When fishermen are using trawl nets, not only the acquired fish are caught. More than that, 70% of the catch is thrown back overboard, because the fish are too small to be legally justified for sale, or because the admitted quota is reached, or even because the fish is not interesting, commercially speaking. The fish that are thrown overboard often are already crushed to death, choked, or died in another way. Anyone who isn't touched by the suffering of fish during the catch, is referred to the additional catch of e.g. mammals like dolphins, which are often caught at the tuna catch.
The trawl nets destroy the bottom of the sea, as a result of which the ecological system is completely out of balance, and lost for a long time. The seas are almost emptied and left completely disturbed.