Acoustic Telemetry of Fish
Introduction
Changes in trophic levels of fish landed by global fisheries have been evident over the last
50 years. Pauly et al., (1998) coined the phrase ‘Fishing down the food web’ to describe this
on-going trend. He explains how landings from global fisheries have shifted from mainly
large piscivorous fish to smaller invertebrates and planktivorous fish. A well-known example
of this is the decline of bluefin tuna, Thunnus thynnus in the Western Atlantic (Myers &
Worm, 2003). This species has been described by Safina, (1993) as being one of the most
vulnerable and over-exploited of creatures. When further investigated the decline of this
species can be explained by looking at how landings of this species has increased over the
years, as shown in catch records (Myers & Worm, 2003). As more fisheries look toward
lower trophic level species, the total catch from global fisheries keeps rising (Pauly et al.,
2000). With the continually added pressure on fish populations from humans, Ecosystem-
Based Fisheries Management (EBFM) have made ocean zoning, a scheme for dividing oceans
into sectors with a defined set of rules for managing each sector (Courtney & Wiggin, 2003),
a critical element of their protocol to establish what kind of human activity is sustainable
within each sector, considering both the geographical area the sector falls within and the
region of the world it occupies i.e. small area in the tropics or large area in Antarctica
(Pikitch et al., 2004). This leads us to ask the following questions; Why does all this matter?
Why is it important to study fish and recognise these trends? The main answer to these
questions is global fisheries management. It is important to control catch quotas and fishing
areas or else with continued over fishing, more and more large predatory fish and other fish
of high trophic levels could become depleted (Myers & Worm, 2003).
By studying life history traits of fish and the range of behaviours they exhibit, global fisheries
can be better managed. Understanding the movements of fish, both broad scale and fine
scale movements and being able to predict where certain species will be at different times
of the year and what size they are at that time can help set catch quotas and ensure fishing
is carried out in the most efficient and sustainable manner. This will also help to identify
important hotspots and migration routes. Sanchirico et al., (2006) explains how the
identification of these hotspots can promote the idea of a no-take zone in which fishing is
prohibited. This will allow the increase of conserving biodiversity and protecting sources of
larvae and biomass.
Understanding how the impacts of man have affected fish communities is also useful to
judge how conserved a certain species is. Historically, an example of how man has impacted
fish communities has been shown by the collapse of the Grand Banks in Canada. In a study
by Myers et al., (1997) they describe how six fisheries of Atlantic Cod, Gadus morhua all
collapsed within a time space of 4-6 years of each other leading to a ban on fishing between
1992-1993, but the population levels had become so low they were unable to recover.
Hutchings & Myers, (1995) proposed the reason for this was the advancement in fishing
technologies through the years. Fishing vessels are now equipped with bigger and stronger
nets, accompanied with sonar technology for finding where fish are (Misund et al., 1997;
Horne, 2000).
Historically, the movements of fish were recorded using simple methods such as mark and
recapture. This was a useful technique for answering simple questions such as does fish
population 1 move between site A and site B. A disadvantage of this method is that it can be
quite time consuming and it can be hard to recapture individuals which may limit the
accuracy of data interpretation (Hobson, 1999; Cunjack et al., 2005). Simple methods such
as mark and recapture can prove to be more useful that modern methods such as acoustic
telemetry depending on the study. Of all methods used to study fish movements and
behaviour, Time Depth Recorders (TDR’s) are the most widely used, not only on fish but
other shallow diving animals (Hays et al., 2007). Advances in technology have allowed for
different types of studies to be carried out than previously thought of, using other
techniques. Radio transmitters, TDR’s and archival loggers are examples of a more
technological approach to studying animal behaviour. These devices work on the basis of
sending data to a receiver or storing data on a type of medium (Cooke et al., 2004). In more
recent years acoustic telemetry has been used to describe fine scale movements of fish by
recording the acoustic signal sent from a fish fitted with a transmitter using a number of
different ‘active’ and ‘passive’ methods (Heupel & Hueter, 2001). Using this approach we
can build up an idea of residency patterns (Chapman et al., 2005), foraging ecology (Milne et
al., 2005) and resting areas (Johnson et al., 2009). Acoustic telemetry focuses more on the
movements and behaviour of individuals rather than populations, but when used with other
equipment such as sonar whole populations can be tracked. Also acoustic tracking can be
used hand in hand with mark and recapture methods. Individuals that are marked can also
be fitted with an acoustic transmitter and followed so that if they are not recaptured, it will
be evident where they did go. These are examples by which the two methods complement
each other, as shown in a study by Egil and Babcock, (2004) when they described how using
large-scale mark and recapture along with acoustic telemetry can be useful in estimating
fisheries yield enrichment within a marine reserve.
The use of acoustic telemetry can also be used to give an insight into questions that still
pose a subject of debate such as how different factors affect fish at both species and
population level. Changes in migration routes and times have been recorded over the last
decade and have shown a change in both aquatic and terrestrial animals and the reason can
often be attributed to climate change (Sims et al., 2001; Newson et al., 2008). These
changes can often be linked with other factors which occur as an indirect effect of climate
change such as the alteration of prey distributions, as seen changes in migration of bluefin
tuna shown by Walther et al., (2002).
This review aims to discuss how acoustic telemetry has been used to study fish movements
and from this, better understand certain behaviours such fine scale movements (foraging
ecology, residency patterns, species interaction) and broad scale movements (migration,
reproduction, geographical dispersion). In addition to this, understanding how
environmental influences can have an effect on the distribution can help improve
population assessments (Brill & Lutcavage, 2001).
Fine Scale Movements
These types of movement are described within a fine geographical resolution. Individuals
fitted with acoustic transmitters can provide insights into how life history traits differ
between species. Collecting data for fine scale movements can be achieved in a variety of
ways. Methods of ‘active’ and ‘passive’ tracking of marine fish have been described by
Johnston et al., (2009). Active tracking involves following a tagged fish from a distance in a
boat using receiving equipment tuned into the correct frequency, such as a directional
hydrophone and recording fixed GPS position as demonstrated in a study by Morrissey &
Gruber, (1993). Passive tracking involves an array of receivers dispersed within a geographic
range which records every time a tagged fish passes within its range as shown by Johnston
et al., (2009). To determine if a fish is in transient, foraging or resting we can consider
changes from the rate of travel it exhibits and attribute differences in behaviour. Block et
al., (1992) showed how using acoustic telemetry, depth and swimming speed of blue marlin,
Makaira nigricans could be recorded. They concluded that the individuals that were tagged
were not part of a resident group and were actually passing along a migratory route through
the islands of Hawaii. This study is an example of how acoustics can be used to study
residency patterns, species interactions and migration routes.
Movements within the home range of a fish can attribute to its fine scale movements. Home
range has been described as the relative frequency distribution of an animal over defined
geographic range in a given period of time (Seaman & Powell, 1996). By determining home
range we can obtain a more accurate definition of fine scale movements as it can be defined
differently between species. Heupel & Hueter, (2001) demonstrated how acoustic telemetry
can be used to record these behaviours during fine scale movements which supports
research carried out by Bolden, (2001) showing that acoustic telemetry can be used to
determine the home range of certain species of fish. Fine scale movements can be
attributed to foraging for food or reproductive activities, and not confusing this with a
passing species on a long migratory route. These movements can give an idea of areas of
high prey density, as shown by Josse et al., (1998) where by acoustic surveys in French
Polynesia showed the movements of two species of tuna and their prey.
Movements connected with reproduction may be observed at both fine and broad scales.
On a smaller scale, Flavelle, (2001) was able to identify fish that exhibited spawning habitat
selection through acoustic telemetry studies. Knowing this information can be crucial to the
survival of many species and play a major role in global fisheries management and
effectiveness of marine reserves (Rowley, 1994).
Broad Scale Movements
The use of acoustic telemetry has been quite limited when studying broad scale movements
as technological advances have moved more towards satellite tags, restricting acoustics to
short term tracking (Bruce et al., 2006). It can however help understand broad scale
movements taken by fish. By using acoustic telemetry, early ocean migration and where the
beginning of a migration route occurs can be recorded before moving to satellite tracking,
as shown in a study by Lacroix et al., (2004). Such data can be of great importance for
fisheries management of migratory species by defining migration patterns. As technology
has developed we can now track acoustically tagged fish for a period of up to one year
through the use of self-contained submersible receivers. This development shows how
autonomous acoustic tracking systems can give insights into the early and vulnerable life
history stages of juvenile fish (Welch et al., 2002).
Moving beyond home range, listening stations can be set in place over a broad spectrum
that a fish utilises. These types of studies can be useful when looking at reproductive areas,
which is closely related to early migration of juveniles as discussed before. Robichaud &
Rose, (2002) showed how acoustic telemetry can be used to depict behavioural differences
between males and females during spawning period. Differences in behaviour both pre- and
post-spawning have been identified in different species of fish. Acoustic studies have shown
how the degree of site fidelity changes between species. Humston et al., (2005) was able to
contradict previous studies of bonefish Albula vulpe and conclude that these fish did not
undertake extensive movements between different habitats but instead utilised shallow
habitats in the northern Florida Keys.
Species Interaction
As discussed in the previous section, acoustic telemetry can explore a number of different
types of behaviours, many of which conventional methods such as mark and recapture
cannot do. It is clear from previous studies that through tracking we can identify foraging
grounds (Dagorn et al., 1999), breeding and spawning grounds (Robichaud & Rose, 2002),
migration routes (Lacroix et al., 2004) and residency patterns (Collins et al., 2007). Bégout &
Lagardére, (1995) go on to show how through the use of acoustic telemetry, species interaction and
behavioural alteration can be observed. They showed that individuals that remained as part of a
school were generally more active and displayed a diurnal pattern, whereas individuals that
remained solitary were not as active and displayed nocturnal activity.
Conservation
Through studying behaviour of fish and using acoustic studies to see where they go and why, we can
help better conserve species. We can also help identify trends as to why some species are more in
decline than others and put in place legislation and quotas to help resolve this problem. Species
identification by acoustic telemetry is a long term goal aimed by fisheries and biologists. The
purpose of identifying fish by acoustic telemetry and not visually is to be able to increase the
amount of information collected through an increase in frequency bandwidth or number of acoustic
beams. In order to do this reliably, a set of statistical metrics that can discriminate between a wide
range of similar body shapes and sizes. When identifying species a number of factors need to be
considered. Position of fish in the water column and type of habitat it normally utilises can have an
effect on the accuracy of identification (Horne, 2000). Being able to identify fish species without
catching them or causing stress provides a much more efficient method of carrying out certain types
of research that require knowledge of where a fish is at a certain time as it will not alter their
behaviour.
To implement areas of conservation and no-take zones, various studies have been carried out to see
where certain species of fish go around their home range or place of capture. Chapman et al., (2005)
showed how two species of shark were caught and tagged with an acoustic transmitter, and when
released stayed within the proposed conservation zone. This study provided substantial evidence of
an area in which biodiversity can flourish if human activity is restricted. Klimley et al., (1988) further
showed how with the aid of acoustic telemetry, scalloped hammerhead sharks, Sphyrna lewini were
able to be tracked revealing that they aggregated close to a seamount during the day, then at night
moving separately into the surrounding pelagic areas. At dawn they all returned and remained at
the same seamount until the following night. This temporal pattern was related to the light-dark
cycle where individuals remained during the day and departed at night. The fact that these sharks
returned repeatedly to the tagging site provides evidence for any future proposals that this area be
considered as a no-take zone. In order to implement such areas, biologists need to have knowledge
of fish’s spatial ecology (Cooke et al., 2005). To aid the conservation of a species or number of
species, continuous knowledge of their ranges within their life span is essential and can be achieved
through continual tracking, as described by O’Dor et al., (1998).
Ocean Tracking Network
Ocean Tracking Network (OTN) comprises of hundreds of researchers from OTN’s 14 ocean regions,
together studying how marine life and climate are changing worldwide. Their range of study spreads
over all five of the world’s oceans. There were many reasons for setting up such a network, some of
which included the rapid decline in large pelagic fish such as bluefin tuna and blue marlin (Myers and
Worm, 2003; Polachec, 2005). With the use of acoustic telemetry combined with archival tags, OTN
has set up one of the world’s largest tracking networks, one which can sample 2000 times more
frequently and with ten times more accuracy and consistency, for example the Halifax Line. A recent
study undertaken in February of this year by the Commonwealth Scientific and Industrial
Research Organisation (CSIRO) tagged 230 juvenile southern bluefin tuna with acoustic
transmitters on the south coast of Australia. Using three OTN cross-shelf monitoring lines,
each fitted with 20 receivers, the seasonal movements of these juvenile fish were able to be
recorded over three years, indicating any change in response to climate change. The way in
which data are transmitted through OTN networks are unique. Acoustic receivers are place
within each other’s range. Once a tagged fish passes within a receivers range, the signal
picked up is able to be passed along other receivers until it reaches a listening station on
shore. This type of data transmission is referred to as a ‘Daisy-chain’ data flow. Underwater
fibre-optic cables can also relay data from a receiver to a listening station. Along with
transmitting a signal received from a passing fish, environmental sensors along the line relay
the physical state of the ocean at the time. From the results of past and present research
from OTN’s global network, it has been possible to help address climate change, ocean
modelling, and marine resource management.
Conclusion
To conclude, it has been shown that acoustic telemetry has evolved from simple methods of
sampling and tracking such as mark and recapture, leading to advances in technology such as
VEMCO V16 acoustic tags (http://www.vemco.com/pdf/v16cont.pdf) which can be used as simple
tag to show the location of an animal or they can be depth and temperature sensors. This has
opened a window to different types of questions to be answered. Although acoustic data have
proven to be extremely useful to fisheries management and migration studies as shown in previous
sections of this review, it still has some limitations. Active tracking requires a lot of manual labour
and can be very time consuming, when compared to other tracking methods such as satellite
tracking and passive tracking in which an array of listening stations are set up, methods of which are
demonstrated by OTN such as receivers attached to buoys and on the underside of ships. Acoustic
telemetry has opened doors into aspects of animal’s lives and evolutionary advances in this field are
becoming increasingly evident. One of the main objectives faced by producers of acoustic telemetry
products to produce a tag with the ability to identify species and record environmental data. Just as
acoustic tags are limited in this way, satellite tags are limited as the animals they are attached to
need to break the surface in order to transmit a signal. Future research and technology may lead to a
more complex range of tags, able to identify all these factors and identify more in depth information
regarding life history traits and behaviours related to species and by doing so, better manage global
fisheries and aid in conservation strategies.
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