19 



Of course, the third alternative — I am sure the steamship compa- 

 nies would go absolutely bonkers over this — but if any thought was 

 given to a possible quarantine station-type site at a port of entry, 

 be it below the city of New Orleans on the Mississippi River, be it 

 at some point near San Francisco Bay, where when you are enter- 

 ing an area with a designated ballast spot that has either a high 

 enough river flow, high enough salinity and yet it is still a safe 

 place, getting back to what this gentleman was talking about, since 

 it is going to involve some time and, obviously, you want to do it in 

 a place where the vessel would not be subjected to very high seas, 

 very high winds. 



I am curious if anyone would like to comment on that. 



Dr. Carlton. I will start with your first question about why 

 now? Why do invasions continue? It is one of the more vexing ques- 

 tions and one of the problems in making predictions about what 

 the next invading species will be. 



Why the mussel did not invade North America before the 1980's, 

 we just do not know. There are a number of possibilities. Ballast 

 water from Europe, from zebra mussel source areas, was being 

 brought over here for many decades and yet the zebra mussel 

 failed to appear. That has led to speculation about other sorts of 

 species that if they were going to be here, they would be here by 

 now. And, clearly, the zebra mussel shows us that that is simply 

 something we cannot say. 



In May of 1988, we might have rounded up a number of biolo- 

 gists in this country who would have said that about the zebra 

 mussel. "If it was going to be here, it would be here by now." And 

 of course in June of 1988 it was discovered. 



But things change along these corridors, these conveyor belts. It 

 may be that the source area changed. It might be that zebra mus- 

 sels became more abundant in some European harbors because the 

 European harbors were becoming cleaner and more conducive to 

 large populations of zebra mussels and they were ballasted up. 

 That is a donor area hypothesis. 



Another hypothesis is that the receiver area changes. The Great 

 Lakes were not the same in the 1950's, and they changed in the 

 1960's, and they changed in the 1970's, and by the 1980's the Great 

 Lakes became a different kind of environment, in part perhaps be- 

 cause of environmental amelioration. We made a tremendous at- 

 tempt to clean up the Great Lakes and this is a more hospitable 

 environment for more species. 



A third hypothesis would be that the vector is changing. There is 

 more ballast water. The ships are faster. The survival getting 

 across the oceans is greater and, therefore, more things are surviv- 

 ing. 



A fourth hypothesis would be that it was a massive inoculation 

 event. It simply was a stochastic sort of thing; that one ship came 

 over with hundreds of millions of zebra mussel larvae and dropped 

 those on just the right day in Lake St. Clair and it was a sort of a 

 stochastic inoculation. 



A fifth hypothesis is called the invasion window hypothesis, and 

 that is that everything has to be right on a certain time for a spe- 

 cies to be established, and all the variables rarely come together at 



