Fisheries-Induced Adaptation Under Influence of Climate Change: a Reaction-Norm Perspective for Resident Salmonids

There is an increasing body of documentation demonstrating fisheries-induced adaptational change (FAC) in important fish life-history traits. An often documented adaptation outcome, especially in fisheries targeting the larger individuals, is altered age- and size at maturation pattern so as to maturing earlier and/or at smaller sizes than before harvesting was initiated. Also, reduced individual growth rate is commonly documented. Most FAC examples come from marine ecosystems, but examples from lake systems also exist. Because lake fish have little opportunity to disperse away from unfavorable conditions, whether being fisheries or climate-related conditions, they will have to adapt to them in order to prevail. The water temperature has increased in most Nordic lake systems in over the last 2-3 decenniums and is predicted by IPCC to do so for decenniums to come. In isolation, such a temperature-increase scenario will result in increased individual growth in most salmonid species, provided access to sufficient food resources. This growth-rate increase can be a purely plastic response following thermal norms of reaction (TRNG). However, in the face of ongoing size-biased harvesting increased growth may not be favorable. One may therefore expect TRNGs to be subject for selection and ultimately leading to adaptational change. Because the maturation process is closely linked to individual growth, one may expect changes in the growth-related maturation reaction norms (GRNM) to occur as well. In this study, we explore what will ultimately result from these seemingly conflicting processes of climate warming and fisheries in lake-dwelling salmonids in Nordic lakes. In particular, we use data from a grayling (Thymallus thymallus) system located in central parts of Norway, for which we have information about FAC that has occurred during the last 100 years, to estimate contemporary GRNMs and TRNGs. Under various IPCC temperature-change scenarios, we simulate (Leslie-matrix modeling) and explore the fitness consequences for different altered TRNGs and GRNMs. We also explore the ultimate TRNG and GRNM outcome of different harvesting regimes under each climate scenario.