Comparison of baited longlines and baited underwater cameras for assessing the composition of continental slope deepwater fish assemblages off southeast Australia

Abstract

Expansion of demersal fisheries into the deep sea has driven a need for methods to provide information on the status of deepwater fish assemblages, especially for vulnerable species. For this purpose, we compared co-located sampling by baited longlines and baited remote underwater video systems (BRUVs) off southeastern Australia and found these techniques observed different elements of the continental slope fish assemblage, but were complementary for monitoring needs. Of the 94 species surveyed in total, only 32% were common to both survey techniques, with BRUVs sampling fewer species, with 51 species detected compared with 73 species captured by longlines. Species detected exclusively by BRUVs (21 spp) included those not vulnerable to longline hooks, and those with large-body size—including several chondrichthyans. One group of chondrichthyans, gulper sharks (Centrophorus) were the focus of this study because they are listed as conservation dependent under Australia’s Environmental Protection and Biodiversity Conservation Act, and are subject to a management plan that requires their recovery to be monitored. Very few gulper sharks were observed by BRUVs (n=10). Longlines captured 773 gulper sharks suggesting this technique provides a more effective means of sampling. However, most longline caught sharks were either dead or in poor condition on capture, further depleting this vulnerable species. BRUVs are non-lethal, but a high sampling intensity is likely to be needed to detect changes in gulper shark abundance within typical management timeframes (years).