THE THERMOREGULATORY FUNCTION OF DIEL VERTICAL MIGRATION FOR A JUVENILE FISH, COTTUS-EXTENSUS

Juvenile sculpin (Cottus extensus) less than 30 mm long exhibit a diel vertical migration in the limnetic zone of Bear Lake (Utah-Idaho). Using mid-water and bottom trawls we found that these fish inhabit the bottom of the lake (5 degrees C) during the day but migrate 30-40 m into the water column at night where they reside in the metalimnion or epilimnion at temperatures near 13-16 degrees C. Larger fish do not migrate into the water column. Stomach analyses demonstrated that the young-of-the-year fish do not migrate into the water column to feed: from July to October their diet is 70-93% benthic ostracods and copepods, and pelagic prey are rarely consumed. Furthermore, gut fullness of the sculpin increases through the daylight period and decreases through the night, reaching minimum levels just before the dawn descent. Laboratory experiments demonstrated that the diel migration would increase digestion rate from 3%/h at profundal temperatures, to 22%/h in the warmer surface water, thus allowing the fish to empty their guts overnight and permit feeding the following day. Additionally, sculpin held in a temperature and feeding regime that mimicked that experienced by migrating fish grew 300% faster than those reared at 5 degrees C. Given the overwhelming importance of fast growth for juvenile fishes, a post-feeding thermotaxis that increases digestion may be a common phenomenon increasing growth, and affecting the distribution and bioenergetic relationships of fish.