The Role of Food for Overwintering Juvenile Eulachon (Thaleichthys pacificus)

According to the critical size hypothesis, winter survival is a function of fish size at the end of the growing season. The basis for the hypothesis is the observation that there is a threshold size at which a fish’s energy storage capacity exceeds its metabolic demand over winter.  This suggests fish below this threshold must forage during winter and their survival depends on food availability. To test this hypothesis we examined food availability, size and energy content of young-of-the-year and juvenile eulachon (Thaleichthys pacificus) in two locations (Fritz Cove and Berners Bay) in southeastern Alaska during the winter of 2005-2006. Fish were located during hydroacoustic surveys and sampled by trawling. In January, YOY (< 77mm) were the same average length in both bays, but food supplies were twice as dense in Fritz Cove as in Berners Bay. Consequently, YOY eulachon in Berners Bay experienced greater energy losses over winter than those in Fritz Cove.  Moreover, YOY from Berners Bay experienced size dependent mortality while there was no such evidence in Fritz Cove. Similarly, age-1 eulachon in Berners Bay lost more energy than those in Fritz Cove. In both locations YOY lost more energy than age-1 fish.  These data demonstrate that while size was an important determinant to winter survival increased food availability compensated for the limited energy storage capacity in small fish in Fritz Cove.  Consequently, overwintering success for juvenile forage fish is likely to be highly localized.