The use of otter guards in fyke net surveys and their effect on catches of European Eel Anguilla anguilla

Abstract The European eel is critically endangered and European member states are required to undertake measures to ensure the recovery of eel stocks. The otter is also a protected species under international legislation. As a predator of the eel the otter is often caught in fyke nets located close to shore and in shallow water and otter guards can be used to reduce this unwanted by-catch. This study was undertaken to investigate the effect of otter guards in fyke nets on 1) eel catch and 2) eel length frequency and to compare with eels caught in non guarded fyke nets. In seven lakes at each net location a combination of guarded and non-guarded fyke nets were set. A generalized linear mixed model analysis (GLMM) was applied to the eel catch data with treatment (guarded/non-guarded) as the fixed effect. The catch model predicts that on average the catch per unit of effort (CPUE) for a guarded fyke net is 0.5 (95 % CI [0.34−0.73]) and the CPUE for a non-guarded net is 0.7 (95 % CI [0.48–1.0]). Converting the CPUE to numbers of eels, the model predicts that a guarded net will catch 5 eels (95 % CI [3–7 eels]) and a non-guarded net will catch 7 eels (95 % CI [5–10 eels]). A second GLMM analysis investigated the effect of treatment on the length of eels captured in fyke nets. The predicted mean length of an eel for a guarded net is 48 cm (95 % CI [44–52 cm]), and 49 cm (95 % CI [45–54 cm]) for a non-guarded net. Considering the degree of variation within the lake, these results show that there is no impact on the numbers or the length of eels captured in guarded fyke nets. There are more important variables influencing the eel catch within lakes.