06 August 2002: Mudcrabs recently found in the estuaries and rivers of Western Australia's south-west are natives of the Shark Bay to Broome region that were almost certainly swept down the west coast by the strongest Leeuwin Current in more than 70 years, a Department of Fisheries study has found.

Mud crabs are normally found in waters from Shark Bay northwards and are a prized catch for recreational fishers in WA's north-west, but there have been an increased number of captures along the south-west coast over the last couple of years.

Mud crabs have even been caught in Denmark's Wilson Inlet, more than 1000km from Shark Bay, their previously reported most southern location.

Several theories as to how mud crabs came to be in the south-west were put forward, including the transport of larvae in the ballast water of ships, translocation by recreational fishers, or adult crabs being released from restaurants and fish markets.

However, Department of Fisheries crab scientist Dr Lynda Bellchambers said research, using samples collected by members of the public, showed a more natural explanation.

"There were a number of possible explanations, but our conclusion was that the mud crabs arrival in the states south-west was most likely due to the Leeuwin Current," she said.

"Genetic analysis indicated that the mud crabs caught in the south-west were all the same species - there are two species found in WA - and that they were from populations between Exmouth and Broome.

"It also suggested that all the mud crabs were from a single colonisation event, leading to the conclusion that mud crab larvae were carried down the coast by the Leeuwin Current in a particularly strong year.

"The size range of the mud crabs captured suggests that they are approximately 2-3 years old meaning that the larvae would have been carried down the coast of WA in the Leeuwin current in 1999-2000"

Dr Bellchambers pointed out that in 1999 and 2000 the Fremantle sea level, which is used as an indicator of the strength of the Leeuwin Current, was the highest it has been since 1930.

"It all seems to fit with the explanation of larval transport in the Leeuwin Current," she said.
"Whether these populations can be self sustaining remains to be seen."

Dr Bellchambers said the WA Museum had a number of records of other tropical crustaceans appearing in the rivers and estuaries of the State's south-west, but not in the same kind of numbers that the mud crabs seem to have achieved.

The Department of Fisheries reminds recreational fishers that gear specifications and bag limits apply for mud crabs across the entire State.

All berried (egg carrying) females must be returned to the water unharmed.

Mud crabs may be caught legally using drop nets, scoop nets or metal hooks and there is a bag limit of 10.

MEDIA CONTACTS:

Dr Lynda Bellchambers Crab scientist Ph: 92468405

Jenny Hodder Senior Public Relations Coordinator Ph: 9482 7235 (Mob): 0418 901 767