low flow

Instream cover and shade mediate avian predation on trout in semi-natural streams
Brooke E. Penaluna, Jason B. Dunham, David L.G. Noakes

Piscivory by birds can be significant, particularly on fish in small streams and during seasonal low flow when available cover from predators can be limited. Yet, how varying amounts of cover may change the extent of predation mortality from avian predators on fish is not clear. We evaluated size-selective survival of coastal cutthroat trout (Oncorhynchus clarkii clarkii) in replicated semi-natural stream sections. These sections provided high (0.01 m2 of cover per m2 of stream) or low (0.002 m2 of cover per m2 of stream) levels of instream cover available to trout and were closed to emigration. Each fish was individually tagged, allowing us to track retention of individuals during the course of the 36-day experiment, which we attributed to survival from predators, because fish had no other way to leave the streams. Although other avian predators may have been active in our system and not detected, the only predator observed was the belted kingfisher Megaceryle alcyon, which is known to prey heavily on fish. In both treatments, trout >20.4 cm were not preyed upon indicating an increased ability to prey upon on smaller individuals. Increased availability of cover improved survival of trout by 12% in high relative to low cover
stream sections. Trout also survived better in stream sections with greater shade, a factor we could not control in our system. Collectively, these findings indicate that instream cover and shade from avian predators can play an important role in driving survival of fish in small streams or during periods of low flow.

DISCIPLINE: Fisheries    STUDY:    TYPE: Journal Articles    TAGS: trout survival, refuge, piscivory, body size, low flow, experiment
The Effect of Timber Harvest on Summer Low Flows, Hinkle Creek, Oregon
Surfleet, C.G., A.E. Skaugset

Changes to summer low flows from forest harvesting were measured for a gauged fourth-order stream in the Hinkle Creek Paired Watershed Study. At the gauged stream, August streamflow increased an average of 1.9 mm/year (45%) for the three summers following forest harvest of 13% of a 1,084 ha watershed. Following a second harvest of an additional 13% of the watershed the August streamflow increased by 4.5 mm (106%) the first summer and 2.0 mm (47%) the second summer. Master recession curves were fit to the gauged watersheds and the resulting recession coefficients were used to predict low flows from small watersheds nested within the gauged watersheds. The estimated low flows were used to evaluate changes in summer low flows associated with forest harvest for the small watersheds. Using recession curve analysis, the estimated range of the increase for average August streamflow for the four small watersheds in the Hinkle Creek Paired Watershed Study was 1.7 mm to 4.4 mm the first summer following forest harvest. August streamflow in the small watersheds was not distinguishable from preharvest levels within 5 years for all but one watershed, which had the highest proportion of watershed area harvested.

DISCIPLINE: Hydrology & Water Quality    STUDY: Hinkle Creek    TYPE: Journal Articles    TAGS: low flow, recession curve, Forest management, water yield, Forest Hydrology
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