Black Bream are more localised than once thought with a new study deepening our understanding of their stock structure.

Understanding how fish populations are connected is key to sustainable management. This is especially true for one of southern Australia’s most iconic estuarine species, the Black Bream (Acanthopagrus butcheri).  

Black Bream is found in estuaries and inshore waters from central New South Wales to the central west coast of Western Australia, including Tasmania. They are slow-growing, long-lived and highly valued by recreational anglers and commercial fishers, particularly in Western Australia. Despite their wide distribution, the degree of connection between populations in different estuaries has remained uncertain, until now.

Led by Professor Bronwyn Gillanders from the University of Adelaide, FRDC project 2019-012 brought together researchers from the South Australian Research and Development Institute (SARDI) Aquatic Sciences and the University of Adelaide to examine Black Bream stock structure and connectivity across southern Australia. Using a combination of genomic tools and otolith (ear bone) analysis, the team set out to understand whether fish in different estuaries form part of the same population or are largely isolated.

“We wanted to know whether Black Bream in different estuaries are connected, or if they’re more like local residents,” says Bronwyn. “That kind of knowledge is essential if we’re going to manage each fishery at the right scale.” 

What otoliths can tell us

Otoliths, small calcium carbonate structures in a fish’s head, are more like rocks than bones. By taking a thin cross-section, scientists can read the story of a fish’s life, much like tree rings reveal a tree’s age and growth. Each ring records growth patterns and chemical signatures that reflect the fish’s environment, revealing whether it spent time in freshwater, estuarine or marine habitats.

Using these natural archives, the research team examined how Black Bream growth responded to environmental conditions and whether fish from different estuaries showed distinctive patterns. The results from genetic testing revealed strong local structuring, with limited gene flow between estuaries, meaning most populations are self-sustaining and not mixing with their neighbours. 

"We found that within each state there’s very limited movement between estuaries,” explains Bronwyn. “That fine-scale structure means we can’t assume all populations are connected.” 

Training the next generation of fisheries scientists

A core focus of the project was building research capabilities in South Australia. PhD student, Koster Sarakinis undertook much of the work, supervised by both university and SARDI scientists in a mentoring model that paired early and senior career researchers. Koster has since completed his PhD and joined SARDI as a research scientist - a success story that demonstrates the value of investing in people as well as science. 

"It’s fantastic to see the next generation of researchers coming through,” says Bronwyn. “Projects like this not only generate new knowledge but also help develop the scientists who’ll lead future work.” 

The research has confirmed that Black Bream populations are more locally defined than previously thought. This has important implications for how the species is managed, particularly as climate change and altered freshwater flows affect estuarine habitats. Some estuaries occasionally close off from the sea, trapping fish inside until rainfall reopens the connection.

Understanding these dynamics helps managers identify which populations might be more vulnerable to environmental change or overfishing. It also supports adaptive management strategies that consider differences between estuaries rather than treating all Black Bream as a single, uniform stock.

A foundation for future research

Beyond its immediate findings, the project has established a framework for future studies of connectivity and stock structure in other species. The team is already exploring similar approaches in Snapper (Chrysophrys auratus) and Mulloway (Argyrosomus japonicus), using otolith and genomic techniques to assess movement and population resilience.

“Black Bream provided a perfect model for testing these methods,” says Bronwyn. “Now we can apply what we’ve learned to other estuarine species that face similar challenges.”

By linking cutting-edge science with practical management needs, and by training the researchers who’ll carry that work forward, the project exemplifies FRDC’s commitment to sustainable, evidence-based fisheries.