A wide variety of resource management activities can affect surface discharge in small streams. Often, the effects of variation in streamflow on fish survival and growth can be difficult to estimate because of possible confounding with the effects of other variables, such as water temperature and fish density. We measured the effect of streamflow on survival and growth of rainbow trout Oncorhynchus mykiss in a small stream in northwestern California by manipulating the flow entering four of eight enclosed stream sections (9.0-15.3 m long) containing one pool and 2.5-4.0 m of upstream riffle habitat. In the four manipulated experimental units, we reduced inflow by 75-80% over a 6-week period in summer 2003. Flow diversion substantially decreased water velocity in riffle-pool transition areas but did not strongly affect habitat volume or water temperature. Fish in control units grew about 8.5 times as much as those in units with reduced streamflow; however, discharge manipulation did not affect survival, which exceeded 90% in both treatments. The input of invertebrate drift to pools within control units greatly exceeded the input into units with reduced streamflow; the concentrations of drifting invertebrates differed to a lesser extent between control and manipulated units. Managers concerned about salmonid populations in small streams should consider dryseason streamflow to be a key variable affecting fish growth.

To improve understanding of the significance of large woody debris to stream fishes, we examined the influence of woody debris on fall and winter movement by adult coastal cutthroat trout (Oncorhynchus clarki) using radiotelemetry. Fish captured in stream pools containing large woody debris moved less than fish captured in pools lacking large woody debris or other cover. Fish from pools lacking cover commonly moved to habitats with large boulders or brush, particularly during the day. Movements by fish over 1-day periods were strongly influenced by large woody debris or other elements providing cover. Fish initially found in habitats lacking large woody debris, large boulders, or brush cover moved the most extensively, while fish initially found in pools with large woody debris moved the least. Fish did not move extensively in response to a bankfull flood, although some moved to habitat downstream of large woody debris in tributaries or secondary channels. Habitat downstream of woody debris in the main channel was not used during the flood, apparently because of extreme turbulence. Overall, these observations provide additional evidence for the value of habitat complexity to some stream fishes and support previous observations of minimal effects of flooding on adult fish.Résumé : Afin d'améliorer notre connaissance de l'importance des gros débris ligneux pour les poissons des cours d'eau, nous avons examiné, par radiotélémétrie, l'influence des débris ligneux sur les déplacements d'automne et d'hiver de truites fardées côtières adultes. Les poissons capturés dans les fosses de cours d'eau où se trouvaient de gros débris ligneux se déplaçaient moins que ceux capturés dans les fosses exemptes de tels débris ou d'un autre couvert. Les poissons des fosses sans couvert se déplaçaient généralement vers des habitats où se trouvaient de grosses pierres ou des broussailles, cela surtout pendant le jour. Les mouvements des poissons au cours d'une même journée étaient fortement influencés par la présence de gros débris ligneux ou d'autres éléments offrant un couvert. Les poissons des habitats exempts de gros débris ligneux, de grosses pierres ou de broussailles sont ceux qui se déplaçaient le plus tandis que ceux des fosses à gros débris ligneux se déplaçaient le moins. Les poissons ne se sont pas déplacés de façon importante suite à une crue affectant toute la rive, bien que certains se soient déplacés vers des habitats se trouvant en aval de gros débris ligneux, dans des tributaires ou des chenaux secondaires. Les habitats en aval des débris ligneux du chenal principal n'étaient pas utilisés pendant les crues, sans doute à cause de la turbulence extrême. De façon générale, ces observations confirment l'importance de la complexité de l'habitat pour certains poissons de cours d'eau et complètent des observations antérieures du peu d'effets des crues sur les poissons adultes.[Traduit par la Rédaction] Harvey et al. 2166

We studied the concurrent effects of riparian canopy opening and salmon carcass addition on salmonid biomass, density and growth rates in small streams over 2 years. In each of six streams in the Smith and Klamath River basins in northern California, red alder (Alnus rubra) and other hardwoods were removed along both banks of a 100-m reach to increase incident radiation. A second 100-m reach, with an intact canopy, was established in each stream as a light control. Salmon carcasses were added each year to both cut and uncut riparian sections in three of the six streams. Differences in total density and biomass of cutthroat trout (Oncorhynchus clarki) and rainbow trout (Oncorhynchus mykiss) from pretreatment levels responded positively to canopy removal but were not detectably affected by carcass addition. Differences in specific growth rates of the fish between open and closed canopy reaches were greater in sites without carcasses than in sites with carcasses. In light-limited settings where temperature gains associated with canopy opening are not problematic for aquatic resources, gains in salmonid production might be achieved by selective trimming of riparian hardwoods.

While two cyprinid fishes introduced from nearby drainages have become widespread and abundant in the Eel River of northwestern California, a third nonindigenous cyprinid has remained largely confined to ≤25 km of one major tributary (the Van Duzen River) for at least 15 years. The downstream limit of this species, speckled dace, does not appear to correspond with any thresholds or steep gradients in abiotic conditions, but it lies near the upstream limits of three other fishes: coastrange sculpin, prickly sculpin, and nonindigenous Sacramento pikeminnow. We conducted a laboratory stream experiment to explore the potential for emergent multiple predator effects to influence biotic resistance in this situation. Sculpins in combination with Sacramento pikeminnow caused greater mortality of speckled dace than predicted based on their separate effects. In contrast to speckled dace, 99% of sculpin survived trials with Sacramento pikeminnow, in part because sculpin usually occupied benthic cover units while Sacramento pikeminnow occupied the water column. A 10‐fold difference in benthic cover availability did not detectably influence biotic interactions in the experiment. The distribution of speckled dace in the Eel River drainage may be limited by two predator taxa with very different patterns of habitat use and a shortage of alternative habitats.