Distribution and abundance of Glyphis and sawfishes in northern Australia and their potential interactions with commercial fisheries

Abstract

The distributions of Glyphis and sawfishes are poorly known around most of the coast of the Northern Territory (NT). Previous studies have only found Glyphis in a few of the freshwater habitats around northern Australia and as such have been considered extremely rare. Our study has expanded what is known of the distribution of Glyphis and shown that these species may be more abundant than previously thought. Three species of sawfish (narrow, green and dwarf sawfish) were also found across large areas of the Northern Territory. The distribution of freshwater sawfish remains poorly understood. The analysis of different catch records from NT Fisheries and survey data indicate that a standard reporting and logbook scheme should be adopted by NT Fisheries that interact with these Threatened, Endangered and Protected (TEP) species. Of these, the NT Offshore Net and Line (NTONL) fishery provides the greatest level of detail in logbooks (catch and TEP reporting) and allows for species-specific estimates of catch. In addition, observer coverage is essential to verify catch records and collect biological information for these data-deficient species. We suggest that the reporting requirements for this fishery be adopted for all commercial fisheries in the Northern Territory. Local-scale information from catch records of TEP species and observer programmes has added to our knowledge of distributions identified from TEP catch and observer data. Furthermore, the presence of Glyphis garricki in the Daly River has been confirmed by our independent surveys. Spatial and temporal patterns in abundance of these species, however, are still unclear. The NT Barramundi Fishery (NTBarr) has the greatest interactions with these species: they represent about 12% of the elasmobranch bycatch compared to only ~0.2% of the total shark catch of the NTONL fishery. This may represent a greater local abundance of these globally threatened species inshore, especially compared to other Glyphis and sawfishes populations in the region, but the biological consequence of this interaction is unclear and requires further investigation.