Shipping lanes that carry about 20 cargo and cruise vessels a day in and out of San Francisco Bay are being revised in an effort to reduce fatal collisions with whales, federal officials said Thursday.

The proposal developed by the U.S. Coast Guard and the National Oceanic and Atmospheric Administration modifies shipping lanes that head north, west and south from the Bay, limiting their overlap with areas frequented by endangered blue, humpback and fin whales.

Five whales were killed by confirmed or likely ship strikes in the San Francisco area in 2010, and scientists say the mortality rate may be much higher because most dead whales sink.

The northern shipping lane, which runs along the Marin County coast, will be narrowed to 3 nautical miles wide and extended by nearly 17 miles, sweeping past the Point Reyes Peninsula.

It will also be turned slightly to keep ships away from Cordell Bank, known as a "destination feeding ground" for blue and humpback whales, giant mammals that eat krill, a small crustacean.

Narrowing the western lane is intended to shift vessel traffic away from the seabird colony at the Farallon Islands.

"This is a very positive step," said Lance Morgan of Glen Ellen, president and CEO of the Marine Conservation Institute, a nonprofit organization.

Morgan served as co-chairman of a working group that included environmentalists, whale experts and shipping industry representatives that assessed vessel strikes on whales and recommended, in a June report, modification of shipping lanes.

The idea, Morgan said, is to "reduce the probability" of whale vs. ship collisions.

Shipping lanes handle vessels over 300 gross tons, including tankers, container and cruise ships.

The northern and western lanes will be extended to the edge of the continental shelf, the area where migrating whales feed and congregate.

Blue whales, the largest animals on the planet, are of prime concern because their northeast Pacific population of about 2,000 has "flat-lined" since commercial whaling in the United States ended in the 1970, said Michael Carver, deputy superintendent of the Cordell Bank National Marine Sanctuary, a federal agency.

The blue whales' vulnerability is heightened by their tendency to surface in response to stress, bringing them into "striking range" of large vessels, Carver said.

The proposed shipping lane modifications were approved last month by the International Maritime Organization, which governs shipping worldwide, and will be implemented in June, he said.

Adjustments will also be made to the shipping lanes in the Santa Barbara Channel and approaching the ports of Los Angeles and Long Beach.

The next step, Morgan and Carver said, is to establish real-time monitoring of whales and to actively direct vessel traffic away from them.

You can reach Staff Writer Guy Kovner at 521-5457 or