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 statistics, 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). Consequently, there is a need to develop methods for obtaining reliable fishery-independent estimates of biomass for Snapper that can inform stock status.
A variety of fishery-independent approaches have been used to estimate the biomass of exploited fish stocks. Several of these methods were developed and established for small pelagic fish species and have been adapted for demersal species such as Snapper, including the daily egg production method (DEPM) (Steer et al. 2017, Drew et al. 2022) and hydroacoustic techniques (Scoulding et al. 2023). There are also other recently developed, novel approaches to estimate abundance that could be considered for Snapper, such as close-kin mark-recapture (CKMR) (Bravington et al. 2016). This project will investigate and refine multiple fishery-independent approaches to estimate biomass for Snapper and provide recommendations based on feasibility, cost, and applicability to guide future assessments.
This research proposal has been developed to address four research priorities:
• Review the existing methodology used to estimate spawning biomass for Snapper using the DEPM and investigate methodological and statistical approaches to reduce uncertainty associated with individual parameters and refine estimates of spawning biomass.
• Evaluate the feasibility and suitability of alternative methods, such as hydroacoustics and CKMR, to generate a fishery-independent estimate of abundance and biomass for Snapper.
• Compare estimates of spawning biomass of Snapper obtained using the DEPM and hydroacoustic techniques.
• Review the existing Snapper stock assessment model (‘SnapEst’) and assess the suitability of new datasets to improve annual estimates of fishable biomass, exploitation rate, and recruitment. There is also the need to develop forecasting capability to predict how fishable biomass will respond under various recruitment scenarios. Such projections would assist the development of appropriate recovery and management strategies.
Consequently, Research Theme 2– Estimates of Biomass involves four projects:
2.1 Refinement of DEPM methodology for Snapper
2.2 Development and application of hydroacoustic techniques for Snapper in South Australia
2.3 Evaluation of close-kin mark-recapture (CKMR) for Snapper
2.4 Enhancement of the stock assessment model ‘SnapEst’
2.1 Refinement of DEPM methodology for Snapper
The daily egg production method (DEPM) has been used to estimate the spawning biomass of Snapper in New Zealand (Zeldis and Francis 1998), South Australia (McGlennon 2003, Drew et al. 2022), and Western Australia (Jackson et al. 2012). The underlying principle of the DEPM is that spawning biomass can be determined from the mean number of pelagic fish eggs produced per day over the spawning area (i.e., total daily egg production) divided by the mean number of eggs produced per unit mass of adult fish (i.e., mean daily fecundity) (Parker 1980, Lasker 1985). Total daily egg production is the product of mean daily egg production (P0) and total spawning area (A), while mean daily fecundity is estimated from the adult parameters of spawning fraction (S), batch fecundity (F), female weight (W), and sex ratio (R) (Parker 1980, Lasker 1985).
Difficulties differentiating eggs of Snapper from other fishes based on their morphology had precluded the application of DEPM for Snapper, until this issue was addressed through the development of a molecular technique to validate the identity of Snapper eggs (Oxley et al. 2017, Steer et al. 2017). Thereafter, the DEPM has been used to provide fishery-independent estimates of spawning biomass that have become an integral input to the stock assessment for Snapper in SA (Fowler et al. 2019, 2020, Drew et al. 2022). Recent applications of the DEPM for Snapper have identified several potential areas for method development and experimentation to reduce uncertainty associated with individual parameters and refine estimates of spawning biomass (Drew et al. 2022).
This experimental study involves two components that relate to refining the estimates of (1) total daily egg production and (2) mean daily fecundity. The first component related to total daily egg production includes a comparison of field techniques used to sample Snapper eggs, assessment of the spatial design of plankton surveys, and a temporal comparison of total daily egg production within a spawning season. Three field sampling techniques for plankton (i.e., vertical, oblique, and horizontal tows) will be compared to determine the most appropriate methodology to sample Snapper eggs for the DEPM. Concurrently, the spatial design of plankton surveys will be explored by conducting stratified plankton sampling at multiple intensities (i.e., 4 × 2 nm2, 2 × 2 nm2, and 1.4 × 1.4 nm2) to identify the most appropriate spatial scale to sample Snapper eggs from aggregations of spawning fish. The experimental field study will be repeated twice in a single spawning period (i.e., December 2023 and January 2024) over the same survey area to evaluate within-season variation in total daily egg production. The data collected from the field study will be explored using various traditional and geostatistical approaches to estimate total daily egg production (i.e., P0 × A). In doing so, the study will improve the understanding of how P0 and A spawning area are estimated for an aggregating demersal species.
The second component relates to mean daily fecundity and will examine how methods to estimate adult parameters (i.e., S, F, W, and R) can be refined. Representative samples of adult Snapper will be collected throughout the survey area concurrent with plankton surveys and processed for biological information. Initially, the aforementioned adult parameters will be calculated using current methods established for Snapper and compared to previous applications of the DEPM to Snapper in SA (e.g., Drew et al. 2022). Then, approaches developed in applications of the DEPM to other species will be used to estimate adult parameters and their variance, including the estimation of relative fecundity (F’) which is calculated by dividing batch fecundity (F) by female weight (W) to estimate the number of eggs produced per gram of total female weight (Ward et al. 2021). For each parameter, field, laboratory, and analytical methods will be evaluated to develop recommendations for future applications of DEPM for Snapper.
2.2 Development and application of hydroacoustic techniques for Snapper in South Australia
Hydroacoustic surveys have been used extensively around the world to survey pelagic fish species that form monospecific schools. They exploit the long-range propagation of underwater sound to survey large areas relatively quickly at high resolution. This technique relies on the proportional relationship of acoustic backscatter to abundance when the scattering properties of the target species are known. Individual species can be identified based on acoustic characteristics of the aggregations. However, additional optic methods can be incorporated to provide additional evidence on species composition, fish size, and orientation.
Researchers from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Western Australian Department of Primary Industries and Regional Development (WA DPIRD) have developed a conceptual method to quantify abundance and estimate the biomass of Snapper that incorporates traditional acoustic surveys with underwater video techniques (Scoulding et al. 2023). The optical component enables the species composition of the school to be quantified and the approximate size of the fish to be determined, which contributes to improved estimates of biomass.
Several limitations were identified in the recent study which prevented the estimates of biomass from being recommended for incorporation into formal stock assessment (Scoulding et al. 2023). However, it is likely that some of these limitations are not relevant to Snapper in SA’s gulfs (e.g., aggregations of mixed species, presence of other large teleosts), and others could be addressed through further method development (e.g., measurements of target strength for other species, benthic habitat maps).
This study will apply the methodology developed by Scoulding et al. (2023) to a targeted survey area in Gulf St Vincent in December 2023 and January 2024 to evaluate the applicability of hydroacoustic methods to quantify the abundance and biomass of spawning aggregations of Snapper. Furthermore, the hydroacoustic study will be completed over the same survey area and at the same time as the DEPM refinement study, therefore enabling direct comparison of biomass estimates between methods.
2.3 Evaluation of close-kin mark-recapture (CKMR) for Snapper
CKMR is a form of mark-recapture experiment in which the size of a spawning population (and fecundity at age, and survival) can be estimated based on the number of closely related individuals in a sample (i.e., parent-offspring pairs and half-sibling pairs). Intuitively, given a sample of individuals from a population, a greater number of closely related pairs are expected to be observed in a smaller population, whilst fewer related pairs are expected from a larger population. Close-kin mark-recapture can be used to estimate abundance, natural mortality (if catches are known), and fecundity with high precision (given sufficient sampling) and is independent of the fishery. Close-kin mark-recapture is a relatively new fisheries assessment tool that is increasing in popularity and has been successfully applied to two large-scale and economically important commercial fish and shark species (e.g., Southern Bluefin Tuna – Davies et al. 2020, School Shark – Thomson et al. 2020) and several species of conservation importance (e.g., Speartooth Shark – Patterson et al. 2022, Grey Nurse Shark, Bradford et al 2018, White Shark, Hillary et al ).
This scoping study will provide the expected precision for estimates of abundance of Snapper stocks in South Australia from a CKMR study (given a range of samples sizes) provided the true stock abundance is as estimated by the base case stock assessment model. If the stock is in fact smaller, then the precision from a CKMR model will be greater than forecast, and if bigger, then precision will be lower but catches will likely be sustainable. The information on population demographics, stock structure, and biological parameters will be drawn from the base case stock assessment for SA snapper.
2.4 Enhancement of the stock assessment model ‘SnapEst’
The SA Snapper fishery stock assessment model, ‘SnapEst’, was developed over two decades ago with FRDC support as a dynamic, spatial, age- and length-structured model (McGarvey and Feenstra 2004). The model integrates multiple data sources including biological information (i.e., length and age) and fishery-dependent data (i.e., catch and catch rate) to produce annual estimates of fishable biomass, exploitation rate, recruitment, and egg production. Significant modification and improvements have been applied to the model in recent years, which were necessitated by significant changes to management arrangements for the commercial sector from 2012 onwards and the updated understanding of population dynamics and stock structure for Snapper in SA (i.e., SG/WCS, GSVS, and the SE Region; Fowler et al. 2017). The most recent change to the stock assessment model was driven by the replacement of catch rate by DEPM biomass estimates as the fitted index of abundance for the SG/WCS and GSVS from 2013 onwards as a consequence of the extensive management changes and known catch rate hyperstability, which increased the need for a fishery-independent estimate of biomass.
The aim of this study is to evaluate, enhance and extend the stock assessment modelling capability for Snapper in SA. This will involve four main components. Firstly, the existing model (i.e., the version that was used in the most recent stock assessment; Drew et al. 2022) will be reviewed by a leading independent fishery modeller. It is anticipated that the external review will provide a series of recommendations and suggestions to improve confidence and transparency in model outputs. The second component involves integrating new data inputs into the model developed in the Snapper Science Program, such as an index of juvenile recruitment and improved estimates of biomass. The various data inputs will be assessed for their suitability for incorporation into the model.
The third component is the development of a recruitment forecasting sub-model that will use relative estimates of juvenile (i.e., age 0+) abundance to predict potential future trends in recruitment to fishable biomass 4 or 5 years later. The primary data source for the recruitment forecasting sub-model will be the time series of juvenile abundance for each stock that is being developed through a concurrent project (Snapper Science Program: Theme 1 – Biology and Ecology). The sub-model will relate the relative abundance of age 0+ juveniles in each year to the model-estimated recruitment of adults to the fishable stock.
The fourth component is to design, develop, code and implement a projection model, ‘SnapProj’. Once validated, the projection model will be a powerful tool to forecast population trends and inform management decision making.
