A gear component approach to trawling impact and sediment mobilization assessments

Increased awareness of the impacts of bottom trawling has led to the development of various methods to assess the extent and impact of trawling-induced seabed abrasion. These methods typically classify bottom trawls into broad categories and rely on whole-gear averages to evaluate their spatial distribution and seabed impacts. However, such approaches overlook variations in gear design within fisheries and are unable to capture changes resulting from gear innovations. Here, we present a Gear Component Approach (GCA), which, using data from literature and industry-surveys, considers the individual contribution of each seabed-contacting gear component (doors, clumps, sweeps, and ground gear) within a fishery. We demonstrate the GCA's ability to assess the extent and impact of three Danish otter trawl fisheries and compare these results with those obtained using the currently practiced Whole-Gear Approach (WGA). The GCA produced different estimates of gear width and penetration depth compared to WGA estimates. This led to lower predictions of the relative benthic state (RBS) indicator, which represents the proportion of the original benthic community biomass remaining under a given fishing pressure. These results imply higher fishing impacts for all three otter trawl fisheries compared to the WGA estimates. The difference was most pronounced for the Nephrops fishery, where the RBS indicator was 23% lower. Additionally, the GCA enabled the quantification of sediment mobilization, with up to 3.2 kg per m2 swept by the Nephrops trawl. The GCA improved the representativeness of fishing gear and the accuracy of impact assessments, thereby supporting sustainable marine fisheries management. Moreover, owing to its focus at the gear-component level, the GCA facilitates the evaluation of innovative gear modifications aimed at reducing seabed impact.