Long-term dynamics ofLigula intestinalisand roachRutilus 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 ofLigula intestinalisin roachRutilus rutilusin a small lake, and the relationships of these cycles to the densities of rudd,Scardinius erythrophthalmus, and Great Crested Grebes,Podiceps cristatus, over 31 years. The parasite was introduced to the lake byP. cristatusin 1973 at a time when the roach population had increased in response to eutrophication to a level at which individual fish growth was stunted and the hithero dominant rudd population had declined in numbers as a consequence of inter-specific competition with roach.Ligulaprevalence peaked at 28% in only 2 years: thereafter parasite-induced host mortality caused a decline in the roach population, releasing fish from stunting and allowing the rudd population to recover. The consequent improved growth of roach individuals and their short life-span reducedLigulatransmission rates and prevalence levels declined to approximately 1% althoughLigulanevertheless persisted for a further 10 years. Following a massive winter-kill of the fish populations in 1984–1985, fish andLigulanumbers declined to barely detectable levels and the parasite disappeared from samples. Rudd recovered first, then roach and interspecific competition again led to a decline in rudd numbers. This increase in roach numbers led to a decrease in roach growth rates, which coincided with the re-colonization of the lake byLigula. This second epizootic ofLigulapeaked within 2 years in 1991–1992, when up to 78% of roach were infected with a maximum abundance of 2.2 parasites and intensity of 21 parasites. Heavy parasite-induced mortality of roach led to a decline in numbers, an improvement in individual growth rate and a reduction ofLigulatransmission rates such 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 events over the second cycle was so similar to that of the first that it confirms the interpretations of that cycle. Comparison with other localities shows that epizootics ofLigulaalways coincide with rapid increases in roach numbers, for whatever cause, and stunted growth, which together attract piscivorous birds. At the start of a cycleLigulais 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 regulated and the roach–Ligulasystem is inherently unstable.