Evaluation of Current and Historical 10-inute-count screens at the Tracy Fish Collection Facility, Tracy, California

The U.S. Department of the Interior, Bureau of Reclamation's Tracy Fish Collection Facilities (TFCF) 10-minute-count screen is a critical tool used to acquire sub-samples and provide an estimate of fish entering the facility. The introduction of a new screen in 1999, prior to fish screen retention comparisons, could possibly have altered TFCF salvage estimates. Three experiments were conducted during 2003 and 2004 to evaluate the retention efficiencies of the current and historical screens for juvenile delta smelt (Hypomesus transpacificus): (1) Wild Juvenile Delta Smelt Retention Comparison, (2) Evaluation of Bead Loss, and (3) Cultured Juvenile Delta Smelt Retention Comparison. In experiment No. 1 there was no significant difference between the mean number of juvenile delta smelt (20 to 30.5 mm in fork length [FL]) retained using the current (40.1 percent +/- 7.4; mean +/- standard error [SE], n = 6) and historical (34.5 percent +/- 7.9; n = 8) screens (p = 0.70). However, delta smelt with a greater maximum body depth than the maximum hole width were recovered outside of both screens. Experiment No. 2 was conducted to determine where, aside from screen holes, these fish may have been lost. The lowest success rate for retention of beads was achieved when no seal was used (8.6 percent for 4 mm, 41 percent for 10 mm). Retention of particles was highest (100 percent for beads > 5 mm) when seals were used on the top and bottom of the screen, demonstrating that loss was occurring at both locations. In experiment No. 3 eight conditions were tested, using the two screen types. Retention was evaluated with and without seals with two class sizes of delta smelt (small, 20 to 25 mm in FL and large, 25 to 30 mm in FL). Retention of the small class size was significantly lower using the current screen with seals, compared to all three other treatment types (current + seal, 3 percent; current, 18 percent; historical + seal, 13 percent; historical, 15 percent, P = 0.002). However, no significant differences were detected among treatments of the large class size (current + seal, 23 percent; current, 33 percent; historical + seal, 59 percent; historical, 44 percent, P = 0.06). Experiments Nos. 1 and 3 support the hypothesis that there is no difference in retention between current and historical screens when seals are not used. SInce seals were not used historically, we conclude that the current and historical salvage data sets are comparable.
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