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Keywords:

  • Anguilla ;
  • downstream migration;
  • DIDSON ;
  • population model;
  • seasonality;
  • silver eel production

Abstract

The European eel (Anguilla anguilla) is in decline throughout its distribution and accurate regional information, particularly on the silver eel life-stage, is an essential component of stock recovery plans. Therefore, silver eel population dynamics and production were investigated on the Irish River Shannon. Size frequency analysis during the annual seaward migration showed that between 2008 and 2011, seasonal trends in both sex ratio and female size occurred. Catch analysis and mark-recapture experiments were undertaken during the same period to determine silver eel production. Due to extreme discharge in 2009, no catch data was available for 17 nights, and a novel protocol was developed using a Dual Frequency Identification Sonar (DIDSON) to quantify silver eel migration. In 2008, silver eel production was estimated at 57.3 t (1.35 kg·ha−1), but an additional 18.3 t of potential migrants were estimated to have been removed by the summer yellow eel fishery. In 2009, following closure of the yellow eel fishery, silver eel production increased to 68.6 t (1.62 kg·ha−1), before declining to 62.7 t (1.47 kg·ha−1) in 2010 and 61.6 t (1.45 kg·ha−1) in 2011. Modelling, based on long-term stocking data (1959–2011) and retrospectively estimated silver eel production (1992–2007) suggests that despite commercial fishery closure on the river, the decline in River Shannon silver eel production will continue for at least the coming decade.