Browsing by Author "Evans, Derek W."
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ItemThe changing times of Europe's largest remaining commercially harvested population of eel Anguilla anguilla L(Wiley, 2021-06-04) Aprahamian, Miran W.; Evans, Derek W.; Briand, Cedric; Walker, Alan M.; McElarney, Yvonne; Allen, Michelle M.This study quantifies the processes involved in regulating the European eel population of Lough Neagh, a lake in Northern Ireland. The relationship between glass eel input and silver eel output for the 1923–1997 cohorts was best described by a Beverton–Holt stock recruitment model. Glass eel input time series was not complete and was thus derived from the relationship between catches elsewhere in Europe and Lough Neagh, together with the addition of stocked glass eel. Silver eel output was the sum of silver eel escapement, catch and yellow eel catch converted to silver eel equivalents. Natural mortality increased with glass eel density, ranging from 0.017 to 0.142 year−1. The mean carrying capacity increased from ≈3.25 M silver eels (≈26 kg ha−1) for the 1923–1943 cohorts to ≈5.0 M (≈40 kg ha−1) for the 1948–1971 cohorts before regressing back to ≈3.25 M. The total silver eel output was highest during the late 1970s/early 1980s at 35–45 kg ha−1 year−1 and lowest during the early years of the 20th century and is currently at 10–15 kg ha−1 year−1. The findings are discussed in relation to (a) the ecological changes that have occurred within the lough, associated with eutrophication and the introduction of roach (Rutilus rutilus L.), and (b) the decline of the wider European eel stock across its distribution range. The findings from this study have relevance for the wider management of the European eel stock. ItemLong term retention of dummy acoustic transmitters in adult brown trout(Wiley, 2020-08-26) Kennedy, Richard; Evans, Derek W.; Allen, Michelle M.A group of 36 1+ age class Salmo trutta were surgically implanted with dummy acoustic tags and monitored for 370 days. In total 13 tags were expelled through-out the experiment with an overall tag loss rate of c. 0.035 tag/d. Fish length was the only explanatory variable which had a significant association with subsequent tag expulsion. The estimated probability of retaining a tag for a year for a fish of length 32 cm was 0.76, 34 cm was 0.60 and 36 cm was 0.38. The long term tag loss patterns were examined and discussed. ItemRiver lamprey present an unusual predation threat to Atlantic salmon smolts in Lough Neagh, Northern Ireland.(Wiley, 2020-09-18) Kennedy, Richard; Campbell, W.; Gallagher, K.; Evans, Derek W.A new monitoring programme on the Lough Neagh catchment has documented a high incidence of river lamprey, Lampetra fluviatilis L., predation on Atlantic salmon smolts, Salmo salar L.,. In total 470 smolts were examined during the 2020 emigration period with 168 fish (36%) exhibiting lamprey scars of which 57 were lightly scarred and 111 were classed as heavily scarred. Lamprey predation was not size selective on Lough Neagh S. salar smolts.