| Summary: | Cold water species living at high latitudes are expected to respond rapidly to climate change due to their sensitivity to increasing temperature. Water temperature strongly affects the ecology of fish, influencing food intake and conversion, and ultimately somatic growth rate. In a lake, somatic growth rates will also be contingent on food availability, which is negatively affected by fish abundance. Abundance plays a key role in competitive interactions and the scope for food acquisition and is hence an important constraint on temperature-dependent growth. The current field study addresses gradients of both water temperature and relative abundance affecting somatic growth in juvenile fish at high latitudes over nearly 40 years. A sclerochronological analysis was conducted using sagittal otoliths extracted from Arctic charr annually from the early 1980s up to 2016. Otolith growth measurements from individual fish were used to obtain length-at-age data, via statistical models, to investigate whether water temperature variability and abundance translates into changes in somatic growth. Results revealed a clear positive effect of water temperature on somatic growth in juvenile individuals and a clear negative effect of abundance. Temperature-dependence in growth was significant for average and fast-growing individuals across all investigated age classes with the only exception of 2-year-old fish. Abundance had a consistently negative effect on growth rates for all age classes considered. These findings suggest that as global temperatures rise, somatic growth of Arctic charr will correspondingly increase in high latitude lakes, up to an optimum, past which they will likely decline. This will have direct consequences for cold-water fish populations at high-latitudes, affecting fish length at age, maturation schedules and ultimately fish demographics.
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