Stock assessment of the Queensland Gulf of Carpentaria Spanish mackerel (Scomberomorus commerson) fishery

Spanish mackerel (Scomberomorus commerson) is a pelagic species that forms genetic populations (stocks) around northern Australia. For this stock assessment, we have investigated fish harvested from eastern waters of the Gulf of Carpentaria, Queensland. This stock assessment was set up as an age-structured model with a yearly time step and length-based selectivity. The annual data inputs included total fish harvest, standardised catch rates, length structures and conditional age at length data. The model used data from 1940 to 2018. The assessment suggested the stock in 2018 was between 30 and 40 per cent unfished spawning biomass. The recommended biological catch was estimated at 21 t, which is below the 2018 harvest of 181 t. This initial catch estimate aims to allow the stock to rebuild to the target 60 per cent spawning biomass. Smaller harvests are estimated at first to be taken in early years, which increase through time as the fish population builds higher

Stock assessment of Australian east coast Spanish mackerel (Scomberomorus commerson)

Spanish mackerel, Scomberomorus commerson, are large offshore pelagic fish. On the east coast of Australia, Spanish mackerel form a single genetic stock between Cape York Peninsula in north Queensland and Newcastle on the New South Wales mid-coast. This stock assessment used an annual time-step, two-sex, age-structured population model. The model incorporated data from 1911 to 2020, including estimated commercial, charter and recreational harvest for Queensland and New South Wales, Queensland commercial standardised catch rates, fish age-length frequencies, and key long-term fishery information on fishing power changes and catch rates. Eight model scenarios were run, covering a range of modelling assumptions and fixed parameters. The stock assessment estimates the spawning biomass of east coast Spanish mackerel in 2020 was between 14 and 27% of unfished levels in 1911. The base case model estimate was most likely at 17% of unfished biomass in 2019–2020. This report provides estimates of sustainable harvests for all sectors—commercial, charter and recreational across Queensland and New South Wales—and supports the harvest strategy defined in the Queensland’s Sustainable Fisheries Strategy 2017–2027 (Department of Agriculture and Fisheries 2017)

Stock assessment of Australian east coast Spanish mackerel : Predictions of stock status and reference points

The stock assessment describes Australia’s east coast Spanish mackerel fishery operating in ocean waters between Cape York Peninsula and northern New South Wales. The species are a large offshore pelagic fish that can live for up to 26 years and weigh in excess of 30 kg. Spanish mackerel reach maturity above the minimum legal size of 75 cm at between two and four years of age. The stock assessment was conducted at the whole stock level and included commercial, charter, recreational and research data from both New South Wales and Queensland. The data included estimates of Spanish mackerel harvest from logbook systems and recreational fishing surveys, catch rates from commercial logbooks and historical surveys of long term commercial fishers, and annual fish age-length compositions. The fishery for east coast Spanish mackerel commenced in the early 1900s. Annual east coast harvests taken by commercial, charter and recreational fishing steadily built to peak at 1000–1300 tonnes (t) per year during the 1970s and 1000–1150 t per year between 1998 and 2003. The estimated annual Spanish mackerel harvest since 2005 reduced to 500–760 t per year. Most fishers harvest Spanish mackerel using line-fishing techniques. Net fishing for Spanish mackerel is illegal. In recent years about 47 per cent, 6 per cent and 47 per cent of annual Spanish mackerel harvests were taken respectively by commercial, charter and recreational fishing across all east coast waters. Population modelling of the data estimated that Spanish mackerel population size in 2016 was between 30–50 per cent of original biomass estimated at the start of the fishery in 1911 (best estimate around 40 per cent biomass). Estimates of recommended sustainable annual harvest of Spanish mackerel for all fishing sectors and east-coast waters ranged between 400 and 800 t. There is presently substantial unfished commercial quota. The current Queensland total allowable commercial catch quota is 574.6 t. If this were to be largely utilised, together with current or increased charter, recreational and New South Wales commercial harvests, then the biomass of the Spanish mackerel population may decline. Such high harvests would result in overfishing and reduce average catch rates longer-term. The results suggested annual harvests of around 550 t (across all sectors) will build the biomass towards the 60 per cent level, consistent with the 2027 biomass targets set in the Queensland Government’s Sustainable Fisheries Strategy. If there is a desire to operate the fishery closer towards 60 per cent biomass for better economic yield and quality of fishing (higher catch rates), then fishing pressure will need to reduce for a period of time to build the fish population to a higher biomass