The Overlooked Evolutionary Dimension of Modern Fisheries

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The Overlooked EvolutionaryDimension of Modern
Fisheries
Ulf Dieckmann Laxenburg, Austria Mikko
Heino Bergen, Norway Adriaan Rijnsdorp IJmuiden,
The Netherlands David Conover Stony Brook,
USA David Reznick Riverside, USA Richard
Law York, UK
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The Overlooked Evolutionary Dimension

• To be shown in this session
• Modern fishing results in such substantial
  changes of mortality patterns that evolutionary
  responses of stocks are inevitable.
• Such changes are not as slow as is widely
  believed Significant evolution can occur within
  10 or 20 years.
• Evolutionary changes are not necessarily
  beneficial, neither to the stock nor to the
  exploiting agents.
• Once evolutionary changes have occurred, they may
  be very difficult to reverse.
• In short Fishing does not only change the
  numbers, but also the traits of exploited fish.

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Session Overview
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Background
1
5
World Fisheries Have Reached a Ceiling
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75 of Stocks Are Maximally Exploited

75
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Fisheries-induced Evolution
Initial composition
After fishing
After reproduction
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Which Traits Are at Risk?

• Age and size at maturationReproducing late may
  not be a viable option.
• Reproductive effortSaving for future
  reproduction may be futile.
• Growth rateStaying below mesh size pays.
• BehaviorReducing exposure to fishing is
  selected.

Focus ofmy talk
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The Case of Northeast Arctic Cod
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Northeast Arctic Cod Stock Structure
With ca. 2 million tons per year, Northeast
Arctic cod is one of the most important gadoid
stocks worldwide.
Feeding grounds(mature juvenile fish)
Spawning grounds(only mature fish)
Northeast Arctic cod
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Northeast Arctic Cod Fishing History
Northeast Arctic cod
Immature
Mature
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Northeast Arctic Cod Stock Response
Continual decline in key life-history characters
Length at maturation
Age at maturation
Northeast Arctic cod
Year
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Two Hypotheses

• Compensatory Response (Plastic Effect)Decreased
  biomass gt Increased growth gt Earlier maturation
• and/or
• Reaction Norm Evolution (Genetic Effect)Shift
  in maturation reaction norm gt Earlier maturation
  at smaller size

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Maturation Reaction Norms Definition
Size
75 maturing
Envelope
50 maturing
Maturation
Midpoint
25 maturing
Neither the growth trajectories nor the reaction
norm need to be straight.
Growth trajectories
Age
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Maturation Reaction Norms Utility
Baseline
Faster growth (compensatory response)
Length
Reaction norm
Growth trajectories
Age
Earlier maturation
Faster growth earlier maturation
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Northeast Arctic Cod Evolutionary Change
Northeast Arctic cod
100
1923
Length (cm)
1990
Significant shift in maturation reaction norm
50
5
12
Age (years)
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Understandingthe Past, Predictingthe Future
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Two Salient Questions
Given the mounting empirical evidence for
fisheries-induced adaptive change, are the
observed rates of selection compatible with the
predictions of genetic models?
A

• How can we anticipate the future course of
  fisheries-induced adaptive change and evaluate
  the impact of potential management measures?

B
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Model Structure

• Five characteristics of individuals are tracked
  through time

Age
Maturationstatus
Reaction normposition
Size
Reaction normangle
StockDynamics
Evolution
Demography
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Modeling Northeast Arctic Cod

• Demography
• Linear growth before maturation
• Growth increments negatively correlated with
  stock biomass
• Growth increments of mature individuals depend on
  size and gonadosomatic index
• Natural mortality of 0.2
• Density-dependent newborn mortality
• Density-dependent cannibalism on age classes 1
  and 2
• Linear maturation reaction norm of constant width
  
• Fecundity allometrically depends on size

• Fishing Mortality
• Historical regime
• Fimmature 0.05 and Fmature 0.2
• Contemporary regime
• Fimmature 0.4 and Fmature 0.3

Immature
Mature

• Estimated size selectivity of fishing gear taken
  into account

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Modeling Evolutionary Speed 1
Heritability 0.2
Today
Age at maturation (years)
ca. 40 years
Historical regime
Current regime
0
100
Time (years)
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Modeling Evolutionary Speed 2
Heritability 0.2
Today
Age at maturation (years)
ca. 250 years
Historical regime
Current regime
0
100
Time (years)
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Conclusions

• Fisheries-induced evolution has been with us for
  several decades without having been recognized.
• The speed of such evolution is much faster than
  previously believed .
• Fisheries-induced evolution often reduces yield.
• Models suggest that each year during which
  current exploitation continues requires several
  years of evolutionary recovery
• A Darwinian debt to be paid by future
  generations.