adaptive changes in harvested populations: plasticity and evolution of maturation

Bruno Ernande, NMA Course, Bergen

Adaptive Changes in Harvested Populations:Plasticity and Evolution of Maturation

Bruno Ernande

Fisheries Department

IFREMER

Port-en-Bessin, France


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.


Environment

Phen

otyp

e

Plasticchange

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.


Environment

Phen

otyp

e

Evolutionarychange

∎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.

Phenotypic plasticity or evolution


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


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)}

E

gi

1 2 3 4 5

zi1

zi2

zi3

zi5

zi4

zE1

E2E3

Phen

otyp

e y

Phenotype x

gi

grow

th 1

grow

th 2

growth

3

agesi

ze

e.g., maturationreaction norm


Age

E1 E2 E3

Environment

Larval stage

Stock life cycle



Larval stage

Immature stage

Metamorphosis

Age

E1 E2 E3

Environment

Stock life cycle



Larval stage

Immature stage

Metamorphosis

Mature stage

Maturation

Age

E1 E2 E3

Environment

Stock life cycle



Larval stage

Reproduction

Immature stage

Metamorphosis

Mature stage

Maturation

Age

E1 E2 E3

Environment

Random distribution

Habitat selection

Stock life cycle



Random distribution

Larval stage

Reproduction

Immature stage

Metamorphosis

Mature stage

Maturation

Age

E1 E2 E3

Environment

Variation in growth and mortality rates

Habitat selection

Stock life cycle



Δ

migration to a new environment

growth trajectory

Trade-off between reproduction and

somatic growth rate

metamorphosisEnvironmental variability

in growth trajectories

maturation reaction norm

juveniles

larvae

adults

Maturation process

∎Maturation process: maturation occurs when the growth trajectory intersects with the maturation reaction norm



0

1

Stock Biomass

Fish

ing

Mor

talit

y

positivedensity-dependence

negativedensity-dependence

density-independence

Quotas

Stock Size

Harvesting and management rules

∎ Mortality rates increase because of harvesting. Three management rules:Fixed Quotas: positive density-dependence Constant Harvesting Rate: density-independenceConstant Stock Size or Constant Escapement: negative density-dependence



)( PLS e,e,an m

0

0

)(

)()( )(

mam

mmmammS

S,δεSf

S,SfS,δεSflimaDf

m

mm

'm

'mSm

'mmSSmm daaga,anaS

dt

dmm )()( )( 2

1

PLS

PLSPLS,SPLS,SS,S

dededan

e,e,ane,e,ade,e,abf

'm

'm

m'mm

'mm

'm

)(

)()( )(

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



Evolution under state-dependent harvesting

Quota Constant Rate Constant Stock Size

Immature

Mature

Q

CRCSS

age (a)

harvesting mortality H0

size

(a)

H0(Mature)


Evolution under size-dependent harvesting


age (a)

size

(a)

Unfished sizesUnfished sizes

Unfished sizes Unfished sizes Unfished sizes

Unfished sizes Unfished sizes Unfished sizes

Unfished sizes

H0


Sens

itivi

ty

natural morality growth rate trade-off strength

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.



Consequences for demographic characteristics

∎Evolutionary induced decrease in population biomass due to a decrease in adult mean size and population density.


mean adult size

population biomass

population density

mortality

Evolutionary time

Pro

port

ion

of

orig

inal

val

ue

Fishing m

ortality


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.


Trade-off growth-reproduction

expressedearlier R

ela

tiv

e b

iom

ass

evolutionarytime, t

FixedQuotas

Negativedensity-dependence

evolutionarytime, t

Lo

cal

ha

rves

tin

g m

ort

alit

y

Evolutionary feedback



Trade-off growth-reproduction

expressedearlier R

ela

tiv

e b

iom

ass

Evolutionarysuicide

Rel

ati

ve

den

sit

y

evolutionarytime, t

ecological time

ecological time

FixedQuotas

Negativedensity-dependence

evolutionarytime, t

Lo

cal

ha

rves

tin

g m

ort

alit

y



∎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.

Conclusions

adaptive changes in harvested populations: plasticity and evolution of maturation

Documents