California's most abundant salmon run suddenly dropped this season to an historic low. Fishing groups and many environmental organizations were quick to point the finger: The pumps in the Sacramento-San Joaquin Delta that move water to grow half of the nation's fruits and vegetables and provide a key water supply for two out of every three California residents. "It's proof that the operation of these water projects is harming salmon," one environmentalist told the Associated Press.

But what if this treasured salmon run is in trouble for other reasons? What if government scientists were increasingly suspecting changing conditions in the ocean as the primary factor? And what if environmental groups were publicly reluctant to blame another human activity - recreational and commercial salmon fishing - because the groups were allies in court skirmishes against the water projects?

When it comes to figuring out why any given fish species is thriving or struggling at any given moment, usually the experts point to a variety of factors. First, a little background on salmon. There are three different "runs," or populations, of salmon that migrate from the Pacific Ocean, through the Sacramento-San Joaquin River Delta and up the Sacramento River to spawn in various tributaries. There is the winter run (the adults swim upstream in winter months), the spring and the fall. Out in the ocean, where fishing is allowed, the various species all mingle and can't be regulated separately. In the delta, the winter and spring runs have been protected for years under the federal Endangered Species Act so that pumping operations have a limited impact on the species.

Protections have not been extended to the more abundant fall run. Its population has gone up and down over the last 10 years like the stock market, varying between 300,000 and 800,000 fish. But this past fall, the official run was 90,414 returning salmon, the lowest in a quarter century. When the information was released, the blame-the-delta-pumps theory was once revived.

Looking at the human impacts on salmon, here are three to consider. Yes, there are the delta pumps of the State Water Project and Central Valley Project. But, their operations are carefully regulated when salmon are migrating to minimize possible impacts. These same pumping restrictions were in place when the salmon run skyrocketed to record highs too.

A second human activity is the state's nurturing of a destructive, artificial fishery in the delta, a population of non-native striped bass. According to a 1999 study by the California Department of Fish and Game, the bass consume a significant number of the fall salmon run as the fingerlings (called smolt) try to swim through the delta to the ocean.

Third, there is fishing of salmon in the delta and upstream for pure recreation. What percentage die this way before they spawn? Perhaps 25 percent, according to a 2006 report by the Pacific Fishery Management Council.

And then there's a fourth "culprit"- Mother Nature. Changing ocean conditions have dramatically lowered food sources for salmon in recent years. "The ocean environment has a strong influence on how many survive the initial period at sea and how many come back to spawn three to four years later in the Sacramento River," a biologist with the Farallon Institutes for Advanced Ecosystem Research told The Chronicle. Climate change will exacerbate these problems in the ocean. Indeed, this is not a problem unique to the Sacramento salmon runs, populations have crashed in rivers all the way up to Alaska.

Despite all these factors at play, single-focused environmental and fishing groups are blaming the water systems in the delta for the salmon's problems. We need to have a more candid and complete conversation about how to minimize all human impacts facing the salmon and other important delta species. Instead of wasteful litigation, an approach based more on science and cold, hard facts is the only way to create a better water system that provides California with safe, reliable drinking water supplies and safe passage for salmon through the delta to a changing ocean.