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 net 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 efficiency of multimesh gill nets was compared with that of gill net series by a method that is based on net selectivity curves and the pooled relative efficiency of mesh size combinations. The net selectivity curves were defined for perch and roach by fitting a modified normal probability density function, No statistically significant differences were found between the net selectivity curves of different gill net types. It was essential that the model fitting was done over the same length-range of fish and as much as possible over the same mesh size range of the gill net types that were compared. The models cannot be extrapolated outside the length range for which they were originally fitted. The catching efficiency of the multimesh gill net was greater than that of the gill net series after correcting for the selectivity of different mesh sizes. The catching efficiencies for perch in the multimesh gill nets and gill net series were different in the two lakes studied. This suggests that the catching efficiencies of different gill net types are not necessarily the same in different lakes. More studies are needed in lakes of different sizes and morphometry.