Shoaling behaviour of fish under parasitism and predation risk

Shoaling is a common antipredatory adaptation in several fish species. However, parasite infections may alter shoaling behaviour of fish by impairing fish sensory/motor systems and by reducing the net benefit of shoaling. In an experimental study, we investigated whether Diplostomum spathaceum (Trematoda) eye flukes alter shoaling behaviour of rainbow trout, Oncorhynchus mykiss, and if this has an interaction with predation risk. The parasite reduces the vision of fish by inducing cataract formation, which in previous studies has been shown to alter fish escape responses and crypsis. We found that the shoals of infected fish contained fewer individuals and they divided into separate groups more often compared with controls. The difference between infected and control fish in their shoaling behaviour further increased undersimulated avian predation, which increased the cohesiveness of groups in control fish but not in infected fish. These findings suggest that reduced shoaling of D. spathaceum-infected fish may predispose them to predation by birds, and thus provide a potential mechanism to enhance the transmission of this trophically transmitted parasite.