The Midrash, as cited by Rabeinu Bachya, teaches us that Klal Yisrael suffered the terrible tragedy of the Asarah Harugei Malchus, Ten Martyrs, who were murdered by the Roman emperor as a result of the sin of mechiras Yosef, the sale of Yosef. It is only implied by his statements. That is not sufficient. This is why the tragedy of the Ten Martyrs occurred many centuries later. All creatures include species, some of which are kosher and some of which are not kosher.
Fish are no exception.
Even after fowl and animals are ritually slaughtered, however, they still require a process of checking that the shechitah went well, followed by a process of koshering the meat. Fish, on the other hand, observes Horav Mordechai Cohen, zl, are kosher immediately, as long as they manifest the required natural kosher sign. Yaakov blessed his children that they should remain on the same level of purity throughout their lives as they held when they were born. The Baal Shem Tov comments that when Moshiach arrives, every Jew will be preoccupied and engaged in his daily endeavor. The sudden advent of Moshiach will be a sudden and totally unexpected occurrence.