Transcript Fisheries-induced evolution of maturation reaction norms

Bruno Ernande, NMA Course, Bergen
Adaptive Changes in Harvested Populations:
Plasticity and Evolution of Maturation
Bruno Ernande
Fisheries Department
IFREMER
Port-en-Bessin, France
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 microevolutionary 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.
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.
Phenotype
Plastic
change
Environment
Bruno Ernande, NMA Course, Bergen
Phenotypic plasticity or evolution
Phenotype
∎ 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
Environment
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
Bruno Ernande, NMA Course, Bergen
Bivariate reaction norm
∎ The historical view:
∎ univariate reaction norms
∎ Another view:
∎ bivariate reaction norms
gi
zi5
zi4
zi3
zi2
zi1
1
2
3
4
5 E
∎ {zi1, zi2, zi3, zi4, zi5}
size
Phenotype y
z
E1
E2
E3
gi
e.g., maturation
reaction norm
Phenotype x
age
∎ {yi(xi1), yi(xi2), yi(xi3)}
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment
E1
E2
E3
Larval stage
Ernande et al. 2004. Proc Roy Soc B
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment
E1
E2
E3
Larval stage
Metamorphosis
Immature stage
Ernande et al. 2004. Proc Roy Soc B
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment
E1
E2
E3
Larval stage
Metamorphosis
Immature stage
Maturation
Mature stage
Ernande et al. 2004. Proc Roy Soc B
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment
E1
E2
E3
Random distribution
Larval stage
Metamorphosis
Habitat selection
Immature stage
Maturation
Mature stage
Reproduction
Ernande et al. 2004. Proc Roy Soc B
Bruno Ernande, NMA Course, Bergen
Stock life cycle
Environment
E1
E2
E3
Random distribution
Larval stage
Metamorphosis
Habitat selection
Immature stage
Maturation
Variation in growth and
mortality rates
Mature stage
Reproduction
Ernande et al. 2004. Proc Roy Soc B
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
juveniles
larvae
Environmental variability
metamorphosis
in growth migration
trajectoriesto a new
environment
Ernande et al. 2004. Proc Roy Soc B
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
Fishing Mortality
1
Quotas
positive
density-dependence
density-independence
negative
density-dependence
0
Stock
Size
Stock Biomass
Ernande et al. 2004. Proc Roy Soc B
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.
nSm (a ,eL ,eP )
∎ Invasion fitness of a mutant: long term growth rate of a mutant Sm’ in a resident
population with reaction norm Sm
nS' (a ,eL ,eP )
f S' ,S   (bS' ,S (a ,eL ,eP )  d S' ,S (a ,eL ,eP )) m
da deL deP
m m
m m
m m
nS'
m
∎ Selection gradient: functional derivate of invasion fitness
Df Sm (am )  lim
 0
f ( S m  ε  δam , S m )  f ( S m , S m )



f ( S m  ε  δam , Sm )

 0
∎ Evolutionary dynamics: Canonical equation for infinite dimensional traits
d
S m (am )  12  Sm nSm   (am , a'm ) g Sm (a'm ) da'm
dt
Ernande et al. 2004. Proc Roy Soc B
Bruno Ernande, NMA Course, Bergen
Evolution under state-dependent harvesting
Constant Rate
Constant Stock Size
size (a)
Quota
age (a)
Q
Immature
CR
CSS
Mature
harvesting mortality H0
Bruno Ernande, NMA Course, Bergen
Evolution under size-dependent harvesting
Quota
Constant Rate
Constant Stock Size
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
Unfished sizes
size (a)
H0
age (a)
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
Sensitivity
the average natural mortality rate decreases,
the average growth rate increases,
the strength of the trade-off between growth and reproduction weakens.
natural morality
growth rate
trade-off strength
Ernande et al. 2004. Proc Roy Soc B
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
Evolutionary time
population biomass
mortality
Fishing mortality
Proportion of
original value
population density
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.
Bruno Ernande, NMA Course, Bergen
evolutionary
time, t
Trade-off growthreproduction
expressed
earlier
Relative biomass
Consequences for population sustainability
Evolutionary
feedback
Local harvesting mortality
Fixed
Quotas
Negative
density-dependence
evolutionary
time, t
Bruno Ernande, NMA Course, Bergen
Relative density
expressed
earlier
ecological
time
Evolutionary
suicide
ecological
time
Fixed
Quotas
Local harvesting mortality
evolutionary
time, t
Trade-off growthreproduction
Relative biomass
Consequences for population sustainability
Negative
density-dependence
evolutionary
time, t
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.