Species selectivity in different sized topless trawl designs: Does size matter?

Most demersal trawl fisheries are conducted in a multispecies setting, and the catch consists of severaldifferent species. An inherent challenge in such fisheries is to provide both biologically and economicallysustainable exploitation of individually fluctuating stocks and vessel- or fleet-specific quotas. The toplesstrawl design was developed to improve species-specific selectivity in such fisheries. In a topless trawl,the foot rope is located more forward than the headline to allow fish to escape upwards, whereas theheadline is located in front in traditional trawl designs. In this study we conducted twin trawls with atopless trawl towed parallel to a similar standard trawl; we tested a topless trawl design on a small trawlwith a low headline height and on a larger trawl with a high headline height. We conducted the towsin the Nephrops (Nephrops norvegicus) directed mixed fisheries. For both the small and large trawls, wefound a significant topless effect for haddock (Melanogramus aeglefinus) and no effect for Nephrops. ForAtlantic cod (Gadus morhua) we found a significant topless effect for the low headline trawl but no effectfor the high headline trawl. In both the eastern and western Atlantic, topless trawls have been introducedas legal cod-selective trawl designs. However, this study demonstrates that identical gear modificationsmade to similar trawls of different sizes and used in the same fishery can lead to different results.