"What’s Black and White (and Black and White) All Over?"
By Mary Jane Schramm
The broad expanse of surf was frothed with whitecaps. Below the surface, currents interlaced with cross-currents, and the push of wind and surge created ideal conditions for beachcombers to discover whatever flotsam and jetsam the sea might surrender. An object broke the scalloped arc of the receding waves, and an onlooker, drawing closer, saw it was a shark! It’s robust body, dark back and contrasting white sides and belly, its fathomless black eyes and fearsome set of pointed teeth indicated it was a white shark’s! But - was it? At only three feet, this was no giant monster of the deep. A baby white shark, then? Acting quickly, he noted its exact location and snapped photos, sending them to Greater Farallones National Marine Sanctuary, which determined it was a Lamna ditropis, a salmon shark; close cousin and near dead-ringer to the white shark, Carchardon carcharias. State biologists collected the specimen, and conducted a necropsy to determine its cause of death: encephalitis, a disease pathologists are investigating, as it’s increasingly found in juvenile salmon sharks.
KISSING COUSINS: Salmon sharks and white sharks are closely related, look incredibly alike, and overlap geographically. Salmon sharks get big, usually six to eight feet but sometimes 10 ft., - still no match for the 20+ ft. white shark. This fish’s white “suspenders” above its dark gray pectoral fin gave it away. Other distinguishing marks are more subtle or are behavioral. For example, in our sanctuary, whites are almost all mature –and BIG – meat eaters, whereas salmon sharks eat mostly fish.
BUILT FOR SPEED: Most sharks are cold-blooded, their body temperatures reflecting that of the sea around them. But salmons and whites are among only five sharks worldwide that regulate their body temperature internally (endothermism). In fact, the salmon shark may be the world’s most hot-blooded: able to maintain a temperature of 77ºF, even in near-freezing Arctic waters. Warm blood permits it to function within a wider range of depths and temperatures. With pre-warmed muscles, it can move quickly from at-rest, to speeds of 40 or 50 mph in pursuit of mackerel, salmon and other “fast food.”
HUNGRY HUNTER: Their hunting styles differ, too, like the lion and the cheetah: The white shark (lion) waits in ambush, then attacks. If it misses, instead of pursuing its prey over any great distance; it regroups for the next opportunity. But the “cheetah” salmon shark is both champion sprinter and marathon runner, capable of achieving and sustaining full-throttle velocity. Their dental arsenals differ greatly, too: Both begin life with conical, pointed teeth for grasping and swallowing fish. But as young whites mature and transition to eating seals and sea lions, they grow large, flat, triangular serrated teeth for pinning, gripping, and slicing.
RANGE AND MIGRATION: Salmon sharks are highly migratory, distributed across the North Pacific from southern Japan north to Russia, the Bering Sea and Gulf of Alaska, and south to Baja California. Adults and sub-adults cold northern waters, following schools of Pacific salmon and other fish and small sharks, hunting individually or in groups of up to 40.
“Farallones” whites feed on seals off North-central California in fall and winter, but migrate far offshore in spring and summer to feed on squid and other invertebrates, and possibly to mate. They likely breed off Mexico and Southern California.
REPRODUCTION & LONGEVITY: Both species give live birth to small numbers of fully independent pups after a long gestation. Salmon sharks may live to 30 years -- about the age when white shark females sexually mature! Whites, however, live longer: up to 70 years for males, but only around 40 for females: a very short reproductive window.
CONSERVATION: Some salmon sharks are caught accidentally in fisheries, but are a threatened species. White sharks are less at risk of entanglement, but both are vulnerable to longlines, climate change, ocean noise and pollution. But whites especially, being slow to mature, bearing so few young, and being uniquely susceptible to disturbances from cage dive tourism and other human causes, warrant greater concern.
Learn about the white sharks of our sanctuary and the measures we take to protect them at https://farallones.noaa.gov/eco/sharks/ and more about both species at: https://www.floridamuseum.ufl.edu/discover-fish/sharks/
