Considerations for the management of hatchery salmon: Stream characteristics associated with attractiveness to strays

Dispersal ("straying") of hatchery-origin Pacific (Oncorhynchus spp.) and Atlantic salmon (Salmo salar) complicates fishery management and can erode wild salmon productivity and resilience, yet relatively little is known about what makes certain sites apparently attractive to strays. In this study, we sought to identify stream characteristics associated with higher numbers of stray hatchery-origin chum salmon (Oncorhynchus keta; téel' in Łingít) in Southeast Alaska. We compiled a dataset of approximately 45,000 chum salmon sampled from 57 streams in the region during the 2008-2019 spawning seasons. Approximately 8300 fish, or 18% of the total number of fish sampled on the spawning grounds, were confirmed to be first-generation hatchery strays based on the presence of otolith thermal marks. A generalized linear mixed-effects model confirmed the role of distance in patterns of straying, whereby a 1-SD increase in hatchery releases (an additional 27.6 million juveniles) within 40 km of a stream resulted in a 44% increase in the predicted number of hatchery-origin strays. Beyond distance, a nonlinear effect of stream discharge was detected, with the numbers of strays being particularly low in streams with intermediate coefficients of variation of water flow (0.48-0.55). High and low levels of flow variation were primarily associated with snowmelt-fed streams, indicating a possible role of temperature regulation during warm summer spawning seasons as an attractive stream characteristic, but such a mechanism has yet to be tested. In addition to providing new insights into habitat-mediated dispersal and colonization dynamics by straying salmon, our results allow for the incorporation of stream attractiveness in spatial planning of release locations for hatchery fish beyond the sole effect of distance.