Effect of parallel-bar spacing on Walleye and Muskellunge escapement through a simulated reservoir spillway barrier

ABSTRACT Objective Parallel-bar barriers are used throughout the Midwest to reduce escapement of sport fish. Our objective was to assess how parallel-bar spacing affects the number and sizes of adult Walleye Sander vitreus and fingerling Walleye and Muskellunge Esox masquinongy that escape through a simulated reservoir spillway barrier. Methods Adult Walleye were captured from Rathbun Lake, Iowa, and fingerling Walleye and Muskellunge were cultured at Rathbun Fish Hatchery, Iowa. We used parallel-bar barriers (either no barrier [control] or 25-, 51-, and 76-mm [fingerlings only] and 102- and 152-mm [adults only] bar spacing) to divide a U-shaped raceway into upstream and downstream sections. A set of 2-h behavior trials were conducted in a covered raceway with water flows maintained at 0.1 m/s. Trials were conducted during spring using adult Walleye and fall using fingerling Walleye and Muskellunge (n = 10 fish per trial, n = 10 trials per treatment). We then used logistic regression to compare escapement probability and analysis of variance to compare fish body depth among bar-spacing treatments for each of the three groups (adult Walleye, fingerling Walleye, and fingerling Muskellunge). Results Adult Walleye escapement was highest in the control (no barrier) and tended to decrease with reductions in barrier bar spacing, while no adult Walleye escaped through the smallest 25-mm barrier. Escapement probability was similar across adult Walleye lengths and between sexes. Fingerling Walleye escapement was reduced by the 25-, 51-, and 76-mm barriers compared with the control, while fingerling Muskellunge escapement was reduced by the 25-mm barrier but was similar among all other treatments. Fingerling Walleye and Muskellunge escapement probability declined with fish length, as smaller fish were more likely to escape than larger fish. Body depth of fingerling Walleye and Muskellunge escapees was similar among treatments. Conclusion Our results demonstrate that parallel-bar barriers can reduce the number of fish that escape from spillway reservoirs. Barriers with smaller bar spacing are more physically restrictive and will likely lead to the greatest reduction in escapement; however, even the barrier with the smallest bar spacing we tested was unable to completely prevent escapement of fingerling Walleye or Muskellunge. Our results suggest that parallel-bar barriers with ≤51 mm of bar spacing may be an effective and inexpensive solution to spillway reservoir escapement, but we encourage further field evaluations to verify the efficacy of these barriers under a range of environmental conditions.