Changement de rgime dans le rseau alimentaire du bank Georges et rtablissement des stocks de poisson

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Changement de régime dans le réseau alimentaire
du bank Georges et rétablissement des stocks de
poissons surexploités
Jeremy Collie Graduate School of
Oceanography University of Rhode
Island jcollie_at_gso.uri.edu
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Sommaire

• Changements à long terme dans les communautes de
  poissions, causes par lexploitation et le
  climat
• La variation climatique éffectue des changements
  dans le réseau alimentaire
• Example ecosystème du banc Georges
• Modèle du réseau alimentaire linéaire
• Modèle biomass-dynamique plurispécifique
• Modèle structuré daprès lâge plurispécifique
• Modèle multi-équilibre
• La dynamique mono-spécifique de léglefin.
• Conclusions génerales

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Gradient of Model Complexity
Gradient of Model Complexity
Single-species models useful for stock
assessment and management Can add explicit
predators, habitat or climate considerations.
Age structured multispecies assessments Multispec
ies production models.
Whole ecosystem models forget population
dynamics.
Food-web or energy-budget models May be
mass-balance or dynamic.
Modified from Link (2002) Fisheries 2718-21
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Example Georges Bank Ecosystem
Image from Peter Wiebe (2002) Oceanography 1514.
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Integrated production (gC m-2 yr-1) Zooplankton 2
7.0 Benthos 10.5
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Data from Stockhausen, NMFS
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Balancing the upper and lower food webs
Fish
Feeders
Four Stanzas 1963-2002
Plankton
Benthos
Feeders
Feeders
Carnivorous
Carnivorous
plankton
benthos
Benthos
Plankton
GLOBEC 1995-1999
Lower Web
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Fish consumption accounts for all the production
from the lower food web
Steele, Collie, Gifford, et al. (In press)
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Conclusions from the food webs

• The lower food web accounts for recycling,
  spatial and seasonal differences, and advective
  losses
• Commercially-important species have been replaced
  with non-commercial species
• Plankton consumption has increased relative to
  benthos
• There is some evidence of bottom-up control of
  fish production in the 1960s
• Outputs from the lower food web limit fish
  production
• Therefore, recovery of the commercial fish
  species requires reductions in non-commercial
  species.

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Is there enough food for a recovered cod stock?
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Food-web modifications

• Scenarios
• Make cod the dominant species in the piscivore
  guild
• Make piscivores the dominant guild
• Eliminate carnivorous zooplankton (jellyfish)
• Eliminate carnivorous benthos
• Increase suspension-feeding benthos
• Reduce microzooplankton
• Reduce flux of phytoplankton to the benthos.

J.H. Steele (Personal Communication)
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Multispecies production models of the Georges
Bank fish community
Model Formulation (gadoids as example)
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Simplified Georges Bank Food Web
gadoids
Type III interactions

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Type I interactions
flatfish
pelagics
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-
-

elasmobranchs
Collie and DeLong (1999)
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Single and multispecies model fits
Gadoids
observed single-species muti-species yield
Flatfish
1000 metric tons
Pelagics
Elasmobranchs
Year
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Gadoids
observed model fit yield positive effect of
pelagics competition with elasmobranchs
1000 metric tons
Year
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Gadoid equilibrium yield depends on the harvest
rates of the other groups
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Conclusions from multispecies production models

• Multispecies production models can be fit
  statistically to time series of abundance data
• In this example the multispecies models fit
  significantly better than single-species models
• Gadoids and elasmobranchs compete with each other
  and prey on pelagics and flatfish
• Trade-offs exist between predator and prey yields
  (May et al. 1979)
• Biological reference points depend on the
  abundances of other taxa (Collie Gislasson
  2001).

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Multispecies Virtual Population Analysis of the
Georges Bank fish community
Tsou Collie (2001a)
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from Per Sparre (1991)
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Predation mortality rates are high
Tsou Collie (2001b)
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Biomass of prey consumed by year and predator
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Total prey biomass over time
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Species composition of the diet of six predators

 
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Predation affects year-class size for some species
Silver hake
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Regimes shifts can be illustrated with a
catastrophe manifold
Points on the manifold represent equilibrium
values of the fast variable (Abundance) for
possible combinations of the slow variables,
Internal Structure and External Forcing
Collie, Steele, and Richardson (2004)
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Simple models can have multiple equilibria
P population abundance c consumption rate of
predator H r rate of increase n exponent of
functional response K carrying capacity D
half-saturation constant
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Criteria for distinguishing regime shifts
A. Shift in time series
B. Bimodal distribution
C. Dual relationship
D. Trajectory depends on initial state
F. Hysteresis
E. Triggered shift
Scheffer and Carpenter (2003)
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Example Georges Bank haddock
Haddock was the most abundant demersal fish
species from 1930-1965
H. L. Todd, Illustrator
Overfished in the 1960s, the stock is now
recovering dramatically.
Photo by Howard Schuck (1953) NEFSC Photo archive
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Georges Bank Haddock
2004
Fishing mortality
Winter NAO
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Implications for rebuilding depleted stocks
Yields are restricted by the climatic regime, or
the stock can rebuild to prior high abundance.
?
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Why do some fish stocks recover
while others dont?
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Control Rule for Georges Bank Haddock
Bmsy
Fmsy
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General Conclusions

• Bottom-up forcing of marine food webs alters
  community structure and fish productivity
• Strong trophic interactions imply trade-offs in
  fisheries yields
• Fishing can induce regime shifts and delay the
  recovery from climate-driven shifts
• Single-species are assessed and managed within
  this ecological milieu
• Fish stocks can recover from overexploitation, if
  management sets the stage.

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Acknowledgements
Funding Agencies National Science
Foundation Rhode Island Sea Grant NOAA Coastal
Ocean Program

• Collaborators
• John Steele
• Michael Fogarty
• Jason Link
• Allison DeLong
• Tien-shui Tsou
• Dian Gifford
• Russell Brown