Nicholas Scheidt | USGS Science

USGS2024

Assisted migration of coho salmon: Influences of passage and habitat availability on population dynamics

Assisted migration is a means of introducing a species into a previously unoccupied area. Although this idea is relatively new for many species, there are many extant examples involving fish that can be instructive. We studied a case of assisted migration where upstream access of migrating adult coho salmon Oncorhynchus kisutch over a naturally impassible barrier was established through construction of fish ladders. Although these passage structures have successfully allowed coho salmon to colonize upstream locations, managers had concerns regarding how efficiently these structures passed fish, as well as questions regarding access to specific upstream habitats, and passage barriers further upstream. To address these concerns, we developed a stage-based population model to explore: (1) influences of passage over structures, (2) rearing habitats upstream of the structures, and (3) consequences of additional barriers to passage in the system. Model simulations suggest high fish passage at the ladders was associated with the highest smolt and adult abundance of coho salmon. The importance of passage was strongly influenced by juveniles rearing in a lake, where increased lake rearing at each passage scenario increased abundance of smolts and adults. Opening habitat further upstream was estimated to increase adult and smolt abundance up to 12%. Results of model simulations also helped to identify uncertainties that could be evaluated further (e.g., juvenile rearing in the lake). In general, our findings point to the importance of considering a full range of processes that can drive expected outcomes for assisted migration.

Oregon↗

USGS2022

Passage of adult coho salmon (Oncorhynchus kisutch) over Lake Creek Falls, Oregon, 2019

Across the Pacific Northwest, there are many examples of artificial structures created to allow passage of upstream-migrating salmon over natural barriers. We studied upstream passage across three structures installed in 1989 to allow passage of salmon over Lake Creek Falls, a series of three natural waterfalls at the outlet of Triangle Lake on Lake Creek, in the central Oregon Coast Range (lat 123.57508°; long 44.15735°). To track upstream passage by adult coho salmon ( Oncorhynchus kisutch ), 87 fish were tagged using gastrically implanted radio tags. Tracking was accomplished with a series of stationary receivers installed to detect crossings at each of three structures—over Lake Creek Falls using two upstream Denil-type ladders and a bypass downstream constructed to mimic a natural side channel. Tracking spanned the upstream migration and spawn timing for adult coho salmon in the basin and extended from October 2019 to February 2020. A total of 15 coho salmon (17 percent) were tagged in October, 30 coho salmon (35 percent) were tagged in November, and 42 coho salmon (48 percent) were tagged in December. Later-than-normal precipitation and associated low discharge delayed upstream migrations. Accordingly, most fish arrived late in the season (late November and December) and in sudden flushes with the erratic rain events. Fish that were tagged earlier were more likely to cross all three ladders, with more than 93 percent of fish tagged in October compared to 46.7 and 19.0 percent of November and December fish passing, respectively. The decline in passage rate could be attributed to the overlapping influences of stream discharge and advanced stage of maturation (lower energy reserves) of fish later in the season. Near the end of the study, both fish that crossed and fish obstructed by barriers were observed in tributaries known to be used for spawning by coho salmon. Without a much longer-term study involving many more fish than the current study, more intensive tracking, and coverage of different flow years, firm conclusions are difficult to draw regarding the overall influences of the passage structures on the likelihood of upstream passage by adult coho salmon. However, substantial numbers of fish are capable of crossing during certain conditions. The population-level consequences of the barriers on spawning distribution and the production of coho salmon in the watershed are not clear. Additional empirical study or population modeling could be used to address this question in more detail.

Oregon↗