The decision was made Monday by the California Department of Fish and Game after agency biologists determined that commercial gill nets were inflicting substantial harm to common murres -- a seabird -- and the southern sea otter.

The closure covers all the areas where it is still legal and practical to gill-net for halibut, including all of Monterey Bay and waters off parts of Marin County, the entire San Francisco Peninsula and portions of Santa Barbara County.

Although the order allows commercial fishermen to set gill nets in water deeper than 360 feet, the nature of gill-net gear and the configuration of California's offshore sea floor make it impractical to fish at such depths.

Fishing industry officials said the order would be devastating to the state's remaining gill-net fishery. ``It's basically going to put these guys out of business,'' said Michael Stiller, president of the Santa Cruz Commercial Fishermen's Association.

About a dozen commercial boats gill-net for halibut in the affected waters. Gill nets are wide-meshed nets that are generally weighted on the bottom. As fish try to swim through them, their gill plates are entangled in the mesh, entrapping them.

The closure comes at the end of the summer season, when most halibut are caught in California waters. It will be in effect for 120 days and will then be reviewed. At that time, regulators can either modify the order or make the closure permanent.

A public meeting on the closure will be held by Fish and Game from 4 to 6 p.m. Sept. 18 at Seaside City Hall in Monterey County.

The closure comes after studies by state and federal biologists concluded that the commercial gill-net fleet was entangling and drowning large numbers of both common murres and otters.

Murres have undergone steep declines in recent years because of oil spills and warm water currents that have reduced the fish they prefer to eat. To counter the trend, state biologists are attempting to encourage new nesting colonies on coastside cliffs.

The closure is likely to fuel the controversy over commercial fishing in national marine sanctuaries. Offshore waters from Marin to Monterey counties constitute the Gulf of the Farallones, Cordell Bank and Monterey Bay National Marine Sanctuaries.

The closure is the most restrictive measure the state has ever taken over the problem of ``bycatch'' -- the capture of nontarget species. Restrictions on commercial fishing over bycatch are extremely unusual. The last major bycatch decision by the Fish and Game Department occurred about a decade ago.

``We determined that gill-netters fishing in shallow water were killing about 80 to 100 otters a year, so we made them move to beyond 30 fathoms (180 feet),'' said David Bunn, a deputy director for Fish and Game.

But bycatch closures may become increasingly common as environmentalists sue to protect threatened wildlife species.

While Monday's closure followed lengthy evaluation of the situation by state biologists, it also appears to have been at least partially motivated by a notice of intent to sue by the Center for Biological Diversity, an environmental group known for its lawsuits on behalf of endangered species.

Earlier this month, the U.S. Fish and Wildlife Service recommended use restrictions on 5.4 million acres of land to protect the California red-legged frog. That decision was driven by a court order that resulted from a Center for Biological Diversity suit.

``On a fundamental level, we believe there should be no bycatch of endangered or threatened species,'' Cummings said. ``There may be situations where certain types of nets are appropriate in certain places, but generally hook-and-line fisheries, such as salmon trolling, (are preferable).''

From a larger perspective, said Cummings, a hard look must be taken at commercial fishing in national marine sanctuaries, such as the waters off Monterey Bay and the San Francisco Peninsula.

``A marine sanctuary is pretty hollow if commercial fishing is allowed,'' said Cummings, who also said his group doesn't have a problem with recreational angling.

News of the closure did not sit well with commercial fishermen.

``It's a reactive rather than a proactive way of managing fisheries,'' said Mike Ricketts, a Monterey area crab, salmon and albacore commercial fisherman and a board member of the Pacific Coast Federation of Fishermen's Associations, a commercial fishing lobbying group.