Which Way Home? Route Selection and Movement of Returning California Central Valley Adult Chinook Salmon near Their Upper Thermal Limit

California’s Central Valley watershed represents the southern limit of Chinook salmon freshwater distribution. River and estuary temperatures exceed lethal levels in the summer and early fall, constraining the return of maturing adults.  Some of the highest temperatures along the migratory corridor are found in the Sacramento-San Joaquin Delta, a complex network of sloughs and wetlands. In addition to high temperatures, water pumping and the resulting irregular hydrodynamics of the California Delta have long been suggested to disorient and delay returning Chinook salmon.

In October 2010, an unusually warm fall, 81 adult Chinook salmon were caught and acoustically tagged in the San Francisco Bay estuary, just downstream of the Delta. These tagged individuals experienced temperatures ranging from 13 to 22 degrees Celsius on their subsequent migration through the Delta, the upper end at or above published rates at which migration is inhibited and potentially lethal. Based on time of year, fish were likely fall or late-fall run, with final confirmation awaiting results of DNA analysis and scale aging. Using an extensive network of acoustic monitors spread throughout the watershed, the small-scale movements and ultimate success of each individual’s migration will be determined, as well as the route taken in the delta. Preliminary data returns suggest a sizeable portion (~10%) of tags migrated downstream from release site, likely evidence of marine mammal predation or evidence of individuals seeking thermal refugia.

This is one of the first studies to follow the movement behavior and navigational success of adult Chinook salmon in excessively warm waters. Of special interest are the differences in navigational success between individuals attempting to return to different headwaters, requiring pathways of varying temperatures and navigational complexity through the delta. The data should also provide insight into temporal changes in movement behavior through the migration season, especially once temperatures start to decline. This investigation provides insight into how Chinook salmon populations are currently coping with warm and increasing temperatures.