It is asserted that reduction or elimination of hatchery stocking will increase natural‐origin salmon Oncorhynchus spp. and steelhead O. mykiss production. We conducted an analysis of steelhead population census data (1958–2017) to determine whether elimination of summer steelhead stocking in the upper Clackamas River in 1998 increased the productivity of natural‐origin winter steelhead. A Bayesian state–space stock–recruitment model was fitted to the adult steelhead data set, and productivity was estimated as a function of hatchery‐origin spawner abundance as well as other environmental factors. When used as a predictive variable in our model, the abundance of hatchery summer steelhead spawners (1972–2001) did not have a negative effect on winter steelhead recruitment. However, spill at North Fork Dam (the gateway to the upper Clackamas River basin) and the Pacific Decadal Oscillation (an index of ocean conditions) were both negatively associated with winter steelhead recruitment. Moreover, winter steelhead abundance in the upper Clackamas River basin failed to rebound to abundances observed in years prior to the hatchery program, and fluctuations in winter steelhead abundance were correlated with those of other regional winter steelhead stocks. Our assessment underscores the need for studies that (1) directly quantify the effects of hatchery fish on the production of natural‐origin salmon and steelhead, (2) empirically test published theories about mechanisms of hatchery fish impacts on natural‐origin populations, and (3) document population responses to major changes in hatchery programs.

Providing passage at man‐made barriers is important to preserving migratory fish species. Fishways used to bypass barriers have historically been designed to provide passage for economically important species, with little regard given to nontarget species. Consequently, native fish species whose swimming behaviors are not well suited to these fishways have suffered from restricted access to habitat. For example, poor passage at dams has contributed to declines in populations of Pacific Lamprey Entosphenus tridentatus in the Columbia River basin. Pool‐and‐weir fishways typical of Columbia basin dams are well suited to salmonids but not to anguilliforms like the Pacific Lamprey. We evaluated the passage efficiency of Pacific Lampreys through a new pool‐and‐weir fishway at River Mill Dam on the Clackamas River, Oregon, that was designed to facilitate Pacific Lamprey passage. Rounded corners at the fishway entrances, flush‐mounted weir gates, chamfered corners on orifices and weir walls, and orifices flush with the floor were all included in the fishway design specifically for Pacific Lampreys. In 2013 and 2015, Pacific Lampreys were radio‐tagged and PIT‐tagged to assess passage success. Dam passage efficiency estimates ranged from 84% to 98%, roughly 10–50% higher than Pacific Lamprey passage efficiency estimates at other dams in the Pacific Northwest. The median passage time through the fishway was 0.87 d in 2013 and 0.71 d in 2015. The high passage efficiency of Pacific Lampreys at River Mill Dam is likely due to the design elements incorporated specifically for Pacific Lampreys. These features could inform future fishway designs and modifications at locations where Pacific Lamprey passage is a consideration.

Quantifiable estimates of predator–prey interactions and relationships in aquatic habitats are difficult to obtain and rare, especially when individuals cannot be readily observed. To overcome this observational impediment, imaging sonar was used to assess the cooccurrence of predator‐size fish and juvenile salmonids, Oncorhynchus spp., at the entrance to a floating surface collector (FSC) in the forebay of North Fork Dam on the Clackamas River, Oregon (USA). Imaging sonar can be used to transform active sound waves into visual data, making it possible to obtain continuous underwater observations on the presence and interspecific interactions between predator‐size fish and prey (juvenile salmonids). Hourly counts of smolt‐size fish tracks, diel phase, water clarity and river discharge were used as covariates within a zero‐inflated Poisson model to determine how these factors may influence the number of predators in front of the FSC. Both the number of smolt‐size fish tracks and diel phase had the strongest effects on the number of predator‐size fish tracks, with more predator‐size fish tracks observed during the daytime, and as the number of smolt‐size fish tracks increased. Additionally, the presence of predator‐size fish may affect the abundance and direction of travel of juvenile salmonids, as fewer smolt‐size fish were observed when predators were present, and a greater proportion of smolt‐size fish were observed travelling away from the FSC when predator‐size fish were present. This study provides estimates of predator and prey fish abundance in the vicinity of surface collection systems at moderate‐sized hydropower projects and could help resource managers better understand mechanisms that can influence the survival and passage behaviour of juvenile salmonids using surface collection structures at dams.

Global climate change is shifting the timing of life-cycle events, sometimes resulting in phenological mismatches between predators and prey. While phenological shifts and subsequent mismatches may be consistent across populations, they could instead vary unpredictably across populations within the same species. For anadromous Pacific salmon ( Oncorhynchus spp.), juveniles from thousands of locally-adapted populations migrate from diverse freshwater habitats to the Pacific Ocean every year. Both the timing of freshwater migration and ocean arrival , relative to nearshore prey (phenological match/mismatch), can control marine survival and population dynamics. Here, we examined phenological change of 66 populations across six anadromous Pacific salmon species throughout their range in western North America with the longest time series spanning 1951 -2019. We show that different salmon species have different rates of phenological change, but that there was substantial within-species variation that was not correlated with changing environmental conditions or geographic patterns. Moreover, outmigration phenologies have not track ed shifts in the timing of marine primary productivity, potentially increasing the frequency of future phenological mismatches. Understanding population responses to mismatches with prey are an important part of characterizing overall population-specific climate vulnerability.