The cost of copepod reproduction: increased susceptibility to fish predation.

1. We describe a laboratory investigation to determine how the possession of egg sacs by a freshwater copepod influences the likelihood of its capture by both efficient (bream) and inefficient (roach) zooplanktonivorous fish. 2. For both predators the reaction distance was greater for the larger, more visible ovigerous prey than for nonovigerous copepods. 3. Copepods spent more time stationary, in contact with the substrate, when a fish was present. The more susceptible ovigerous individuals were generally less active than nonovigerous individuals even in the absence of predators. The likely adaptive significance of this behaviour in the natural environment is discussed. 4. The inefficient zooplanktonivorous fish had a significantly increased attack efficiency on ovigerous prey because their egg sacs reduced acceleration and manoeuvrability. Attack efficiency was only marginally enhanced in the case of the more efficient predator. 5. The energy value of ovigerous prey, expressed in terms of biomass consumed per unit handling time, was greater than for non-ovigerous individuals.