Refinement of bycatch reduction devices to exclude freshwater turtles from commercial fishing nets

Abstract

The capture of non-target species is a conservation issue in many commercial fisheries. Bycatch reduction devices (BRDs) are commonly used as mitigation tools to improve selectivity of fishing gear and thus reduce bycatch. The aim of this paper was to refine a simple BRD to exclude 4 species of freshwater turtles from commercial fyke nets in a fishery in eastern Ontario, Canada, that targets a variety of fish species. We tested the efficacy of modified exclusion devices (vertically oriented exclusion bars and a constriction rectangle) using an adaptive approach including in situ observations, controlled behavioural experiments and field trials. In situ observations made by camera were used to estimate turtle catchability and to document turtle behaviour during net interactions, which was used to inform BRD design and placement. In controlled behavioural experiments, the passage rates of target fish (i.e. sunfish), bycatch fish (e.g. game fish) and turtles across a modified net throat suggested that a 5 cm constriction rectangle should be suitable for reducing bycatch in this fishery; turtles readily turned sideways to pass through larger openings. Paired field trials indicated that a 5 cm constriction rectangle reduced turtle bycatch for all species. The constriction rectangle also reduced captures of non-target game fish. In controlled behavioural experiments, there was little evidence of a reduction in catches of target sunfish; however, in paired field trials, there was a 23.4% reduction in sunfish catches. We recommend the use of a 5 cm constriction rectangle for fisheries targeting sunfish in areas where freshwater turtles are present.