Stock assessment of Queensland east coast dusky flathead (Platycephalus fuscus), Australia, with data to December 2020

Abstract

In Queensland, dusky flathead (Platycephalus fuscus) are found in inshore waters of east coastal regions from south of Cairns to the New South Wales (NSW) border and targeted by commercial, charter and recreational fishers. Dusky flathead are a gonochore (born male or female and does not change sex) and spawn primarily in the summer months. Female dusky flathead grow much larger and live longer than males. For Queensland’s east coast, they generally attain maximum average length of 85 cm (total length) and live for 11 years. Dusky flathead are believed to be a single population on the east coast of Queensland. Eighteen scenarios were run to cover a range of modelling assumptions and sensitivity tests for the stock model. Base case (most plausible) results suggested that the dusky flathead biomass experienced a decline in the period of 1901–2000 to reach 30% unfished spawning biomass (Figure 3). At the beginning of 2021, the stock level was estimated to be 46% unfished biomass with the 95% confidence interval between 31% and 62%. The East coast inshore fishery harvest strategy: 2021–2026 identifies a target reference point (Btarg) of between 50 and 60 % for tier 2 species within the fishery which applies to dusky flathead (Fisheries Queensland (2021b)). This variation in biomass targets for tier 2 species recognises different biological and economic characteristics among target species in the fishery. The equilibrium yield curve produced as part of this assessment suggests that a 50 % target reference point would maintain the stock in a more productive state than a 60 % target, and is therefore likely to be the most reflective of MEY. The harvest consistent with maintaining a spawning biomass of 50% was estimated at 138 t and for rebuilding the stock back to 50% target, the assessment recommends a biological catch of 106 t for 2021. The harvest consistent with maintaining a spawning biomass of 60% was estimated at 112 t. For a 60% target, the assessment recommends a biological catch of 60 t for 2021(Table 1).