What is the blood-like material found along the backbone in the body cavity of most fishes?

The kidney. It is usually removed when the fish is cleaned.

Can shark meat be used as food for humans?
Is it true that the meat of the hammerhead shark is poisonous?

dogfish Yes to the first question. Shark meat is palatable and nutritious if properly prepared. In some countries shark meat is marketed under its common name, in others it is marketed under various names. The fish in England's "fish and chips" is sometimes dogfish or school shark (although this would be considered low quality "fish & chips". True "fish and chips" consists of cod, haddock (the 2 most popular) and also plaice). The prejudice against shark meat arises from a distaste for the scavenging habits people attribute to sharks, and to the fact that the meat spoils quickly. The meat of certain species is apt to be strongly flavored, a characteristic that may be reduced by icing for 24 hours, then soaking for two hours in brine Dry salted shark has become a staple food in some countries where salt cod was formerly popular. But shark liver should never be eaten - its high concentrations of vitamins can cause illness in humans. It is only a rumor that the hammerhead is poisonous.

What is the true description of a sardine?

Commercially, the name has come to signify any small herring-like ocean fish. In the United States, it is mandatory that when the name "sardine" is used on a can, the country or state of origin be listed, and a statement must appear that identifies preserving and flavor supplements.

How many kinds of tuna are there, and which kind makes up the biggest catch?

There are seven commercial and sport-caught tunas, as well as several related species, all of which are members of what is called the scombrid family. Commercially caught tunas consist of albacore, bigeye, blackfin, bluefin, bonito, skipjack, and yellowfin. Yellowfin, taken in the eastern Pacific and tropical Atlantic, makes up the biggest U.S. commercial catch. Albacore, caught in the eastern Pacific, is the true "white-meat" tuna; skipjack, caught throughout the world in tropical and subtropical waters, makes up the second largest U.S. commercial catch; bigeye is caught mostly in tropical waters; blackfin is caught commercially only in the Caribbean and off South America; the very large bluefin (rod-and-reel record, 1,040 pounds) is a highly prized sport catch in the Atlantic and Pacific; and the widely distributed bonito is used largely as pet food.

albacore tuna

bigeye tuna

skipjack tuna

Do tunas have scales?

Yes, all species do, but scales are so small over most of the body as to be nearly invisible. Prominent scaling appears only around the head, on the cheeks, and in a triangular area on each side of the body near the head.

Tuna images courtesy of "Regulatory Fish Encyclopedia, Office of Seafood and Office of Regulatory Affairs, Food and Drug Administration, 1993-1996."

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(File Modified Jun. 16 2011)