Salmonids frequently adapt their feeding and movement strategies to cope with seasonally fluctuating stream environments. Oncorhynchus mykiss tend to drift-forage in higher velocity habitat than other salmonids, yet their presence in streams with seasonally low velocity and drift suggests behavioral flexibility. We combined 3-D videogrammetry with measurements of invertebrate drift and stream hydraulics to investigate the drivers of O. mykiss foraging mode and movement during the seasonal recession in a California stream. From May to July (2016), foraging movement rate increased as prey concentration and velocity declined; however, movement decreased in August as pools became low and still. In May, 80% of O. mykiss were drift-foraging, while by July, over 70% used search or benthic-foraging modes. Velocity and riffle crest depth were significant predictors of foraging mode, while drift concentration was a poor univariate predictor. However top ranked additive models included both hydraulic variables and drift concentration. A drift-foraging bioenergetic model was a poor predictor of foraging mode. We suggest that infall and benthic prey, as well as risk aversion, may influence late-summer foraging decisions.

Duration and temporal stability of resource subsidy largely affect the response of recipient communities. Factors that influence the temporal dynamics of resource subsidy from aquatic-to-terrestrial habitats by emerging aquatic insects were examined in this study. By measuring the flux of aquatic insect

In the Western United States, juvenile salmon and steelhead are especially vulnerable to streamflow depletion in the dry season. Releasing water from off-channel storage is a method of streamflow augmentation increasingly used to offset impacts of anthropogenic flow alteration. However, to date, no studies have evaluated the effects of these small-scale flow augmentations on salmonids. Here we quantify the effects of one such augmentation project on habitat connectivity, water quality, invertebrate drift, juvenile salmonid movement and survival. Our study took place in a Northern California stream and included an unusually wet summer (2019) and a more typical dry summer (2020). We found that differences in ambient streamflows between the two years mediated the physical and ecological effects of a 13.9 L/s augmentation treatment. In the dry year, flow augmentation significantly improved dissolved oxygen and habitat connectivity at sites > 1.5 km downstream from the point of augmentation and had a marginal warming effect on stream temperature. During the wet year, both dissolved oxygen and water temperature effects were negligible. In both years, augmentation had a small but positive effect on invertebrate drift. Inter-pool movement of juvenile steelhead (Oncorhynchus mykiss) and stocked Coho Salmon (O. kisutch) increased due to augmentation during the dry summer. Flow augmentation also increased the survival probability for salmonids, with a larger effect during the dry summer (24% higher survival for Coho and 20% higher for steelhead), than during the wet summer (when no effect was observed for steelhead survival and Coho Salmon survival increased by 11%). This study indicates that appropriately designed and timed flow augmentations can improve conditions for rearing salmonids in small streams, particularly during dry years. More broadly it provides empirical evidence that efforts to restore summer streamflow in small, salmon-bearing streams can yield significant ecological benefits.