traditional lands and waters is

whether the cultural and economic needs of Aboriginal peoples and Torres Strait Islanders can be met

(Caughley ef a/. 1996; Davies et a/. 1999; Baker et a/. 2001). For Aboriginal people and Torres Strait

lslanders to maintain their spiritual and cultural values associated with wildlife and other natural resources,

access to land and sea and their resources is fundamental. Access to 'country' (land and sea) is essential

to their spiritual, cultural and political aspirations and sense of self and community identity (Caughley ef al.

1996; Davies et a/. 1999; Baker et a/. 2001). However, it is also essential that wildlife use be sustainable

under contemporary lndigenous lifestyles (Caughley et a/. 1996; Baker et a/. 2001). This poses an

important challenge for lndigenous peoples, wildlife managers and governments (Caughley et a/. 1996).

This challenge is particularly pertinent in Torres Strait because of the mounting national and international

pressure to ensure that the subsistence consumption of globally threatened species such as the dugong is

sustainable. Furthermore, Torres Strait Islanders are demanding greater political and economic autonomy

(Kwan ef a/. 2001). Thus, the development of effective co-management strategies should be a major

priority for government management agencies.

Research for this study was conducted as a process of active participation and negotiation with community

members, especially the hunters. This approach provided an opportunity to establish a relationship based

on mutual trust, cooperation and commitment between the communities and myself as a scientist. Such

relationships have considerable potential to enhance the development of effective community-based

management of dugongs in Torres Strait as they increase the likelihood of the Islanders trusting the

scientist's empirical data.

1.3.3 Life History Characteristics and Sustainable Hunting

Reliable estimates of life history parameters are essential as an empirical basis for sustainable wildlife

exploitation. Knowledge of the level of mortality that a population is able to sustain without the intrinsic

growth rate declining provides the basis of establishing sustainable harvest rates (Caughley 1994).

Hunting can be expected to increase the mortality rate above the level of natural mortality. Concerns that

hunting of dugongs in Torres Strait may have been unsustainable at times highlight the importance of

quantifying hunting mortality in the context of dugong population dynamics.


Approaches to evaluating the sustainability of hunting included evaluations of the population trends of

hunted species, comparison of age structures and hunting yields over space and time and the use of

sustainability models (Novaro ef a/. 2000). There are several approaches to sustainability models. Some

estimate population production using data on population densities and reproductive rates or estimate

maximum population growth with data on reproduction and survival (i.e., a Maximum Sustainable Yield)

(Musick 1999; Bodmer 2000; Novaro eta/. 2000). Others develop models for optimal harvesting which

factor in the risk of extinction or severe depletion from harvesting andlor by accounting for environmental

stochasticity (Barlow et a/. 1995; Lande et a/. 1995; Wade 1998). Catch rates estimated as sustainable,

derived by one of the standard approaches, are then compared to actual harvest rates to determine if

harvest is sustainable

With the exception of the optimal harvesting models, most approaches assume a steady state, non-

fluctuating environment or do not consider the impact of environmental fluctuation on the population

dynamics of the species of interest (Lande ef a/. 1995; McCarty 1996; Musick 1999). Knowledge of the life

histories of prey species and the influence of environment on their reproductive rates provides essential

information about the dynamics of populations, Information that helps predict which environmental

changes are likely to regulate populations and the population processes that are likely to operate have led

to considerable improvement in our ability to detect the circumstances that will lead to a decrease or

increase in the abundance of a population (Boyd ef a/. 1999; Wade 1998).

The life history of the dugong is characterised by a long life span, high adult survivorship, high female

parental investment and low reproductive rates (Marsh 1995a). Consequently, the dugong is slow to

recover from reductions in population size. As such, dugongs are very susceptible both to environmental

perturbations and over-exploitation and the interactions between these two factors (Boyd et a/. 1999).

There is anecdotal evidence that dugongs (and manatees) are able to adjust their reproductive rates in

response to environmental fluctuations (Boyd et a/. 1999). As a specialist seagrass feeder, dependent on

a food resource susceptible to large-scale disturbance caused by extreme weather events, the dugong is

vulnerable to episodic losses of seagrass (dieback events) (Johannes and MacFarlane 1991; Preen and

Marsh 1995; Preen et a/. 1995; Poiner and Peterkin 1996). Large-scale dieback events have been

reported in Torres Strait (Johannes and MacFarlane 1991; Williams 1994; Long et a/. 1997) but prior to

this study there has been little scientific evidence that links food availability and dugong reproductive rates.

lnformation from this study has enabled comparisons with other studies using similar techniques to show

that the life history parameters of dugongs exhibit considerable spatial and temporal variability, possibly in

response to changing conditions of food availability.


This lack of a demonstrated link between the food resource and reproductive rates is the likely result of the

difficulties arising from the long lifespan and the inaccessibility of dugongs in the wild. A feasible option to

obtain the necessary data is from studies such as this based on carcasses of dugongs caught for food by

hunters. Such research relies on the active participation of hunters and community members. This

cooperation increases the exposure of such people to western scientific methodology. Given the concerns

of Islanders about possible adverse impacts from consumption of dugongs, I also collected specimens

from the carcasses of dugongs caught for food by hunters to measure the heavy metal levels in dugong

meat. Although the results of that study are not included in this thesis (see Haynes and Kwan 2001), this

provided an additional opportunity to enhance the trust-building process between Torres Strait Islander

communities and myself.

The research process undertaken by this study thus provided invaluable data and specimens, which are

the basis of empirical information to ensure ecological sustainability of the traditional dugong fishery in

Torres Strait.

1.4 THESIS AIMS

This study provides information relevant to addressing the complex and difficult task of ensuring the

sustainability of the subsistence dugong fishery while meeting the wide range of the ecological, biological,

cultural, socio-political and international obligations for the conservation and management of the dugong

population in Torres Strait.

The future of the subsistence use of dugongs in Torres Strait poses a question of great relevance for all

stakeholders:

How can harvesting be managed to ensure the sustainability of the dugong fishery in Torres Strait?

To contribute to the answer to this question and with the conviction based on the review in Section 1.3.2

that community-based management is the only practical way forward, my thesis had the following specific

aims:

(1) To provide information relevant to the development of community-based management for the dugong

fishery in Torres Strait by:

describing and quantifying the major factors that affect hunting pattern, hunting effort, hunting

success and hawest levels in the major dugong hunting community of Mabuiag Island, and


estimating the life history parameters of Torres Strait dugongs at the time of my study, and (i)

comparing them with parameters obtained from parallel studies of dugong life history and

reproductive biology in order to gain insights into the factors influencing these aspects of dugong

ecology, and (ii) incorporating them into population models to predict the natural rate of change

in the size of the population in the absence of hunting mortality.

(2) To assist in the development of community-based management in Torres Strait through contributing to

capacity building by actively involving Torres Strait Islanders in my research and training them in the

collection of catch statistics and biological samples from dugongs.

1.5 THESIS OUTLINE

Figure 1 .I is a conceptual model of my research. This model is provided as a guide to the reader and I will

refer to it repeatedly throughout this thesis, which comprises ten chapters as follows:

Figure 1.1. A conceptual model of my research and the contents of this thesis. The current chapter is coloured yellow.


Chapter 1 (this chapter) describes the rationale for my research in the context of the threatened status of

dugongs, with particular emphasis on Torres Strait. I then review some of the main issues inherent in

ensuring the sustainability of the Torres Strait traditional dugong fishery, a fishery endorsed by the Torres

Strait Treaty between Australia and Papua New Guinea. Given the cultural and socio-economic values of

dugongs to Torres Strait Islanders, successful management is reliant on development of a process for

community-based management as well as empirical data. This chapter provides a background for this

approach.

The literature review in Chapter 2 synthesises the information on the importance and effects of nutrition on

the population dynamics of large herbivores. I review the relative importance of environmental

stochasticity and density dependence on the population dynamics of large herbivores, with particular

reference to the dugong. I then review seagrass ecology in the context of the feeding ecology of dugongs.

I conclude that long-term weather patterns and extreme events resulting in stochastic and cyclic patterns

of seagrass abundance have important impacts on the distribution, fecundity and survivorship of dugongs.

Chapter 3 provides an overview of the biophysical environment, demographic profile, institutional and

governance structures, and Islander participation in environmental management in the Torres Strait region

where this study was undertaken. I then describe my study sites in Torres Strait and explain the rationale

for their selection.

Chapter 4 describes the general methodology used to collect data and specimens including the

development of a protocol to ensure that sampling was culturally appropriate. It describes some of the

specific challenges in undertaking fieldwork in a remote location in the context of the sensitivities

surrounding the traditional hunting of dugong in Torres Strait as evidenced by both local communities and

stakeholders external to Torres Straits. I conclude that such an approach to research has provided an

opportunity to obtain empirical data as well as a valuable contribution to the process for community-based

management because of the opportunity to build trust between the scientist and the community.

Chapter 5 describes the traditional fishery for dugongs at Mabuiag Island as it operated during 1997-99.

Chapter 6 provides information on the major biological, environmental, social, cultural and economic

factors that affected the pattern of hunting, hunting effort and harvest levels of dugongs in Mabuiag Island

in 1997-99. I conclude that this information has potential to improve existing management arrangements if

it is integrated with the cultural and socio-economic perspectives of Torres Strait Islanders.

Chapter 7 provides information on the life history parameters and reproductive biology of female dugongs

based on the analysis of 127 carcasses landed at Mabuiag lsland during 1997-99. These results are


compared and contrasted with other information on reproduction in female dugongs using similar

techniques. I conclude that the life history parameters of age and size at sexual maturity and first

reproduction and calving intervals of dugongs exhibit considerable variability in both space and time. This

variability has important effects on the population dynamics of dugongs. The major factors affecting the life

history parameters, reproductive biology and population dynamics of dugongs are discussed further in

Chapter 9.

Chapter 8 uses data from 51 carcasses of male dugongs landed at Mabuiag Island during 1997-99 to

expand on the information on the life history and reproductive biology of male dugongs. I conclude that the

life history parameters of age and size at sexual maturity and first reproduction in male dugongs exhibit

considerable variability in both space and time. The breeding pattern of male dugongs is diffusely

seasonal with most reproductive activity occurring in the latter part of the year. These results parallel that

discussed in Chapter 7 for female dugongs.

Chapter 9 synthesises information on the importance and effects of nutrition on the population dynamics of

large herbivores. I review the relative importance of environmental stochasticity and density dependence

on the population dynamics of lage herbivores, with particular reference to the dugong. I then review

seagrass ecology in the context of the feeding ecology of dugongs and explore evidence of resource

dependency in dugong reproduction. I suggest that the links between food availability and dugong's life

history parameters have very important implications for management given the dugong's reliance on

seagrass and long lifespan.

Chapter 10 synthesises the results of my research and examines their implications for the community-

based management approaches being developed for dugongs in Torres Strait. I conclude that the

processes and information obtained in this study have considerable potential to contribute to the

sustainable management of the Torres Strait dugong fishery.

CHAPTER 2

FACTORS AFFECTING THE POPULATION DYNAMICS AND REPRODUCTIVE ECOLOGY OF LARGE

HERBIVORES, WITH PARTICULAR REFERENCE TO DUGONGS

This chapter synthesises the information on the importance and effects of nutrition on the population

dynamics of large herbivores. I review the relative importance of environmental stochasticity and density

dependence with particular reference to the dugong. I then examine seagrass ecology in the context of the

feeding ecology of dugongs. I conclude that long-term weather patterns and extreme weather events

resulting in stochastic changes in seagrass abundance have important impacts on the distribution and

population dynamics of dugongs.

Chapter 2 Factors Affecting Population Dynamics and Reproductive Ecology of Large Herbivores 18

2.1 INTRODUCTION

An understanding of the variability in the size of populations requires evaluation of the mechanisms that

regulate population size. The availability of resources such as food, space or potential mates has been

shown to affect the life history parameters of most animals (Caughley and Sinclair 1994). The dynamics of

populations of large mammals, including many herbivores are regulated by mechanisms that commonly

operate through the reproductive process and which are expressed in various vital parameters including

age at sexual maturity and first reproduction, fecundity of adults and juvenile survival (Caughley and

Sinclair 1994).

Both population density perse and density independent factors (i.e., environmental stochasticity) affect life

history parameters through their influence on the amount of food available to each individual in a

population. Density dependence refers to any factors that change in response to population density.

Environmental stochasticity refers to unpredictable or random variations that affect all individuals in a

certain group and may arise from the influence of various climatic factors (McCarty 1996; Annes etal.

2000). These two factors may interact or act independently to influence the population dynamics of large

herbivores.

Studies of plant-herbivore interactions have established that the population dynamics of large terrestrial

herbivores are variously influenced by the com bined effects of environmental stochasticity and density

dependence (Caughley and Gunn 1993, Siether 1997; Solberg et a/. 1999; Aanes ef a/. 2000; Gaillard ef

a/. 2000). There is also evidence to suggest that environmental stochasticity and possibly density

dependence are important regulatory influences on the population dynamics of a mari ne herbivore, the

dugong. As a specialist feeder on seagrass, the dugong is vulnerable to environmental perturbations such

as seagrass dieback events that can destroy its food supply over hundreds of square kilometres (Preen

and Marsh 1995; Preen etal. 1995; Poiner and Peterkin 1996). These variations in food supply have

important effects on dugong populations and thus, very significant implications for their management (see

Chapter 9).

In this chapter, I assess the effects of nutrition on the population dynamics of large herbivores. I review the

relative impacts of environmental stochasticity and density dependence on population dynamics, with

particular reference to the dugong. I then review seagrass ecology in the context of the feeding ecology of

dugongs. I conclude that long-term weather patterns and extreme weather events resulting in stochastic

changes in seagrass abundance have important impacts on the distribution, fecundity and survivorship of

dugongs.


2.2 MECHANISMS THAT AFFECT THE POPULATION DYNAMICS OF LARGE MAMMALS

2.2.1 Relationship Between the Parameters That Determine the Rate of Population Change

The ability of a population to persist is dependent on the rate at which it changes in size (Caughley 1994).

The fundamental equation of population dynamics (Lotka 1907) is:

C / ,e - -~m~ l Equation 2.2.1

where I,= probability of surviving to age x, mx = fecundity at age xfor a population changing exponentially

at a rate r overx years. This equation describes the dynamic nature of a population, which is detemined by its age specific mortality and fecundity rates, interacting with its age distribution (Caughley 1994).

For most mammals, the rate and direction at which a population changes are determined by the amount of

food or some other limiting resource and the intrinsic ability of the species to maximise energy resources

to enhance fecundity and reduce mortality (Caughley 1994). Thus for large mammals, age structure, age

specific mortality and age specific reproduction are key factors influencing the ability of a population to

persist. However, long-term population studies, which enable these parameters to be estimated, are rare

for marine mammals, including sirenians (see Mamontel et a/. 1997; Wade 1998).

2.2.2 Effects of Environmental Stochasticity and Density Dependence on Nutrition in Large Herbivores

A central question in population biology has been the relative importance of density dependent versus

density independent factors in fluctuations in the size of populations. As recently reviewed by Gaillard et

a/. (2000), temporal variability in the abundance of large terrestrial herbivores is influenced by several

factors including density dependent and environmental stochastic food limitation or control by humans,

predation and disease. In the context of my study, I will concentrate on density dependent and

environmental stochastic effects on the population dynamics of large herbivores (see Chapter 9).

The most extensive studies involving large herbivores have involved ungulates. These studies have

shown that the dynamics of some populations are influenced by a combination of predation (including

human exploitation), density dependent food limitation and stochastic environmental variation (Seather

1997; Solberg et a/. 1999). These factors change life history parameters by influencing body condition in

individual animals ( S ~ t h e r 1997; Solberg et a/. 1999; Aanes ef a/. 2000).

In a recent review of temporal variations in the population dynamics of large terrestrial herbivores,

environmental variation and density dependence were shown to co-occur and to have similar effects on

individual life history parameters (Gaillard eta/. 2000). Thus, it is often very difficult to distinguish between


the relative contributions of density dependence and environmental stochasticity to population growth. The

relative importance of these factors has generated considerable controversy in the literature (see Turchin

1995). Not surprisingly then, attempts to assess the role of harvesting compared with density dependent

and density independent factors or indeed their interactions in affecting temporal variability in population

size can also be very difficult. This is largely because such efforts are further hampered by a lack of

information on the responses of life history parameters to changes resulting from harvest rates (Solbert ef

a/. 1999; see Chapter 9).

Several studies have shown that most parameters that affect the population dynamics of large mammals

vary with density (see Fowler 1981,1987). Evidence of density dependence in large mammals (Fowler

1984) has been detected in fecundity, age at first reproduction and juvenile and adult survival and often

seems to involve resource levels as a cause (Fowler 1990; Caughley and Sinclair 1994). Although adult

survival is an important population regulatoly mechanism for large mammals, there are few species for

which there is empirical evidence for its being density dependent. This is at least partly because of

difficulties in estimating survival with sufficient accuracy and precision (Fowler 1984; Gaillard ef a/. 2000).

Studies of large mammals have shown that when populations are below carrying capacity because of

exploitation, colonisation or a natural event, age at first reproduction decreases and population growth rate

increases. As populations approach carrying capacity, population growth rate declines; fecundity and

recruitment increase and females are older when initially recruited to the breeding population (see Fowler

1981,1984).

Models based on density dependence strongly emphasise intraspecific competition and equilibria that

function to maintain populations at or near levels that their habitat is able to support (i.e., at carrying

capacity) (Fowler 1990; Caughley and Sinclair 1994). An alternate theory suggests that fluctuations in

large mammal populations are regulated by stochastic variations in environmental conditions. The

regulatory mechanisms in these models are based on environmental stochasticity rather than density and

are commonly regarded as density independent factors. These models suggests that a population is

regulated by the dynamics of its limiting resource expressed as a functional response of a large mammal

to the level of this resource and ultimately as a change in population growth rate (Caughley and Gunn

1993; Caughley and Sinclair 1994). In tropical and temperate ungulates, environmental stochasticity may

strongly influence population fluctuations by altering fecundity through its effect on body condition (Siether

et a/. 1996; Siether 1997). Reduced food availability in moose (Alces alces) resulted in a decrease in

fecundity due to an increase in the calving interval and delayed sexual maturity associated with poor

nutrition (Szther ef al. 1996).


Thus while population density may affect life history parameters because intraspecific competition reduces

food per capita availability (Begon et a/. 1996), in many populations, density independent environmental

factors may also be important because they also influence the resource base (Langvatn et a/. 1996).

Changes in life history parameters generate delays in the response of populations that may create

complex population fluctuations (Szther 1997). If regulation of population size occurs through food

limitation, a rapid resource-dependent response in life history parameters is required (Szther et a/. 1996).

However, such feedback mechanisms between resource level and population growth rate may be delayed

in some large herbivores because of the effects of stochastic environment variation andlor the time it takes

to accumulate nutrients to convert to energy (Szther et al. 1997; Langvatn etal. 1996; see below). This

contributes to the considerable difficulty in separating the relative contributions of density dependence and

environmental stochasticity to fluctuations in population growth (Turchin 1995; Solberg et al. 1999; Aanes

ef a/. 2000).

2.2.3 The Influence of Environmental Stochasticity on Populations: When it is likely to be more important than Density Dependence on population dynamics?

As described above, there has been considerable debate within population biology as to the relative

effects of density dependent or density independent factors on populations (Seather 1997; Solberg et a/.

1999; Gaillard etal. 2000). In Szthets review (1997), he concluded that as a result of the strong influence

of environmental stochasticity, a stable equilibrium between a large herbivore and its food plants is

unlikely to be maintained in some environments. In other environments a stable equilibrium is maintained

only by frequent and perhaps constant adjustment of population density by predators.

Ungulates living in Arctic ecosystems are exposed to an extreme and temporally unpredictable climate

(Caughley and Gunn 1993; Aanes et a/. 2000), which affects their mortality rates (see Szther 1997;

Gaillard et a/. 1998). Severe winters result in extensive mortality of in Svalbard reindeer (Rangifer tarandus

platyrhynchus) and muskoxen (Ovibos moschatus) apparently because animals are deprived of access to

food as a result of terrestrial icecrusts formations or snow accumulation (Aanes et a/. 2000).

For other terrestrial herbivores, stochasticity is also likely to be the more important influence on population

growth in extreme environments such as arid regions of mainland Australia where unpredictable pulses of

rainfall, separated by extended periods of drought determine pasture growth and biomass (Bayliss 1985a).

The biomass of high quality forage determines the population dynamics of red kangaroos (Macropus

rufus) (Bayliss 1985b; Caughley and Gunn 1993).

Many investigations of density dependence assume that populations are at equilibrium and that the

environment is constant. There is now increasing consensus that, although density dependent responses


are a biological reality (at least occasionally), they may be of only minor importance in explaining particular

observed population sizes (Turchin 1995; Szther 1997; Solberg ef a/, 1999; Aanes ef a/. 2000).

Furthermore, because all environments (including marine environments) are variable, it is likely that no

natural population is ever truly at equilibrium (Begon et al.1996). Thus, populations have evolved to

respond to stress caused by environmental perturbations such as events caused by the El Nino Southern

Oscillation (ENSO), fresh water intrusions from flooding or fluctuations in prey availability. As discussed in

greater detail later, the seagrass systems upon which dugongs depend are particularly prone to such

stochastic events (Preen ef a/. 1995; Poiner and Peterken 1996).

Bowen and Siniff (1999) have noted that accumulating information about how populations, resources and

environments interact and change over time suggests that for some marine mammals, it is unlikely that

their populations reach specific sizes at equilibrium. The concept of a stable equilibrium is now questioned

given the stochastic effects of both global and local environmental change. This conclusion supports

McLeod's (1 997) suggestion, that while the concept of carrying capacity may be valid for deterministic low

variance environments, it is not useful for stochastic environments that are characterised by a high degree

of unpredictable environmental variation such as those exhibited by many plant-herbivore systems. The

importance of accounting for environmental stochasiticity in the population dynamics of large mammals, is

reflected in the increasing number of empirical studies that quantify the variances of life history parameters

as well as their mean values (Gaillard et a/. 2000; Grant and Benton 2000).

Thus the influence of density dependence that depends on intraspecific competition to regulate population

size at carrying capacity is likely to be limited in environments with a high degree of stochasticity.

2.2.4 Importance of Nutrition to Reproductive Success

For most animals, feeding and searching for food are their most time-consuming activities (Robbins 1983).

The behavioural and energetic components of the foraging strategies of many animals are determined by

a series of highly predictable interactions between an individual animal's requirements and its perception

of the distribution of resources for meeting those requirements. An efficient animal will minimise time and

energy expenditure for food gathering while maximising digestible energy intake (Bergman et a/. 2001).

There is increasing evidence that many herbivores may adopt foraging strategies that minimise foraging

and maximise the long-term rate of energy intake (Belovsky 1984: Bergman ef al. 2001).

As discussed in more detail below, in large herbivores, particularly those living in stochastic environments,

storage of fat reserves is closely linked to their reproductive success (Owen-Smith 1990; Caughley and

Gunn 1993; Szether 1997; Mduma et a/. 1999). Hence, fats or lipids, which are the primary mode of storing

energy in vertebrates, play an important role in reproduction. Deposited fat reserves have been quantified


as an index of physiological condition in animals in many species of reptiles (Congdon ef a/. 1982), birds

and several mammals (Hanks 1981). Measures of fatness are also used as indicators of condition in

marine mammals, both on an individual and population level (Fowlerand Siniff 1992; Beck ef a/. 1993;

Ward-Geiger 1997; Pitcher et a/. 2000).

The body condition of a herbivore reflects its nutritional state and thus indicates past environmental

circumstances, i.e., the current and recent balance between food requirements and food availability

(Caughley and Sinclair 1994; Moss and Croft 1999). It follows that body condition, reproductive capacity

and reproductive success and the survivorship of some large mammalian herbivores, are closely coupled

to serial changes in environmental conditions (Caughley and Sinclair 1994; Moss and Croft 1999).

However, in some species there may be a lag period that corresponds to the time that it takes an

individual to harvest new plant growth and convert this to body growth and fat deposition (Shepherd 1987).

The central role of nutrition, particularly the relationship between nutritional status and reproductive

capacity of wildlife populations, has been well documented for many species (Hanks 1981; Harder and

Kirkpatrick 1994). As for other slow growing K-selected species such as large terrestrial herbivores (Van

Soest 1994; see Szther 1997), the nutritional composition (i.e., quality) and quantity of food plants may

also critically influence survivorship, fecundity and distribution of sirenians (Boyd ef a/. 1999). Although

the importance of fat reserves to individual survivorship and reproduction is well recognised in mammals,

there have been few studies in sirenians (but see Ward-Geiger 1997). Evaluation of body condition in

sirenians may provide valuable insights into the energetic requirements of individual animals for

reproduction and the habitat requirements of populations, as well as the carrying capacity of specific

habitats (Ward-Geiger 1997). In this study, I quantify the fat reserves of dugongs for the first time and link

them to reproductive status (see Sections 7.3.12 and 8.3.6).

2.2.5 Effect of Nutrition on Population Dynamics

As food availability affects the balance of energy available for reproduction versus growth and

maintenance (i.e., survival), it is an important regulating or limiting factor for populations of large

mammalian herbivores (Owen-Smith 1990; Caughley and Gunn 1993; Szther 1997; Mduma ef a/. 1999).

As discussed above, reproduction in most organisms is closely tied to the availability of resources, which

is determined in part by environmental conditions.

~utritibn and thus body condition affect all the life history parameters in the fundamental equation of

population dynamics (see Section 2.2.1), including the onset of sexual maturity (Flowerdew 1987; Szther

1997). There are many advantages to early reproduction and selection can operate to push the age at first

reproduction to its physiological minimum in growing populations (Steams 1976) or those not limited by

resources such as food. However, many animals postpone reproduction because of the associated costs


that either lower survival andlor decrease future fecundity (Gadgil and Bossert 1970; Steams 1976; Bell

1980). It is assumed that resources directed to reproduction reduce those available for growth and

survival. This trade-off imposes a greater strain on young or inexperienced individuals because a higher

level of reproductive effort may be needed for a given level of reproductive success (Reiter and Le Bouef

1991). Given the putative costs of early reproduction, increases in fecundity or in the quality of offspring

may occur with age, making delayed maturation advantageous (Bell 1980).

In food-limited populations of ungulates, fecundity is strongly influenced by the effects of climate, which

are probably reflected in turn in the quality or quantity of their food (Glutton-Brock et al. 1997; Szther

1997). Gaillard etal. (2000) found that in large herbivores, the fecundity of young females is more

sensitive to adverse environmental conditions than the fecundity of prime-age females in both temperate

and tropical ungulates. Climatic conditions during spring or winter affected the age at maturity in red deer

(Cewus elephas) (Langvatn et a/. 1996), mountain goats (Ovis aries) (Jorgenson et al. 1993) and moose

(Szther et a/. 1996).

The highest mortality in red deer in both Europe and North America occurs amongst calves during their

first winter because of food limitations (see Szether 1997). In a tropical species, the wildebeest

(Connochaetes taurinus), the main cause of mortality is undemutrition in both calves and adults (Mduma

et al. 1999). Differential mortality rates as a result of drought- induced starvation have also been

documented in the red kangaroo (Macropus mfus) (Moss and Croft 1999). Nutritional stress was found to

affect male more than female kangaroos probably because of the costs associated with competition for

mates, dispersal and increased food demands when food availability was scarce (Moss and Croft 1999).

In prolonged drought conditions, suckling red kangaroos suffer particularly high mortality rates presumably

because of the lactational costs of maintaining a young exceeds the mother's daily intake (Moss and Croft

1999).

In long-lived herbivores where fecundity and mortality vary with age, temporal and spatial variation in food

availability can have a strong influence on population fluctuations (Szther 1997; Gaillard et al. 2000) but

estimation of these parameters is very difficult in many species (see Section 2.2.1). Nonetheless,

variations in food resources have been shown to have a profound effect on population dynamics of

herbivores through both reproductive rates and the fitness of individual animals (Langvatn ef al. 1996;

Gaillard etal. 2000), making understanding the mechanisms that influence nutrition in a population crucial.

2.3 SEASONALITY IN BREEDING

For most animals, the timing of reproduction is crucial to the survival of both parents and offspring. The

timing of births in mammals is governed by the need to maximise offspring survival. This is achieved by


reducing the risk of predation and by maximising food availability to support the high metabolic demands

of lactation and growth of offspring (Oftedal 1985; Flowerdew 1987). Thus, in many species of mammals,

it is likely that parturition and lactation are coincident with the most favourable time from a nutritional

perspective for the lactating female and weaned young. The breeding season of many large terrestrial

herbivores is thus synchronised with seasonal patterns in vegetation quality andlor availability. This

pattern may vary both spatially and temporally (see Gaillard et a/. 2000; Rubin et a/. 2000).

For example, seasonal forage availability is an important factor in regulating breeding season in the

northern range of big-horn sheep (Ovis Canadensis) where most births coincide with a short predictable

period of high plant productivity (Festa-Bianchet 1988; Rubin et a/. 2000). In their northern range, the

harsh climatic conditions and limited forage availability appear to select against calves that are born late in

summer (Rubin ef a/. 2000). In contrast, bighorn sheep in desert environments have been reported to

have longer breeding seasons (Hass 1997) or to give birth throughout the year (see Rubin et a/. 2000).

The period of lambing or lactation in these desert populations generally coincides with periods or high

plant productivity or diet quality (Hass 1997) suggesting that forage availability or quality are very

important factors selecting for seasonal breeding in the desert environment (Rubin et a/. 2000).

This pattern is similar in donkeys (Equus asinus) in northern Australia where conception and birth is

seasonal, peaking prior to the annual monsoonal wet season. Synchronous foaling and lactation in

November-December are coincident with the predictable annual flush at the beginning of the wet season

(Choquenot 1991). This is contrasted to populations of donkeys in western North America where animals

are able to feed on forage year round and breeding also occurs throughout the year (see Choquenot

1991).

In all sirenians, calving peaks appear to be coincident with the time of maximum plant productivity

presumably to enable each mother to meet the high energetic demands of her offspring during late

pregnancy and early lactation (Best 1982). Both Florida manatees and dugongs (at least in some parts of

Australia) generally mate in spring and summer with calving occurring throughout the year but with peaks

recorded in spring and early summer (Marmontel 1995; Rathbun ef a/. 1995; Reid et a/. 1995; Boyd et a/.

1999).

The apparently diffuse seasonal birth peaks reported for the Amazonian manatee (Trichechus inunguis)

(Best 1982), West Indian manatee (T. manatus manatus) (Odell ef a/. 1995) and Florida manatee (1

manatus latirosfris) (Marrnontel 1995; Rathbun eta/. 1995) and the dugongs (Marsh 1995a; Boyd ef a/.

1999) have been related to environmental factors in various studies. Best (1982) suggested that

Amazonian manatees take advantage of flooding in their riverine habitats to disperse into inundated areas

to gain access to a greater abundance and diversity of nutritious plant foods. Marmontel (1995)


considered that Florida manatees similarly breed and give birth in spring and summer because adult

females and young have access to more nutritious forage. Marmontel(1995) further suggested that the

milder temperatures in spring and summer may reduce the energetic stress on lactating females and

newborn calves of the Florida manatee, a hypothesis supported by Rathbun ef a/. (1995). Evidence of

diffusely seasonal reproduction in captive Florida manatees in the Florida oceanaria where circulating

water tracks the ambient air temperature but food supply is constant adds further weight to this suggestion

(see Rathbun et a/. 1995).

Researchers studying the interannual variation in the proportion of both males and females breeding and

the diffusely seasonal breeding pattern of dugongs also implicate food availability as a major factor

influencing the timing of reproduction (Marsh 1995a; Boyd et al. 1999). Marsh et a/. (1984~) suggest that

for dugongs, the timing of calving coincides with the start of the period of maximum plant productivity. This

would serve the high energetic demands of lactation, which is estimated to be up to 18 months. As calves

also begin feeding on seagrass soon after birth (Marsh et a/. 1982), they can also take advantage of

abundant high quality forage. Marsh (efal. 1984c, 1995a) further suggest that the observed pattern of

diffuse breeding in northern Australia results from dugongs timing their reproduction to take advantage of

year to year differences in the timing of the seagrass growth spurt. In contrast to the observation in captive

manatees, a captive nine-year-old female dugong (Wakai et a/. in review) cycles throughout the year. Her

food supply is also constant but she is kept in a heated pool.

2.4 DUGONGS AND SEAGRASSES

2.4.1 Dugongs as Seagrass Specialists

With the exception of the extinct Stellets sea cow (Hydrodamalis gigas), which fed primarily on kelp

(Domning 1978), modern sirenians feed almost exclusively on angiosperms (Heinsohn ef a/. 1977;

Hartman 1979; Best 1981; Marsh ef a/. 1982). Marine algae are also eaten but in significant quantities only

when angiosperms are scarce (Spain and Heinsohn 1973; Lewis ef a/. 1980). Florida manatees are

opportunistic, generalist herbivores feeding on a variety of freshwater and marine angiosperms (Best

1981; Reynolds and Odell 1991; Lefebvre ef a/. 2000,2001). Florida manatees are broadly distributed in

estuarine, riverine and coastal waters, reflecting these generalist food preferences (Reynolds and Odell

1991; Rathbun ef a/. 1995; Lefebvre eta/. 2000, 2001). The capacity of manatees to eat a wide range of

food plants probably allows them easy access to a variety of species that may be of higher quality and

more abundant than the food species of the dugong which is a seagrass specialist (Rathbun et al. 1995;

Lefebvre efal. 2000,2001).


The highly specialised food requirement of dugongs is reflected in their functional anatomy, which is

adapted to feed on the leaves and rhizomes of small seagrasses (Domning 2001). The strong rostra1

deflection (70") and wide muzzle of dugongs is adapted for bottom feeding and allows them to uproot

whole plants when they are accessible and to feed only on the leaves when the whole plant is not

accessible (Anderson 1982; Marsh et al. 1982, 1999; Domning 2001).

Dugongs commonly favour 'pioneer' species of seagrasses, especially those of the genera Halophila and

Halodule (Preen and Marsh 1995). Like many herbivores, dugongs (Preen 1995a) and probably manatees

(e.g., Rathbun et al. 1995) appear to optimise their diet by selecting food species that maximise digestible

nutrients (Aragones 1996). Although restricted to what is generally considered relatively low quality forage,

dugongs optimise their diet by selecting seagrasses with low fibre content, high digestibility (Halophila)

and high nutrients (Halodule) (Lanyon 1991; Aragones 1996; Massini et a/. 2001).

However, under circumstances of nutritional stress, dugongs may expand their diet beyond seagrass.

Dugongs from southern Western Australia and Queensland have been reported to deliberately eat macro-

invertebrates to supplement nutrients, particularly nitrogen, apparently in response to seasonal reductions

in seagrass availability (Anderson 1989; Preen 1995b). Large amounts of algae have been reported in the

stomachs of dugongs observed to be in poor condition (Spain and Heinsohn 1973; Nietschmann and

Nietschmann 1981; Preen and Marsh 1995).

Dugongs have a huge grazing impact on seagrass habitats (Aragones 1996). It has been estimated that

an adult dugong consumes approximately 40 kg wet weight of seagrasses per day (Preen 1992; Aragones

1994). Based on an estimated population of 85 000 dugongs in Australia (Marsh et al. 1999), dugongs

consume about 2 400 tonnes (wet weight) of seagrass per day, about 876 000 tonnes per year in

Australian waters (Aragones 1996). As seagrass specialists (Lanyon 1991; Aragones 1996; Preen 1995a,

1995b) reliant on vast quantities of seagrass, the population parameters of dugongs would be expected to

be responsive to environmental variability affecting their food supply. Thus any attempts at interpreting

variations in dugong population size must also consider spatial and temporal variability in their seagrass

food.

2.4.2 IndoPacific Tropical Seagrass Communities with Particular Reference to Torres Strait

Seagrasses occur in nearshore, shallow, sheltered tropical and subtropical marine waters throughout the

world. The IndoPacific region encompassing Indonesia, Papua New Guinea and northern Australia

(including Torres Strait) is believed to be the centre of diversity for seagrasses (Mukai 1993). A large

proportion of species occur in tropical Australia (Mukai 1993; Short et a/. 2001), which is probably the

result in part, of the overiap of tropical and temperate seagrass floras and the high degree of endernism

present in particular bioregions (see Carruthers ef a/, in press).


Torres Strait supports the largest known area of seagrass (17 500 km2) in Australia (Poiner and Peterken

1996) (Figure 2.1). The region is characterised by several seagrass communities occupying a variety of

habitats that are a feature of tropical seagrass systems: river estuaries (on the south-west coast of PNG),

coastal waters, deep water (>30 m) and reef (Carruthers ef a/. in press). Complex interactions between

abiotic and biotic factors including exposure and hydrology, impacts from seasonal and stochastic events

as well as macroherbivores such as dugongs and green turtles, influence all seagrass habitats in Torres

Strait to varying degrees. These interactions maintain the environmental heterogeniety, which

characterises tropical seagrass systems of north-eastern Australia, particularly those of Torres Strait (Long

and Poiner 1997; Carruthers ef a/, in press).

THIS IMAGE HAS BEEN REMOVED DUE TO COPYRIGHT RESTRICTIONS

Figure 2.1. Areas of inter-reefal seagrass habitats (stippled) in Torres Strait based on surveys undertaken by CSlRO in Western, Central and Eastern Torres Strait (adapted from NSR 1997). Note areas to the west of 142" have not been comparatively sampled.

The reef-flat, subtidal depth-zoned coastal and open ocean seagrass communities in Torres Strait are very

diverse consisting of complex assemblages that are mostly dominated by Thalassia hemprichi, Enhalus

acoroides and Halophila ovalis and commonly include species of Halophila and Halodule favoured by

jc151654Text Box

THIS IMAGE HAS BEEN REMOVED DUE TO COPYRIGHT RESTRICTIONS


dugongs (Poiner and Peterken 1996; Long and Poiner 1997). Seagrasses in the genera Thalassia and

Enhalus are not normally eaten by dugongs but are reportedly abundant in the stomach of dugongs in

poor condition in Torres Strait (Nietschmann 1984) and other parts of Queensland (Preen and Marsh

1995).

The extensive inter-reefal seagrass communities in Torres Strait are dominated by Halophila spinulosa

and by H, ovalis in moderately deep waters (= 12.1 m). The deep-water (more than 30 m) open ocean

seagrass communities in Torres Strait are also characterised by Halophila spinulosa and H. ovalis Long

and Poiner 1997). These communities occur in areas of coarse sediments and clear water and are

restricted to central, western and south-western Torres Strait (Poiner and Peterken 1996) (Figure 2.1).

In Torres Strait, biological and environmental processes that operate at small spatial scales (in the order of

metres) maintain the characteristically complex community structure of seagrasses in the region (Long

and Poiner 1997). Seagrass communities dominated by species such as Halophila and Halodule are more

variable (in terms of distribution and biomass) at smaller spatial scales in Torres Strait compared with the

Gulf of Carpentaria where species of these genera occur in monospecifc communities (Long and Poiner

1997). The biomass and percentage cover of Halophila and Halodule species in Torres Strait are generally

low and patchy even though they have the broadest range in region (Long and Poiner 1997).

2.4.3 Tropical Seagrasses as Resources for a Specialist Herbivore

The preferred food of dugongs, species of the genera Halophila and Halodule (Marsh ef al. 1 982; Lanyon

ef a/. 1989; Lanyon 1991; Preen 1995a; Argones 1996), are small fast growing seagrasses characterised

by high reproductive potential for rapid recolonisation and high productivity (Walker et al. 1999). These

seagrass communities are thus well adapted to environments of high disturbance and high grazing

pressure. They are also subjected to dieback events, which are unpredictable in space and time (Walker

et a/. 1999). Members of the genus Halophila occur in deeper water than other species of tropical

seagrasses and appear to be very sensitive to light reductions (Longstaff ef a/. 1999). Reduction in light

conditions has been implicated in large-scale loss of deep-water seagrasses in Torres Strait (Poiner and

Peterken 1996) and Hervey Bay (25O0O1S, 152O43'E) (Preen ef a/. 1995).

The seagrass species preferred by dugongs are generally intertidal and subtidal growing to depths of up to

8-10 m. The biomass of intertidal and subtidal seagrass in northern Australia is generally low (Wake

1975) and feeding by dugongs is curtailed by low tides in some areas. In Torres Strait, with a 3.5 m tidal

range and 4 m depth on the reef flats, feeding by dugongs on reef flats can be severely curtailed by tides.

Studies of dive depths of foraging dugongs reflect the generally shallow distribution of seagrasses in the

study areas. Lanyon and Sneath (2001) reported that the majority of dives of six dugongs in Moreton Bay


(270301S, 153O30'E) were short shallow foraging dives. This is supported by the data from Chilvers et al.

(unpublished data, 2002) who studied the diving behaviour of 15 dugongs at three locations in Western

Australia and Queensland (but not Torres Strait) representing over 39 500 dives. Overall, these dugongs

spent 72% of their time at depths of 8 m) and

deepwater zones suggesting that seagrass communities in these areas are also likely to be very important

foraging habitats fordugongs (Marsh and Saalfeld 1989, 1991; Lee Long ef a/. 1996). Dugongs have been

reported diving to maximum depths ranging between 20.5 m (Chilvers et a/. unpublished data, 2002) and

39.8 m (Lanyon and Sneath 2001). Although the ecological role of deep-water seagrass communities is

poorly understood, the importance of the deep-water seagrass communities to dugongs is suggested by

reports of significant numbers of dugongs sighted more than 10 km from land in Torres Strait (Marsh and

Saalfeld 1989,1991). Dugongs have been reported -58 km from the Queensland coast in waters of

depths up to 37 m (Marsh and Saalfeld 1989) and dugongs feeding trails have been sighted in depths of

up to 33 m off north-eastem Queensland (Lee Long et a1 1996).

Very little is known about how the preferred food of the dugong (Halophila and Halodule species) varies in

quality and quantity over the spatial scales relevant to individual animals (Ivan Lawler, pers, comm. 2001).

However, the complex patterns of large-scale spatial and temporal variability in food supply described in

more detail below presumably mean that dugongs must be able to monitor the quality and availability of

different patches of food. Aragones (1996) and Mellors (unpublished data) have found the seagrass

nutrients varied spatially on both local and regional scales.

The complexity of the community structure and composition of tropical seagrass such as those in Torres

Strait suggests that dugongs are sometimes unable to maximise their intake by restricting their diet to

seagrasses with the highest nutrient content (Aragones 1996). The highly specialised dietary requirements

of dugongs means that only certain seagrass meadows may be suitable as dugong habitat (Preen ef al.

1995a). Dugongs have to sample at both plant and patch level (sensu lllius ef a/. 1992; O'Rea

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