Nedarim, Chapter 6, Mishnah 4. The Mishnah discusses one who takes a vow against fish, and the question of which foods are included in the wording of his vow and which fall outside it.
"Dag dagim she'ani to'em":
One who says "dag dagim she'ani to'em" - he has vowed not to taste fish. This wording uses two forms, singular and plural, and therefore "asur bahen bein gedolim uvein ketanim" - he is forbidden to eat them, whether large or small.
The Ran explains: "dag" in the singular refers to a large fish, since large fish are sold individually; small fish are sold in groups and in abundance, and it is of them that the plural "dagim" is said. Since the one taking the vow used both forms, both the large and the small are included in his vow.
Not only that, but the Mishnah extends the prohibition to every manner of preparing them:
"bein meluchin bein tefeilin" - whether salted fish or unsalted.
"bein chayin bein mevushalin" - whether raw fish or cooked fish.
On the other hand, two things are not included in this vow:
"tarit terufah" - tarit is a species of fish that they would cut into pieces, and it is not considered part of "dag dagim".
"tzir" - the liquid that comes out of salted fish, and it too is not included in the wording "dag dagim".
One who vows against tzachanah:
Tzachanah is a mixture of cut-up fish. One who vows against it - "asur b'tarit terufah" - he is forbidden to eat tarit terufah, since "tzachanah" is a general term for cut-up fish, and tarit terufah is a particular type included in it.
But on the other hand: "u'mutar b'tzir uv'morayes" - he is permitted tzir and morayes. Tzir is the liquid that comes out of salted fish, and morayes is the liquid, something like a sauce, that comes out of pickled fish. Neither of these is included in the wording "tzachanah".
One who vows against tarit terufah:
With one who vows against tarit terufah the ruling is different: "asur b'tzir uv'morayes" - both kinds of liquid that come out of salted or pickled fish are forbidden to him, because anything mixed together with the fish is included in the wording "tarit terufah".
In summary: One who vows against "dag dagim" is forbidden large fish and small, salted and unsalted, raw and cooked, and is permitted tarit terufah and tzir. One who vows against tzachanah is forbidden tarit terufah, since it is included among cut-up fish, and is permitted tzir and morayes. And one who vows against tarit terufah is forbidden even tzir and morayes, since anything mixed together with the fish is included in his vow.