Death traps lurking off WA shores

A vast field of whirling eddies offshore of Western Australia may hold the key to the local fishing industry according to a recent study by WA marine scientists.

Researchers from the University of Western Australia, Murdoch University, CSIRO and Department of Fisheries spent three weeks aboard the research vessel Southern Surveyor investigating the functioning of large eddies spun off the Leeuwin current.

Working some 500 km offshore of Fremantle, the multi-disciplinary team, led by Dr Anya Waite of UWA, studied the complex oceanography and biology of the eddy field and revealed some startling results.

"The convention is that the warm Leeuwin current suppresses upwelling off the coast of WA but cyclonic eddies provide a possible mechanism to bring nutrients to the surface and promote production by plankton," said Murdoch University researcher Dr Lynnath Beckley.

"However, opposing these are eddies spinning in an anti-clockwise direction which force warm, nutrient-poor water down to the depths. So, depending on their dynamics, these eddies can be either deathtraps or nurseries for the larvae of fish and rock lobsters."

During the team's 3-week study they have discovered complex interactions with layers of warm surface water that also seem to suppress the upwelling.

"Our observations during the voyage have suggested that eddy perimeters are highly productive, but that the patterns are complex and will take some time to untangle. Luckily we have a dynamic, multidisciplinary team including physical oceanographers, biological oceanographers and fisheries scientists, who are up to the challenge," said Dr Anya Waite of the Centre for Water Research at UWA.

Previously only examined through surface measurements made by satellites, the eddies were examined first-hand by the scientists, who were able to investigate the amounts of nutrients, plankton and fish larvae at depths down to 1000m by using the latest advances in marine technology.

These included a remotely controlled "Seasoar" which is an underwater plane laden with sensors that undulates up and down in the water column, and a large EZ plankton net, capable of being opened and closed at discrete depths.

The region is so unstudied that the investigators have already observed rare and possibly undescribed species. The team expects that unravelling the secrets of these eddies will contribute to sustainable use of Western Australian fish resources.