Marine life heavily depends on forage fish populations. Seeing the consequences of their decline, such as weekly sightings of dead sea lions on the California beaches, is what prompted me to give up eating seafood.
Iconic marine species like Dall's porpoises as well as sea lions, tuna and puffins depend on forage fish to survive. Global expansion of aquaculture and agriculture industry is threatening the forage fish population. What's worse is that forage fish catch isn't even for direct human consumption--it's sold as food for farmed fish, chickens and pigs.