Long-term dynamics of Ligula intestinalis and roach Rutilus rutilus: a study of three epizootic cycles over thirty-one years

Data are presented on 2 full epizootic cycles and the start of a third of L igula intestinalis in roach Rutilus rutilus in a small lake, and the relati onships of these cycles to the densities of rudd, Scardinius erythrophthalm us, and Great Crested Grebes, Podiceps cristatus, over 31 years. The parasi te was introduced to the lake by P. cristatus in 1973 at a time when the ro ach population had increased in response to eutrophication to a level at wh ich individual fish growth was stunted and the hithero dominant rudd popula tion had declined in numbers as a consequence of inter-specific competition with roach. Ligula prevalence peaked at 28% in only 2 years: thereafter pa rasite-induced host mortality caused a decline in the roach population, rel easing fish from stunting and allowing the rudd population to recover. The consequent improved growth of roach individuals and their short life-span r educed Ligula transmission rates and prevalence levels declined to approxim ately 1% although Ligula nevertheless persisted for a further 10 years. Fol io-wing a massive winter-kill of the fish populations in 1984-1985, fish an d Ligula numbers declined to barely detectable levels and the parasite disa ppeared from samples. Rudd recovered first, then roach and interspecific co mpetition again led to a decline in rudd numbers. This increase in roach nu mbers led to a decrease in roach growth rates, which coincided with the re- colonization of the lake by Ligula. This second epizootic of Ligula peaked within 2 years in 1991-1992, when up to 78% of roach were infected with a m aximum abundance of 2-2 parasites and intensity of 21 parasites. Heavy para site-induced mortality of roach led to a decline in numbers, an improvement in individual growth rate and a reduction of Ligula transmission rates suc h that the epizootic died out in 1996. Similar conditions of roach numbers and growth prevailed at the start of a third cycle in 1998. The course of e vents over the second cycle was so similar to that of the first that it con firms the interpretations of that cycle. Comparison with other localities s hows that epizootics of Ligula always coincide with rapid increases in roac h numbers, for whatever cause, and stunted growth, which together attract p iscivorous birds. At the start of a cycle Ligula is a major determinant of the population dynamics of the roach, but at the end of the cycle the fish population dynamics determine those of the parasite. The cycles are not reg ulated and the roach-Ligula system is inherently unstable.