Estimating quantitative gear and taxa specific encounter thresholds for commercial fisheries bycatch of vulnerable marine ecosystem indicator taxa

Abstract

Corals and sponges are often a component of vulnerable marine ecosystems (VMEs) in the deep sea. These taxa can be impacted and removed by bottom contacting fishing gear and protecting VMEs is an important component of managing ecosystems. One of the tools that is routinely used to manage VME impacts from fishing gear is move-on rules triggered by bycatch thresholds (encounter thresholds) of VME. Usually, these bycatch thresholds are set with little information regarding the level of impact on the benthic habitat. The objective of this analysis was to develop and apply methods for quantifying threshold catches of VME indicator taxa by gear type and VME indicator taxa grouping. Three previously used methods based on cumulative bycatch distributions and one novel method based on percentile regression of fishery bycatch and density from underwater camera surveys were applied to data from the northeast Pacific Ocean to determine data-based encounter thresholds that could trigger spatial fishery closures. The percentile regression method suggested encounter thresholds of ∼40–65 kg of Antipatharia, <20 kg of gorgonians and 78–131 kg of Porifera would equate to a density of 0.2 VME indicator taxa per $m^2$ for bottom trawl bycatch. Threshold values were lower for longline and pot gear (generally <10 kg per set). Using the percentile regression method allowed for the definition of VME encounter thresholds to be expressed in terms of density of the taxa of interest, an improvement over examination of break points in the cumulative bycatch data alone. This improvement allows the ecological importance (e.g. density of VME) to be defined and used to estimate encounter thresholds, rather than assuming that the natural break points in cumulative bycatch represent an ecological break point.