Estimating the sustainability of towed fishing-gear impacts on seabed habitats: a simple quantitative risk assessment method applicable to data-limited fisheries

Abstract

Impacts of bottom fishing, particularly trawling and dredging, on seabed (benthic) habitats are commonly per-ceived to pose serious environmental risks. Quantitative ecological risk assessment can be used to evaluate actualrisks and to help guide the choice of management measures needed to meet sustainability objectives.2.We develop and apply a quantitative method for assessing the risks to benthic habitats by towed bottom-fish-ing gears. The method is based on a simple equation for relative benthic status (RBS), derived by solving thelogistic population growth equation for the equilibrium state. Estimating RBS requires only maps of fishingintensity and habitat type–and parameters for impact and recovery rates, which may be taken from meta-ana-lyses of multiple experimental studies of towed-gear impacts. The aggregate status of habitats in an assessedregion is indicated by the distribution of RBS values forthe region. The application of RBS is illustrated for atropical shrimp-trawl fishery.3.The status of trawled habitats and their RBS value depend on impact rate (depletion per trawl), recovery rateand exposure to trawling. In the shrimp-trawl fishery region, gravel habitat was most sensitive, and though lessexposed than sand or muddy-sand, was most affected overall (regional RBS = 91% relative to un-trawledRBS = 100%). Muddy-sand was less sensitive, and thoughrelatively most exposed, was less affected overall(RBS = 95%). Sand was most heavily trawled but least sensitive and least affected overall (RBS = 98%).Region-wide, >94% of habitat area had >80% RBS because most trawling and impacts were confined to smallareas. RBS was also applied to the region’s benthic invertebrate communities with similar results.4.Conclusions. Unlike qualitative or categorical trait-based risk assessments, the RBS method provides a quanti-tative estimate of status relative to an unimpacted baseline, with minimal requirements for input data. It could beapplied to bottom-contact fisheries world-wide, including situations where detailed data on characteristics ofseabed habitats, or the abundance of seabed fauna are not available. The approach supports assessment againstsustainability criteria and evaluation of alternative management strategies (e.g. closed areas, effort management,gear modifications).