By Natasha Prokop
Research to investigate unexpected and relatively dramatic changes in the distribution of South Australia’s Snapper (Chrysophrys auratus) population has revealed three separate fish stocks across the state, leading to changes in the way the fishery is managed.
It has not been uncommon for catches in South Australia’s Snapper fishery to fluctuate from year to year. The fishery had historically relied on significant catches from the Spencer Gulf. But from 2007 onwards, things started to change. Catches in the northern Gulf St Vincent and south-east regions increased exponentially, while those in the traditional fishing grounds of Spencer Gulf declined to record lows.
This redistribution of biomass was not necessarily a concern. The statewide catch actually increased over that time, so much so that between 2007 and 2011 the highest-ever annual commercial catches were recorded. But significant changes were happening over very short periods of time.
“The problem was that we didn’t understand the processes that were responsible for those rapid changes,” says Anthony Fowler, principal research scientist at the South Australian Research and Development Institute (SARDI).
The changes that began in 2007 drove an investigation to better understand processes within the fishery, including recruitment, mortality and migration between regions.
An FRDC-funded project, initiated in 2012, has helped to address this knowledge gap. Researchers at SARDI, a division of Primary Industries and Regions South Australia (PIRSA), used comparisons of regional data to get a better picture of complex population dynamics. These included otolith (fish ear bone) chemistry, acoustic tracking of fish (telemetry) and morphological studies.
They found that the key driver of the dramatic spatial changes in Snapper biomass was the year-to-year variation in recruitment success in the three major nursery areas – northern Spencer Gulf, northern Gulf St Vincent and Victoria’s Port Phillip Bay. The number of fish surviving to enter the fishery changed each year in the three nursery areas in response to environmental conditions.
Anthony Fowler says these variations and the subsequent migration of fish have the capacity to alter the biomass distribution of the entire fishery. “We found that it was really the high recruitment of fish in Port Phillip Bay in 2001 and 2004 – two really strong year classes – which led to the increase in biomass in the south-east region of South Australia.
“The south-east population is sustained by fish that move about 600 kilometres from Port Phillip Bay,” Anthony Fowler says. “That’s in contrast to what happens in the northern Gulf St Vincent, where the population has increased over time due to local recruitment.” Anthony Fowler says these findings have given researchers an appreciation for the population dynamics in the different regions.
Having a better handle on the movements of Snapper in South Australia also helped the researchers to determine the biological stock structure of the fishery – another important component for management.
Biological stocks are discrete, self-sustaining groups of fish with similar life history characteristics. If the Snapper in SA were found to move large distances and mix with fish from other regions, there would be only one well-mixed stock throughout the state. But if they were found to mix very little, regional differences would arise, causing stock differentiation.
Several techniques were used to determine the degree of movement and connectivity among South Australia’s regional Snapper populations. The biggest clues came from the chemical fingerprints found in the ear bones, or otoliths, of Snapper, which told the researchers where the fish were residing throughout their life. This, along with differences in head shape, helped the researchers to determine that across South Australia’s waters there are three biological stocks. They were identified as the Spencer Gulf/West Coast stock, the Gulf St Vincent stock and the Western Victorian stock, which spans south-east South Australian and western Victorian waters.
Anthony Fowler says these findings have changed the way researchers and managers think about the structure of the fishery and triggered a rethink of research strategies. “We used to carry out stock assessments at the regional scale. But now we know that we’re dealing with three biological stocks, we’re reporting at this larger spatial scale.”
This will mean more accurate assessments and fewer surprises in the future, if things begin to shift once more.
Over the same period that the distribution of biomass was changing within the fishery, so were the dynamics of the fishery’s operation. In 2009, there was a shift from hand-lining to more efficient longlining technologies. Consequently, catches, fishing effort and catch rates increased dramatically.
Then a 2011-12 review of management arrangements for the South Australian Snapper fishery saw several changes implemented the following year. This included extending the statewide seasonal Snapper fishing closure by an additional 15 days, introducing spawning spatial closures in both gulfs, reducing catch limits, and additional gear restrictions, including long-line hook limitations.
This meant that traditional means of assessing the sustainability of the fishery, such as catch and effort, were difficult to compare year to year. Michael Steer, subprogram leader of finfish fisheries at SARDI, says this was the motivation for fisheries managers to initiate an FRDC-funded project to develop an innovative, and fishery-independent, method for estimating the biomass of South Australian Snapper.
“Understanding biomass in fisheries is critically important because you need to know how large the resource is to ensure you’re harvesting at appropriate levels,” he says.
The daily egg production method (DEPM) does not rely on fishing to estimate populations. It involves determining how many eggs are in the water column and how many eggs each spawning female is producing. This can then be used to back-calculate the number of fish required to produce that number of eggs.
Adapting DEPM to estimate the spawning biomass of Snapper involved overcoming several challenges. The first was that Snapper eggs look much like any other demersal fish egg and are spawned at the same time of year as many other species. “We needed a way to identify and validate the Snapper eggs within a mixed sample,” Michael Steer says. “It’s really important to get that right because if you don’t, you’ll overestimate or underestimate the spawning biomass.”
Andrew Oxley, a SARDI molecular biologist, says they did this using a molecular probe (a radioactively labelled fragment of DNA or RNA) that seeks out a Snapper-specific sequence in the DNA of a fish egg, dyeing it blue. “This effectively removes all of the human error because we no longer have to go through and identify the eggs by eye,” he says.
Andrew Oxley says the Snapper eggs initially proved to be really tough to crack. “The egg chorion, or egg shell, has been so robust that we’ve had trouble getting the probe to penetrate the developing embryo,” he says.
They developed a mechanical method for breaking the egg, so that they could get the molecular probe to penetrate the egg and do its work. Now with the probe working, Michael Steer is confident this approach will have wide-ranging applications, and the technique has been successful in identifying Snapper eggs collected annually between 2013 and 2015.
He says the results from the DEPM will be used to develop improved harvest strategies and to evaluate management strategies that are already in place, such as assessing the effectiveness of spawning closures. And he says other fisheries could also benefit.
“It’s something that other jurisdictions can potentially use, such as the Victorian and West Australian Snapper fisheries. There’s also potential to extend the use of this method to other commercial species where we’ve had difficulties in identifying the eggs,” he says.
SARDI researchers are already working to extend the technique to King George Whiting, through the FRDC Project 2016-003.
FRDC Research Codes: 2012-020, 2014-019
Anthony Fowler, 08 8207 5432,
Michael Steer, 08 8207 5435