Cooperative Fish and Wildlife Research Units Program: Utah
Education, Research and Technical Assistance for Managing Our Natural Resources

Size dimorphism in fish populations, both causes and consequences, has been an area of considerable focus; however, uncertainty remains whether size dimorphism is dynamic or stabilizing and about the role of exogenous factors. Here, we explored patterns among empirical vital rates, population structure, abundance and trend, and predicted the effects of climate change on populations of arctic char (Salvelinus alpinus) in two lakes. Despite differences in underlying geology, population and lake size, the density of adult char was extremely similar between lakes (0.002-0.003 m-2), and both populations cycle between dominance by small (≤ 300 mm) and large (> 300 mm) char. Survival and growth rates were relatively high (40-96 %; approximately 0.1 g.day-1) and comparable to lower latitude relatives. Climate change scenarios resulted in temperatures closer to optimal and for a longer growing season. An increase in consumption rates (28-34 %) under climate change scenarios led to much greater growth rates (23-34 %). Higher growth rates predicted under climate change resulted in an even greater predicted amplitude of cycles in population structure as well as an increase in Ro (reproductive output) and decrease in Go (generation time). Collectively, these results indicate that arctic char populations are extremely sensitive to small changes in the number of ice-free days. We hypothesize that, years with a longer growing season, predicted to occur more often under climate change, produce elevated growth rates of small char and act in a manner similar to a “resource pulse”, allowing a sub-set of small char to “break though”, thus setting the cycle in population structure.