One mesh does not fit all: A dual compartment codend provides flexible selectivity opportunities to manage mixed fisheries

Abstract

Fisheries management is moving from a single-species to an ecosystem-based approach to better balance the technical, biological, and economic aspects of the many mixed-species fisheries in the world. Most mixed fisheries are conducted with trawl gears, which have a hereditary challenge of providing a species and size selectivity that meets the development in management objectives of reducing unwanted catch. When fishing on sympatric species, a codend that allows separation of species into different compartments during towing can provide separated catch fractions with contrasting selectivity opportunities. In a case study, we quantified the processes of vertical separation and length-based selectivity for a dual compartment codend having an upper, large mesh compartment intended for cod (Gadus morhua) and a lower, small mesh compartment intended for Nephrops (Nephrops norvegicus). This allowed identification of which processes to improve to reduce unwanted catch. The gear concept delivered a complex selectivity profile while enabling high flexibility to adjust selectivity, also at sea, and provides a tool for the industry to ensure better compliance with increasing management ambitions. An unusual selectivity curve for cod resulted from the contrast in mesh size between the two compartments. Cod and Nephrops smaller than and just above the minimum conservation reference size (MCRS) were efficiently released through the 120 mm square meshes in the upper and the 60 mm square meshes in the lower compartment, respectively. Cod (47%) had a low probability of escaping when caught in the lower compartment, and Nephrops (9%) were lost when caught in the upper compartment. A compartment in the dual compartment codend may easily be changed at sea to adjust the selectivity according to the mix of species encountered, available quota portfolio, and management regime.