1. Port-en-Bessin, France
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Adaptive Changes in Harvested Populations: Plasticity and Evolution of Maturation
Bruno Ernande
Fisheries Department
IFREMER
Port-en-Bessin, France

2. The potential for fisheries-induced adaptive changes
Ernande et al.
Bruno Ernande, NMA Course, Bergen
The potential for fisheries-induced adaptive changes
The commercial exploitation of fish stocks may not only have demographic consequences on the target species, but may also induce adaptive changes in their life history because fishing is by essence selective (Stokes et al. 1993, Palumbi 2001, Ashley et al ).
Adaptive changes can have two different origins (Rijnsdorp 1993, Law 2000):
Phenotypic plasticity: most species can modify their phenotype in the short term in response to environmental variation;
Evolution: the prerequisites for contemporary fisheries-induced evolution are met:
Fisheries selective pressure is strong: fishing mortality on average 2 to 3 times higher than natural mortality (Law 2000)
most life history traits have sufficient heritability to evolve and micro-evolutionary changes have been proven to occur within a few generations in controlled and field experiments (Reznick et al. 1990; Conover & Munch 2002)
Phenotypic plasticity and evolution have very different implications for management purposes: plasticity can be reversed within a generation whereas to mitigate adverse evolutionary changes requires many such generations.

3. Phenotypic plasticity or evolution
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Phenotypic plasticity or evolution
With empirical data, one has to disentangle plastic and evolutionary response. Evolutionary changes in life history traits can be assessed by modifications in their reaction norms.
Plastic
change
Phenotype
Environment

4. Phenotypic plasticity or evolution
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Phenotypic plasticity or evolution
With empirical data, one has to disentangle plastic and evolutionary response. Evolutionary changes in life history traits can be assessed by modifications in their reaction norms.
Evolutionary
change
Phenotype
Environment

5. Ernande et al.
Bruno Ernande, NMA Course, Bergen
Objectives
Modifications of reaction norms have been recently shown for age and size at maturation in commercially exploited fish stocks, e.g., North East Artic cod (Heino et al. 2002), North Sea plaice (Grift et al. 2003), Georges Bank cod (Barot et al. 2003), and Nothern cod (Olsen et al. 2003).
We propose a theoretical approach for modelling the evolution of maturation reaction norms in exploited populations in order to tackle three specific points:
Can harvesting be really responsible for evolutionary changes in maturation reaction norms?
Can we evaluate the evolutionary impact of different harvesting practices and the potentiality of different management policies?
What are the consequences of evolutionary changes on population abundance and sustainability?
Ernande et al Proc Roy Soc B

6. Bivariate reaction norm
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Bivariate reaction norm
The historical view:
univariate reaction norms
{zi1, zi2, zi3, zi4, zi5}
Another view:
bivariate reaction norms
{yi(xi1), yi(xi2), yi(xi3)} z E gi
growth 1
growth 2
growth 3
age
size
e.g., maturation
reaction norm E1 E2 E3
Phenotype y
Phenotype x gi
zi5
zi4
zi3
zi2
zi1 1 2 3 4 5

7. Stock life cycle Environment Larval stage Age E1 E2 E3 Ernande et al.
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment E1 E2 E3
Larval stage
Age
Ernande et al Proc Roy Soc B

8. Stock life cycle Environment Larval stage Metamorphosis Age
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment E1 E2 E3
Larval stage
Metamorphosis
Age
Immature stage
Ernande et al Proc Roy Soc B

9. Stock life cycle Environment Larval stage Metamorphosis Age
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment E1 E2 E3
Larval stage
Metamorphosis
Age
Immature stage
Maturation
Mature stage
Ernande et al Proc Roy Soc B

10. Stock life cycle Environment Random distribution Larval stage
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment E1 E2 E3
Random distribution
Habitat selection
Larval stage
Metamorphosis
Age
Immature stage
Maturation
Mature stage
Reproduction
Ernande et al Proc Roy Soc B

11. Variation in growth and mortality rates
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment E1 E2 E3
Random distribution
Larval stage
Metamorphosis
Habitat selection
Age
Immature stage
Variation in growth and mortality rates
Maturation
Mature stage
Reproduction
Ernande et al Proc Roy Soc B

12. Trade-off between reproduction and somatic growth rate
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Maturation process
Maturation process: maturation occurs when the growth trajectory intersects with the maturation reaction norm
Trade-off between reproduction and somatic growth rate
maturation reaction norm Δ
adults
Environmental variability
in growth trajectories
growth trajectory
juveniles
metamorphosis
migration to a new environment
larvae
Ernande et al Proc Roy Soc B

13. Harvesting and management rules
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Harvesting and management rules
Mortality rates increase because of harvesting. Three management rules:
Fixed Quotas: positive density-dependence
Constant Harvesting Rate: density-independence
Constant Stock Size or Constant Escapement: negative density-dependence
Quotas
positive
density-dependence
Fishing Mortality
density-independence
negative
density-dependence
Stock Biomass
Stock Size
Ernande et al Proc Roy Soc B

14. Evolutionary dynamics
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Evolutionary dynamics
Structured population dynamics with age and environmental trajectory as individual state variables. Size is fully determined by age and environmental trajectory.
Invasion fitness of a mutant: long term growth rate of a mutant Sm’ in a resident population with reaction norm Sm
Selection gradient: functional derivate of invasion fitness
Evolutionary dynamics: Canonical equation for infinite dimensional traits
Ernande et al Proc Roy Soc B

15. Evolution under state-dependent harvesting
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Evolution under state-dependent harvesting
Quota
Constant Rate
Constant Stock Size
size (a)
H0(Mature)
age (a)
Immature Q CR
CSS
Mature
harvesting mortality H0

16. Evolution under size-dependent harvesting
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Evolution under size-dependent harvesting
Quota
Constant Rate
Constant Stock Size H0
Unfished sizes
Unfished sizes
Unfished sizes
size (a)
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
age (a)

17. Control of the sensitivity of the evolutionary response
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Control of the sensitivity of the evolutionary response
The sensitivity of the evolutionary response of maturation reaction norms to harvesting is controlled by three life history parameters: it increases as
the average natural mortality rate decreases,
the average growth rate increases,
the strength of the trade-off between growth and reproduction weakens.
Sensitivity
natural morality
growth rate
trade-off strength
Ernande et al Proc Roy Soc B

18. Consequences for demographic characteristics
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Consequences for demographic characteristics
Evolutionary induced decrease in population biomass due to a decrease in adult mean size and population density.
Quota
Constant Rate
Constant Stock Size
population density
mean adult size
Proportion of original value
Fishing mortality
Evolutionary time
population biomass
mortality

19. Consequences for population sustainability
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Consequences for population sustainability
The previous insights are qualitatively the same for the three management policies.
The main difference between the three management policies lies in the consequences of evolutionary changes of the maturation reaction norm on population abundance.

20. Consequences for population sustainability
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Consequences for population sustainability
Trade-off growth-reproduction
expressed
earlier
Relative biomass
evolutionary
time, t
Fixed
Quotas
Negative
density-dependence
evolutionary
time, t
Local harvesting mortality
Evolutionary feedback

21. Consequences for population sustainability
Ernande et al.
Bruno Ernande, NMA Course, Bergen
Consequences for population sustainability
Trade-off growth-reproduction
expressed
earlier
Relative biomass
evolutionary
time, t
ecological
time
Fixed
Quotas
Negative
density-dependence
evolutionary
time, t
Local harvesting mortality
Evolutionary
suicide
Relative density

22. Ernande et al.
Bruno Ernande, NMA Course, Bergen
Conclusions
Fishing can induce evolutionary modifications in the position and the shape of the maturation reaction norm.
The direction of these changes actually depends on the life history stage which is harvested when harvesting depends on maturity status
According to the sensitivity analysis, these changes could be minimized by fishing mainly adults and by focusing on species characterized by high natural mortality, low growth rate, and a strong trade-off between growth and reproduction.
The prevalent system of management currently, quotas, seems to be the worse management practice in terms of fisheries-induced evolution
The consequences of these evolutionary changes on stock abundance and sustainability may be dramatic as suggested by the example of extinction through evolutionary suicide. Simple population dynamics models would overlook this possibility, which highlights the necessity to take evolutionary trends into account in responsible management practices.