Stock enhancement process

The decline of the Spencer Gulf, West Coast and Gulf St Vincent Snapper stocks is related to extended periods of poor juvenile recruitment – low numbers of baby Snapper entering the population.

The volume of Snapper that are produced, survive, grow older, and contribute to the adult population each year can vary greatly and is difficult to predict. Given this slow pace of life where populations are replenished by irregular recruitment events, it may take several years for the stocks to recover.

One way to help the process is stock enhancement, or restocking, which involves supplementing natural recruitment with juvenile Snapper that are reared in captivity.

Snapper stock enhancement in South Australia is led by the Aquaculture Program at SARDI Aquatic and Livestock Sciences, which has delivered annual releases of juvenile Snapper since 2021. Funding from the Snapper Recovery Package will support the production and release of up to 900,000 juvenile Snapper in Spencer Gulf and Gulf St Vincent over the next few years.

Snapper broodstock

Initially, the restocking program involved collecting adult Snapper from Spencer Gulf and Gulf St Vincent in early 2020 that were retained as broodstock. Approximately 40 fish of various sizes were collected from each gulf and are maintained at the South Australian Aquatic Science Centre at West Beach.

The adult broodstock are conditioned to spawn as the water temperature increases throughout spring. This aligns with the natural reproductive cycle of Snapper in South Australia. The fish begin to spawn when the water temperature reaches about 20°C and can continue to spawn daily for several weeks. Snapper eggs are buoyant and float to the surface of the spawning tank, where they are collected and transferred to a controlled rearing tank.

Snapper eggs are very small (less than 1 mm diameter) and hatch after approximately 30 hours, releasing larvae of approximately 2.5 mm total length. The larvae initially absorb their yolk sac, then feed on small zooplankton as they develop. Over the next 30 days, the larvae begin to develop fins, scales, and the general appearance of juvenile Snapper.

Juvenile Snapper marking

Once they have developed sufficiently, the juvenile Snapper are transferred from the nursery to larger grow-out tanks.

At this stage each fish is marked so that it can be identified if it is captured in the future. This is done using a fluorescent compound that is permanently retained in the otoliths (ear stones) of each fish. The compound (alizarine complexone) is non-toxic and has been used globally to mark the otoliths of reared fish for over 50 years.

When the otolith is sectioned to estimate the age of the fish, researchers can identify the mark and determine its origin – whether the fish was wild or reared.

The juvenile Snapper are maintained in grow-out tanks as they continue to develop.

Releasing Snapper

By 60 to 70 days, the fish have reached around 40 mm length and are ready to be released. The fish are released into or directly adjacent to recognised nursery areas for Snapper in the 2 gulfs.

For Spencer Gulf, the primary nursery areas for Snapper are near Western Shoal (about 20 km south of Whyalla) and in the vicinity of Ward Spit (about 20 km north of Port Pirie).

For Gulf St Vincent, the nursery areas are near Ardrossan, Long Spit and adjacent to the metropolitan coast.

On the day of release, the fish are transported from the Science Centre at West Beach to their destination in specially designed fish transport tubs. Each tub has an in-built oxygen supply and is regularly monitored to ensure that the appropriate conditions such as temperature, dissolved oxygen, and nitrogen compounds are maintained. The fish are mildly sedated during transport to reduce stress and potential injuries.