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.
非法捕鱼可以是： 使用被禁止的工具捕鱼； 在禁捕区捕鱼； 或者是，捕获错误的尺寸或种类的鱼。