Just last month Redmap verified a report of a 1.4 kg mud crab in Perth’s Swan River and in the past few years The Bunbury Herald detailed two accounts of unsuspecting fishermen catching more than they had bargained for (Bunbury Herald, 2012 and Bunbury Herald, 2013). Clearly, mud crabs in Western Australia are on the move. What is less clear is why?

Two species of mud crab are found in Western Australia – the brown mud crab (Scylla olivacea) and the green mud crab (Scylla serrata). Fishers consider the common names reflect the colours of the waters in which they are usually found – “brownish” estuarine and “greenish” oceanic waters respectively.  To date, its believed that only green mud crabs have been recorded in the south-west.

While adults can tolerate an impressive gamut of environmental conditions, juvenile mud crabs are more sensitive to changes in their environment, particularly fluctuations in water temperature and ocean currents like the Leeuwin Current. This current flows southward along Western Australia’s coastline and is generally weakest during the summer months, when reproductive activity in green mud crabs peaks.

In 2011, a very strong Leeuwin Current event occurred in conjunction with record-high sea surface temperatures along the West and South coasts of Western Australia.

Scientists believe that these type of abnormal events might have allowed green mud crab larvae to hitch a ride south in the warmer waters as they flowed downwards and inshore along the West Australian coastline. Indeed, unusual distribution patterns for a variety of coastal marine species in the south-west are linked with La Ninã-related fluctuations in the Leeuwin Current.

Scientists studying the distribution of mud crabs in the south-west point out that mud crabs take approximately 1.5 years to mature and that, perhaps not so coincidentally, reports of adult crabs in the south-west began about 2 years after the unusually strong summer Leeuwin Current.

Clearly mud crabs are on the move, but important questions regarding their specific distribution in the south-west remain and whether they can self-sustain southern populations. More confirmed reports would help scientists tackle these complex questions. Please take a photograph and submit it to Redmap, along with details of the location, if you happen to spot a mud crab south of Shark Bay.

A Department of Fisheries WA fact sheet can be downloaded for more information on mud crabs and how to identify them.