Divergent growth patterns between juveniles of two sympatric Arctic charr morphs with contrasting depth gradient niche preferences

Abstract

Seasonal growth patterns have been investigated in juveniles of two reproductively isolated Arctic charr [Salvelinus alpinus (L.)] morphs, which coexist in distinct ecological niches (upper-water and deep-water environments) in a subarctic lake. Offspring (F1-generation) of each of the littoral spawning charr (LO) and the profundal spawning charr (PB) were reared individually over a period of 12 months under simulated natural photoperiod (at 69°N) and two different temperature regimes to mimic the profundal zone conditions with constantly low temperature (~4 °C) and the littoral zone conditions with seasonally cycling temperatures. All charr increased in weight during the experimental period. The LO-morph performed generally best at naturally fluctuating temperature, whereas the PB-morph grew best at a constantly low temperature (4 °C). The LO-morph showed a typical seasonal growth pattern found earlier for anadromous Arctic charr, with elevated specific growth rates (SGR) during June–October and profoundly reduced or negative SGR-values during November–February suggesting that the fish ceased feeding during this period. In contrast, the PB-morph maintained a steady growth rate, regardless of temperature regime, and showed less signs of reduced appetite during the polar night period and exhibited significantly higher SGR-values during late autumn and early winter compared to the LO-morph. This study suggests different heritable adaptations in somatic growth trajectories where the deep-water PB-morph do not show a down-regulation of appetite and voluntary fasting during winter conditions with typically low light intensity and low temperatures, and up-regulation of appetite with hyperphagia during summer, as characteristically found for the upper-water LO-morph and anadromous Arctic charr.