Near-record numbers of Chinook salmon returned successfully to the Shasta River last fall despite daunting, drought-related environmental conditions and a large number of migrating fish that increased the threat of disease.
The California Department of Fish and Wildlife (CDFW) counted more than 29,000 adult Chinook salmon at video camera monitoring sites and fish weirs, making the return the largest on the Shasta River since 1962.
“Irrigation districts and individual landowners stepped up and contributed water to reduce disease risks to returning salmon,” said Neil Manji, CDFW Regional Manager. “The increased flow helped cool the river water and avert disease and a potential salmon kill."
Drought conditions combined with large numbers of salmon, low stream flow and high temperatures posed a potential disease threat in the lower reaches of the Shasta River in September.
In 2002, similar environmental conditions caused more than 30,000 salmon and steelhead to die downstream in the lower Klamath River due the outbreak of two diseases, columnaris and Ichthyophthirius multifiliis or “Ich.”
With flows low in early September, large numbers of salmon began returning to the Shasta River. Irrigation season was slated to continue until Oct. 1, which could have caused further reductions in river flows.
Over a half dozen key landowners and irrigators with water rights recognized the threat and decided to reduce diversions from the Shasta River to increase flow in the river. At one juncture their contributions increased the river flow to 44 cubic feet per second more than doubling the flow. This helped move salmon through the system and reduce the vulnerability to disease outbreaks that are a threat when fish are highly concentrated. Increased flow also helped disrupt the life cycles of lethal disease pathogens.
CDFW environmental scientists closely monitored the health of returning salmon and reported no significant disease outbreaks.
The impressive return of Chinook salmon last fall year is attributed to good conditions in the ocean, where salmon live and feed for up to three years, and excellent juvenile reproduction and out-migration success to the ocean.
Environmental improvement projects throughout the watershed, including water diversion screening and riverbed enhancements, may also have contributed to this year’s return.