GILLNET SELECTIVITY AND A MODEL FOR CAPTURE PROBABILITIES FOR A STUNTED BROWN TROUT (SALMO-TRUTTA) POPULATION

The gillnet selectivities for a lacustrine population of allopatric brown trout (Salmo trutta) were estimated directly, using maximum girth as the parameter for fish size. A model for capture probabilities separating the of encountering the net and being retained in case of an encounter was developed. Swimming distances and retention probabilities were estimated from the model. Size selectivities decreased with increasing mesh size and fish size. This was explained by decreasing swimming distances with increasing fish size. The retention probabilities are consistent with a geometrical similarity hypothesis. Serious errors in the assessment of population size structure by gillnetting may arise when variations in swimming distances are not accounted for.