Influence of food presence in the zone of temperature optimum upon behavior of young perch Perca fluviatilis L.

In experimental conditions it has been shown that absence of food objects in the zone of optimal temperatures may lead to change in thermoregulatory behavior of perch young. Thus, at addition of food into the chambers of the experimental installation where fish was present at the momentof feeding (control group) the values of the final preferred temperatures (FPT) were 26.0 °С. At addition of food objects into the chambers with low temperatures (15.0–19.0 °С) the values of FPT were significantly (p < 0.05) lower by 2.9 °С (23,1 °С) (experimental group). Such reduction of FPT was caused by regular "foraging migrations" of young from the optimum zone to the chambers with food. Besides, fish did not stay long in the "foraging" areas. Therefore, perch young would stay in locations with temperature closest to their species’ optimum most of time in a thermally heterogeneous environment. At the same time, absence of food in these areas encourages fish to perform short "foraging migrations" into adjacent areas. It seems, that such interaction of thermoregulatory and feeding behavior allows fish young to maintain higher growth rate than they would be if fish constantly stayed in the "foraging" areas with lower-than-optimal temperatures.