Is there a limit to the potential effects of shortening lastridge ropes on the size selectivity of diamond mesh codends?

Abstract

Shortening codend lastridge ropes can be an effective fishing gear modification to improve the size selection properties in diamond mesh codends. Lastridge ropes attached to codend selvedges withstand the longitudinal forces created by the catch building up and therefore, prevent the codend meshes from closing. However, the extent to which the lastridge ropes should be shortened to maximize the effect of this measure is unclear. Besides opening codend meshes, shortening lastridge ropes can also lead to net folding, which can potentially have negative consequences for size selectivity. In the present study, we tested the size selective properties of a 129 mm diamond mesh codend in three different configurations: 0 \%, 15 \% and 30 \% shortened lastridge ropes. Selectivity data were collected for cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and redfish (Sebastes spp.) in the Barents Sea gadoid bottom trawl fishery. Shortening the lastridge ropes by 15 \% had a significant effect on the release efficiency of haddock between 35 and 50 cm, whereas to obtain a similar result for cod, the lastridge ropes had to be shortened by 30 \%. However, the use of shortened lastridge ropes significantly increased the retention of fish below 35 cm for both species, especially when the lastridge ropes were shortened from 15 \% to 30 \%. The effect on redfish size selectivity was in general limited. Exploitation pattern indicators showed that there was no added benefit from shortening them further from 15 \% to 30 \% for any of the three species. This study concludes that, while shortening lastridge ropes can contribute to improved size selection of diamond mesh codends, reducing them beyond 15 \% is not recommended because it can substantially increase the retention of undersized fish, probably due to net folding.