One of the highest research priorities for Snapper is the development of reliable independent methods to monitor population trends and inform management decisions. This is part of the focus of the Snapper Science Program – a three-year, $5 million initiative co-funded by the South Australian Government and FRDC.
By Dr Sophie Dolling
Monitoring population size and demographics is crucial for managing fisheries and developing reliable independent methods for estimating the numbers of catchable fish. This need is driven by the aggregating nature of Snapper, which not only makes them susceptibility to being caught, but can also make it look like there are greater numbers than there really are, masking significant declines in the population until it is too late.
Snapper Science Program researchers are evaluating two fishery-independent methods to estimate Snapper biomass: the daily egg production method (DEPM) and hydroacoustic surveys. As a part of FRDC Project No. 2023-091, researchers undertook several experimental field studies in summer 2023-24 to refine the two methods and inform the most appropriate approach to estimate biomass for the 2025 Snapper stock assessment in South Australia.
Daily egg production method (DEPM)
Since 2013, the DEPM has been applied periodically to estimate the spawning biomass of Snapper in South Australia, where it has become an integral tool to monitor population trends and inform stock status. Last year, the Snapper research team undertook trials to improve the DEPM survey design by assessing sampling intensity and three different plankton sampling methods (vertical, oblique, and neuston tows).
‘The daily egg production method (DEPM) refinement project has been an important step in enhancing the application of this method for Snapper to provide the most precise and robust estimate of spawning biomass for the 2025 stock assessment’ says Dr Michael Drew.
Results indicated that despite significant differences in the volume of water filtered by each sampling method, the concentrations of Snapper eggs collected were very similar. Furthermore, the high intensity plankton sampling identified discrete patches of Snapper eggs that aligned with aggregations of spawning fish. These results were consistent between surveys conducted in consecutive months.
Last year’s trial has informed the design of surveys conducted in December 2024 and January 2025 as part of the 2025 stock assessment for Snapper in South Australia. This resulted in a greater spatial intensity of plankton sampling and a vertical tow was used to sample for Snapper eggs at over 500 stations in Spencer Gulf and Gulf St Vincent. Snapper eggs are identified in each sample using a molecular probe developed through FRDC Project No. 2014-019, which contributes to estimating spawning biomass and assigning stock status.
Click to continue reading on the FRDC website.