Parasite infestations can render a fish more vulnerable to predation, a recent study shows.
In a paper published in the most recent issue of the journal Animal Behaviour, Otto Seppl, Anssi Karvonen and Tellervo Valtonen show that rainbow trout (Onchorhynchus mykiss) infected with the eye fluke Diplostomum spathaceum are less able to shoal properly and are thus more vulnerable to predation.
The authors obtained juvenile rainbow trout from a commercial fish farm, and then infected randomly chosen individuals with eye fluke.
The infected and uninfected fish were placed in separate tanks and them analysed for shoal cohesiveness before and during simulated predator attacks (a white plate moving over the tanks at regular intervals to simulate an attack by gulls/terns).
The authors measured the following to assess shoal cohesiveness: (1) the number of fish shoaling (i.e. within two body lengths from another individual, which represents a typical inter-individual distance for shoaling fish), (2) the number of separate shoals, and (3) the distance from each shoaling fish to its nearest neighbour.
The results showed that fish infected with eye flukes formed looser, smaller shoals that divided more frequently compared to uninfected fish.
This difference was further emphasized when subjected to simulated predator attacks, as the uninfected fish formed tighter shoals than the infected fish, leaving the infected fish more vulnerable to predator attacks.
The authors surmise that he exact mechanism that brings about the above changes in fish behaviour is not known, but it is likely to involve reduction in fish vision through parasite-induced cataract formation...
For more information, see the paper: Seppl, O, A Karvonen and ET Valtonen (2008) Shoaling behaviour of fish under parasitism and predation risk. Animal Behaviour 75, pp. 145"150.