Characteristics of east Australian demersal trawl elasmobranch bycatch as revealed by short-term latitudinal monitoring

Abstract

Elasmobranchs are being depleted on a global scale, caused mainly by fisheries. Demersal trawling is a component of mortality but is often not assessed. This could pose risk to benthic/demersal elasmobranchs which are often endemic and therefore vulnerable to fisheries when species ranges are within (or mainly within) trawl footprints. Northern New South Wales (NSW) is an area with endemism but also an area with fisheries such as the ocean prawn trawl (OPT) (penaeid sector). The OPT may interact with elasmobranchs, but this has never been comprehensively studied. To identify high assessment-priority species, determine spatiotemporal stratification for designing future monitoring, and to report catch rates of individuals caught during a trip (i.e. form baseline), we implemented an observer programme (2017 to 2019). To test for stratification of assemblages, we used model-based multivariate analysis. On 435 trawl trips, observers identified elasmobranchs from ~54 species, 13 orders and 34 families from variable catches. Only 2 elasmobranchs were protected in NSW, ~7% qualified for conservation listing, and ~33 and ~17% were endemic and lifeboat (listed elsewhere) species, respectively. Models suggested common elasmobranch assemblages were significantly affected by all strata (geographic zone, season and depth). Elasmobranch catch rates were low compared to other taxonomic groups (e.g. teleost fish), with 2 species captured at >10, 5 species at >2, and the remaining species <2 individuals per trip. The occurrence of endemism and spatiotemporal assemblage variation was explained by mesoscale climate transitions and oceanography. This study forms a timely baseline which can be used to assess the impact of the OPT on elasmobranchs in the future.