A comprehensive understanding of the general biology and ecology of any fishery species is fundamental to determine its response to exploitation and inform appropriate fishery management. Despite the significant body of research into the biology of Snapper, there remain considerable knowledge gaps regarding the underlying factors that drive interannual variation in juvenile recruitment and the demographic processes that maintain populations. Furthermore, there is uncertainty in how these processes will be influenced by changing environmental conditions associated with climate change. As such, understanding drivers of recruitment variability was identified as one of the highest research priorities for Snapper at the most recent National Workshop (FRDC Project No. 2019-085; Cartwright et al. 2021). Given the strong relationship between episodic recruitment and fishery production described above for the SG/WCS and GSVS, this recommendation was also strongly endorsed by fishery researchers, managers, and industry stakeholders in SA (Drew et al. 2022).
This research proposal has been developed to address four research priorities:
• To understand the biological and environmental factors that affect recruitment of Snapper in SA and evaluate the potential influence of climate change.
• Provide a contemporary understanding of stock structure for Snapper on the west coast of Eyre Peninsula to inform the appropriate spatial scale for fishery management.
• Develop a contemporary series of biological parameters for each stock of Snapper in SA to be used as inputs in the stock assessment model.
• Evaluate changes in the physical environment that may affect Snapper recruitment.
Consequently, Research Theme 1 – Biology and Ecology involves four projects:
1.1 Investigating recruitment variability and evaluating the potential effects of climate change for Snapper in South Australia
1.2 Contemporary demographic processes and stock structure for Snapper on the west coast of Eyre Peninsula
1.3 Review of biological parameters for Snapper in South Australia
1.4 Benthic habitat survey for Gulf St Vincent.
1.1 Investigating recruitment variability and evaluating the potential effects of climate change for Snapper in South Australia
The population dynamics and fishery productivity for Snapper in SA are fundamentally driven by highly variable interannual recruitment, i.e., the number of age 0+ juveniles that enter the population each year (Fowler et al. 2017, Fowler and Jennings 2003). As such, a relative index of annual age 0+ juvenile abundance would be a powerful, fishery-independent tool to predict future trends in fishable biomass. To address this need, a recent project was undertaken to identify the most appropriate sampling methodology for age 0+ Snapper in SA’s gulfs and to develop a pre-recruit index (FRDC Project No. 2019-046). The first component of the present study involves the continuation of annual surveys for age 0+ Snapper for each stock to monitor trends in juvenile recruitment. The surveys will be repeated annually at the recognised nursery areas for each stock, i.e., northern Spencer Gulf (NSG) for the SG/WCS and northern Gulf St Vincent (NGSV) for the GSVS (Fowler et al. 2017). In addition, age 1+ juvenile Snapper will be sampled from annual fishery-independent surveys for the Spencer Gulf and Gulf St Vincent prawn fisheries, which will provide further information to determine relative year class strength.
The second component of the study involves investigating the relationships between environmental parameters and recruitment. The datasets for juvenile abundance will be considered with annual population age structures to develop a time series of recruitment for the two stocks (i.e., late 1960s to 2020s). Long-term time series of environmental parameters (e.g., temperature, salinity, productivity, wind stress) will be developed and compared to the time series of recruitment for each stock. In conjunction with the pre-recruit index, understanding the environmental influences that drive recruitment variability would provide even greater predictive capability to forecast trends in recruitment and fishable biomass, particularly under changing environmental conditions associated with climate change.
The third component of the study will investigate the potential effects of environmental change for Snapper in SA. Using the environmental datasets previously developed, a high-resolution oceanographic model for SA will be hindcast to determine the intensity of local environmental change and identify potential climate ‘hot spots’. The model will then be forecast with different climate change scenarios to predict changes in ocean conditions over the next 5, 10, and 50 years. Based on the physiological tolerance ranges for Snapper spawning and larval development, these predictions will be used to evaluate potential shifts in the availability of suitable environmental conditions for Snapper in SA. Such responses will be considered in terms of potential implications for future trends in recruitment and fishable biomass.
1.2 Contemporary demographic processes and stock structure for Snapper on the west coast of Eyre Peninsula
There are three recognised stocks of Snapper in SA waters: the SG/WCS, the GSVS, and the Western Victoria Stock (WVS) (Fowler 2016, Fowler et al. 2017). The population of Snapper on the West Coast of Eyre Peninsula (WC) is a regional component of the SG/WCS. It is hypothesised that in most years, the WC population is replenished by local demographic processes that maintain the population at a relatively low level. However, episodically in years of exceptionally strong recruitment in northern Spencer Gulf (i.e., 1991, 1997, and 1999), the WC population is replenished through the density dependent emigration of fish of a few years of age that disperse from northern Spencer Gulf, through southern Spencer Gulf and to the WC. These fish then remain on the WC for the remainder of their lives.
As a consequence of the prolonged period of poor recruitment in northern Spencer Gulf since 1999 and the subsequent depletion of the population in Spencer Gulf, it is unlikely that this density dependent movement from Spencer Gulf to the WC has occurred to a major extent for a number of years or will occur until the Spencer Gulf population has recovered. Furthermore, age structures for the WC developed in 2020 and 2021 showed that only a very small number of fish from the strong 1997- and 1999-year classes in northern Spencer Gulf remained in the population, and there were several other year classes in the age structures for the WC that were not present in northern Spencer Gulf (Drew et al. 2022). Consequently, there is a need to understand the relative contributions of local population processes and emigration from northern Spencer Gulf to the WC population. This is particularly important following the regionalisation of the fishery through the Marine Scalefish Fishery (MSF) Reform (Smart et al. 2022).
This study will investigate the contemporary demographic processes that maintain the population of Snapper on the WC through the regional comparison of population age structures, elemental chemistry of otoliths, and population genomics. The findings will provide insight into the relative contributions of local recruitment and emigration to the WC population, that will be compared to the existing conceptual model of stock structure for Snapper in SA (Fowler 2016, Fowler et al. 2017). The proposed study will build on several previous projects that have investigated the stock structure of Snapper in SA (i.e., FRDC Project No. 2002-001, FRDC Project No. 2012-020, ARC Linkage Project No. LP180100756).
1.3 Review of biological parameters for Snapper in South Australia
The biology of Snapper in SA has been studied over the past 40 years, with particular focus on northern Spencer Gulf (e.g., Jones 1981, 1987, McGlennon 2003). Since 2000, a weekly market sampling program has been undertaken by SARDI researchers that has provided biological data for Snapper caught by commercial fishers across SA. The sampling program has been augmented with periodic trips to regional areas, research cruises, and targeted research projects. Since the closure of the SG/WCS and GSVS in November 2019, biological samples from the two stocks have been accessed through a targeted sampling program which involved contracting commercial fishers. The data collected through these projects and programs is maintained in a MS Access database which currently contains biological information (i.e., capture date, location, length, weight, sex, reproductive stage, and age) for >27,000 Snapper and length information for >75,000 individuals.
This study will investigate potential changes in the biological characteristics of Snapper throughout SA over the past 40 years in response to temporal changes in environmental conditions and stock abundance. This will involve spatial and temporal comparisons of length, age, growth rate, and length at maturity for Snapper from each region of SA. The study will also consider various approaches to estimate natural mortality. A key output of the study is a summary of contemporary biological parameters for each stock of Snapper in SA that will be incorporated into the stock assessment model (‘SnapEst’).
1.4 Benthic habitat survey for Gulf St Vincent
Snapper utilise a diversity of different benthic habitats throughout its life history, ranging from soft sediments that are favoured by recently settled juveniles to high relief structures that act as aggregation sites for spawning adults. As a result of the significant interannual variation in recruitment observed for Snapper in SA and the subsequent development of a pre-recruit index, there is particular interest in the spatial distribution and relative abundance of benthic habitats that act as nursery areas for age 0+ juvenile Snapper. In order for the pre-recruit index to provide reliable estimates of annual recruitment, it is essential that the key areas which support 0+ Snapper are sampled consistently each year.
In the recent project to develop a pre-recruit index (i.e., FRDC 2019-046), sampling for 0+ juvenile Snapper was targeted at the hypothesised nursery areas for each stock, i.e., northern Spencer Gulf (NSG) for the SG/WCS and northern Gulf St Vincent (NGSV) for the GSVS (Fowler et al. 2017). For NSG, the sampling design was informed by the results of annual surveys in the region from 2000 to 2010, which identified a strong relationship between the spatial distribution and abundance of 0+ Snapper and localised areas of soft, silty benthic substrate (Fowler and Jennings 2003, Fowler et al. 2010). There were no previous surveys for 0+ Snapper in NGSV, and therefore sampling locations were determined by the presence of suitable benthic substrate from existing habitat studies (Shepherd and Sprigg 1976, Tanner 2002). However, from 2021 to 2023, the catches of age 0+ Snapper in NGSV were very low in each annual survey and it cannot be determined if this reflected poor juvenile recruitment in these years, or if key nursery areas were not adequately sampled.
The aim of this study is to quantify the spatial distribution and relative abundance of benthic habitats in GSV, with particular emphasis on localised areas of soft substrate that may support age 0+ juvenile Snapper. The study will use towed underwater video and particle size analysis of sediment samples to quantify habitat types at ~150 sites throughout GSV following the methods recently applied in Spencer Gulf (FRDC 2020-002; Grammer et al. in prep.). The spatial distribution and relative abundance of benthic habitat types identified in this study will be compared to previous surveys in 1964-69 (Shepherd and Sprigg 1976) and 2000-01 (FRDC Project No. 1998-208; Tanner 2002) to assess changes in benthic habitats in GSV over the past 50 years, and how such changes may relate to trends in recruitment and stock abundance for Snapper over this time.
