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

Gillnets are commonly used for monitoring fish communities and assessing the ecological quality inEuropean lakes. However, the relationship between gillnet CPUE and fish abundance is not linear anddepends on several factors affecting the catchability of gillnets. We investigated the effects of high watercolour, naturally anoxic hypolimnion, water temperature and sex ratio on the catchability of perch Percafluviatilis in gillnets. Data were collected during 10 years in nine forest lakes in southern Finland. Suchlakes are very common in Finland. The density and size structure of perch populations were estimatedby mark and recapture method during spring and standard gillnet fishing (CEN, 2005. Water qualitySampling 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. Catchabilityincreased with water colour, water temperature, and the volume of the anoxic hypolimnion. The sex ratiowas also important because late summer gillnetting overestimates the proportion of female perch. If thefactors that affect the catchability of perch in gillnets are taken into account, gillnet CPUE using standardmethods can be used as rough index of fish density in small forest lakes.