Factors associated with fish modify life history traits of the cladoceran Ceriodaphnia cf. dubia

This study examined the effect of water that previously contained fish on several life history traits of the small-bodied cladoceran Ceriodaphnia cf. dubia. Four fish density treatments were tested (i.e. 16.7, 5.9, 2 and 0.9 l fish^-1). A significant (P < 0.05) increase in the mean generation length occurred at a fish density of 16.7 l fish^-1. Fish densities of at least 5.9 l fish^-1 had a significant (P < 0.05) positive effect on mean brood sizes, population growth rate and net reproductive rate. Increased fish density appeared to increase the length of time during which the cladocerans reproduced significantly (P < 0.05). None of the tested fish densities affected (P > 0.05) cladoceran survival. The observed effects are ascribed to the presence of unidentified substances in the water that previously contained fish. These substances are thought to be 'fish kairomones'. While life history changes occur in the laboratory, it remains unclear whether similar changes would occur in the field.