Lecture 1. Overview: Population Ecology & Sustainable Fisheries Resources

OCEAN 5105

Population Ecology and Sustainable Fisheries Resources

Hui-Yu Wang1

Outline

• Population ecology

• Fisheries resources

• Sustainable management

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Inter-dependence

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Population Ecology

Dynamics of fisheries resources

Management & conservation

Aims

• Gain knowledge on population ecology and fish biology

• Practice basic programming, data analysis and modeling

• Understand the concepts of population dynamics and management of fisheries resources

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How will we proceed this course?

• A lecture and a exercise session each week

• Exercises include: field sampling, computer and lab exercises, and group discussion

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Syllabus & instructors

• Course website:https://ceiba.ntu.edu.tw/1061Ocean5105_fish/

• Instructors: Hui-Yu Wang (海洋所 415) and Yi-Jay Chang (海洋所 411)

• Office hours: Wed 1:30-2:30 pm and by appointments

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Tools for our course

• Theory and concepts: from lectures and readings

• Data: from laboratory and field experiments

• Mathematical models:– What is a mathematical model?

– What does it mean by testing a model?

– How to do modeling?

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Structure of our course

• Pop ecology involves interactions between organisms and habitats, at the levels of individuals, populations, communities, and ecosystems

• We will specify the essential processes of pop ecology at each of these levels

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Processes at the individual level

• Growth (development)– by time: larva juvenile adult

– by mass: bioenergetics

• Maturation: immature vs. mature

• Reproduction: quantity (fecundity) and quality (offspring size)

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Processes at the population level

• Population density

• Population growth

• Population demography

• Lifespan

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Processes at the community level

• E.g., trophic cascade

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Cury et al. 2001

Outline

• Population ecology

• Fisheries resources

• Sustainable management

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Scopes of fisheries resources and ecology

Ecology/biology

Habitat (physical and chemical)

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Related fields

• Ichthyology (identification, evolution)• Fish ecology (interactions with

environments, including other fish)• Population dynamics (trends of

populations in space/time)• Aquaculture (raising fish)• Oceanography, limnology, fluvial systems

(fish habitats)• Economics, sociology, etc.

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Factors influencing fisheries production

• Primary productivity

• Human impact

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Primary productivity

• Primary production is plants’ photosynthetic fixation of carbon

• In shallow coastal areas plants include algae and flowering plants; in open ocean primary production is mainly due to phytoplankton

• The limiting factors for aquatic primary productivity include light, temperature, nutrients (e.g., upwelling), and dissolved gases (CO2 and O2)

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• The sea covers 71% of the earth’s surface, but most of it is deep and dark

• Light intensity falls rapidly with depth

Primary productivity

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Primary productivity

• Compensation depth: depth at which respiration rate equals photosynthetic rate In shallow water, light

is high, photosynthesis exceeds respiration

In deeper water, less light penetration, respiration is in excess of photosynthesis

Compensation

depth (Pc=Rc)

Critical depth

(Pw=Rw)

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Primary productivity

• Temperature also decreases with depth

• Thermocline shifts seasonally (depending on wind intensity, too)

• Stratification prevents sinking of plankton below compensation depth, but also prevents nutrient transfer from deep waters

• Leading to seasonal and spatial variation in production

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Latitudinal variation in production

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Upwelling leads to global patterns in productivity

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Food web• The energy produced by plants is

transported to higher trophic levels; depending on the processes of obtaining energy, organisms can be categorized into:

Autotrophs: obtain energy from inorganic material (e.g., plants)

Heterotrophs: obtain energy through consuming organic material

Predators

Prey 22

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Marine food web

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Carnivorous fish

Forage fish

Zooplankton

Phytoplankton

Energy is

transfered

between trophic

levels

Food web

• Each step in the web is called a trophic level

• Some energy (carbon) is lost at each step

• The proportion of consumed prey carbon converted into predator carbon is called gross growth efficiency (GGE)

• GGE x predation efficiency is transfer efficiency (which is < GGE)

• Biomass decreases up the food chain

• Fish production depends on levels of primary production, length of the food chain, and transfer efficiency

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Primary production required (PPR)

• Pauly and Christensen (1995) estimate the primary production required (PPR) to sustain fisheries in different environments

• Primary production required (PPR) to sustain the world fish catch by assuming that the average fish feeds two trophic levels above the primary production

• In the late 90’s PPR estimated as = 8 % of global aquatic primary production (compared to 2.2% in the 80’s)

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Differential Primary production required (PPR) for different ecosystem types

D. Pauly & V. Christensen, Nature 374: 255-257 (1995)

Ecosystem typePP

(gC m-2yr-1)Catch

(g m-2yr-1)Discards

(g m-2yr-1)TL PPR (%)

Open ocean 103 0.01 0.002 4 1.8Upwellings 973 22.2 3.36 2.8 25.1Tropical shelves 310 2.2 0.671 3.3 24.2Non-tropical shelves

310 1.6 0.706 3.5 35.3

Coastal/reef systems

890 8 2.51 2.5 8.3

Rivers and lakes 290 4.3 n.a. 3 23.6Total 126 0.26 0.07 2.8 8.0

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Human impact on fishery production

• Habitat alteration• Fisheries exploitation• Invasive species• Fish farming• Climate change • …

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Changes in mixed-layer temperature in 2050 (with present day as reference)

Barange et al. 2014

3C!

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Increased occurrence of hypoxia

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Poleward range shifts for fishes in Australia

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Sunday et al. 2015

Model predicts increased fisheries catch at higher latitudes, but the opposite at lower latitudes

32Barange et al. 2014

Projected changes in fisheries catch in 2055

Cheung et al. 201033

Overall, global fisheries production is projected to increase 3.4%

Summary

• Two important drivers for resource levels of exploited fishes: primary production and human impact

• But, how exactly these drivers affect fish?

– We will explore these effects step-by-step

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Outline

• Population ecology

• Fisheries resources

• Sustainable management

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Why manage fisheries?

• Shared resources

• UN-FAO: “75% of world fisheries are either over-harvested, fully harvested or depleted”

• Important to economy and human health

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Why is fisheries management challenging?

• High degree of natural variation

• High level of uncertainty

• Broad space and time scales

• Interface of natural systems and human social systems

• Conflicting uses 37

Overfishing

• Common sense: the opposite of sustainable fishing

• More formal sense: a state when empirical fishing intensity > the model-derived level of sustainable fishing, or, when fisheries yield > the model-derived amount of sustainable yield

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Maximum sustainable yield (MSY)

• MSY is the highest catch that allows fish population to maintain fast population growth

39Tsikliras and Froese 2018

Additional reference points for fishing regulation

• 𝐹𝑀𝑆𝑌, 𝐹𝑚𝑎𝑥, and MEY are indices derived from models with different assumptions and structures

40Tsikliras and Froese 2018

Conflicts on fisheries resources between countries

41https://www.newsmarket.com.tw/blog/12360/

Conflicts between fishing and conservation

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• High risk under bycatch mortality: Yellow-eyed

penguin, Humboldt penguin, and Magellanic penguin

• A map of penguin bycatch data (Crowford et al. 2017)

Summary

• Because of difficulty in estimating suitable levels of sustainable fishing and conflicting uses, it is difficult to manage fisheries resources

• However, one can tackle the uncertainty with knowledge on pop ecology and data analysis

• Thus, we are hopeful to enhance sustainable fisheries management

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