Scientists investigating the cause of a recent decline in several Delta fish species said today they are analyzing an array of system “stressors” but have yet to identify any one factor responsible for the problem.

Members of an interagency team released initial results gleaned from six months of intensive research and analysis of the sudden decline in species such as Delta smelt, striped bass and threadfin shad. While some patterns have emerged, researchers said more work is needed to explain what is happening in a very complex situation.

For now, Bruce Herbold of the U.S. Environmental Protection Agency said the team is looking at two possible “narratives” that may be contributing to the decline. The first describes changes in Suisun Bay that have diminished its suitability as a habitat for young smelt and other species. The changes include a decline in copepods that serve as a key food source and increased abundance of an invasive Asian clam and other introduced species.

The second narrative involves an apparent increase in “salvage” of Delta smelt and other species at the water export pumps during winter months. The high number of fish salvaged at the pumps is unusual given the low number of fish found elsewhere in the estuary at that time, and coincides with an increase in water exports during the same months.

Herbold and others cautioned that more data is needed to understand what relationship the salvage numbers may have with fish abundance in the Delta. The researchers are not recommending specific changes in water project operations or policy at this point, they said.

The report will be available at http://science.calwater.ca.gov/workshop/workshop_pod.shtml.