Whale of a problem: Marine giants are losing weight

Whales depend on cocktail-shrimp-size crustaceans to bulk up for their long southerly migration. As Arctic ice recedes, fat-rich crustaceans that flourished on the Bering Sea floor are becoming scarce.

Skinny whales were first spotted this year in the protected waters of San Ignacio Lagoon in Baja California, where gray whales spend the winter breeding and nursing their calves before returning every summer to the Arctic.

That’s where a team led by Steven Swartz of the National Marine Fisheries Service in Silver Spring, Maryland, and Jorge Urban of the Autonomous University of Baja California Sur noticed that about 10 per cent looked more bony than blubbery, a telltale sign of malnutrition.

Instead of making steady progress during their long migrations, the whales have been stopping often to eat along the way. They have been seen straining mysid shrimp from kelp beds off California and British Columbia, sucking up mouthfuls of sand in Santa Barbara Harbour and skimming surface waters for krill-like crustaceans all along the West Coast.

Such opportunistic feeding has its risks. Switching to new food can expose the whales to harmful parasites as well as other hazards. There have been at least two fatal accidents this spring near San Francisco’s Golden Gate Bridge. Gray whales, surfacing to breathe after dining on sea-floor snacks, have been ripped apart by propellers on cargo vessels.

To find food, gray whales have also been expending more energy by extending their 5,000-mile northerly migration beyond the Bering Strait into the Chukchi and Beaufort seas north of Alaska.