Since the late 1980s marine scientists and others with an interest in Tasmania's marine environment have noticed significant increases in the population of a large and distinctive species at sea urchin along the north-east coast of Tasmania and around some of the islands in Bass Strait.

This increase may lead to massive environmental damage, economic loss to the fishing industry and may be a result of bad fishing practices.

The sea urchin, Centrostephanus rogersii, is native to the waters of northern Tasmania and mainland Australia, but in Tasmanian waters this species has never been particularly common.

Something appears to have changed over the last 20 years or so and has resulted in the major increase in the numbers of this species of sea urchin that we are now seeing.

Does this matter? After all Centrostephanus is a native species.

Unfortunately, problems arise if urchin numbers and grazing increase to such an extent that the normal leafy plants are removed from large areas of rocky reef and replaced by encrusting coralline algae. The kelp and other marine plants that are so typical of Tasmania's shallow rocky reefs disappear and are replaced by "urchin barrens".

Urchin barrens offer less food and shelter to a whole range of reef species, and are much less productive. Just from a fisheries perspective, this impact on the reef habitat is disastrous. It is estimated that the impact of urchins may result in a 70% loss of productivity on affected reefs for the lucrative abalone and rock lobster fisheries alone.

Urchin barrens also appear to be a very stable kind of habitat once they are formed. It may take a high concentration of urchins to actually form a barren, but one formed they are easily maintained by very low urchin population densities.

Anecdotal evidence suggests that this process has occurred in the past along sections of the NSW coast, where Centrostephanus is very common. The creation of extensive areas or relatively unproductive reef appears to have been the result.

The phenomenon has stated to become significant along the north east coast and around the Bass Strait islands. The last decade or so has seen a major increase in urchin barrens, and alarm bells are finally beginning to ring.

The reasons for this major environmental change are still not clear. One theory is that global warming has suited Centrostephanus and encouraged a population explosion in local populations of Centrostephanus. Another more likely possibility is that overfishing of rock lobster and/or abalone is at least partly to blame.

For some years, especially for the years immediately before quota was introduced into the rock lobster fishery, there was enormous pressure on rock lobster. Animals tended to be caught in the year that they grew above the legal size limit. Large animals became comparatively rare.

As Centrostephanus is a large and robust sea urchin, it takes a large and robust rock lobster to eat one. It is very likely that removing most of the large rock lobsters, one of the most important of the predators that feed on Centrostephanus, has allowed the urchin population to explode. The remaining small rock lobsters are just not big enough to break open and eat an average Centrostephanus.

The abalone fishery may have compounded the human impact caused by the rock lobster fishery. It is possible that small abalone compete for food and living space with small Centrostephanus. Overfishing of abalone in some areas may have assisted the population explosion of this species of sea urchin.

It is important to remember that all the real reason for the increase in numbers of sea urchins and consequent urchin barrens remains unknown. There is an urgent need for research to find out what is really going on. The findings could have a huge impact on the way Tasmania's fisheries are managed.

The TCT raised this issue several years ago at one of the first Abalone Fisheries Advisory Committees meetings, and also in written submissions in 1997. Unfortunately not much came out of this at the time, but others have independently recognised the problem and have been doing important research in this area.

Professor Craig Johnson from the Zoology Department at the University of Tasmania has been investigating Centrostephanus and its relationship with urchin barrens, and has received at least one major grant to support this work. Unbelievably, a second application to FRDC for funding was rejected. It is unbelievable because of the potential threat a population explosion of Centrostephanus poses to the Tasmanian marine environment, not to mention major Tasmanian fisheries. It is also work that could easily result in simple management changes that would totally solve the problem.

This may be one of the most important environmental issues in Tasmania's marine environment at the moment. The cause/s of expansion of the Centrostaphanus population may not be related to human activities, or may be related to global worming, and therefore outside the ability for local management to have any effect.

However, there is a reasonable chance that fishing activities are behind the population expansion. If this was the case, a great opportunity to do something about this problem will be lost unless the research is done now and the cause/s positively identified.

Jon Bryan
jonbryan@southcom.com.au