Relative Effects of Mortality versus Emigration on a Reservoir Walleye Population

Abstract

Mechanisms influencing population fluctuations are an important component of fisheries ecology. Fishes can be lost from populations due to mortality or emigration, potentially resulting in population declines. Although emigration in large, open-river systems is well recognized, emigration may also occur in reservoirs that are conventionally considered closed systems, potentially resulting in important population-level effects. However, little is known regarding annual variation in reservoir emigration rates, the influence of reservoir discharge on emigration, or the influence of emigration compared to mortality in regulating populations. We used tagging data collected from 1990 to 2000 to evaluate the effects of various discharge patterns on reservoir Walleye Sander vitreus emigration and the importance of emigration versus mortality for regulating the population. Walleye annual mortality was variable, ranging from 0.05 to 0.46, but was similar between sexes. Walleye emigration ranged from 0.02 to 0.26 and was best explained by variation in mean April discharge and the number of days discharge exceeded 14 m 3 /s. Emigration rates were similar to mortality rates in 5 of 9 years but lower than mortality rates in the remaining 4 years, suggesting that both processes likely contribute to fluctuations in population abundance. Combined, our results indicate that wet years requiring prolonged reservoir discharge, even at relatively low rates, result in increased Walleye emigration, with up to 26% of the population emigrating. Larger, more severe precipitation events are predicted to occur in the future as a result of climate change, thus altering reservoir management plans and threatening the sustainability of reservoir fisheries.

Muskellunge Survival, Interbasin Movement, and Emigration in a Simple and a Complex Interconnected Glacial Lake

“…Multistate models estimate transition between possible states and survival separately within each state. For the Iowa Great Lakes, the multistate model allowed Muskellunge to occupy one of four possible locations: Spirit Lake, East Okoboji Lake, or West Okoboji Lake that were annually sampled during spring or downstream in the Little Sioux River that was not sampled and treated as an unobservable state (p = 0; Kendall and Nichols 2002;Kendall 2004;Weber and Flammang 2019). Similarly, for Clear Lake, Muskellunge could exist in one of two possible states: Clear Lake that was annually sampled or downstream in Clear Creek that was not sampled.…”

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

“…However, our estimates are comparable to those from an unexploited reservoir in Pennsylvania that ranged from 32% to 41% (Kocovsky and Carline 2001a). Other studies in Midwestern reservoirs estimated that total annual mortality of Walleye ranged from 5% to 46% in an Iowa reservoir (Weber and Flammang 2019) to 40–60% in eight Kansas reservoirs (Quist et al 2004). Walleye prefer water temperatures ranging from 21°C to 23°C, with an optimum temperature of 22°C and an upper lethal limit near 31°C (Hokanson 1977; Hasnain et al 2010).…”

“…Beyond mortality, an important but often overlooked factor that may threaten the sustainability of reservoir sport fish populations is escapement over spillways and through outlet structures (Louder 1958; Lewis et al 1968; Paller et al 2006; Weber et al 2013; Weber and Flammang 2019) that complicates the maintenance of populations (Louder 1958; Wahl 1999). For instance, approximately 10,000 fish (nine different species) escaped from an Illinois, USA 65‐ha lake during a 23‐month period (Louder et al 1958).…”

“…Likewise, 31% of a Largemouth Bass Micropterus salmoides population in a new impoundment escaped within a year (Lewis et al 1968). Escapement of Walleye may also be problematic, with annual estimates of escapement from a large reservoir in Iowa, USA approaching 26%, with the economic loss of escaped Walleye near US$400,000 (Weber et al 2013; Weber and Flammang 2019). Although a few studies have quantified Walleye escapement from reservoirs, these took place in large systems with water control structures that operate differently and have contrasting water releases relative to smaller spillway reservoirs.…”

“…Escapement can vary among seasons, with greater escapement occurring in spring during periods of higher water levels and discharge rates (Lewis et al 1968; Powell and Spencer 1979; Wolter et al 2013). For example, Weber and Flammang (2019) found that Walleye escapement from a bottom‐draw water control reservoir was positively related to mean April discharge, suggesting that most escapement occurs during spring spawning periods. Water levels may also affect Walleye escapement rates from spillway reservoirs, but the timing, magnitude, and relationship with water levels may be different than those described in reservoirs with control structures.…”