Studies on the biology of juvenile European eels (Anguilla anguilla L.) in Irish rivers
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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 infection was determined. Relationships between host length and condition and the infection intensity were also investigated. Juvenile eels sampled from the River Lee (N = 73) were found to be uninfected. Prevalence recorded for a sample from the Erne (N = 44) was 14% and mean intensity of infection was 1.0. On the River Shannon, samples (N = 498) were collected at the Ardnacrusha dam and Parteen regulating weir during 2008, 2009 and 2010. At Ardnacrusha elvers were more frequent in the trap catch and mean prevalence ranged from 23-66% with mean intensity ranging from 1.35 to 1.94. At Parteen, where larger juvenile eels are predominant in the trap catch, mean prevalence ranged from 59-66% and mean intensity ranged from 1.90 to 2.00. There was no significant correlation between the number of Anguillicoloides present and the condition factor of the eels sampled. Anguillicoloides infection of migrating juvenile eels used for stocking represents a mechanism of dispersal for the parasite to upstream habitat. Measures to mitigate the effect of hydroelectric dams and regulating weirs on juvenile eel migration include implementation of trap and transport programmes. Evaluation of trap efficiency is a necessary step in improving the effectiveness of these programmes. In this study the size selectivity of traps with different substrate types was investigated at the Parteen regulating weir. Brush substrates of varying densities were shown to facilitate the capture of a wider size range of migrating eels. The importance of the location of a trap to its effectiveness was also demonstrated. Mark and recapture experiments were conducted at the Parteen regulating weir using visible implant elastomer (VIE) marking. Recapture rates of marked eels (N=1814) ranged from 0-16.25% and varied depending on release location. This low and variable recapture rate reflects the variable tendency to migrate exhibited by juvenile eels and the importance of the location of a trap to its effectiveness. Juvenile eels entering the trap at the Parteen regulating weir were monitored by closed-circuit video over three consecutive 24 hour periods. A pronounced diel pattern in activity was observed with 98% of juvenile eels entering the trap between dusk and dawn.