When people think of environmental impacts of fishing, they usually think about overfishing (which endangers fish stocks) and unintended bycatch (which is ridiculously wasteful). What should also be considered is all the fishing gear out there, either operative or drifting through the seas, that entangles marine mammals.
Entanglement is one of the major threats to large whale species (and turtles) and the numbers just keep going up (despite all the inspiring whale disentanglement videos you love on Facebook). It’s just one more problem with the human love of fish protein.
There are many places on the planet where human populations really do rely on the bounty of the sea to eat (though most of those fishing operations are more subsistence than industrial). But most of the entanglement damage is being done by fisheries that are feeding people who could easily choose not to eat fish. Sure, people like crab, lobsters, and swordfish. But they are a matter of taste and preference, not of nutritional requirement. At some point it is important to consider whether simple taste and culinary preferences are worth the collateral damage. Because they really aren’t.