Fish Bulletin No. 64. The Biology of the Soupfin Galeorhinus zyopterus and Biochemical Studies of the Liver
The use of fish liver oils as a dietary supplement has been practiced for hundreds of years but not until a comparatively recent date has it been known that the virtue of these oils lies in their vitamin content. Because of this increased knowledge of vitamins, manufacturers of pharmaceutical products analyzed many species of fish in a search for new sources of this much desired food fortifier. Additional stimulus to the search for vitamins in other fish was also furnished by the second World War which curtailed the production of cod liver oil and eventually cut off European supplies. Laboratory tests showed that the soupfin shark along the Pacific Coast of North America has a liver richer in vitamin A than any other fish yet analyzed. As a result, the California fishery for this shark increased at a spectacular rate.
By 1942 an investigation of the fish and its fishery was deemed necessary. This required study of the biology of the fish to determine if the population could continue to support the fishery, and an analysis of the vitamin A content of the livers both to understand the causes for the variation in yield between individuals and to ascertain whether methods in practice by the industry produced the highest possible vitamin yield.
The soupfin shark (Galeorhinus zyopterus) is classified with the family of true sharks, Galeidae, in which are included the tiger, great blue, smooth-hounds, leopard, and bay shark. The family has a wide distribution in the oceans of the world. The species Galeorhinus zyopterus is found from Cedros Island, Lower California, to northern British Columbia.
The soupfin attains a size of slightly over six feet. Its head is flattened dorso-ventrally with the snout projecting well beyond the eyes. It has five gill openings and an anal fin. The first dorsal fin is located about midway between the pectorals and ventrals. The tail, not lunate in shape, is considerably shorter than the body and is deeply notched. There is no lateral keel on the caudal peduncle. The teeth are sharp, placed in several rows and are notched on the outer edge below the point, with the lower part of the notch divided into two to five points.
The position of the second dorsal and anal fins offers the simplest means of distinguishing soupfin from other species of sharks found in California. These fins are inserted opposite each other just anterior to the caudal peduncle and are of about the same height. In lateral view an extension of the anterior and posterior edges of the two fins would intercept to form an almost perfect diamond.
To the casual observer, small specimens of soupfin may be confused with brown and grey smooth-hound sharks. However, the second dorsal fin of the smooth-hounds is inserted in advance of the anal fin and exceeds this fin in both length of base and in height. If close attention is given to the insertion and the relative height of the second dorsal and anal fins, the distinction between small soupfin and smooth-hounds is quite marked and should offer no difficulties.