Spillway Escapement of Muskellunge

Many of the quality recreational fishing opportunities in Illinois occur in reservoirs. Fish emigration from reservoirs during periods of overflow is termed “dam” or “spillway escapement.” Escapement can detract from recreational fishing opportunities in the reservoir and can disturb downstream ecosystems by creating an influx of unwanted and/or non-native fish. Previous research on spillway escapement has been limited, but existing studies highlight the magnitude of the issue. In a two-year period, 10,000 largemouth bass, sunfish, and bullheads were estimated to have escaped from a 65-hectare lake in Illinois. Another study showed losses of up to 30% annually for a largemouth bass population. Muskellunge are stocked by the Illinois Department of Natural Resources (IDNR) into over 30 lakes across Illinois. Escapement of muskellunge is observed and reported from many of these reservoirs. In some instances, high-density muskellunge populations persist in tailwaters below spillways, presumably supported by annual influxes of escaping fish, and are popular angling destinations. In smaller tailwaters, however, prey and oxygen resources are often not adequate to support these large predators and high rates of mortality can occur. The Illinois Natural History Survey (INHS) recently began a research project aimed at gathering data that would further our understanding of muskellunge spillway escapement and lead to management solutions. A laboratory study incorporating a simulated reservoir and spillway was conducted to observe escapement under varying levels of ambient light, turbidity, habitat, and flow. Conventional thought going into this project was that escapement predominantly occurred at night when muskellunge are largely inactive and situate high in the water column. However the laboratory study revealed that escapement was more likely to occur during the day when muskellunge activity rates are high. The INHS was also interested in collecting real-time data on escapement of muskellunge in the field. Passive integrated transponder (or PIT) tags are often used in fisheries research and management to differentiate among individuals. These tags can be as small as a grain of rice and are inserted into the body of the fish. Typically, PIT tags are read when a fish is recaptured using a handheld wand to “excite” the tag, which then transmits a unique numeric code. An antenna INHS Fisheries Technician Dan Schermerhorn with a muskellunge. Photo by Max Wolter, INHS