Climate change impact on high latitude freshwater fish populations

Abstract

<p>Climate change is one of the greatest threats to animal wildlife in high latitude freshwater ecosystems. Climate warming is rapidly increasing water temperatures in these areas, affecting biological processes of ectotherms such as growth, maturation and reproduction, which in turn trigger population responses. The magnitude of the effects of climate warming will vary depending on the thermal niche and phenotype of species. Climate change will continue to redistribute species, and fish species from warmer temperature guilds will invade and possibly take over areas where cold water fish currently dominate. Hence, it is important to establish the performance of cold vs warmer water species in a warming Arctic. The aim of this thesis is therefore to provide novel insights and predictions on population level implications of climate change for both cold- and cool water fish at high latitudes. The primary focus is on climate effects mediated by direct and indirect individual-level responses to increasing water temperatures, addressed using long-term empirical investigations and modelling in retrospective and prospective studies. In addition, the thesis addresses interactions between climate change and size-selective harvesting, a main pressure on high latitude fish populations, by modelling their cumulative effects to evaluate risks and reveal potential synergistic threats. <p>The thesis documents how both cold- and cool water fish at their northern range edge have increased their somatic growth rates during the last three decades of warming. However, the cool-water adapted vendace and perch displayed a higher increase in juvenile somatic growth with warming compared to cold-water Arctic charr and whitefish, stressing how the thermal niche modulates the magnitude of warming effects. The individual based models developed for this thesis predict a further increase in somatic growth towards year 2100 under warming scenarios (RCP-4.5, -8.5), with cool water fish displaying a greater increase in somatic growth rate than cold water fish. The documented and projected climate driven increase in somatic growth rate mediates changes in survival rates and life history, including a likely increase in juvenile survival, and earlier maturation, the latter being contingent on species’ maturation reaction norm. The demographic implications of these individual effects were investigated via modelling and long-term empirical studies. <p>The population level response to climate warming, mediated by individual effects, was evident in the cool water adapted perch, which experienced a substantial increase in density and importance relative to the cold-water adapted whitefish, which is dominant in the investigated lakes. The population response of this cool water fish was mediated by an increase in juvenile growth rate which resulted in larger size at age and earlier maturation, but also a likely increase in survival through the first critical winter. The modelled populations displayed higher biomass and yield as size at age increased with warming, but this effect was larger in the cool water specie than in the cold water species. In sum, cool water fish will benefit more from climate warming than cold water fish at high latitudes, and where they coexist, cool water fish may become the dominant player in the fish community. <p>The climate driven increase in size at age affects the age-specific exposure to size-selective harvesting, increasing the risk of younger individuals being caught by gillnets. The population level effect of earlier gillnet exposure is an increased age truncation, as illustrated by individual based model outcomes. Also, larger size at age increased the proportion of immature individuals being caught, with the magnitude of the effect being contingent on growth trajectories, their temperature dependence, and orientation of the maturation reaction norm. The increased juvenile mortality and more pronounced age truncation reduce recruitment, increasing the vulnerability of exploited populations to environmental stressors. Fish species with large size, slow growth, and late maturation like Arctic charr were more vulnerable to warming and harvesting than species with a faster life history, like vendace. <p>In conclusion, the stronger positive effects of warming on the performance of cool-water adapted species relative to cold-water salmonids, and the greater vulnerability of the latter when exposed to size-selective harvesting, warn of incipient reorganizations of Arctic fish communities, and invite climate adaptation in the management of high latitude populations.