Sustainable fishing

Is sustainable fishing even possible? 

How is commercial fishing a threat to dolphins and sea bird life? By catch in the fishing industry is when fish or other marine species is caught unintentionally while trying to catch one specific specie (tuna). Commercial fishing conducted within the coastline of French has a large impact on dolphins. The reason being is because dolphins play an important roll in keeping their environment balanced. They eat other animals - mainly fish and squid themselves are a source of food for some sharks and other creatures. without dolphins, the animals they pray on would increase in numbers, and their predators wouldn't have as much to eat.