Rotary screw traps are one of several traps which may be employed to sample a portion of juvenile salmon and steelhead as they migrate downstream to the ocean. A rotary screw trap consists of a cone five to eight feet across with large rotating blades which funnel fish into a live box. A cone is mounted to a floating platform with the large end facing upstream. The screw trap is anchored at a fixed point in the stream. The live box is emptied at least once daily, more often if fish numbers are high. If there are high debris loads or the discharge is too high, the trap may be temporarily stopped to prevent damage to the trap and harm to the fish.
Under the Okanogan Basin Monitoring and Evaluation Program, an eight foot screw trap is deployed every year in the middle of the Okanogan River below the Highway 20 Bridge in Okanogan, Washington. The time frame for trapping can span from late March through mid-July, or whenever numbers of migrating smolts decreases to near-zero. When discharge levels exceed 4,800 cubic feet per second, a second 5 foot cone is deployed near the shoreline.
Each time the live box is emptied, all fish are observed for species, life history stage, and presence of tags or marks. In addition, 10 fish per species are anesthetized and measured for fork length. All summer steelhead are checked for PIT tags and fin clips. Fish are allowed to recover and are promptly released back into the river below the trap.
Over the season, groups of salmonids are dyed using Bismark Brown “Y” dye and released upstream of the screw trap. The proportion that is recaptured at the trap is used to calculate the trap efficiency at a given discharge. Once the trap efficiency for any given discharge is known, the number of fish trapped may be used to estimate the total abundance of the population.
As of 2012, the rotary screw trap program was moved to the Chief Joseph Hatchery Monitoring and Evaluation Program. The sampling design was modified with plans to operate the trap from 8:00 pm to 6:00 am, seven days a week. Similar data will still be collected and should still be compatible with previous years.