You have to find out how they're moving around, and in the case of the green crab, we think they arrived in Newfoundland via ballast water. Many of these species produce larval stages that are microscopic and they would be picked up probably on the eastern seaboard of the United States and then transported elsewhere.
In the Great Lakes, our problem is mainly a European one. Historically, we have ships that were picking up ballast water in major freshwater ports such as Rotterdam and Antwerp and coming across the ocean. A study for Environment Canada in 1981 indicated that we were going to get invaded by zebra mussels. They misspelled the name, but they said we were going to get invaded by zebra mussels if we didn't do anything. Fast-forward six years and we have zebra mussels.
Thus, you have to control the vectors. We've heard this a couple of times here, and I want to put in a plug for DFO. There is a very clear need to stop the western spread of zebra mussels. Rather than doing it piecemeal with each province in the west trying to do it on its own, we need to put in sufficient money and quarantine Lake Winnipeg and the lake just north of it that is also colonized. Make sure that people are not bringing boats out of those systems and moving them west. It's going to be a lot easier to manage the problem there than trying to manage it once it starts spreading to many systems.
In Ontario, zebra mussels are in so many lakes that all you can do is stand back and watch. You can't do anything. However, when you have only one or two systems that are invaded, you can quarantine those systems. It's the most effective way to prevent the western spread of these animals.