Susceptibility of eye fluke-infected fish to predation by bird hosts.

Host manipulation by trophically transmitted parasites may predispose infected hosts to predation and in this way enhance parasite transmission. In most study systems, however, the evidence comes from laboratory studies, and therefore knowledge of the effect of manipulation on parasite transmission efficiency in the wild is still limited. Here we examined the effect of Diplostomum spathaceum (Trematoda) eye flukes on the susceptibility of fish intermediate hosts to predation by bird definitive hosts. Our earlier studies have shown that the parasite alters fish phenotype and increases their susceptibility to artificial predation under laboratory conditions. In the present field study, we allowed wild birds to feed on fish from cages placed into a lake, and found that predation vulnerability of infected fish did not differ from that of controls. However, we suggest that the experimental set-up likely affected the result because the cages allowed gulls, which caught most of the fish in the study, to feed on fish in an easy, unnatural manner by standing on the edges of the cages. Nevertheless, considerable predation was observed, which provides important initial evidence of how this question should be addressed in the wild.