The effects of fish accumulation on the catchability of multimesh gillnet

The catchability of a gillnet reduces as the fish accumulate in it. This may skew the estimations of relative fish abundance or community structure. Two lakes with different fish densities and water clarity were sampled diurnally by continuous (12 h) and consecutive (3 x 4 h) gillnetting. The catchability of the NORDIC multimesh gillnet reduced considerably during 12 h set time. The catchability model suggested that even a small catch can reduce the catchability, and the level of zero-catchability is reached with a rather low proportion of occupied meshes. Independent of the catch level, the reduction was stronger during daytime or clear water conditions indicating that avoidance, rather than space limitation, was the main reason for decreased catchability. The validity of gillnet CPUE as an index of fish abundance could be improved by shortening the fishing time, and gillnetting during dark periods. (C) 2004 Elsevier B.V. All rights reserved.