An evaluation of gillnet CPUE as an index of perch density in small forest lakes

摘要

Gillnets are commonly used for monitoring fish communities and assessing the ecological quality in European lakes. However, the relationship between gillnet CPUE and fish abundance is not linear and depends on several factors affecting the catchability of gillnets. We investigated the effects of high water colour, naturally anoxic hypolimnion, water temperature and sex ratio on the catchability of perch Perca fluviatilis in gillnets. Data were collected during 10 years in nine forest lakes in southern Finland. Such lakes are very common in Finland. The density and size structure of perch populations were estimated by mark and recapture method during spring and standard gillnet fishing (CEN, 2005. Water quality Sampling of Fish with Multimesh Gillnets. European Committee for Standardization, Brussels (EN 14757)) in late summer. Perch CPUE was positively but non-linearly dependent on the perch density. Catchability increased with water colour, water temperature, and the volume of the anoxic hypolimnion. The sex ratio was also important because late summer gillnetting overestimates the proportion of female perch. If the factors that affect the catchability of perch in gillnets are taken into account, gillnet CPUE using standard methods can be used as rough index of fish density in small forest lakes. (C) 2015 Elsevier B.V. All rights reserved.