Evolutionary effects of fishing and implications for sustainable management: a case study of North Seas plaice and sole

F.M. Mollet

Research output: Thesisexternal PhD, WU

Abstract

Exploited resources might genetically evolve as a consequence of ex¬ploitation by adapting their life history to the imposed mortality re¬gime. Although evolution favors traits for survival and reproduction of the fittest, human-induced evolution might have negative consequences for the exploiter. In general, a shift towards lower growth rate, earlier maturation and increased reproductive investment might be expected from increased (unselective) mortality and these changes might lead to generally smaller exploited individuals. Hence, the evolution might ne¬gatively affect the productivity of the resource and thus the sustainable exploitation and furthermore, genetic changes might be slow to reverse. If selection forces are high, evolution might occur fast and be observa¬ble within a few decades. Fisheries provide a large scale experiment for fisheries-induced evolution (FIE) since fishing mortality rates, typically being size-selective, exceed natural mortality rates by a multiple and data samples are available for decadal time scales. This thesis aims to assess the potential importance of FIE for sustainable exploitation by empirical evidence as well as evolutionary modeling, illustrated for the North Sea flatfish plaice and sole.
In empirical studies the problem of inferring on genetic changes from phenotypic observations lies in the disentangling of the phenotypic plasticity caused by environmental variations from the potential gene¬tic change. This is at least partly achieved by constructing norms of reaction that account for this environmental variation. The probabi¬listic maturation reaction norm for instance disentangles phenotypic plasticity in maturation caused by variation in growth. Because growth, maturation and reproductive investment are correlated due to tradeoffs on the individual level, a method was developed that fits an energy al¬location model to individual growth trajectories, obtained by the back-calculation of otoliths. This method provides size-specific estimates of the mechanistic individual life history tradeoffs and of the selection differentials imposed by the fishery. Because the correlation of esti¬mated life-history traits is captured, temporal changes could (for the first time) be analyzed conditionally on the correlation and on potenti¬al environmental effectors, thus disentangling not only environmental variability but also effects from changes in another trait. The results suggest that maturation shifted to occur earlier, surplus energy and reproductive investment increased partly due to environmental factors, but that all changes also bear a genetic component, indicative for FIE.
Species-specific individual-based eco-genetic models were developed to explore the evolutionary causes of reverse sexual size dimorphism in the case of flatfish. The hypothesis that males are smaller than fe¬males because of an energy loss through behavioural reproductive in¬vestments has to be rejected in this evolutionary perspective, since a higher demand on reproductive investment is compensated by increased energy acquisition. In contrast, the results show that males are smaller because increasing reproductive investment pays off less in males than in females. The finding can likely be generalized to many cases where mating opportunities are limited in space and time. Since eco-genetic models include the inheritance of traits with frequency-dependent se¬lection, they are therefore a powerful tool to study FIE and the model is therefore fitted to the estimated evolution of plaice and the evolu¬tionary impact of different management scenarios is assessed. The so called maximum sustainable yield MSY and the corresponding maximal fishing mortality FMSY evolve along with the population life history and occur both at lower levels after a while. The currently estimated refe¬rence points are thus not sustainable but slipping targets. By a dome-shaped exploitation pattern being protective for larger fish the evolu¬tionary trends could be reversed and with it the negative evolutionary impact. However, the evolutionary impact trades off against the short term loss in yield: by protecting the large fish the evolutionary impact is minimized but the instantaneous yield is decreased too – the optimal strategy for a given time horizon is somewhere in between. In summary, the thesis provides evidence that FIE should be taken into account for sustainable management.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Rijnsdorp, Adriaan, Promotor
Award date7 May 2010
Place of Publication[S.l.
Print ISBNs9789085856139
Publication statusPublished - 7 May 2010

Keywords

  • plaice
  • dover soles
  • fisheries
  • fishing
  • evolution
  • maturation
  • growth
  • reproduction
  • life history
  • traits
  • adaptation
  • fishery management
  • sustainability
  • marine fisheries
  • experimental evolution

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