Anna Salleh ABC News 8 Dec 09;
A combination of overfishing and climate change are triggering catastrophic overgrazing of reefs by sea urchins in eastern Tasmania, say researchers. Professor Craig Johnson of the University of Tasmania and colleagues report their findings today in the Proceedings of the National Academy of Sciences.
"When you get the two things happening together, it enables the urchin populations to build up to the point where they destructively graze," says Johnson.
Scientists believe climate change is causing stronger winds in the Southern Ocean, which speeds up the rotation of the ocean system that drives the East Australian Current.
The faster current has caused warmer water to spread further south, to the waters off eastern Tasmania, says Johnson.
He says this is causing the water in the area to warm nearly four times faster than the global average.
Northern invaders
The current is also carrying with it invaders from the north - the long-spined sea urchin (Centrostephanus rodgersii) from New South Wales waters.
"It is a really aggressive grazing species and it can just completely chew out the seaweeds and many of the other associated animals growing on the seafloor," says Johnson.
He says this can create a habitat called a "sea urchin barren".
"The analogy is like taking a bulldozer to a rainforest - you clear it back to bare earth," says Johnson.
"About 50% of the New South Wales coast looks like that right now."
Johnson says he and colleagues have previously found that the sea urchins are starting to form barrens in Tasmania and the race is on to stop these from spreading.
Research team member Dr Scott Ling also previously discovered that a key predator of the sea urchins is the spiny lobster (Jasus edwardsii), which is worth $50 million to the fishing industry.
The team's latest research looks at the impact on reefs of the interaction between climate change and the decline in the number of lobster predators due to overfishing.
Reef collapse
Johnson and colleagues carried out experiments inside and outside protected marine areas to show that fishing has made reefs more vulnerable to the climate-driven threat of the sea urchin.
"If we just had the climate change without the overfishing of the lobsters then the urchin would be here but probably just as a background species ... It wouldn't cause barrens," he says.
The urchins only become a problem when its key predator is overfished, says Johnson.
He says a major problem is sea urchins can keep reefs denuded indefinitely, using microscopic algae to help them survive.
"Unlike other herbivores they don't just die out," he says.
And he says while a lot of urchins are required to cause the barrens in the first place, reversing the process requires getting rid of nearly all.
Solutions
Johnson says only lobsters of a certain size can eat sea urchins: They must be big enough to get their front pair of walking legs around a sea urchin.
"The problem is these big guys are being fished out. There's basically none left," he says.
"Fishing pressure in some way, shape or form, needs to be reduced."
Johnson says it is not enough just to set limits on the size of lobsters that can be caught because there needs to be enough new animals to replenish the big ones when they die.
He says the $50 million Tasmanian lobster industry is worried about sea urchins and the Fisheries Research and Development Corporation is helping to fund research into how to control the sea urchins.
"We're trying to determine the best way of increasing the numbers of these large lobsters that has the minimum pain to the industry," says Johnson.
