| Summary: | Over the past three decades recruitment of European eel (Anguilla anguilla L.) to inland waters has declined by more than 95%. The causes of this decline are still unclear although a number of factors are thought to have a contributory effect including pollution, habitat loss, climatic changes in the ocean, overfishing and the spread of non-indigenous parasites and disease. In this study, undertaken from 2008 to 2010, the abundance, timing and seasonal duration of eel recruitment was investigated on the Shannon, Erne and Lee river systems, where the natural connectivity of these rivers is interrupted by hydroelectric dams and regulating weirs. Catches of juvenile eels at fixed traps downstream of these structures were monitored and the length, weight and age of eels sampled were analysed. Total catch varied between years at each site and were generally low. The timing of the migration showed considerable interannual variability as well as differences between sites. Mean eel length was greater for eels trapped further upstream from the tidal limit of estuaries. At the Parteen regulating weir on the River Shannon variation in the juvenile eel catch was analysed in relation to environmental factors using multiple regression appropriate to time series data. Factors included in the regression model which explained a statistically significant portion of the variation in juvenile eel catch were: day length, flow, water temperature, moon fullness and year. An exceptional peak in the eel catch was recorded in August 2008 following very heavy rainfall. The catch recorded during August 2008 accounted for 51% of the total catch for the period 2008 to 2010. Juvenile eels sampled at fixed traps were examined for the presence of Anguillicoloides crassus, an introduced Asian parasitic nematode of eels that can affect swimbladder function. Investigations on the infection levels in yellow and silver eels are frequently reported but there is a paucity of information regarding infection of juvenile eels. Prevalence and intensity of ...
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