Blackwell Publishing Journal Backfiles 1879-2005
Investigations into the biology of the roach, Rutilus rutilus (L.), and Ligula intestinalis (L.) populations at Slapton Ley, Devon were carried out between October 1982 and December 1984, and additional data collected from the lake since 1977 have been re-analysed. The Ligula population exhibited some unusual features: a limited specificity, a persistently low abundance and a scarcity of multiple infections. The population dynamics of the roach were also unusual in that the individual growth rate was one of the fastest in Britain, the survival was very poor and the year classes tended to alternate in strength. Whether the unusual epidemiology of Ligula could be explained by the unusual population dynamics of the roach is investigated.The transmission period of Ligula to the roach was limited to their first few months of life, probably due to a limited period of feeding on copepods by the fry. This narrow transmission window was almost certainly the major factor that has prevented the abundance of Ligula plerocercoids attaining high levels in the fry or of increasing in subsequent years, and has resulted in their abundance being determined entirely by transmission events to the fry. The limited transmission period has also caused the life-cycles of the roach and Ligula populations to become synchronized. At other localities, the abundance of Ligula is typically high, and can have a significant effect on the mortality of the host population, but, with the low abundance at Slapton, Ligula-induced roach mortality was insignificant. The rapid growth of the roach fry was the most likely explanation for the limited period of feeding on copepods, and the large size, short life-span and low abundance of the roach probably also constrained the build-up of the Ligula population in the lake. The random frequency distribution and scarcity of multiple infections was not considered unusual in view of the very low plerocercoid abundance. The very low abundance of Ligula, the separation of the spawning of roach and rudd in time and space, and the scarcity of rudd may account for the absence of infection in this species. It was concluded that the Ligula population in Slapton Ley was being constrained by the atypical population dynamics of the roach, and not vice versa as theory predicts.
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