Answer:
The statement means that the number of whales hunted is so large that it is impossible or hard to get it back to normal or tolerate the extent of the reduction in the whale population.
Step-by-step explanation:
The term "unsustainable" means something that is not bearable, viable, acceptable, tolerable, renewable, etc. This means that an issue cannot be replaced or refilled easily.
In the context of the whaling industry in Japan, the anti-whaling activists expressed the "unsustainable" nature of the extent to which the Asian nation hunted and killed whales. In arguing that "the number of whales hunted by the Japanese each year is unsustainable", they mean to emphasize the large extent to which the whales were killed and the difficulty in replacing or tolerate the act. In short, the activists' argument means that the number of whales hunted will be impossible to regain or filled, or even accepted.