Homing is a key feature of salmon life histories. Less understood and, perhaps, less appreciated is the tendency of salmonids to stray into new environments. Straying has allowed salmonids to colonize new environments (e.g., once glaciers recede) as well as newly accessible habitats (e.g., once migration barriers are removed). Because strays are difficult to identify, much of the research on straying has focused on populations colonizing new environments where only strays are expected. Another body of research has focused on the recovery of fish marked and released as juveniles and later recovered as adults. This research has allowed a comparison of rates of straying as a function of, for example, release site (e.g., in-river vs. estuary release of hatchery fish). Streams draining into California’s San Francisco Bay potentially provide fitting examples of this. In the last 20 years, there has been an apparent “invasion” of Chinook salmon (Oncorhynchus tshawytscha) into this area. Our objectives are to: (1) review information on the current distribution of Chinook in this area, (2) examine the evidence for successful reproduction in these streams (e.g., presence of juveniles), and (3) determine whether these fish represent self-sustaining runs or hatchery strays originating from California’s Central Valley.
Results/Conclusions
Limited data suggests that these fish originate, in large part, from Central Valley hatcheries because several recovered adult carcasses contained a hatchery mark (i.e., adipose fin clip, presence of a coded-wire tag). The presence of juvenile Chinook salmon in several of these streams provides evidence of successful reproduction. What remains unknown is whether these juveniles then return (or home) to these sites as adults to breed – a topic that we are currently investigating. Concomitant with invasions has been an increased hatchery practice of trucking and releasing fish directly into San Francisco Bay. Although known to enhance survival through California’s Sacramento-San Joaquin Delta, this technique also is known to increase straying rates by disrupting the imprinting process. We discuss the possible influences of different management strategies as factors leading to this apparent “invasion” of Chinook into streams flowing into San Francisco Bay. We will also explain potential negative ecosystem effects with a focus on impacts to native fish, like steelhead trout (O. mykiss), and discuss potential techniques to further understand this situation.
