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Exotic shellfish posing major threat -

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(generated from captions) waged by a foreign invader. like dandelions They're sort of dispersing and we suspect that their impact to something like rabbits. is probably closer noticed on this side of the Tasman The New Zealand screwshell was first back in the 1920s, but in recent years into plague proportions, its population has exploded from Tasmania to New South Wales. stretching along the sea floor are worried Both scientists and fishermen is swamping native marine species that the screwshell invasion in its path. for space and food The screwshell probably competes on the bottom. with a very large number of species into a mass of shells, It just transforms the sandy bottom

and other kinds of effects so the competition are likely to be quite high. 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. 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. But one thing is for concern,

it's going to be tough slowing down the nation's most invasive species. REPORTER: Does the screwshell have a natural predator? 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. How much of a concern is that? Well, you can see that there is no shortage of shells. There certainly isn't any predator control in this part of the world. And how hardy is the screwshell? 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. 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. 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. All the currents from southern Australia go from west to east, so there is no really 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. 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. 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. 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 more exotic marine pests. 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 a thousand oysters. Now all we've got is crushed shell, European shore crab. simply through one is planning to spend $40,000 In the short term, Col Dykes on crab-proof cages, about exotic pests, but he's confident more research like the New Zealand screwshell, the longer-term outlook will improve under siege. of businesses and environments and know what we're dealing with. We need everybody to be focused but we can minimise it. We're never going to have zero risk, can hope for Right now, the best scientists marching west is to stop the screwshells across the Great Australian Bight. is here to stay. But elsewhere, the marine plague Jocelyn Nettlefold reporting there. And that's the program for tonight. tomorrow, but for now, goodnight. We'll be back at the same time International. Captioning and Subtitling Captions by