If there are not enough fish, cetaceans will starve. The more fish that are caught, the more cetaceans die. Fishermen also sometimes kill cetaceans, to stop them "stealing" their catch. Of course, overfishing is not good for fish either. It affects how the ones that are left behind reproduce and where they breed. If too many fish are caught, the world's fish stocks could slowly die out. Cetaceans would die with them.