SAN FRANCISCO (CBS SF) — Dozens of migrating blue whales have stopped to feed in the krill rich waters off the Farallones Islands, triggering a warning from federal maritime officials to commercial shipping companies to be on the lookout for the massive marine mammals.

Maria Brown, Superintendent of the Greater Farallones National Marine Sanctuary, issued the warning after biologists sighted at least 47 blue whales over a single one-hour period on June 13. Observers the day before spotted 23 whales.

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“The sanctuaries are working with the U.S. Coast Guard to notify mariners to be on the lookout for the whales, and maintain slow speed and maximum possible distance to avoid deadly collisions,” said Brown in a press release.

It’s extraordinary. It may be an absolute record,” said NOAA Greater Farallones spokeswoman Mary Jane Schramm. “Essentially, the driver here is krill and that’s the food they feed upon this time of year and we have it in such great abundance in the marine sanctuary that the blue whales have apparently targeted our water specifically to feed.”

Blue whales — the largest animal on the planet — are listed as endangered under the Endangered Species Act. The Eastern North Pacific population, the West Coast stock of blue whale populations, have seen no significant increase since the early 1990s, despite being protected.

“Their enormous size dictates that they maximize feeding effort when food is available, and this sometimes takes them into dangerous waters,” the NOAA statement read: “While focused on feeding, they are not likely to recognize or evade a ship’s approach.”

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It been a busy migration season off the Northern California coast. In March, Park rangers at the Point Reyes National Seashore reported sighting over 200 grey whales during a three-day span.

Embarking in the longest annual migration of any mammal, the California gray whale travels in the neighborhood of 10,000 miles every year, spending about one third of its life swimming from the chilly nutrient-rich waters of Alaska to the warm lagoons of Baja California and back. During the trip, they are often visible from the shores of Point Reyes.

The primary threats currently facing whales off the Northern California coast are vessel strikes and entanglements in fishing gear.

In May, a U.S. Coast Guard crew was able to help a team from NOAA untangle a whale that had gotten caught in fishing gear near Moss Landing.There was no word on what species of whale was found or the type of fishing gear the sea mammal got entangled by.

Last year, at least four whale carcasses washed up on Bay Area beaches, the victims of fatal collisions with freighters and tankers.

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Maritime officials said to report a dead, injured or stranded marine mammal, please call the West Coast Marine Mammal Stranding Network: 1-866-767-6114. For law enforcement, harassment, and other violations, please call the NOAA Enforcement Hotline: 1-800-853-1964.