BAY AREA -- A pair of environmental organizations filed two lawsuits against the U.S. Fish and Wildlife Service Friday to seek greater protection for two species of smelt found in the Sacramento-San Joaquin Delta.

The lawsuits over the longfin smelt and Delta smelt were filed in federal courts in San Francisco and Sacramento by the Center for Biological Diversity and the Bay Institute.

The groups say both of the small fish are "critically imperiled" because of water diversions from the Delta, pollution and harm from non-native species.

Jeff Miller, a conservation advocate with the Center for Biological Diversity, said, "Formerly abundant fish at the base of the food chain in the San Francisco estuary are being driven to near extinction."

The Delta smelt, which is currently listed as a threatened species, is 2 to 3 inches long and the longfin smelt is 5 inches long. Both are eaten by larger fish, birds and marine mammals.

In the Sacramento lawsuit, the groups are seeking an order requiring the Fish and Wildlife Service to consider upgrading the Delta smelt from threatened to endangered status.

The suit says the fish population is falling to record lows and "an emergency situation clearly exists for the Delta smelt." It charges the agency violated the U.S. Endangered Species Act by failing to act within a year on a petition the groups filed in 2006.

In the San Francisco lawsuit, the groups are challenging an April decision in which the service said the San Francisco Bay-Delta population of longfin smelt is not a distinct species and declined to consider listing it as a threatened species.

The lawsuit alleges that decision "ignored the best available science."

Steve Matarano, a spokesman for the service's Bay-Delta Office in Sacramento, said the agency stands by that decision, but has established an open-ended public comment period "to seek additional information for a broader assessment and possible agency action" on the longfin smelt.

Matarano said the Delta smelt is "under status review."

The lawsuits do not directly challenge a package of water bills affecting the Delta and a water bond that were passed by the Legislature last week.

But Miller said, "It may not directly affect it, but we want to make sure the smelts are going to get protection under the Endangered Species Act while we deliberate the future of the Delta."