Fishery-independent estimate of biomass for Spencer Gulf and Gulf St Vincent
Fishery-independent estimates of biomass have become an integral part of the stock assessment for Snapper in South Australia. This is due to the compromised integrity of fishery-dependent statistics and the recent fishery closures.
This project will apply methods that were developed during the refinement study to estimate biomass in Spencer Gulf (SG) and Gulf St Vincent (GSV) in 2024–25. The results will be incorporated into the next stock assessment report.
Background
Since 2018, the spawning biomass of Snapper in SG and GSV has been estimated using the daily egg production method (DEPM).
Several areas for method development were identified in the latest Snapper stock assessment, to refine the estimates of spawning biomass using the DEPM (Drew et al. 2022 ( )). This led to the following concurrent projects:
- Refine DEPM methodology – a study to assess the DEPM methodology for Snapper
- Develop and apply hydroacoustic surveys – a survey to inform us about Snapper aggregations and provide an estimate of biomass.
The 2 methods will be assessed and potentially integrated to provide the most robust estimate of biomass.
DEPM surveys 2024–25
The design of the DEPM surveys for 2024–25 was informed by the recommendations from the refinement study completed in 2023–24. The recommendations for plankton sampling included:
- Use a vertical tow to sample for Snapper eggs at each plankton station.
- Collect plankton samples at a higher spatial resolution (i.e., 1.5 × 1.5 nm).
- Increase the number of plankton samples collected.
All plankton sampling was done from MRV Ngerin, with a team of 6 research staff that operated 24 hours a day.
Plankton samples collected using a vertical tow
Plankton sampling and egg identification
The survey in Spencer Gulf was completed from 9 to 20 December 2024. Plankton samples were collected at 501 stations throughout the northern and central regions of the gulf, with environmental information collected using a CTD cast at 55 stations.
The survey in Gulf St Vincent was completed from 8 to 18 January 2025. Plankton samples were collected at 560 stations throughout the gulf, with environmental data collected at 60 stations.
Spatial distribution of plankton sampling stations in Spencer Gulf (left) and Gulf St Vincent (right)
The mixed plankton samples are sorted following a multi-stage process to identify the Snapper eggs, which includes morphological and molecular techniques.
With over 1,000 plankton samples collected across the 2 gulfs, laboratory processing and analysis will require several months to complete.
Plankton samples being preserved in ethanol for subsequent laboratory analysis
Snapper reproductive parameters
The second component of the DEPM surveys is collecting representative samples of adult fish to calculate reproductive parameters. Adult samples were collected by 7 contracted Marine Scalefish Fishery (MSF) fishers under the direction of a SARDI observer.
For Spencer Gulf, a total of 295 fish were collected from 17 samples throughout the survey area. For Gulf St Vincent, 331 fish were collected from 27 samples. When possible, adult sampling was done in the afternoon and evening to align with the daily spawning cycle and increase the probability of sampling fish about to spawn.
To determine if a female Snapper had spawned recently or would have spawned in the next few hours, a section of the ovary is sampled and processed for histological analysis. This process enables microscopic analysis of the ovary, which is needed to determine reproductive condition.
Preparation of an ovary sample for histology to determine reproductive condition
Acoustic-optical survey
Per the DEPM surveys, the design of the acoustic-optical survey was informed by the results of the experimental study in summer 2023–24. These included:
- Aggregations of Snapper were strong associated with structure on the seafloor.
- Almost all Snapper aggregations were within 100 m of the structure.
- Snapper were often identified in mixed species aggregations that also included Silver Trevally, Yellowtail Scad, and Blue Mackerel.
- Snapper demonstrated avoidance behaviour associated with vessel noise.
An acoustic-optical survey was completed in Gulf St Vincent from 8 to 18 January 2025 – the same time period as the DEPM survey. It involved high intensity sampling at 35 recognised aggregation sites throughout the gulf.
Method
At each site, a series of linear transects were arranged in a star pattern, with each one extending 250 m from the centre point. The acoustic equipment included:
- a 38/200 kHz Airmar transducer connected to a laptop through a Simrad WBT Mini
- a Lowrance HDS Pro used for navigation and side imaging.
An example of an echogram of a Snapper aggregation
Snapper were identified at 85–90% of the sites surveyed. Aggregations that contained Snapper were generally within 100 m of structure and contained other species including Silver Trevally and Yellowtail Scad.
Underwater video provided essential data to:
- identify the species composition of the aggregation
- estimate the relative proportions of those species.
Uncrewed electric vessel trial
A trial was conducted between the trailer boat and an uncrewed electric vessel, to check whether vessel noise influenced Snapper behaviour and acoustic observations. The same acoustic equipment was attached to both vessels.
Six aggregation sites were surveyed for comparison, with the electric vessel completing the transects first, followed by the trailer boat.
Potential benefits of automating hydroacoustic surveys include:
- enhanced efficiency and reduced costs
- increased data quality and higher confidence in biomass estimates.