Australian Broadcasting Corporation


LOCATION: http://www.abc.net.au/7.30/content/2005/s1465676.htm

Broadcast: 21/09/2005

Exotic shellfish posing major threat

Reporter: Jocelyn Nettlefold

KERRY O'BRIEN: From the prickly pear to the cane toad, to the rabbit to the fox, even to feral cats, Australia has a long and unfortunate history of introduced pests overrunning the countryside. Now there's another name to add to that unfortunate list - an unwelcome migrant from across the Tasman called the New Zealand screwshell. The exotic shellfish is now smothering parts of the eastern seaboard and scientists warn it poses a major threat to native marine life as well as to the commercial seafood industry. Jocelyn Nettlefold reports.

JOCELYN NETTLEFOLD: All may appear calm in the Dentrecastreaux Channel south of Hobart, but raging underwater is an ecological war, waged by a foreign invader.

PROFESSOR CRAIG JOHNSON, UNIVERSITY OF TASMANIA: They're sort of dispersing like dandelions and we suspect that their impact is probably closer to something like rabbits.

JOCELYN NETTLEFOLD: The New Zealand screwshell was first noticed on this side of the Tasman back in the 1920s, but in recent years its population has exploded into plague proportions, stretching along the sea floor from Tasmania to New South Wales. Both scientists and fishermen are worried that the screwshell invasion is swamping native marine species in its path.

PROFESSOR CRAIG JOHNSON: The screwshell probably competes for space and food with a very large number of species on the bottom - it just transforms the sandy bottom into a mass of shells. So the competition and other kinds of effects are likely to be quite high.

JOCELYN NETTLEFOLD: Zoologist Craig Johnston leads a team of scientists investigating the screwshell's prolific spread. Up close, it's difficult to tell if a screwshell is dead or alive.

PROFESSOR CRAIG JOHNSON: So that one there, if you look in there now, I've just irritated it, and it's sucked itself right back down into there.

JOCELYN NETTLEFOLD: But one thing's for certain, it's going to be tough slowing down the nation's most invasive species. Does the screwshell have a natural predator?

PROFESSOR CRAIG JOHNSON: They may as planktonic larvae - little tiny, little specks floating around in the water column - but the shells like this, we don't actually know of anything that eats them.

JOCELYN NETTLEFOLD: How much of a concern is that?

PROFESSOR CRAIG JOHNSON: Well, you can see that there is no shortage of shells. There certainly isn't any predator control in this part of the world.

JOCELYN NETTLEFOLD: The accumulation of shells at densities of up to 2,000 per square metre of sea floor is ringing alarm bells for fishing and aquaculture industries.

PROFESSOR CRAIG JOHNSON: We are doing some work with commercial scallops. Again, that work is in its infancy, but the preliminary data that we have certainly suggests some impact on the growth rate of scallops.

JOCELYN NETTLEFOLD: While sea currents seem set to take the screwshell further north towards Queensland, breakthrough genetic work by the CSIRO has detected that screwshell larvae can easily hitch a ride in ship ballast water.

DR NIC BAX, CSIRO: All the currents from southern Australia go from west to east, so there is really no way that the screwshell by itself will move into the Great Australian Bight, move into Western Australia. It's only through our own help that the screwshell can move to those areas.

JOCELYN NETTLEFOLD: Currently, ships arriving from international waters must exchange ballast water in a bid to keep out marine exotic pests like the Pacific sea star and the black-striped mussel. Negotiations are under way between the States and Commonwealth for similar controls on domestic ballast by the end of next year. But Teresa Hatch from the Shipowners' Association says its unrealistic to expect ships to stop and exchange ballast during short runs interstate.

TERESA HATCH, AUSTRALIAN SHIPOWNERS' ASSOCIATION: It's very difficult to give a financial impact to those sorts of delays but an example could be that for a large tanker, you could be looking at upwards of US$60,000 a day.

JOCELYN NETTLEFOLD: Oyster grower Col Dyke says industry and government ballast water management measures in the past have been ad hoc. He's trying to protect his business from four exotic marine pests.

COL DYKE, OYSTER FARMER: Well, what we've got here is a European shore crab, one of the introduced marine pests. They certainly give the oysters a touch-up. We've got a situation where this basket was holding 1,000 oysters. Now all we've got is crushed shell, simply through one European shore crab.

JOCELYN NETTLEFOLD: In the short term, Col Dykes is planning to spend $40,000 on crab-proof cages, but he's confident more research about exotic pests, like the New Zealand screwshell, will improve the longer-term outlook of businesses and environments under siege.

COL DYKE: We need everybody to be focused and know what we're dealing with. We're never going to have zero risk, but we can minimise it.

JOCELYN NETTLEFOLD: Right now, the best scientists can hope for is to stop the screwshells marching west across the Great Australian Bight. But elsewhere, the marine plague is here to stay.

KERRY O'BRIEN: Jocelyn Nettlefold with that report.