Reef ecosystems and iconic fisheries along the east coast of Tasmania are being destroyed by expanding urchin barrens.
Urchin barrens form when urchins overgraze the reefs, removing most of the underwater plants and other life that is normally attached to rock surfaces. Once diverse and productive reef habitats, are reduced to underwater wastelands and no longer support fisheries for abalone, rock lobster and over 150 other species of marine life.
Numbers of long-spined sea urchins, Centrostephanus rodgersii, have exploded on Tasmania's east coast, largely as a result of the rock lobster fishery. While warming water due to climate change has made conditions more favourable for long-spined sea urchins, the primary cause of this problem is over-fishing of rock lobster. First recorded in Tasmania in 1978, there are now estimated to be more than 20 million long spined sea urchins.
Large rock lobster, with a carapace length greater than 138 mm, are the only natural predator of these urchins in Tasmanian waters. Current size limits do not protect these larger rock lobster, and fishing has almost completely removed these animals from inshore reefs along Tasmania's east coast.
Established long-spined sea urchin barrens do not spontaneously recover. The barrens persist indefinitely as the urchins that create them are able to survive even after most of their food has disappeared by eating microalgae.
Using divers to harvest long-spined sea urchins for food or for culling is impractical. Occupational Health and Safety concerns and large expense, limits diving to water shallower than 20 m, while urchin barrens exist up to 60 m.
Recent IMAS studies predict that over 30% of susceptible east coast reefs will be covered by urchin barrens by 2021. Already in some places in the north east, 50% of our reefs have been destroyed by urchin barrens. This is an ecological disaster! More than 150 species of marine life, including sea-horses, weedy sea-dragons and many other fish, depend on rocky reefs for survival and will simply disappear from large sections Tasmania's marine environment.
Urchin barrens are a disaster for local fisheries. They pose a direct threat to the iconic east coast recreational rock lobster fisherywhich supports 80% of Tasmania's recreational rock lobster fishing effort.
Experiments in Tasmania show that when sea urchins are removed, the seaweed cover and community structure recovers to that of ungrazed regions within 18 months. But there has to be ongoing control of urchins to prevent barrens returning. While divers may be useful in controlling urchin numbers in some areas, new technologies such as Autonomous Underwater Vehicles (AUVs) might be a way to aid in the recovery of urchin barrens to productive reef. It is clear that the most effective and cost effective way to protect reefs is to manage Tasmania's commercial and recreational rock lobster fisheries to increase the density of large rock lobster to the point where they can control urchin numbers.
The Tasmanian government has allowed rock lobster stocks to be fished down to alarming levels. Rock lobster biomass, the mass of the total population of rock lobster has been reduced to just 13% of original levels along much of the east coast. This makes it harder to catch rock lobster than it should be, making the commercial fishery inefficient and making the urchin barren problem much worse. Better management of rock lobster fishery is needed to increase the numbers of the large lobsters which are a critical part of the solution to the urchin barren crisis.
HOW CAN WE REACH A SOLUTION FOR THIS CRISIS?
It is vital that stakeholders come together to agree on a strategy to prevent further expansion of urchin barrens and explore ways to repair the damage that has already occurred. Possible solutions might involve emergency closures, harvesting and culling urchins, or using quicklime or Autonomous Underwater Vehicles (AUVs) to control urchin numbers.
If no effective action is implemented soon there will be ongoing destruction of our reefs with a devastating impact on this precious ecosystem and an amazing array of underwater life. Urchin barrens also threaten iconic commercial and recreational abalone and rock lobster fisheries that are so important to Tasmanian communities.