Interspecific dominance amongst three species of coarse fish in Slapton Ley, Devon

Marked changes in the relative numbers of roach Rutilus rulilus (L.), rudd Scardinius erylhroplhalmus (L.) and perch Perca fIuviatilis L. , in Slapton Ley, Devon, have been observed over the period 1967–1978. Historically, the lake had been dominated by rudd and perch; significant numbers of roach were not thought to be present until 1967. From then onwards the roach population exhibited a considerable expansion, apparently replacing the rudd, which had virtually disappeared by 1974. In 1975 extensive mortalities of roach were caused by the pseudophyllidean cestode Ligula intestinalis L. During 1976 and 1977 larger number of rudd, belonging to the 1975 and 1976 year classes, were present, suggesting that the roach mortalities might be giving the rudd a chance to recover. However, very few rudd were present during 1978, possibly because lower incidences of ligulosis during 1976–1978 had allowed the roach to recover from the main outbreak of the disease in 1975, and to replace the rudd for the second time. Evidence from trap catches indicated that the perch population had shown a consistent decline between 1970 and 1977. The major phase of this decrease occurred between 1971 and 1973, when the roach population was increasing the most rapidly. Comparisons were made with changes in fish populations in other localities. The major reason for the changes observed was thought to be a competitive relationship between the plankton feeding younger stages, with roach being the dominant species, although other factors may also have been involved.