Stock assessments of bream, whiting and flathead (Acanthopagrus australis, Sillago ciliata and Platycephalus fuscus) in South East Queensland

Abstract

Yellowfin bream, sand whiting and dusky flathead are major target species for both commercial and recreational fishers in south east Queensland. Their fishery and regional social and economic importance prompted stock assessments to inform on the sustainability of fishing. The assessments covered both estuarine and ocean-beach waters between Baffle Creek north of Bundaberg and Coolangatta on the Gold Coast.

Over the last five years (2013 to 2017), the South East Queensland total harvest for yellowfin bream, sand whiting and dusky flathead averaged 242, 272 and 121 tonnes per year respectively.

The catches split for bream was 54 per cent commercial versus 46 per cent recreational, 77 per cent commercial versus 23 per cent recreational for whiting and 36 per cent commercial versus 64 per cent recreational for flathead.
The stock assessments used commercial, recreational, charter and indigenous catch, and research data. Inputs to the model included fish harvest sizes (1945 to present), standardised catch rates from commercial net logbook data (1988 to present), and fish age–length data collected from the fishery (2007 to present).

All three assessments were challenging due to lack of contrast in the data since the commercial logbook system began in 1988. All three species had been subject to high harvests prior to that year, and commercial catch rates had not varied much since then. In addition, the only available catch rates came from net fishing, which can target whole schools of fish. Net catch rates may be ‘hyperstable’ and not sensitive to trends in fish population size.

Bream biomass was estimated to be at 33.8 per cent of unfished biomass. The equilibrium maximum sustainable yield (MSY) was estimated as 420 tonnes per year (commercial and recreational sectors combined, and Moreton and Fraser regions combined). The model indicated that maintenance of a harvest size of 220 t ⁄ yr will recover the biomass to 60 per cent of unfished in about 25 years. A lower harvest of 150 t ⁄ yr would recover to 60 per cent in about 12 years.
Whiting biomass in 2017 was estimated as 28.7 per cent of unfished biomass, which is approximately the biomass corresponding to MSY (denoted BMSY). The model’s estimate of equilibrium MSY was 452 t ⁄ yr. Current combined harvest size is approximately equal to the equilibrium harvest at 60 per cent unfished biomass (B60). Rebuilding of the stock from its current level to B60, however, would require the harvest to be reduced, ideally to about 150 t (commercial and recreational sectors combined, and Moreton and Fraser regions combined) to rebuild within about five years. Yearly harvests between 150 and 270 t ⁄ yr would recover the stock more slowly; the midpoint 210 t ⁄ yr would reach B60 in about seven years.

The status of flathead is more uncertain than bream and whiting. The precautionary estimate of dusky flathead biomass in the Moreton region in 2017 was between 36 per cent and 39 per cent of unfished spawning biomass, approximately equal to or slightly below BMSY. The estimated MSY was 104 t ⁄ yr to 112 t ⁄ yr, approximately equal to current harvests. Recovery of the spawning stock to 60 per cent in the Moreton region would require the harvest to be reduced, ideally to 63 t ⁄ yr (commercial and recreational sectors combined, Moreton region only) which would recover to B60 within eight years. An intermediate harvest level of 73 t ⁄ yr would reach B60 within 16 years. In the Fraser region, fishing pressure on flathead was lower, and 2017 estimated spawning biomass was 70 per cent of unfished.

Although the results for flathead are already precautionary, additional caution may be needed in view of fishing club catch rates which date back to the 1950s and indicate that flathead were already heavily fished by 1988.