Stock assessment of Australian east coast Spanish mackerel (Scomberomorus commerson), with data to June 2024

Abstract

Spanish mackerel (Scomberomorus commerson) is a large pelagic species of fish. On the east coast of Australia they form a single genetic stock, between the Cape York Peninsula in northern Queensland and Newcastle on the New South Wales mid coast. In these waters, Spanish mackerel can live for up to 26 years, grow to over 30 kg in weight and reach maturity between 2 and 4 years of age, as observed in Fisheries Queensland data.

Previous assessments estimated the stock at 39–51% of unfished levels in 2009, 30–50% in 2016 and 17% (14–27%) in 2020. Several key aspects of this assessment differ from past assessments. Firstly, membership of the project team that guides the assessment was expanded, to include fishery stakeholders (one commercial and one recreational fisher) as well as an independent scientist. Secondly, this assessment benefited from updated data through to June 2024, as well as new data provided through the Fisheries Research and Development Corporation (FRDC) funded project ‘Addressing the uncertainties in the assessment and management of Queensland east coast Spanish mackerel’ (project number: 2021-111). Thirdly, the previous assessment underwent two external reviews, and feedback from these has been incorporated. Finally, the key outputs were constructed as an ensemble across multiple model scenarios, rather than selecting a particular preferred scenario.

All assessment inputs and outputs are referenced on a financial year basis— ‘2024’ means ‘July 2023 to June 2024’.

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 Queensland and New South Wales data spanning the period 1911–2024, collected from the commercial, recreational and charter sectors.

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 six scenarios were chosen for inclusion in summary reporting. This ensemble indicates that the biomass ratio at the beginning of 2025 was between 17% and 62% of unfished levels with a median of 34% of unfished levels.