Effects of Harvest, Natural Mortality, and Escapement on Reservoir Walleye Populations

Abstract

Walleye Sander vitreus are commonly introduced to reservoirs and provide valued fisheries. Walleye are lost though harvest and natural mortality in all systems, but escapement may also reduce reservoir populations. However, the effects of harvest, natural mortality, and escapement loss on reservoir Walleye populations are not known. Our objective was to quantify Walleye harvest, natural mortality, and escapement in two Iowa reservoirs (Big Creek Lake and Brushy Creek Lake) to understand how these factors limit populations. We also assessed Walleye behaviors and compared them among escaped, harvested, and in‐lake Walleye to determine if behavior was associated with fate. Walleye were radio‐tagged beginning in October 2016 and tracked through May 2019. Telemetry was used to estimate annual core and home ranges, seasonal depth use, and movement rates. Detection histories were analyzed using a multistate capture–recapture model with environmental covariates to estimate weekly harvest, natural mortality, and escapement. Harvest was greater in Big Creek than Brushy Creek and was higher in April through July compared to other months. Natural mortality was positively related to water temperature but did not differ between systems. No Walleye escaped from Big Creek (fish barrier on spillway), but escapement at Brushy Creek (no fish barrier) was positively related to mean water levels during April. During 2017–2018, annual Walleye escapement ranged from 22% to 47% at Brushy Creek (0% at Big Creek), harvest ranged from 13% to 27%, and natural mortality ranged from 36% to 38%. Depth of habitat occupied by Walleye was shallower during spring and summer when escapement occurred, intermediate in fall, and deepest during winter. Escaped Walleye at Brushy Creek used greater depths and moved less than in‐lake Walleye; however, other behaviors did not differ among Walleye groups. Our results suggest that escapement can have a larger effect than harvest on reservoir Walleye populations in systems without physical barriers, and it should be taken into consideration when managing reservoir Walleye fisheries.

Modeled Walleye Regulation Outcomes in a Midwestern Reservoir with Varying Forage and Walleye Density

“…However, when Gizzard Shad were established, the fishery contracted to primarily encompass the spring and early summer. Harvest rates were seven times greater from April to July than in other months (Weber and Weber 2021), and anglers reported reduced Walleye catch rates during the ice season. Low angler catch during parts of the year can be attributed to the seasonal peaking of Gizzard Shad availability, which in turn affects Walleye catchability (VanDeValk et al.…”

“…At this time, an increased minimum length limit would likely enhance size structure most effectively, but a protected slot limit could become more appropriate if stockpiling recurs or if anglers prefer harvesting smaller fish that would be sublegal under an increased minimum length limit. In addition, a protected slot limit could be more appropriate if natural mortality is higher than we estimated, as implied by a recent tagging study (Weber and Weber 2021). However, any protected slot limit would need close monitoring to ensure that yield does not drop due to growth overfishing.…”

“…We believe that the unexpected recruitment of fry‐stocked Walleye to the fishery and the fish barrier were the primary drivers of increased Walleye densities in Big Creek Lake during periods C and D. Prior to barrier installation, adult Walleye were observed crowding the swift current immediately upstream of the spillway to take advantage of foraging opportunities (Dodd and Otting 2012), as suggested by Paller et al (2006). Monitoring of Walleye emigration over 4 years revealed minimal to no emigration from Big Creek Lake when the barrier was present; in contrast, 21.9–46.5% of tagged Walleye emigrated from a nearby reservoir with no barrier during the same time period (Weber and Weber 2021). We are unaware of any other substantial changes in Big Creek Lake’s fish community or environmental conditions that could explain the changes in Walleye density.…”

“…However, angler complaints continued regarding the lack of legal‐sized Walleye, and stockpiling below the minimum length limit was suspected. After Gizzard Shad were detected the second time in 2015, Walleye growth and condition seemed to improve, while density remained high (Weber and Weber 2021; Table 1).…”

“…For instance, Largemouth Bass population densities have not changed abruptly based on fisheries management sampling (Dodd, unpublished data), which could alter survival of stocked Walleye (Santucci and Wahl 1993;Fayram et al 2005). Emigration can contribute a substantial portion of fish loss in reservoirs (Louder 1958;Lewis et al 1968;Axon and Whitehurst 1985;Paller et al 2006;Kuklinski 2014;Weber and Flammang 2019) and increases exponentially at higher discharges (Weber and Flammang 2019;Weber and Weber 2021) that are typically occur during spring (Paller et al 2006;Weber and Weber 2021). In Rathbun Lake, Iowa, up to 26% of the Walleye population could emigrate in a year with prolonged spring discharges (Weber et al 2013;Weber and Flammang 2019).…”