Sefer HaChinuch ספר החינוך

The commandment to check the signs of fish: To check the signs of fish, meaning to say that one who wants to eat fish first check properly for the signs that the Torah gave about them — and they are fins and scales, as it is stated (Leviticus 11:9), “And this shall you eat from all that is in the water, etc.” We have already written in the commandment preceding this (Sefer HaChinukh 153, in the commandment of checking the signs of beasts and animals) that Ramban, may his memory be blessed, does not calculate the checking of signs in the species that the Torah permitted; and that the truth is with him, according to what appears to us. And even though we have found explicitly in the Gemara (Chullin 66b) with some of them, “This is a positive commandment,” the intention of the matter is to make liable for impure species with a positive commandment [as well as] a negative commandment — from the principle that is well-known to us, [that] a negative commandment that comes from the implication of a positive commandment, is a positive commandment. And [that is] like we shall write with God’s help in the commandment of lending to a gentile (Sefer HaChinukh 173), such that Rambam, may his memory be blessed, counted it as a commandment (Sefer HaMitzvot, Positive Commandments 198), [whereas] according to what it appears, the verse only comes to make liable for a positive commandment [as well] as a negative commandment for the lending to a Jew with interest.

I have also written the root of the commandment above in the signs of the beast. And it is the general principle for the four species.

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Chullin 59a) that fins are what in the fish is like a type of wings with which to fly; and scales are that covering that clings to all of its body. And even if scales are only found on part of its body — and even if it is only one — it is enough for it and it is fit (kosher). And they said in Tosefta Chullin 3:9, “And that is when it is under its jaw or under its tail or under its fins; but on the rest of its body, it is not enough for us with less than two.” And everything in which scales are found — even one in the places mentioned — need not be checked [to ascertain] if it has fins; as it certainly has [them]. But if fins are found on it, it needs to be checked [to ascertain] if it has scales — as there are many impure fish that have fins. And any fish the nature of which is to have scales, even though they are not produced on it when it is still small until it grows up — such as the sultanit and the afian — behold it is permissible. And anything that has scales when it is still in the sea, even though it drops them at the time that it goes out from the sea — such as the akonas, the afonas, the bisifityas, the afnasetiyas and the tunny that is called biret in the vernacular — behold, it is fit (Chullin 66). As the Torah is only concerned with the species that it have scales and fins — since that is what is seemly for the nature of people, and from the reason that we wrote in the prohibition of foods according to the simple understanding.

And [it] is practiced in every place and at all times by males and females. And if one transgressed it and did not check properly, such that he saw only one sign and relied [upon it] for the other, without checking — even though afterwards it was found to be fit — he violated this positive commandment.