Conservation biology of the European eel (Anguilla anguilla) on a hydropower-regulated Irish river
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The European eel (Anguilla anguilla) has a complex lifecycle, involving catadromous migration between marine spawning grounds and continental growth habitat. European eel stocks have undergone a serious population collapse, and recently introduced E.U. legislation (EC 1100/2007) specifies major conservation actions. In particular, the protection of potential spawners (i.e. silver eels) from continental waters is considered essential for stock recovery. Various aspects of silver eel migratory behaviour, population biology and conservation were examined on the hydropower-regulated River Shannon. The primary study site was at Killaloe eel fishing weir, where long-term reliable catch records are available. Ardnacrusha hydropower dam is located 18 km further downstream (near the tidal limit). Since 2000, silver eels captured at Killaloe are released below the hydropower dam as part of a 'trap and transport' conservation strategy. In 2009, Ireland's Eel Management Plan (EMP) specified that 30% of the silver eel production on River Shannon must be captured and released annually. As a result, four additional fishing sites were established in the mid/upper catchment. An estimated 228 013 silver eels, including a high proportion of females (75.9%), have been released to the lower River Shannon (2000-2010), thus avoiding the hazards associated with passage via Ardnacrusha. Analysis of silver eel migration dynamics/population structure on the river has demonstrated how multi-site capture has enabled EMP targets to be achieved (30.2% in 2009, 39.6% in 2010 and 41.9% in 2011), in addition to increasing the quantity of large female eels released. Silver eel trap and transport is ideally an interim conservation measure, pending development of non-intrusive alternatives (e.g. guidance technology, controlled spillage). To ensure effective implementation of these alternatives, accurate prediction of silver eel migration is essential. Therefore, two predictive approaches were evaluated on the lower River Shannon, by reference to silver eel catches at Killaloe: (i) the Migromat® biomonitor, which predicts migration based on the activity levels of captive eels, signalled 19.9-28.9% of the total catch, or 21.4-32.1% of migration events, during the evaluation period; (ii) alternatively, retrospective analysis of catch and environmental data (water temperature, discharge and lunar luminosity) were used to develop logistic regression models for prediction of silver eel migration. The predictive capacity of this approach varied, depending on the year (27.5-86.4% of the total catch, or 66.7-95.0% of migration events), but was generally more effective than the Migromat®. The use of behavioural guidance as a potential hydropower mitigation measure was also evaluated on the lower River Shannon. During an experiment involving an overwater light barrier at Killaloe eel weir, catch patterns were found to be significantly different between lights on and lights off periods. Eel response to the light barrier was size related, reflecting the increased swimming ability of larger individuals. Eel behaviour in the vicinity of the light field was observed using a sonar camera (DIDSON), and comparison of observed and captured eels shows that the population structure was accurately determined using this technology. A preliminary evaluation of infrasound fish guidance technology was undertaken on the Ardnacrusha headrace canal. However, DIDSON¿ observation of silver eels suggested no avoidance behaviour was elicited by the infrasound stimulus during the typical high discharge conditions. The reproductive ecology of silver eels is not considered in current stock recovery plans, which instead focus primarily on increasing spawner escapement biomass. Silver eel fecundity was estimated on the River Shannon and was shown to increase exponentially with body size. These are the first fecundity estimates derived from a wild population of European eels, as previous analyses have related exclusively to artificially-matured individuals. Biologically-appropriate eel fecundity data can now be incorporated into future management policies and modelling of stock dynamics. The long-term management of the River Shannon eel fishery also provided a unique opportunity for analysis of population trends. Four years (2008-2011) of intensive monitoring at Killaloe indicated within-season variation in population structure (i.e. early male migration, increasing female size). Mark/recapture experiments (undertaken during 2008-2011) and fishery catch records were used to retrospectively estimate silver eel production from 1985-2011, and overall a significant downward trend was apparent. Productivity levels reflect management policies (i.e. stocking, fishery reduction/closure), and an input:output model indicates that silver eel production will continue to decline in the coming decade, due to the collapse of recruitment to the river.
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