Six Delta fish species that have been monitored for decades experienced population declines last year, likely marking a return to poor habitat conditions that have plagued the estuary.
The six species, all residents of the Sacramento-San Joaquin Delta, include the Delta smelt, a threatened species, and striped bass, an introduced species that is a popular sportfish. They all experienced a severe population crash starting in about 2002.
The decline halted in 2011, when a wet winter improved aquatic habitat. All six species rebounded, though not to pre-2002 levels.
In the latest trawl-net survey of the estuary, conducted each fall since 1967, the gains of 2011 were lost. All six species returned to their prior low numbers, according to the survey, which is conducted by the California Department of Fish and Wildlife.
"In a nutshell, we just went back to the baseline. And it's not particularly good," said Randy Baxter, a supervising environmental scientist at the department.
One species, threadfin shad, set an all-time record low population in the latest survey. The species is not native to the Delta but, because it shares the same habitat, is considered another important indicator.
State and federal wildlife experts have spent a decade trying to understand why the species remain in a steep decline. They have found no simple answer, but rather a combination of factors, including water diversions, pollution and competition with invasive species.
Baxter noted 2012 was a relatively dry year, which may have contributed to the latest decline. Less rain and snowmelt runoff means less aquatic habitat, less water to dilute pollutants, and changes in the way food is produced and available to fish.