Effects of increasing temperature on social behaviour in groups of yellow bullheads ( Ictalurus natalis)

Abstract

Groups of two yellow bullheads ( Ictalurus natalis), a socially complex fish, were kept in 50-gallon aquaria and subjected to 1°C temperature increments every other day, over the range 24–33°C. Total activity, and with it the incidence of aggressive behaviour, increased with temperature over the entire range, with one notable exception. At about 30°C activity decreased to a level below that observed at the lowest temperature studied. The mode, as well as the frequency, of aggressive behaviour changed with temperature. Aggressive behaviour units were divided into two categories—display or warning units, which do not result in injury to either fish, and damaging units, which frequently do cause injury. Starting at 30°C, the relative frequency of damaging units became much greater. It is suggested that this change in behaviour is due to the loss of ability by yellow bullheads at high temperatures to distinguish individuals of their own species. A possible implication of this sort of phenomenon in natural ecosystems is discussed.