The 152nd mitzvah is that we are commanded regarding the signs of fish. These signs are explained in Scripture in G‑d's statement,1 "This is what you may eat of all that is in the water."
Our Sages said explicitly in Gemara Chullin,2 "One who eats a non-kosher fish transgresses both a positive commandment and a prohibition." This is because the statement "this you may eat" implies that other things you may not eat, and that a prohibition which is implied from a positive commandment is counted as a positive commandment. This shows that the phrase, "This is what you may eat" constitutes a positive commandment.
As mentioned previously, when we say that this is a positive commandment, we mean that we are commanded to judge according to these signs and declare "this may be eaten" and "this may not be eaten." This is clear from the verse,3 "You must separate out the clean animals and birds from the unclean," and this distinction can only be made through [examining] the signs. Therefore, each of these four categories of signs — those of beheimos and chayos, of birds, of grasshoppers, and of fish — constitutes a separate mitzvah. We have already quoted the language of our Sages where each is called a separate positive commandment.
The details of this mitzvah — regarding the signs of fish — are explained in the third chapter of tractate Chullin.
