Praise be to Allah.
I asked my shaykh, Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) about this, and he answered:No, this (i.e., fish) is not merchandise. It is like the case if he were to go fishing every day and eat what he caught. But if he puts it in the freezer and then sells it, in this case, if a whole year passes, then he has to pay zakaah. And Allaah knows best.