Gillnet catchability of brown trout Salmo trutta is highly dependent on fish size and capture site

Use of experimental gillnet fleets is common both in scientific studies of fish populations and in fishsampling for management purposes. Fish catchability may vary considerably with fish and gillnet meshsize, and catches obtained by gillnet fleets composed of nets with different mesh sizes may give lengthand 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 andpelagic habitat of a small lake based on a mark-recapture experiment. The brown trout catchabilityvaried considerably both with fish size and habitat type, probably due to a size-related variation inswimming distance per time unit and a size-related use of the different lentic habitats. The samplingbias in experimental gillnet fishing may be reduced by operating the gillnet fleets in all possible lentichabitats 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 andlength distribution for a given population will be possible.