Rotary screw traps (RST) are used to capture and estimate the abundance of outmigrating juvenile anadromous salmonids. Monitoring of smolt abundance also enables the partitioning of mortality between freshwater life stages (egg-to-smolt) and marine life stages (smolt-to-adult). Operating a downstream migrant trap allows investigators to collect biological information on wild salmonids produced in the watershed over time, including information on the presence/absence of fishes, determination of age and size at migration, condition, migration timing, species, and genetic characteristics.

The rotary screw trap at BRCAMP is a temporary floating structure (4.6 m length x 2.0 m width x 0.9 m height) consisting of two floating pontoons, a rotating corkscrew cone, and a live box (0.8 m width x 1.0 m x 0.5 m height). The trap is attached to a 5 cm braided steel cable drag line, hooked to temporary anchors (i.e., trees) on each side of the river, connected to a pulley hook on the main cable spanning approximately 40 m across the river. The trap is set to operate in or near the stream thalweg with the conical cylinder facing upstream. The conical cylinder rotates with stream flow and captures and funnels downstream migrants into the live box.

On a daily basis, the screw trap live box is emptied at approximately 0800 hours. Non-target species in excess of five are enumerated, recorded, and released directly downstream of the trap. Target fish (Chinook) are transferred to the tagging trailer, anesthetized in a MS-222 solution, processed (length, mass, tissue sample, and PIT-tags), and then allowed to recover prior to release. Juveniles = 69 mm fork length are stained using Bismark Brown (BB) for a minimum of 20 minutes and maximum of 40 minutes to conduct trap efficiency trials when insufficient numbers of juveniles can be tagged with Passive Integrated Transponder (PIT) tags. A sub-sample of juvenile migrants = 70 mm are injected with a PIT tag and released upstream. The first three Chinook collected on any given day are tissue sampled until 300 samples are acquired. Tissue samples are stored on sheets of Whatman 3mm chromatography paper. As requested by Idaho Fish and Game, measured O. mykiss that are = 80 mm are also PIT tagged and scale sampled. All marked fish are measured to the nearest 1.0 mm and weighed to the nearest 0.01 g. Stained and PIT-tagged juveniles are released 0.9 rkm upstream of the trap (includes two riffles and a pool) at Fir Creek Bridge for mark-recapture analyses to estimate trap efficiency. Up to five fish of each non-target species are also anesthetized and measured for permitting purposes. Recaptures and remaining fish by species are enumerated, recorded, and released downstream of the trap. Mortalities are recorded as either the result of trapping or handling. If the mortality was a PIT-tagged individual, the tag is removed prior to disposing of the mortality downstream of the trap and removed from the P4 database. Temperature and staff gauge measurements are recorded daily. The correlation between number of juveniles captured and river discharge or temperature is determined using Spearman Rank Correlation.