Application of a 3-pass removal experiment to assess the yellow eel specific capture efficiency of a 1-ha enclosure

To monitor yellow eel densities in non-tidal coastal waters an enclosure approach was developed. With a boundary net a quadratic area of 1-ha is enclosed. To catch enclosed eels in each corner a fyke is installed and additionally fyke net chains are deployed. However, the unknown capture efficiency of the enclosure within the standard fishing time of 48 h limits the application for quantitative sampling. A 3-pass removal experiment with 10 replications was conducted to determine the yellow eel specific catchability. The removal data were analyzed using a hierarchical Bayesian model approach. The calculated initial population density was 18.1 eels/ha. According to the modelled catchability estimate, 39% of the enclosed eels were caught within the standard monitoring time of 48 h. Accordingly, a correction factor of 2.6 can be applied to transfer eel monitoring data (48 h fishing time) to true eel density estimates. Furthermore, the presented removal experiment in combination with the Bayesian modelling approach can be applied to estimate other fish species specific catchability coefficients of the enclosure.