The Functioning of Marine Ecosystems

A fisheries perspective

Philippe Cury

Lynne Shannon

Yunne-Jai Shin

Definition of Marine EcosystemsDefinition of Marine Ecosystems

• The term “Ecosystem” is recent The term “Ecosystem” is recent (Tansley 1935)(Tansley 1935)

• An ecosystem is defined as “a spatially An ecosystem is defined as “a spatially explicit unit of earth that includes all of explicit unit of earth that includes all of the organisms, along with all the the organisms, along with all the components of the abiotic environment components of the abiotic environment within its boundaries” (Likens 1992)within its boundaries” (Likens 1992)

• A marine ecosystem contains water, A marine ecosystem contains water, detritus, hundreds of kinds of detritus, hundreds of kinds of organisms including bacteria, organisms including bacteria, phytoplankton, zooplankton, fishes, phytoplankton, zooplankton, fishes, mammals, birds … and fishers!mammals, birds … and fishers!

Ecosystem as an ‘Uncomfortable’ large scale Unit for Ecological studies but an Integrative level for

Fisheries Management

• Ecosystems have no apparent boundaries and lack the sort of clear objective or purpose that can be ascribed to other, more tractable, biological or ecological entities (e.g. cell, individual or population): there is no clear objective function to maximize

• However the ecosystem is now viewed as an integrative level, and its overall complexity is perceived as critical to its sustainability

Questions Regarding the Exploitation of Marine Ecosystems :

• Which species are most critical, and which ecological processes are most sensitive to exploitation?

• Does the removal of top predators have a strong impact on lower trophic levels?

• Does heavy exploitation of forage species, such as anchovies and sardines, cause changes in the functioning of upwelling ecosystems?

The key to answering these questions and exploring

whether general principles apply lies in understanding

the energy flow in the ecosystem

Which Energy Flows in Marine

Ecosystems? Who is controlling Whom?

Bottom-Up Top- Down Wasp-Waist

Bottom-up Control Bottom-up Control

The Very Small drive the Very Large

Parallel long-term Parallel long-term trends across four trends across four

marine trophic marine trophic levels and levels and

weather in the weather in the North SeaNorth Sea

(from Aebischer et al, Nature 1990)

westerly weather

phytoplankton zooplankton

herring Kittiwake laydate

Kittiwake clutch Kittiwake chicks

Five ecosystems in the Pacific were affected by the mid-1970s climate change event

(from McGowan et al. 1998)

Regime Shifts within the Peruvian Ecosystem(from Pauly and Tsukayama 1987)

Bottom-up Control• Under bottom-up control, the physical environment

drastically affects the overall productivity (i.e. the carrying capacity) of ecosystems, but more importantly the dynamics of fish assemblages in a more or less predictable way…

• Decadal-scale regime shifts suggest the existence of multiple stable states in fish communities, resulting in sustained or un-sustained fisheries and induced ecosystem changes

• A Bottom-up control offers a comprehensive framework for understanding how different components would react to environmental changes or to changes at the bottom of the food chain

Top-Down ControlTop-Down Control

The Very Large drive the Very Small

Bigger Fish Bigger Fish Eat Eat

Smaller FishSmaller Fish

Predators are constrained by the size of their jaw and prey fish that are less than 1/3 their own size

Who is eating whom?Cannibalism, Omnivory

The relative stability of the total fish biomass compared to that of individual species

Size-based predation provides an explanation for observed size

spectra in marine ecosystems

Trophic Cascade in Alaska resulting in inverse patterns in

abundance or biomass across

trophic links in a food web

(from Science, Estes et al. 1998)

Top-Down Control

• A top-down control can help to understand several observed ecological patterns at an ecosystem level when removing top predators

• Not all cascades propagate to lower trophic levels or have significant impacts on ecosystem processes as numerous compensatory mechanisms dampen or eliminate them

• Fishing usually greatly reduces the abundance of top predators, and it stands to reason that the abundance of prey populations and their effects on marine communities will increase after release from predator

control

Wasp-Waist ControlWasp-Waist Control

Small pelagic fishes drive both the very large and the very small

Birds and Sardine in

South Africa and Namibia

(from Crawford 1999) 0

2

4

1983 85 87 89 91 93 950

0,4

0,8

0

0,3

0,6

1956 61 66 71 76 81 86 91 960

500

1000

1500

Pelagic fish abundance & zooplankton

Ghana

Japan

South-Africa

Black-Sea

0

100

200

0 100 200 300

0

20

40

60

0 2000 4000 6000

0

100

200

300

0 200 400 600 800

3

5

7

100 300 500 700

Pelagic fish abundance

Zo

opla

nkt

on

a

bun

dan

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Wasp-Waist Control

• Under wasp-waist control the collapse of a dominant prey species can generate drastic changes at higher, but most surprisingly at lower trophic levels

• As fisheries remove substantial amounts of small pelagic fish one must carefully consider the implications for the other species in the ecosystem

Bottom-Up Top- Down Wasp-Waist

Who is controlling whom in Marine Food-Webs ?Who is controlling whom in Marine Food-Webs ?

No General Theory can be No General Theory can be ascribed to the Functioning of ascribed to the Functioning of

Marine Ecosystems Marine Ecosystems

the ability to predict ecosystem the ability to predict ecosystem behavior is limitedbehavior is limited

However ecosystems are neither totally However ecosystems are neither totally predictable nor totally unpredictable : predictable nor totally unpredictable :

tentative generalisations can be proposed tentative generalisations can be proposed

• Control by the environment (bottom-up control) Control by the environment (bottom-up control) predominatespredominates

• Control by predators (top-down control) plays a Control by predators (top-down control) plays a role in dampening ecosystem-level fluctuationsrole in dampening ecosystem-level fluctuations

• Trophic cascades are seldom found, except in Trophic cascades are seldom found, except in lakes, or in marine hard substrata ecosystems and lakes, or in marine hard substrata ecosystems and mainly for less complex food-websmainly for less complex food-webs

• Wasp-waist control is most probable in upwelling Wasp-waist control is most probable in upwelling systems.systems.

From ‘common sense’ and ‘pet concept’ toward an operational framework for From ‘common sense’ and ‘pet concept’ toward an operational framework for dealing with responsible fisheries in marine ecosystems?dealing with responsible fisheries in marine ecosystems?

• These difficulties do not mean that an ecosystem approach to These difficulties do not mean that an ecosystem approach to fisheries management should be abandoned or that we should just fisheries management should be abandoned or that we should just wait for more additional results on the functioning of ecosystemswait for more additional results on the functioning of ecosystems

• Major steps are urgently needed, that will define an operational Major steps are urgently needed, that will define an operational framework for dealing with responsible fisheries in marine framework for dealing with responsible fisheries in marine ecosystems, for example by using ecosystem-based indicators and ecosystems, for example by using ecosystem-based indicators and reference pointsreference points

• This is a complex issue that needs to integrate our simplistic and This is a complex issue that needs to integrate our simplistic and disparate views of nature and to reach major steps towards disparate views of nature and to reach major steps towards incorporating our recent and incomplete, but consequential, incorporating our recent and incomplete, but consequential, theoretical background on the functioning of marine ecosystemstheoretical background on the functioning of marine ecosystems

‘Not only is the science incomplete, but the [eco]system itself is a moving

target, evolving because of the impact of management and the

progressive expansion of the scale of human influences on the planet’

Holling C.S. (1995)