It can be when you catch fish using gears that have been prohibited, when you fish in places where you're not supposed to fish, you catch fish that are the wrong size or the wrong species.
禁止された漁具を用いた漁業も 禁止された漁具を用いた漁業も 漁業禁止区域での漁業も 大きさや種類の制限に違反した漁も 違法です