populations in the Northern and Southern Hemispheres appear to be completely separate. Two stocks are in the North Atlantic, east and west, that seldom mix; only one individual is known to have crossed the Atlantic, covering from Ireland to Canada. Several discrete stocks are likely present in the Southern Hemisphere, as well. This species segregates by size and sex in the North Atlantic, and at least by size in the South Pacific. For example, males outnumber females 2:1 off Spain, females are 30% more numerous than males off Scotland, and immature males are predominant in the Bristol Channel. Older, larger sharks may frequent higher latitudes than younger individuals. Seasonal migrations have been observed in porbeagles from both hemispheres. In the western North Atlantic, much of the population spends the spring in the deep waters of the Nova Scotia continental shelf, and migrates north a distance of to spend late summer and fall in the shallow waters of the Newfoundland Grand Banks and the Gulf of St. Lawrence. In December, large, mature females migrate south over into the Sargasso Sea for pupping, keeping deeper than during the day and at night so as to stay in the cooler waters beneath the Gulf Stream. In the eastern North Atlantic, porbeagles are believed to spend spring and summer in shallow continental shelf waters, and disperse northwards to overwinter in deeper waters offshore. Migrating sharks may travel upwards of , though once they reach their destination they tend to remain within a relatively localized area. In the South Pacific, the population shifts north past 30°S latitude into subtropical waters in winter and spring, and retreats south past 35°S latitude in summer, when sharks are frequently sighted off subantarctic islands. Description The porbeagle is a very stout-bodied shark with a fusiform (spindle-like) shape. The long, conical snout tapers to a sharp point, and is supported by enlarged, highly calcified rostral cartilages. The eyes are large and black, without nictitating membranes (protective third eyelids). The small, S-shaped nostrils are positioned in front of and below the level of the eyes. The mouth is large and strongly curved, with moderately protrusible jaws. North Atlantic sharks have 28–29 upper tooth rows and 26–27 lower tooth rows, while Southern Hemisphere sharks have 30–31 upper tooth rows and 27–29 lower tooth rows. Each tooth has a strongly arched base and a nearly straight, awl-like central cusp, which is flanked by