Declining adult body size and contracted life history in an unexploited boreal Lake Trout (Salvelinus namaycush) population during a period of warming

Evidence of declines in fish body size with warming in the absence of exploitation are lacking. During a period of regional climate warming (1986-2016), we examined trends in growth, size-structure, and age-structure of an unexploited population of Lake Trout (Salvelinus namaycush) from a small Canadian Shield Lake. These data were evaluated against long-term annual air and water temperature data, ice-cover data, and prey fish densities for the same period. Over time, the size of young immature Lake Trout did not change, while mature fish became smaller. The age-at-maturity of the population decreased by two years (from 6 to 4 years) and the body condition of immature and mature fish also declined. Though the adult Lake Trout population doubled over time (125 to 280 individuals), and the production (P), biomass (B), and biomass turnover (P/B) was constant, the population shifted to smaller, skinnier, younger individuals. By contrast, the relative abundance, biomass, and mean size of the prey fish community remained unchanged. Together this suggests that warming correlates with the contraction of the population’s life history.