Stock assessment of king threadfin (Polydactylus macrochir) in the Gulf of Carpentaria, Queensland, Australia, with data to December 2022

Abstract

King threadfin is a large, predatory fish species that is found in foreshore areas of turbid coastal waters, estuaries, tidal rivers and mangrove creeks across northern Australia and southern Papua New Guinea. In Australia, its distribution extends from the Ashburton River in Western Australia, across northern Australia, to the Brisbane River in South East Queensland. This assessment focuses on the Gulf of Carpentaria stock. King threadfin is a protandrous hermaphrodite, beginning as male and later changing to female between about 40 cm to 110 cm total length. It can grow to 150 cm total length and 30 kg in weight. The species lives to at least 20 years of age.

A previous assessment estimated the Gulf of Carpentaria stock to be around 5% of the unfished level, using data through to December 2019. This assessment differs from the previous assessment in several respects. Firstly, it considers data through to December 2022, and includes additional historical age and length data sets. Secondly, it operated under the guidance of a project team consisting of multiple domain experts including fishery stakeholders. Thirdly, it was externally reviewed and incorporates feedback from that review. Fourthly, the summary chapter reports biomass and fishing pressure results as a ‘most likely range’ rather than a single value.

All assessment inputs and outputs are referenced on a calendar year basis (that is, ‘2022’ means January 2022–December 2022).

The assessment used an age-structured model with an annual time step, fitted to standardised catch rates, length composition data, and age-at-length composition data. The model incorporated data spanning the period 1955–2022 including mandatory daily commercial logbook data collected by Fisheries Queensland (1989–2022), collated commercial production returns from the Gulf of Carpentaria (1981–1988), recreational and boat ramp survey data (1997–2022), and age and length data (1988–1994 and 2015–2022).

This stock assessment was guided by a project team with a wide range of skill sets. In addition to managers, scientists, monitoring and data specialists from within the Fisheries Queensland, the team included three fishing industry representatives.

Several scenarios were run covering a range of modelling assumptions and sensitivity tests. All scenarios were optimised using Markov chain Monte Carlo (MCMC) to better explore the robustness of the models. From these exploratory scenarios a final ‘ensemble’ of eight scenarios were chosen for inclusion in summary reporting. This ensemble indicates that the biomass ratio at the beginning of 2023 was between 13% and 44% of unfished levels. The proportion of the ensemble that fell below 20% biomass was 29%.