Concurrent Sessions D: Fish Passage Around the World - I - Juvenile Eel Recruitment in the River Shannon, Ireland: Impact of Hydropower, Development of Mitigation Measures and Decline in Eel Numbers

The River Shannon has been regulated for hydroelectricity generation since construction of the Ardnacrusha dam and associated structures began in 1929. Adverse effects on eel fisheries became evident within about two decades and, following compensation payments to fishery stakeholders, the Electricity Supply Board assumed a statutory fishery management role. Stock restoration measures were focused on capture of juvenile eels in the lower section of the river system and in estuarine tributaries. The trapping protocols that were developed and the results of studies on natural upstream migration of eels will be described. Diel, lunar and seasonal migratory cycles by glass eel, elvers and fingerlings (bootlace eels) were investigated using a variety of techniques that included: monitoring trap catches, use of experimental fishing methods, video observations and mark-recapture experiments. The effects of environmental factors, including water temperature, discharge, light and lunar cycles, in upstream juvenile migration were demonstrated. Modelling of long-term trends in recruitment, eel fishery yield and spawner escapement, suggests that the eel populations of the river system will undergo a serious decline within a decade. Facilitation of upstream migration of juvenile eels is therefore being prioritised in current research and management programmes