The establishment and subsequent history of a population of Ligula intestinalis in roach Rutilis rutilis (L.)

The introduction, establishment and subsequent history of a population of Ligula intestinalis in roach, Rutilus rutilus , of a small lake was studied over a period of seven years. The introduction is believed to be a natural, chance colonization achieved by the return of Great Crested Grebes as permanent residents. Within two years the parasite had infected 33% of the roach between 60–120 mm in size. It exhibited a pronounced seasonal cycle in infection. Fish were often infected when only a few months old in autumn, and young fish and parasites over-wintered together. Parasite growth resumed in the following summer, when some further infections occurred. The majority of infected fish died in or before their second winter, possibly as a result of selective predation, but a small proportion survived into a third year. Dispersion of the parasite was close to random for most of the time. Multiple infections and slight over-dispersion occurred only in summer months. Infections were scarcer in large fish, due partly to selective mortality of infected fish and partly to a change in the host diet. The pattern of seasonal infection compared favourably with other localities, but prevalence and intensity of infection were consistently lower, parasite dispersion closer to random and multiple infections much scarcer than elsewhere. Infection levels and parasite growth rates varied from year to year, but these annual fluctuations were irregular and showed no consistent pattern. The Ligula population exhibited both regulatory and destabilizing features and there was no definite evidence that it was stable. It is suggested that Ligula is an opportunist species. It is endemic in some sites but in others, especially small lakes, the populations are unregulated and unstable and so persist for short periods only in a non-equilibrium state.