A COMPARATIVE-STUDY OF THE USEFULNESS AND CATCHABILITY OF MULTIMESH GILL NETS AND GILL NET SERIES IN SAMPLING OF PERCH (PERCA-FLUVIATILIS L) AND ROACH (RUTILUS-RUTILUS L)

The usefulness and catchability of multimesh gill nets and of a gill n et series were compared in two lakes in southern Finland. The results show that a multimesh gill net is much easier to use and its relative cost per sample is much lower than the gill net series. The catching e fficiency of multimesh gill nets was compared with that of gill net se ries by a method that is based on net selectivity curves and the poole d relative efficiency of mesh size combinations. The net selectivity c urves were defined for perch and roach by fitting a modified normal pr obability density function, No statistically significant differences w ere found between the net selectivity curves of different gill net typ es. It was essential that the model fitting was done over the same len gth-range of fish and as much as possible over the same mesh size rang e of the gill net types that were compared. The models cannot be extra polated outside the length range for which they were originally fitted . The catching efficiency of the multimesh gill net was greater than t hat of the gill net series after correcting for the selectivity of dif ferent mesh sizes. The catching efficiencies for perch in the multimes h gill nets and gill net series were different in the two lakes studie d. This suggests that the catching efficiencies of different gill net types are not necessarily the same in different lakes. More studies ar e needed in lakes of different sizes and morphometry.