If people consume less of things that damage the environment, we will produce less of it and therefore damage less of it.
Fish don't damage the environment. Fishing nets do, when thrown into the ocean by irresponsible fishermen. Eating less fish would punish all fishermen, including the environmentally-responsible ones. What we need instead is a better enforcement of existing laws against env. pollution.
Do discarded fishing nets really cause more harm to fish than actively used ones? The whole job of fishermen is to kill fish. How can you say they're environmentally responsible for not losing their nets? The environmental problems caused by fishing already exist and are far more severe than those caused by plastic which so far are mostly only theoretical or imaginary. The UN says that "half the world's fishing fleet could be scrapped with no change in catch." That's how much overfishing they're doing. It's not environmentally responsible.
Fish don't damage the environment. Fishing nets do, when thrown into the ocean by irresponsible fishermen. Eating less fish would punish all fishermen, including the environmentally-responsible ones. What we need instead is a better enforcement of existing laws against env. pollution.