Torres Strait Finfish Fishery: Spanish mackerel stock assessment, with data to June 2021. Year One Report.

Abstract

Spanish mackerel, Scomberomorus commerson, sustain an important finfish line fishery within the Torres Strait and are managed as a single stock. In these waters the species have been recorded to live for up to 13 years, weigh in excess of 20 kg and mature from two years of age.

The Australian Fisheries Management Authority (AFMA) commissioned annual updates to the Torres Strait Spanish mackerel stock assessment for three years 2021–2023. This was to monitor biomass estimates that were close to the 20 percent limit reference point for declaring an overfished stock.

Across analyses of different fish natural mortality and harvest, the median estimated spawning biomass of Spanish mackerel in 2020 was 29 percent of unfished estimates at the start of the fishery in 1940. The low biomass result was due to the high harvests between 1980 and 2006 and the downturn in Spanish mackerel catch rates 2010–2019.

The 2022–2023 recommended biological catch (RBC) of Spanish mackerel for all fishing sectors in the Torres Strait was 95 t based on the median forecast estimate (Table 1). This RBC was forecast to build Spanish mackerel towards a target biomass of 48% within 12 years, and have less than 10% risk of reducing to the 20% biomass limit reference point.

The assessment work also initiated the first comparison of the packaged stock assessment software stock synthesis (SS), which was used for assessing Australian east coast and Gulf of Carpentaria Spanish mackerel. The SS estimate of spawning biomass ratio in 2020 was similar compared against the current model developed by the Department of Agriculture and Fisheries and reported herein. SS performance and results will be further tested in years two and three of this AFMA project, noting that future provision of a streamlined stock assessment is to transition to stock synthesis after TSFFRAG review.