Recruitment variability and the potential effects of climate change
There are considerable knowledge gaps in the factors that drive variation in Snapper recruitment (the number of juvenile fish that enter the population each year) in SA. This project aims to:
- understand the biological and environmental factors that affect recruitment
- estimate the potential influence of climate change on Snapper.
Background
The population dynamics and fishery productivity of Snapper in SA are driven by highly variable recruitment. This reflects the survival rates of Snapper eggs and larvae as they develop during their first few days of life.
Trends in recruitment ultimately drive changes in the abundance of adult Snapper and fishery productivity – strong juvenile recruitment results in a higher abundance of adults in the following years. In contrast, decreases in adult numbers reflect periods of poor juvenile recruitment and the continued exploitation of the population.
For each of the Spencer Gulf and West Coast stocks, and Gulf St Vincent stocks, there were extended periods of poor juvenile recruitment during the 2000s and 2010s. This contributed to the declines in adult biomass – see the Snapper stock assessment 2022 ( ).
Understanding recruitment dynamics and the processes driving recruitment was identified as the highest research priority for Snapper at the most recent National Snapper Workshop 2021 – see previous Snapper research.
Objectives
There are 3 components involved in the project:
- Continuing to monitor the relative abundance of juvenile Snapper in Spencer Gulf and Gulf St Vincent as an early indication of annual recruitment. The baby Snapper are sampled in March and April each year using a small beam trawl, as recommenced in 2022 through FRDC Project No. 2019-046.
- Investigating the relationships between Snapper recruitment and temperature, wind, productivity, and rainfall. This is done by comparing a time series of juvenile recruitment for each stock to the time series of environmental parameters for that region.
- Assessing the potential effects of climate change on Snapper in SA, using the relationships between recruitment and environmental parameters. This includes identifying areas and life stages most susceptible to impact by:
- modelling various climate change scenarios at a regional scale
- developing spatial models for Snapper at different life stages – spawning, egg, larva, juvenile, and adult.
Project updates
February 2025 – Snapper early life history
The otoliths of a juvenile Snapper provide a unique insight into its early life history. They form increments daily that can be interpreted to:
- estimate age (in days)
- calculate other characteristics including growth rate, spawn date, and settlement date.
When this process is repeated for numerous individuals in a cohort, and for multiple cohorts over several years, it builds a comprehensive description of early life history characteristics.
The preparation of an otolith for microstructure examination is tedious and time-consuming, as each tiny otolith is polished by hand to progressively reveal the daily growth increments.
Preliminary findings
The age-0 juvenile Snapper sampled from northern Spencer Gulf over the last few years have ranged in size from 25 to 80 mm fork length. They were estimated to range in age from 49 to 128 days.
By subtracting the estimate age from the date of capture, it is possible to calculate the date each fish was spawned. These dates ranged from early December to early February. However, the majority of fish were spawned in a much shorter period between mid-December and early January.
The early life history characteristics of age-0 Snapper sampled between 2021 and 2024 were similar to those sampled in annual surveys between 2000 and 2010.
The microstructure of an otolith from a juvenile Snapper showing the continuous sequence of daily increments
June 2024 – environmental variables and recruitment
Estimates of annual recruitment for Snapper in Spencer Gulf and Gulf St Vincent, from 1983 to 2019, were developed from the estimated ages of over 20,000 Snapper sampled since 2000. The time series of juvenile recruitment for each gulf was then compared to time series for over 50 environmental parameters, including sea surface temperature, wind, rainfall, primary productivity, and climate indices.
The time series for each parameter was normalised and detrended to remove seasonal effects. Correlation analysis was then used to identify parameters that needed further consideration.
The optimal temperature range for Snapper spawning and egg and larval survival is between 18 and 22°C. For northern Spencer Gulf, preliminary analysis indicated there had been an increase in the mean sea surface temperature (SST) during the summer months over the past 40 years. This resulted in fewer days within the optimal temperature range during the 2000s and 2010s.
There has also been a progressive increase in the frequency and intensity of marine heatwaves since the mid-2000s, which have potential to significantly affect the survival of developing Snapper eggs and larvae.
Trends in SST in northern Spencer Gulf during summer from 1982 to 2024, showing the optimal temperature range for Snapper spawning and larval survival
May 2024 – juvenile recruitment surveys
Researchers monitor the abundance of juvenile Snapper at key nursery areas in Spencer Gulf and Gulf St Vincent during March and April to determine the relative strength of annual recruitment. These fish were spawned during the previous summer and range in size from 3 to 8 cm and in age from 50 to 120 days.
The key nursery areas in each gulf are localised with a fine, muddy bottom, which is the preferred habitat of juvenile Snapper. As they develop, these fish utilise more complex habitats, shifting from open areas of soft bottom to low profile habitats, such as razorfish beds and broken bottom. Eventually they become associated with high profile structures as adults, for example, reefs and shipwrecks.
In Spencer Gulf, the key nursery areas are in the northern part of the gulf near Whyalla and Port Pirie. Similarly, the key nursery areas for Gulf St Vincent are located near Ardrossan and Long Spit.
Baby Snapper are sampled at night using a small beam trawl net that is towed behind a trailer boat. This method has been successfully used to sample juvenile Snapper in SA over the past few years and in Port Phillip Bay, Victoria, since the late 1990s. The number of baby Snapper sampled is compared between years to provide an indication of annual recruitment.
Plumb-staff beam trawl used to sample juvenile Snapper
The 2024 recruitment surveys were completed in Gulf St Vincent from 13 to 25 March and in Spencer Gulf from 26 March to 8 April. The number of baby Snapper sampled from each gulf was much lower than the surveys in 2023 and 2022. This is not unexpected given the highly variable recruitment of Snapper.
In each gulf there are juvenile Snapper (20 to 30 cm, age 2 to 3 years) that have recruited in recent years. At this life stage, the fish occupy similar habitats to King George Whiting and are often encountered as bycatch. This is a positive sign for the future as Snapper develop and reach sexual maturity in the coming years.