THE EFFECT OF HUNGER AND CESTODE PARASITISM ON THE SHOALING DECISIONSOF SMALL FRESH-WATER FISH

Individual minnows Phoxinus phoxinus and three-spined sticklebacks Gas terosteus aculeatus were provided with a mutually exclusive choice bet ween joining a shoal of conspecifics and foraging alone in a maze. The shoaling decisions and foraging behaviour of individual fish were stu died when the fish were satiated and after 24- and 48-h periods of foo d deprivation. Hunger level was found to have a significant effect on shoaling behaviour. When satiated, fish of both species spent a greate r proportion of time within one body length of the shoal and spent les s time out of visual contact with the shoal than after periods of food deprivation. The effect of the cestode parasite Schistocephalus solid us on the shoaling behaviour of stickleback hosts was complex. When sa tiated, infected fish spent less time than uninfected fish within one body length of the shoal. preferring to remain outside the shoal, yet within visual contact, although when food deprived there was no differ ence in the proportion of time spent by infected and uninfected fish c lose to the shoal. The possible ecological significance of this change in behaviour is discussed with reference to the manipulation hypothes is of host-parasite interactions. (C) 1995 The Fisheries Society of th e British Isles