Shadows in the Deep Blue Seas
By Mary Jane Schramm
The Pacific stretched to the horizon in a slow rhythm of swell broken only by the flighty tease of wind and wavelet. The vessel’s engine gave off guttural growls as it broached each swell, subsiding to a steady purr as it slid down the far side. Earlier, humpback whales had put on a lively show near the Farallon Islands: lunge-feeding, breaching, and pectoral fin slapping to the delight of the whale watchers. But radio chatter in the wheelhouse had reported blue whales off the Continental Shelf, so they headed west to the several thousand-foot deep waters of Greater Farallones National Marine Sanctuary. But whales can prove elusive, and after awhile the vessel came about, ready to return to port. Suddenly, a cluster of small fins slicing and dappling the surface came into view: it was a school of long, lithe sharks slowly milling near the surface, sinuous and elegant. Instead of giant whales, the group had encountered arguably the most beautiful shark species in the world: the blue shark, Prionace glauca.
PORTRAIT IN INDIGO: Blue sharks are slender and lissome, their backs and sides a vibrant indigo or deep lapiz lazuli hue, countershaded to a crisp white beneath. Large, round eyes lend them a slightly surprised expression, and their exceptionally long, winglike pectoral (side) fins help buoy them up as they glide through the water, sometimes riding the currents. Adult females often reach 11 ft., with males around 9 ft., though larger individuals have been reported. Being ectotherms, i.e., taking their body heat from their cool surroundings, their movements are slow - that is, until “the game is afoot.” Second only to the mako shark for speed, blues can reach 43 mph and faster in bursts, and are sometimes called “the wolves of the sea.” Their tapered conical snouts are armed with revolving layers of sharp, recurved teeth for grasping squid, octopus and cuttlefish, crustaceans, herring, hake, and occasionally, seabirds and marine mammals. Humans aren’t on their menu.
OCEAN NOMADS: P. glauca are among the widest-ranging sharks, traversing ocean basins in deep temperate and subtropical seas, but preferring the cooler (50 to 68ºF) food-rich waters of the mid-latitudes.
PUP-A-PALOOZA: Blue sharks remain sexually segregated most of the year, but come together in to mate and feed together, often migrating long distances for trysts. Mating is a rambunctious affair involving “love bites,” and mature females often bear many scars from these encounters. They are viviparous - i.e., they give live birth to litters of four to 135 pups. Gestation lasts around 12 months.
SURVIVAL BLUES: Natural predators include killer whales, white sharks and makos, but humans are their major threat. They’re fished directly for fins and meat, but also represent major bycatch in pelagic longline, gillnet and purse seine fisheries, mostly by industrial high-seas pelagic fleets. Globally, their numbers are declining, and the International Union now lists blue sharks as “Near Threatened.”
WHAT YOU CAN DO: Be an informed consumer; visit the Monterey Bay Aquarium’s Seafood Watch site, https://www.seafoodwatch.org/for a pocket card or app listing species to avoid and sustainably caught alternatives. Learn about international protections for blue sharks via the UN Convention on Migratory Species at https://www.sharktrust.org/cms
NOAA’s Greater Farallones National Marine Sanctuary partners with the nonprofit Greater Farallones Association to ensure healthy habitat for all marine species, and offers education programs for all ages. Visit https://farallones.noaa.gov and http://farallones.org for more information.
Photo Credits:
Top: The blue shark’s graceful, elongated shape. Credit: Diego Delso-delso.photo-LicCC-BY-SA.
Middle: Loligo, or market squid, are a favorite entree. Photo: NOAA.
Bottom: Blue sharks are often curious. Credit: MA Marine Fisheries-PD
