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Use of experimental gillnet fleets is common both in scientific studies of fish populations and in fish
sampling for management purposes. Fish catchability may vary considerably with fish and gillnet mesh
size, and catches obtained by gillnet fleets composed of nets with different mesh sizes may give length
and age distributions that deviate considerably from the length and age structure of the population.
We have estimated the absolute catchability of allopatric brown trout (Salmo trutta) in the littoral and
pelagic habitat of a small lake based on a mark-recapture experiment. The brown trout catchability
varied considerably both with fish size and habitat type, probably due to a size-related variation in
swimming distance per time unit and a size-related use of the different lentic habitats. The sampling
bias in experimental gillnet fishing may be reduced by operating the gillnet fleets in all possible lentic
habitats and most fundamentally, by use of catchability data obtained from populations with ‘known’
length and age structures. By reducing this sampling bias, more realistic estimations of the age and
length distribution for a given population will be possible.
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