Research into the dynamics of Rock Flathead (Platycephalus laevigatus) in Victoria’s Corner Inlet fishery has provided surprising new insights into the species which will better inform future fisheries management. 

By Brad Collis 

As a popular eating fish across southern Australia, Rock Flathead has been the focus of an intriguing research project that has made unexpected discoveries about the species’ biology, with the findings having important implications for fisheries management. 

Rock Flathead inhabit reef, sand and seagrass habitats in shallow inshore waters from Greenwell Point in New South Wales, to Tasmania, and across to Geographe Bay in Western Australia.  

Despite evidence of widespread and healthy stocks across southern Australia, the rapid decline of the Rock Flathead population at Corner Inlet in Gippsland during the mid-late 1990s rang alarm bells.  

The introduction of gear restrictions, size limits and compulsory catch and effort reporting has seen the local Rock Flathead fishery recover. However, the episode highlighted the general lack of information about the species, its biology and the connectedness of populations. 

It was unknown, for instance, whether the Corner Inlet Rock Flathead population was sustained by recruitment from outside the inlet or was an isolated population.  

To answer this question, a collaborative multi-disciplinary project was instigated by Deakin University, the Victorian Fisheries Authority (VFA), fisheries stakeholders and FRDC (2020-003). 

Breeding dynamics 

Associate Professor Adam Miller led the project, saying understanding connectivity across fisheries is one of the key elements for setting sustainability measures.  

“Interconnected populations are going to be more resilient to environmental and harvest pressures because there's greater opportunities for recruitment.” 

Conversely, he says an isolated population’s recruitment potential is largely dictated by its local adult population. Knowing this can prevent over-exploiting the larger, mature, reproductive animals. 

Adam says the first task was to build a genetic profile of the Corner Inlet population and compare this with other Rock Flathead populations. 

Researchers sampled fish at Corner Inlet, at Flinders Island, Port Phillip and north-west Tasmania to compare genetic profiles and their otolith micro-chemistry profiles. 

“We used population genomics to first look at the distribution of DNA variation. If you have a high degree of connectivity, you will have similar genetic profiles and frequencies of DNA variants. If, however, the breeding populations are isolated they will have different genetic profiles because they will have been on different evolutionary trajectories over time.” 

Habitat ‘fingerprinting’ 

The otolith micro-chemistry sampling helped to verify findings from the DNA profiling. 

“The otolith is like the earbone in fish,” explains Adam. “As a fish develops from early life stages through to adulthood the calcification process absorbs the chemistry of its local environment.  So, you can look at the otolith chemistry to determine whether or not animals are using common or different habitats, and this can be used as an indicator of population connection or isolation.” 

The researchers also used acoustic telemetry to track fish that had been fitted with acoustic tags, combining this data with the genetic and otolith micro-chemistry profiles. 

Most of the on-the-ground and laboratory work and data analysis was done by Deakin University PhD students Darren Wong and Madeleine Toomey. 

A distinct population 

Genetic data revealed the Rock Flathead in Corner Inlet were distinct from all other fish sampled.  

“In other words, the Rock Flathead from Corner Inlet are not interbreeding or exchanging genes with fish outside the inlet,” says Adam. 

“The Corner Inlet population also had a distinctive otolith chemistry profile compared to those from all other sample locations which had similar profiles. This could be because Corner Inlet is the only truly estuarine system in the study, which is expected to have a different chemistry to more open coastal environments.” 

Acoustic tracking undertaken as part of the study also suggested that individual Rock Flathead movements were highly localised, with movements between Corner Inlet and other locations highly unlikely.  

Adam says the different lines of evidence all point to the Corner Inlet Rock Flathead fishery being an isolated self-recruiting population.  

“It means the recruitment dynamics within that fishery are likely to be heavily dependent on that local adult population. The primary message for fisheries managers is to implement appropriate catch and size limits that don't over-exploit the local reproductive adult population,” says Adam.

Related FRDC Project

2020-003: ‘Understanding the stock structure of Rock Flathead and the role of movement dynamics in influencing the performance of the Corner Inlet fishery’