The reading claims that the introducing zebra mussel to the North America poses serious threat to freshwater fish population there and it provides three reasons of support.

The reading claims that the introducing zebra mussel to the North America poses serious threat to freshwater fish population there and it provides three reasons of support. On the other hand, the professor states that there are ways to control the spreading of the zebra mussel and it is not clear that it has serious problems. He refutes each of author's reasons. First, the article avers the history of the zebra mussel's spread suggests that the invasion might be unstoppable. In contrast, the lecturer opposes this point by saying that it is true because people didn't have the knowledge that time to stop ships carrying the mussel. In fact, he mentions that they could do that by required ships to empty ballast water before North America and fill it with fresh water so salt water would kill the zebra mussel. This disputes the first point in the passage. Second, the passage posits that once zebra mussels are carried to a new habitat, they can dominate it. However, the speaker counters this point by stating that it is true there are no predators for the mussels in the begining. Actually, he argues that in Europe, birds found a new food which is the mussel and they can eat a lot of the zebra mussels so the zebra mussel can't dominate. This appears a contradtiction to the information made in the passage. Third, the article asserts that zebra mussels are likely to cause a decline in the overall fish population in habitats where they become dominant. Conversely, the professor casts doubt on this point by explaining that it is true that the mussels have a negative effect on some fish that eat plankton, but they have a positve impact on other fish population. He says that the zebra mussel creates general nutertion to fish live in the bottom so these fish may increase and thrive.