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Short-term latitudinal monitoring of eastern Australian elasmobranch bycatch from demersal trawling reveals high diversity and endemism, spatiotemporally stratified assemblages, and relatively low catch rates

Thomas C. Barnes*, Daniel D. Johnson

*Corresponding author:

ABSTRACT: Elasmobranchs are depleted globally mainly from fishing. 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 and to assess spatiotemporal stratification for designing future monitoring and to report catch rates of individuals caught during a trip (form baseline) we implemented an observer program (2017-2019). 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 two elasmobranchs were protected in NSW, ~7% qualified for conservation listing, ~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). Catch rates were low compared to other taxonomic groups (e.g. teleost fish) with two species captured at >10 per trip, five >2 per trip and the remainder very low. 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.