Placing sentinel fish in the Sacramento River at Red Bluff Diversion Dam. Photo courtesy of Brendan Lehman.
As organizations and agencies scramble to preserve the Central Valley’s dwindling Chinook salmon runs, a group of scientists believes they may be overlooking a key factor in the decades-long decline of the fish: disease.
In a paper
published in September’s issue of San Francisco Estuary and Watershed Science, a research team proposes that diseases—caused by viruses, bacteria and other microbes—could be suppressing juvenile salmon survival in a river system that once hosted millions of adult spawners each year. According to tracking studies, nearly all juvenile Chinook born from natural spawning die before they reach the Golden Gate Bridge; habitat enhancement efforts have failed to mitigate this mortality rate. Short-term studies of Central Valley salmon have indicated high rates of infectious diseases, which lead author Brendan Lehman of UC Santa Cruz says demonstrates the need for ongoing systemwide monitoring. This could involve sampling fish directly for infections or conducting DNA testing of water samples. A monitoring program might also involve using so-called sentinel fish. Born and raised in the controlled environment of a hatchery, a sentinel fish is temporarily released into the wild, then retrieved and screened for diseases or infections. Lehman notes that pathogens occur naturally in river systems. “But the way we’ve changed their environment can exacerbate the effects on the fish,” Lehman says.