Adaptive Changes in Harvested Populations: Plasticity and Evolution of Maturation

1. 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. 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. 2003 ). • 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. 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. 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. 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. 2004. Proc Roy Soc B

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

7. Larval stage Bruno Ernande, NMA Course, Bergen Stock life cycle Environment E1 E2 E3 Age Ernande et al. 2004. Proc Roy Soc B

8. Bruno Ernande, NMA Course, Bergen Stock life cycle Environment E1 E2 E3 Larval stage Metamorphosis Immature stage Age Ernande et al. 2004. Proc Roy Soc B

9. Bruno Ernande, NMA Course, Bergen Stock life cycle Environment E1 E2 E3 Larval stage Metamorphosis Immature stage Age Maturation Mature stage Ernande et al. 2004. Proc Roy Soc B

10. Randomdistribution Habitat selection Bruno Ernande, NMA Course, Bergen Stock life cycle Environment E1 E2 E3 Larval stage Metamorphosis Immature stage Age Maturation Mature stage Reproduction Ernande et al. 2004. Proc Roy Soc B

11. Bruno Ernande, NMA Course, Bergen Stock life cycle Environment E1 E2 E3 Randomdistribution Larval stage Metamorphosis Habitat selection Immature stage Age Variation in growth and mortality rates Maturation Mature stage Reproduction Ernande et al. 2004. Proc Roy Soc B

12. Trade-off between reproduction and somatic growth rate Environmental variability in growth trajectories Bruno Ernande, NMA Course, Bergen Maturation process • Maturation process: maturation occurs when the growth trajectory intersects with the maturation reaction norm maturation reaction norm Δ adults growth trajectory juveniles migration to a new environment metamorphosis larvae Ernande et al. 2004. Proc Roy Soc B

13. 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 Size Stock Biomass Ernande et al. 2004. Proc Roy Soc B

14. 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. 2004. Proc Roy Soc B

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

17. 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. 2004. Proc Roy Soc B

18. 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 mean adult size population density Proportion of original value Fishing mortality Evolutionary time population biomass mortality

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

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

22. 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.