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'Virtual larvae' point prawn fishers in the right direction

'Stocking up on prawns,' the Advertiser, December 15, 2011 (external website).

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SARDI researchers Dr Mark Doubell and Dr Cameron Dixon with western king prawns

Above: SARDI researchers Dr Mark Doubell and Dr Cameron Dixon with western king prawns

Spencer Gulf in SA supports the largest known population of western king prawn (Melicertus latisulcatus) in the world, but the peak Christmas time harvest clashes with the species’ spawning season.

Scientists from the South Australian Research and Development Institute are using computer models to develop harvest strategies to optimise both the catch and recruitment of prawns for future fishing.

Principal investigator of the Fisheries Research and Development Corporation funded project and SARDI’s head of Inshore Crustacean Fisheries, Dr Cameron Dixon, said the findings will have a significant positive impact on prawn harvest strategies, particularly at Christmas time, ensuring sustainable increases in catch.

“Now we know which regions we can confidently fish to maximise the catch, and which regions to avoid to ensure young prawns have the best chance of survival.”

Dr Mark Doubell, a biological modeller with the SARDI Oceanography group, said data streams from the Southern Australian Integrated Marine Observing System (SAIMOS) have been used to develop detailed circulation models for the South Australian region, including Spencer Gulf.

“Obviously, trying to track the routes travelled by individual prawn larvae out in the field is not achievable. These models allow us to simulate the development, behaviour and transport of prawns in relation to the circulation and hydrodynamic features of Spencer Gulf, including the complex routes travelled by prawn larvae to settlement grounds,” explains Dr Doubell.
“Our results clearly show larval behaviour plays a key role in maximising the number of prawns recruited to the fishery, and most importantly identifies the major spawning and nursery sub-regions to protect,” said Dr Doubell.

The research has revealed some remarkable prawn behaviours.

A key outcome of the model is the prawn larvae’s ability to “sense” the rise in water height on the flood tide and to swim vertically so as to ride incoming tidal currents. The prawns in turn can “hear” the drop in water on the ebb tide and swim to the bottom where friction makes the currents small.  This remarkable ability to surf the tides into the safety of the coastal mangroves (settlement) is also found in another species of prawn in the Gulf of Carpentaria on the ebb tide.

“This is a truly impressive result” says Dr Dixon, “as it shows that the prawns in each gulf have adapted to use the large tides and coastal mangroves to ensure survival of each species”.

Other outcomes from the research will help shape responses to fishery management under climate change scenarios.

Western King prawn larvae cultured at SARDI Aquatic Sciences

Above: Western King prawn larvae cultured at SARDI Aquatic Sciences

Prawn eggs were hatched in tanks at the South Australian Aquatic Sciences Centre by Dr Shane Roberts. Their growth and mortality was studied under various temperature regimes for two months to the post-larvae stage.

At 17oC it took 31 days for larvae to grow to post larvae. At warmer temps around 24oC, it only took 13 days for them to grow,” said Dr Dixon. “The model will help us to better understand the effects of climate change on prawn stocks”.

Much of the success of the Spencer Gulf Prawn Fishery and its sustainable fishing practices can be attributed to the close collaborative relationships developed between industry and Government. Since the early 1980’s, SARDI has led the development of the fishery-independent surveys that underpin management of the fishery. In recent years, SARDI has worked with managers and industry to develop an auditable harvest strategy based on the survey and real time management systems. This has assisted industry in its goal of greater responsibility in co-management. Finally, SARDI scientists have also led a major research program on the effects of prawn trawling on the ecosystem, which was an integral component of the fishery’s successful MSC certification.

The modelling work has been made possible by funding from Marine Innovation SA (MISA).