Effects of helminth infections on the shoaling behaviour of small freshwater fishes
The findings of laboratory and field investigations into various aspects of the effects of helminth parasites on the ecology and behaviour of minnows Phoxinus phoxinus and three spined sticklebacks Gazsterosteus aculeatus are reported. A sampling programme was implemented at Loch Maragan over a 16 month period between October 1992 and January 1994 (Chapter 2). The overall prevalence of L intestinalis in minnows from the loch was 17.8%: plerocercoids were found to be weakly overdispersed within the minnow population, although one plerocercoid generally dominated multiple infections whenever they occurred. Both total parasite weight, and the weight of the largest plerocercoid present in an infection were found to be positively and highly significantly correlated with host length, with older, larger fish exhibiting higher parasite burdens. This suggests that fish become infected with L. intestinalis at Loch Maragan during a temporally-limited period during early life. No significant effect of the parasite on host body condition was detected. A qualitative model for transmission of L. intestinalis at the site, based on the available epidemiological data and the prevailing ecological conditions, is proposed. The shoaling and investigative behaviours of minnows from two sites in central Scotland were found to differ, and possible reasons for this variation, based on the ecological disparity between the two sites are suggested (Chapter 3). Both small and medium-sized fish formed more cohesive shoals and schooled more frequently when water depth was reduced, but small fish polarised more frequently than medium-sized fish for any given water depth. The effects of visual oddity on individual shoaling behaviour were investigated (Chapter 3). Size-oddity had little influence on shoaling behaviour in the experimental trials, since the non-uniformity of individual small minnows, when placed in a tank with a group of medium-sized minnows, did not affect their shoaling tendency.