Fishing within the territorial waters of a state can be regulated by adopting norms and regulations on the subject. At regional and international level fishing can be regulated by the adoption of bilateral, regional and international agreements.
Please consult the address I attach below. As you'll see, within the European Union, fishing is regulated through regulations, directives, and agreements.
A number of specific rules are taken into account (and transposed in the legal systems of the 28 Member States). Among the others, there are also quotas for the different typologies of fishes (for instance tunas) and periods in which fishing is precluded in order to protect fish stocks: