1. The Functioning of
Marine Ecosystems
A fisheries perspective
Philippe Cury
Lynne Shannon
Yunne-Jai Shin

2. Definition of Marine EcosystemsDefinition of Marine Ecosystems
• The term “Ecosystem” is recentThe term “Ecosystem” is recent
(Tansley 1935)(Tansley 1935)
• An ecosystem is defined as “aAn ecosystem is defined as “a
spatially explicit unit of earth thatspatially explicit unit of earth that
includes all of the organisms, alongincludes all of the organisms, along
with all the components of thewith all the components of the
abiotic environment within itsabiotic environment within its
boundaries” (Likens 1992)boundaries” (Likens 1992)
• A marine ecosystem contains water,A marine ecosystem contains water,
detritus, hundreds of kinds ofdetritus, hundreds of kinds of
organisms including bacteria,organisms including bacteria,
phytoplankton, zooplankton, fishes,phytoplankton, zooplankton, fishes,
mammals, birds … and fishers!mammals, birds … and fishers!

3. 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

4. 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?

5. The key to answering these
questions and exploring
whether general principles
apply lies in understanding
the energy flow in the
ecosystem

6. Which Energy Flows in Marine
Ecosystems? Who is controlling Whom?
Bottom-Up Top- Down Wasp-Waist

7. Bottom-up ControlBottom-up Control

8. The Very Small drive the Very Large

9. Parallel long-termParallel long-term
trends across fourtrends across four
marine trophicmarine trophic
levels andlevels and
weather in theweather in the
North SeaNorth Sea
(from Aebischer et al, Nature 1990)
westerly weather
phytoplankton zooplankton
herring Kittiwake laydate
Kittiwake clutch Kittiwake chicks

10. Five ecosystems in the Pacific
were affected by the mid-1970s climate change event
(from McGowan et al. 1998)

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

12. 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

13. Top-Down ControlTop-Down Control

14. The Very Large drive the Very Small

15. Bigger FishBigger Fish
EatEat
Smaller FishSmaller Fish

16. 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

17. 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)

18. 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

19. Wasp-Waist ControlWasp-Waist Control

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

21. Birds and
Sardine in
South Africa
and Namibia
(from Crawford 1999)
0
2
4
1983 85 87 89 91 93 95
0
0,4
0,8
0
0,3
0,6
1956 61 66 71 76 81 86 91 96
0
500
1000
1500

22. Pelagic fish
abundance &
zooplankton
Ghana
Japan
South-Africa
Black-Sea
0
10
0
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
Zooplankton
abundance

23. 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

24. Bottom-Up Top- Down Wasp-Waist

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

26. No General Theory can beNo General Theory can be
ascribed to the Functioning ofascribed to the Functioning of
Marine EcosystemsMarine Ecosystems
the ability to predict ecosystemthe ability to predict ecosystem
behavior is limitedbehavior is limited

27. However ecosystems are neither totallyHowever ecosystems are neither totally
predictable nor totally unpredictable :predictable nor totally unpredictable :
tentative generalisations can be proposedtentative 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 aControl by predators (top-down control) plays a
role in dampening ecosystem-level fluctuationsrole in dampening ecosystem-level fluctuations
• Trophic cascades are seldom found, except inTrophic cascades are seldom found, except in
lakes, or in marine hard substrata ecosystems andlakes, 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 upwellingWasp-waist control is most probable in upwelling
systems.systems.

28. From ‘common sense’ and ‘pet concept’ toward an operational framework forFrom ‘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 approachThese difficulties do not mean that an ecosystem approach
to fisheries management should be abandoned or that weto fisheries management should be abandoned or that we
should just wait for more additional results on the functioningshould just wait for more additional results on the functioning
of ecosystemsof ecosystems
• Major steps are urgently needed, that will define anMajor steps are urgently needed, that will define an
operational framework for dealing with responsible fisheriesoperational framework for dealing with responsible fisheries
in marine ecosystems, for example by using ecosystem-in marine ecosystems, for example by using ecosystem-
based indicators and reference pointsbased indicators and reference points
• This is a complex issue that needs to integrate our simplisticThis is a complex issue that needs to integrate our simplistic
and disparate views of nature and to reach major stepsand disparate views of nature and to reach major steps
towards incorporating our recent and incomplete, buttowards incorporating our recent and incomplete, but
consequential, theoretical background on the functioning ofconsequential, theoretical background on the functioning of
marine ecosystemsmarine ecosystems

29. ‘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)