36 BULLETIN OF THE BUREAU OF FISHERIES 



11. Mackerel shark {Isuriis punctatus StoTer) 

 Blue shark; Porbeagle 



Jordan and Evermann {Lamna cornuhica Gmelin), 1896-1900, p. 49. 

 Garman, 1913, p. 36. 



Descri-ption. — The mackerel sharks (this and the two species following) are 

 easily told from all the sharks so far mentioned by the shape of the tail, for while 

 its lower lobe is sharklike, somewhat smaller than the upper, the difference is slight, 

 the tail being almost evenly forked, with the upper lobe directed so sharply upward, 

 the lower downward, that the tail as a whole is crescentic and much broader than 

 long. In fact it recalls the tails of such pelagic bony fishes as the mackerel tribe 

 or the swordfish in outline, likewise in its firm texture. More precise if less obvious 

 a character is that the root of the tail bears a well marked longitudinal ridge or 

 keel on either side, a feature shared by the white and basking sharks (pp. 39 and 41) . 



This is a stout, heavy-shouldered shark, tapering in front to a sharply pointed 

 snout and behind to a very slim tail root. Its dorsal and pectoral fins are very 

 large; the former, originating over the armpit of the pectoral, is triangular and 

 about as high as long; the latter, broad-based but tapering sicldelike to a narrow tip. 



Fig. 12. — Mackerel shark {Isutu^ punctatus). After Garman 



is only about half as broad as long. The second dorsal and anal fins are very small 

 indeed, and the ventrals but little larger. The second doi-sal stands over the anal. 

 The positions of the dorsal fins are the readiest field mark to distinguish this species 

 from the sharp-nosed mackerel shark (p. 38). The teeth are alike in the two jaws- 

 small, slender, pointed, smooth-edged, and without spurs on the sides — and their 

 structure differentiates this shark from the European porbeagle {Isurus nasus), 

 which it otherwise resembles closely but in which the teeth bear a sharp denticle on 

 either side at the base of the cusp. 



Size. — The larger mackerel sharks are usually about 8 to 10 feet long, growing 

 to an extreme length of about 12 feet. 



Color. — The upper parts are dark bluish gi'ay to bluish brown, changing abruptly 

 to white below. According to Garman the dorsal, pectoral, and tail fins are tipped 

 with black, there is a black area in the armpit of the pectoral followed by a white 

 space on the fin and body, and there is a large and very noticeable black spot on 

 the outer half of the pectoral, which is one of the distinguishing features of this 

 species. 



