Does risk of intraspecific interactions induce shifts in prey-size preference in aquatic: predators?

Interactions between foragers may seriously affect individual foraging effi ciency. In a laboratory study of handling time, prey value and prey-size pr efer ence in northern pike and signal crayfish, we show that risk of intras pecific interactions between predators does not affect handling time or val ue of prey. However, the presence of agonistic intraspecific interactors sh ifts prey-size preference in these predators. Neither northern pike nor sig nal crayfish foraging alone show a prey-size preference, while pike foragin g among conspecifics prefer small prey, and crayfish foraging in groups pre fer large prey. We ascribe the different outcomes in prey preference to dif ferences in susceptibility to interactions: northern pike under risk avoid large prey to avoid long handling times and the associated risk of interact ions, while signal crayfish foraging among conspecifics may defend themselv es and their prey during handling, and thus select prey to maximise investm ent. In addition, the value of pike prey (roach) is low for very small prey , maximises for small prey, and then decreases monotonically for larger pre y, while crayfish prey (pond snail) value is low for very small prey, has a maximum at small prey, but does not decrease as much for larger prey. Ther efore, a large and easily detected snail prey provides a crayfish with as m uch value as a small prey. We conclude that interaction risk and predator d ensity affect prey-size preference differently in these aquatic predators, and therefore has different potential effects on prey-size structure and po pulation and community dynamics.