Before 1992, 20,000 lobsters were harvested (completely legally) every year.
In 1992, illegal harvesting started. So, 20,000 went down to 18,000.
In 1996, fishing activity was the same (so same number of fishing boats for same hrs were used etc), legal catch was 9000 below 1992 levels i.e. legal catch was 11,000.

Conclusion: Hence, illegal activity harvested 9000 tons that year. (to make up the initial 20,000 number)


Which of the following is an assumption on which the argument depends?

(A) The illegal lobster harvesting was not so extensive that the population of catchable lobsters in Belukia’s territorial waters had sharply declined by 1996.

This says that the number of catchable lobsters stayed the same by 1996. So the available population of lobsters which can be caught (perhaps living above certain water level) has stayed the same. We need this to be true for our conclusion to hold.
Note what happens when we negate it - the illegal harvesting WAS so extensive that the population of catchable lobsters sharply declined by 1996.
If the harvesting is so extensive that the lobsters are unable to breed at the required pace to maintain the population, the number of lobsters that would be caught would be less than 20,000. Hence, illegal activity would not have harvested 9000 tons that year.

(B) The average annual lobster catch, in tons, of an outlaw fishing boat has increased steadily since 1992.
We need to make no assumption about one outlaw fishing boat. Perhaps many new outlaw fishing boats join every year.

(C) Outlaw fishing boats do not, as a group, harvest more lobsters than do licensed lobster-fishing boats.
Again, no such information is given. We don't know the comparative numbers and our conclusion does not depend on them.

(D) The annual legal lobster harvest in Belukia in 1996 was not significantly less than 9,000 tons.
Again, we don't know the comparative numbers and our conclusion does not depend on them.

(E) A significant proportion of Belukia’s operators of licensed lobster-fishing boats went out of business between 1992 and 1996.
Not given. Perhaps the annual harvest of each boat reduced by some percentage but nobody went out of business.

Answer (A)