Substrate composition at each study site was determined by Wolman Pebble Count surveys in three riffles. Riffles that fell under or nearest to transects described above were sampled. Population A refers to the downstream most riffle, population B the middle riffle, and population C the upstream most riffle in each study section by stream.
Invertebrate Prey Availability To examine seasonal fluctuations in prey availability, estimates of invertebrate biomass and composition from benthic, drift, and allochthonous sources were obtained four times during the study: summer (July 06 - August 08, 2001), fall (October 01- October 23, 2001), winter (January 10 - February 20, 2002), and spring (April 02 - April 19, 2002). Benthic invertebrates were collected once each season with a 500-µm mesh Surber sampler (0.09 m^2 area) at 6 random locations in riffle habitat of each study section. Invertebrate drift was estimated by concurrently placing one drift net (500-µm mesh; 0.4 m x 0.4 m opening at the mouth) in the thalwag of riffle habitat at each end of a study section for 20 minutes at dawn. Drift nets were positioned to intercept total water column and capture invertebrates floating on the water surface.
During each study period, arthropod inputs were estimated from samples collected in pan traps (0.056 m^2) for 7 days. Twelve pan traps in each study section were suspended 1 m above the water surface on metal rebar stands. Pan traps were filled with 3 cm of water and two to three drops of surfactant to retain captured invertebrates. The wetted channel area was divided longitudinally into 3 subsections (left, center, and right); four pan traps were placed randomly in each subsection. Pan trap contents were sieved (225-µm mesh) at the completion of each 7-day sample period and preserved in 95% ethanol alcohol solution until analyzed. Invertebrates collected from the benthos, drift, and pan traps were sorted under a dissecting microscope, taxonomically identified (primarily to the family level), enumerated, and measured to the nearest 0.5 mm using an eyepiece micrometer. Invertebrates were categorized as terrestrially derived or aquatically derived based on environment type of the larval stage. Macroinvertebrate biomass was estimated with published taxon-specific length-mass regression equations.
Benthic invertebrate biomass estimates from Surber samples (dry mass mg·m^-2) were combined to obtain the mean for all samples in a study section by sample period. Allochthonous invertebrate biomass estimates from pan trap samples (dry mass mg·m-2·day^-1) were also combined to obtain the mean for all samples in a study section by sample period. Drift biomass (dry mass mg·m^-3) was calculated by dividing the weighed dry mass of invertebrates retained per net by the estimated water volume moving through each net during the sample period. Drift biomass rate (dry mass mg·m-3·hour^-1) for each study section in a sample period was estimated by multiplying the total value of the upstream and downstream net biomass estimates (dry mass mg·m^-3) by water volume filtered (m^-3·hour^-1) and combining rates for each to obtain the mean.
Coastal Cutthroat Trout Diet Coastal cutthroat trout were sampled during each study period to collect stomach contents. During each sample period, we allowed 24 h for habitat to recover from disturbance associated with instream invertebrate sampling and coastal cutthroat trout to return to natural foraging behavior. A variable wave-form backpack electrofishing unit (Model 12, Smith-Root Inc., Vancouver, USA) was used to capture coastal cutthroat trout. Electrofishing occurred between 1000 and 1600 hours. Collection proceeded upstream until 20 coastal cutthroat trout were captured in each study section. Fish were placed in 20-L buckets of water and anesthetized with a solution of water and clove oil. Stomach contents were removed by flushing procedure with a narrow pipeted water bottle and strained into paper coffee filters, and placed into small plastic bags filled with 95% ethanol alcohol. Only stomachs of coastal cutthroat trout ? 80 mm (total length) were sampled, and time and location of capture and total length (to nearest 1 mm) and weight (to nearest 0.1 gram) were recorded for each fish. Subsequently, all fish were returned to their original capture location when fully recovered.
Invertebrates from stomach contents were preserved in 95% ethanol solution. In the lab, organisms were taxonomically identified to family when possible, origin was categorized, and each individual was measured (to the nearest mm) for estimating biomass from length-weight regressions. Lengths of partially digested prey were estimated from intact individuals of the same taxon that appeared to be similar in size. The biomass of invertebrates ingested by all individual fish was combined to obtain the mean for each study section and season.