Adaptations to stochastic environmental variations: the effects of seasonal temperatures on the migratory window of Svalbard Arctic charr

Abstract

The circumpolar Arctic charr, Salvelinus alpinus, is ideal for studying how environmental factors affect life history in fishes. Charr populations demonstrate a tremendous ecological plasticity and adaptations to harsh environments. Arctic charr is the only freshwater fish on Svalbard, including anadromous, resident, and landlocked stocks. Fresh- water lake systems on Svalbard are characterized by very low water temperatures, long-term or even permanent ice cover, and low levels of nutrients. Food is thus limited and may lead to growth stagnation and early maturity in Arctic charr. The individual growth pattern may alternatively follow a sigmoid-shaped curve, caused by a shift to either cannibalistic or anadromous (migration to sea) behaviour. In lake systems that include migratory charr, the population may consist of a mixture of parr, postsmolt, and adult migratory individuals, as well as small-sized resident, large-sized resident (cannibals), and large formerly resident individuals transformed to anadromy. Our study in the Lake Dieset watercourse (79◦ N), Svalbard, demonstrates that the annual water flow in the outlet river is strongly correlated to air temperatures and provides a passage to the sea, allowing the charr access to the nutrient- rich seawater environment, during at most two months each year. During one of the years studied, the youngest and small-sized part of the sea-going stock was prevented from ascending the river and probably suffered mortality during winter. The migratory windowof the Arctic charr in Lake Dieset is therefore highly variable among years and thus unpredictable.We hypothesize that inworst case scenarios (cold years, lowwater discharge), climatic variations may occasionally prevent charr from migrating upstream in Svalbard lake systems in late autumn, resulting in high mortality in the population.