Stock assessment of king threadfin (Polydactylus macrochir) in Queensland, Australia

Abstract

King threadfin is a large, inshore, predatory fish species that can grow to 150 cm total length and 30 kg in weight. It feeds mainly on prawns, other crustacea and small fish. It is found in foreshore areas of turbid coastal waters, estuaries, tidal rivers and mangrove creeks across northern Australia and as far south as the is Brisbane River in South East Queensland.

The stock assessment uses an age-structured model with an annual time step and incorporates data from 1945 to 2019, including harvest sizes, standardised catch rates from commercial gillnetting daily logbook records, and length and age information. It assesses five separate Assessment Regions which are assumed to be self-contained stocks, one in the Queensland Gulf of Carpentaria and four on the Queensland East Coast.

Model analyses suggest that spawning biomass in 2019 was between 54% and 69% in each of the East Coast Assessment Regions of the unfished biomass and 5% of the unfished level in the Gulf of Carpentaria. On the East Coast, acceptable annual harvests under Queensland’s Sustainable Fisheries Strategy 2017–2027 exceed current harvests. In the Gulf, it is recommended that no harvest be taken for three years, to allow the stock to rebuild to above 20% of unfished levels. Then Gulf harvest can gradually resume, matching the 2019 level in the eighth year of rebuilding, at which time the spawning biomass will have recovered to 44% of unfished. Gulf harvest and biomass would continue to increase until biomass levels out at 60%.