Post-feeding thermotaxis and daily vertical migration in a larval fish

Abstract

Many aquatic animals make daily vertical migrations, typically ascending into warm shallow strata for the night and descending to cooler, deeper layers of lakes or oceans for the day. Although some organisms may migrate to avoid predation^1–3 researchers have also suggested that daily migration is a thermoregulatory strategy allowing ectotherms to lower their metabolic rates in cold, deep waters, thus conserving energy^4,5. Tests of this hypothesis, however, have been equivocal^6–8. Here we suggest an alternative hypothesis: that fish ascend into warmer water after feeding to stimulate digestion, thereby allowing greater feeding and growth. We tested this hypothesis using the Bear Lake sculpin (Cottus extensus) which feeds on the bed of the lake during the day, and at night migrates into the water column where temperatures are 10 °C warmer. The warmer temperatures promoted digestion and allowed the fish to feed and grow three times faster than if they had remained in the cold hypolimnion. Thus, daily vertical migration in this species is an adaptation allowing them to exploit thermal gradients in their environment⁹ to maximize energetic intake.