Appropriate fishery management requires a fundamental understanding of the demographic processes of a fish population to assess its response to exploitation. Monitoring the demographics of a population through time can provide an indication of its response to ecological processes and varying levels of exploitation, which provides valuable information for fishery management. Such population demographics include the length and age of individuals through time, trends in juvenile recruitment, and estimates of biomass that collectively provide an indication of overall stock status.
There are several aspects of the biology and life history of Snapper that renders the species susceptible to significant changes in abundance and biomass. Snapper is a relatively long-lived, slow-growing species, and its population dynamics and fishery productivity are fundamentally driven by highly variable inter-annual recruitment, i.e., the number of age-0 juveniles that enter the population each year (Fowler and Jennings, 2003; Hamer and Jenkins, 2004; Saunders 2009; Fowler et al., 2017; Langley 2024). This episodic recruitment is manifested in the age structure of the population and results in cyclic trends in biomass and fishery production through time. That is, strong recruitment events lead to an increase in biomass in the following 4-10 years, and declines in biomass are associated with extended periods of poor recruitment and continued exploitation. These biological characteristics have underpinned the significant increase and subsequent rapid decline in fishery production for numerous Snapper stocks throughout Australia and New Zealand over the past 50 years, e.g., SNA7 and SNA8 in New Zealand (Langley, 2024), Shark Bay and Cockburn Sound in Western Australia (Christensen and Jackson, 2014), and Spencer Gulf and Gulf St Vincent in SA (Fowler et al., 2017; Drew et al., 2022).
In SA, the monitoring of Snapper populations has primarily relied on access to commercial catches through a market sampling program, which has provided invaluable data on the length and age of individuals and recruitment variability for each stock over the past 30 years. More recently, estimates of spawning biomass using the daily egg production method (DEPM) have become an important fishery-independent metric to monitor trends in the population. These two independent data sources indicated that the SG/WC and GSV stocks were depleted, and that management intervention was required (Fowler et al., 2019; 2020; Drew et al., 2022). However, as a consequence of the fishery closures, there is a paucity of fishery-dependent information and a significant need to monitor the stocks to detect evidence of recovery.
This research proposal has been developed to address three research priorities:
• Continue the annual time series of length and age structures for each regional population of Snapper in SA. The population age structures are interpreted to retrospectively infer recruitment and are a fundamental input into the stock assessment model.
• As a consequence of the fishery closures, there is a need to use fishery-independent methods to monitor the trends in fishable biomass in SG and GSV. This involves the application of the refined DEPM methodology and hydroacoustic technqiues developed in 2023/24 through FRDC Project No. 2023-091 – Snapper Science Program: Research Theme 2 – Estimates of Biomass.
• Provide updated information and assign stock status for each stock of Snapper in SA that will be used to inform future fishery management (i.e., review the current fishery closures).
Consequently, Research Theme 3 – Monitoring and Assessment involves three projects:
3.1 Biological sampling program
3.2 Fishery-independent estimate of biomass
3.3 Stock assessment
3.1 Biological sampling program
Length and age structures are fundamental to monitor the demographics of a population and are an integral data source to assess Snapper populations. Since 2000, a weekly market sampling program has been undertaken by SARDI researchers to provide biological data for Snapper caught by commercial fishers across SA. The market sampling program has conformed to a two-stage protocol, where each Snapper in a catch is measured and a sub-sample of the catch is processed for biological information (i.e., length, weight, age, sex, maturation). Over 75,000 Snapper have been measured and 25,000 processed for biological information, which has been used to produce a time series of annual length and age structures for the six regional populations of Snapper in SA: the West Coast of Eyre Peninsula (WC), northern Spencer Gulf (NSG), southern Spencer Gulf (SSG), northern Gulf St Vincent (NGSV), southern Gulf St Vincent (SGSV), and the South-East (SE) Region. The age structures for each region are interpreted to retrospectively evaluate recruitment variability and are an essential input to the fishery model.
The closure of the SG/WCS and GSVS since November 2019 prevented the collection of biological information from commercial catches for these stocks. To overcome this, a targeted sampling program was developed to collect representative biological samples and continue the annual time series of length and age structures. This involved contracting commercial fishers of the Marine Scalefish Fishery (MSF) to collect samples of Snapper from the regions closed to fishing. Fishers operated under the direction of SARDI researchers with an observer present, pursuant to a Ministerial Exemption. After researchers collected biological information, the fish were processed by local processors, and the product was donated to Foodbank who distributed them to meet community needs. The SE Region remained open to fishing and biological data were obtained through the market sampling program.
This project will continue the sampling program to collect biological data for Snapper from the SG/WC and GSV Stocks in 2023/24 and 2024/25. Representative samples of Snapper will be collected for the five regional populations currently closed to fishing (i.e., WC, NSG, SSG, NGSV, and SGSV), which will be used to develop annual length and age structures for each region. The biological data and samples collected (i.e., otoliths and genetics) will be used in multiple projects of the Snapper Science Program.
3.2 Fishery-independent estimate of biomass
One of the highest research priorities for Snapper is the development of reliable fishery-independent indices to monitor population trends and inform stock status (Cartwright et al., 2021). This need is driven by the lack of information on stock status provided by fishery-dependent indices, especially catch-per-unit-effort, resulting from changes in management regulations and hyperstability associated with targeting aggregations. Furthermore, fishery-independent estimates of biomass are essential in the absence of fishery-dependent data resulting from fishery closures.
Since 2013, the daily egg production method (DEPM) has been used to periodically estimate spawning biomass for Snapper in SA’s gulfs and has become a fundamental component of the stock assessment. Two FRDC funded projects have been completed to adapt the DEPM methodology to Snapper – 2014-019 (Steer et al., 2017) and 2023-091 (Drew et al., ongoing). As part of the latter project, several fishery-independent methods were investigated for their applicability and economic feasibility to estimate biomass for Snapper, which involved considerable stakeholder engagement and feedback. The recommendations from that project and the Snapper Science Stakeholder Group (SSSG) have informed the methodology to be applied to estimate biomass for Snapper in South Australia’s gulfs in summer 2024/25 – a combination of the DEPM and hydroacoustic surveys.
The design of the DEPM surveys in Spencer Gulf and Gulf St Vincent in summer 2024/25 is based on the results and recommendations of the refinement study (FRDC Project No. 2023-091). The surveys will involve high intensity plankton sampling (i.e., 1.5 × 1.5 nm systematic grid) using vertical tows throughout the key spawning areas in each gulf. Concurrently, representative samples of adult Snapper will be collected by contracted commercial MSF fishers to estimate adult reproductive parameters. Estimates of spawning biomass for each gulf will be compared to retrospective estimates for the same survey area from previous surveys to evaluate relative trends in estimated spawning biomass since 2013.
For Gulf St Vincent, the hydroacoustic survey that was trialed in summer 2023/24 will be expanded in 2024/25 to sample other key spawning aggregation sites. The survey will apply the sampling design developed in the previous study, i.e., high intensity sampling using the ‘star pattern’ design at key aggregation sites. Key areas in Gulf St Vincent that each contain multiple aggregation sites will be surveyed at the same time as the DEPM survey. The absolute biomass of Snapper at each aggregation site will be estimated and cumulative estimates for each survey area will be calculated. These estimates will be incorporated into the stock assessment and will act as a baseline for future surveys to monitor trends in relative abundance using acoustic methods.
3.3 Stock assessment
Stock assessments for Snapper in SA were first undertaken in the early 1980s (e.g., Jones 1984), and have been completed every 2-3 years since the late 1990s. Initially, these assessments considered trends in commercial MSF statistics (i.e., catch and effort) and length and age structures developed for fish captured from the main region of the fishery (i.e., NSG). Over time, stock assessments for Snapper became increasingly comprehensive as the time series of fishery statistics extended and annual length and age structures were developed for each regional population (since 2000). Furthermore, additional data sources were incorporated for interpretation including the abundance of 0+ juveniles in NSG (from 2000 to 2010), outputs from the stock assessment model (since 2003), evaluation against prescribed fishery performance indicators (since 2007), and more recently, estimates of spawning biomass using the DEPM (since 2018). These data were considered in a weight-of-evidence approach to assign stock status following the National Fishery Status Reporting Framework (Piddocke et al., 2021).
The next stock assessment for Snapper will be the most comprehensive to date. It represents a culmination of research from the Snapper Science Program and will involve multiple data sources to assess the status of each stock in SA, i.e., SG/WCS, GSVS, and the SE Region. The data sources include fishery statistics, regional length and age structures, fishery-independent estimates of biomass (i.e., from the DEPM and hydroacoustic surveys), and trends in juvenile recruitment that are integrated into a fishery model to produce a series of biological performance indicators. The assessment will be delivered in November 2025 and will underpin future fishery management.
