Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Natural Resource Ecology and Management


Fisheries Biology

First Advisor

Michael J Weber


Walleye Sander vitreus is a highly valued sportfish in North America. In 2001, 3.8 million anglers spent approximately 51.9 million days angling for Walleye (USFWS-USCB 2002). The popularity of Walleye has resulted in situations where demand exceeds supply, which has led to the development and implementation of stocking programs across the United States and Canada to enhance fishing opportunities (Lathrop et al. 2002), rebuild depleted stocks (Johnson et al. 1996; Li et al. 1996), and mitigate poor year classes from variable natural recruitment (Mitzner 1992; Johnson et al. 1996; Jennings et al. 2005; Logsdon et al. 2016; Reed and Staples 2017). However, mortality rates of stocked fishes can vary widely (27-95%; Stein et al. 1981; Buckmeier et al. 2005; Freedman et al. 2012; Weber et al. 2020) and small changes in survival can result in large differences in year-class strength and success of stocking initiatives.

Numerous biotic and abiotic factors can influence survival during early life stages of fish, such as transportation and stocking practices (Forsberg et al. 2001; Barton et al. 2003), predation (Santucci and Wahl 1993; Buckmeier et al. 2005; Thompson et al. 2016), available forage (Johnson et al. 1996; Hoxmeier et al. 2006), competition (Le Pape and Bonhommeau 2015; Chase et al. 2016), fish origin (Kellison et al. 2000; Jonsson and Jonsson 2003; Pollock et al. 2007), body size (Litvak and Leggett 1992; Meekan et al. 2006; Grausgruber and Weber in press), and water temperatures (Akimova et al. 2016; Wagner et al. 2017). Furthermore, the aforementioned factors do not act independently of each other, making it challenging to determine their importance. The growth-predation hypothesis predicts that selective mortality should decline as individuals grow and increase in size (Anderson 1988). Increases in size are also associated with decreased predation risk (Post and Evans 1989; Miranda and Hubbard 1994), where larger body size can reduce the chances of predation due to improved maneuverability and swimming speed (Videler 1993). The argument of "bigger-is-better" (Butler 1988; Miller et al. 1988; Litvak and Leggett 1992) has led hatcheries to raise progressively larger fingerling Walleye (Halverson 2008). However, hatchery production is an expensive and labor-intensive process, where production costs are generally positively related to rearing duration and fish size (Wedemeyer 2001). Therefore, it is advantageous to evaluate factors hypothesized to limit post-stocking Walleye survival (e.g., effects of transport duration and handling practices as well as post-stocking predation and starvation) to assess whether rearing larger fingerling Walleye (hereafter referred to as Walleye) is justifiable. The objectives of this dissertation included 1) evaluating relationships between Walleye transport duration with changes in water chemistry parameters, Walleye physiology, and short-term (48 hr) mortality; 2) evaluating whether consumed Walleye total length was related to predator total length, predator gape height, or the probability of predation, as well as assessed whether length distributions varied among stocked, recaptured, and consumed Walleye, and estimate cumulative consumption for up to two months by a suite of piscivores; 3) evaluating diet composition and shifts of stocked Walleye; and 4) use mark-recapture techniques to evaluate the influence of predation, stocking environment, and Walleye physical characteristics on apparent weekly survival of Walleye from stocking until ice cover.

For my first objective, I used two approaches (i.e., field and experimental evaluation) to evaluate relationships between Walleye transport duration with changes in water chemistry parameters, Walleye whole blood glucose and plasma cortisol concentrations, and short-term (48 hr) mortality. For the field evaluation, Walleye were transported between 3.5 to 6 hours and stocked into holding cages located at one of six systems. Water quality and Walleye stress (via whole blood glucose and plasma cortisol) and mortality were evaluated prior to, during, and at 0, 2, 24, and 48 hours post-transportation. During transport, water temperatures generally decreased while carbon dioxide concentrations fluctuated between 2.7 and 22.5 mg/L. Walleye whole blood glucose and plasma cortisol concentrations varied by system and time since transport. Changes in carbon dioxide concentrations were associated with changes in whole blood glucose concentrations. However, cumulative survival rates and plasma cortisol concentrations were not associated with water quality parameters or transportation duration. For the experimental evaluation, Walleye were transported either 0, 0.5, 3, or 5 hours and stress and mortality were evaluated up to 48 hours post-transport. Unlike, the field evaluation, the experimental evaluation used a staggered loading protocol to transport all Walleye on the same truck during the same day and allowed us to keep fish densities in each of the transportation truck compartments consistent throughout the experiment. Furthermore, in the experimental evaluation, I evaluated additional water quality parameters, not included in the field evaluation. In the experimental evaluation, total ammonia nitrogen, carbon dioxide, pH, and water temperature increased with transportation duration while the total alkalinity of the transport water decreased. Plasma cortisol and whole blood glucose concentrations of Walleye transported longer durations took longer to decline relative to those not transported. However, water quality parameters were not associated with changes in whole blood glucose and plasma cortisol concentrations. Despite increases in stress, Walleye mortality was low (2.5%) 48 hours after transportation. Understanding Walleye tolerance to transportation induced stress has the potential to enhance stocking programs by providing the opportunity for managers to make informed Walleye transportation practices.

My second objective was to evaluate predation on Walleye, including whether consumed Walleye total length was related to predator total length, predator gape height, or the probability of predation, as well as assessed if length distributions varied among stocked, recaptured, and consumed Walleye. I also estimated cumulative consumption by a suite of piscivores (Largemouth Bass Micropterus salmoides, Smallmouth Bass M. dolomieu, Northern Pike Esox lucius, adult Walleye, and Muskellunge E. masquinongy) for two months post-stocking. During fall 2015-2017, 301 Walleye were recovered from 3,514 predator stomachs. Quantile regression models (mean = 50th percentile, maximum = 85th percentile, and minimum = 15th percentile) indicated that consumed Walleye total length was not related to predator total length or gape height (P > 0.05), but the probability of predation decreased by 0.02 for every 10 mm increase in Walleye total length. Length distributions indicated consumed Walleye were generally smaller, whereas recaptured Walleye tended to be larger than stocked fish. To estimate cumulative consumption of Walleye by Largemouth Bass, Northern Pike, and adult Walleye, I used Schnabel models to estimate predator population size and bioenergetics models to determine the biomass of consumed Walleye from stocking through ice-up (mid- November) during fall 2017 and 2018. Across years and systems, 3,272 predators were collected and 257 Walleye were recovered from predator stomachs. Northern Pike (0.12 Walleye per predator ± 0.32 SD) had the highest proportion of Walleye in their diets followed by Largemouth Bass (0.11 ± 0.30 Walleye per predator) and adult Walleye (0.04 ± 0.20 Walleye per predator). Largemouth Bass, Northern Pike, and adult Walleye collectively consumed between 2.4-27.2% of the stocked Walleye within two months of stocking, with higher proportions of smaller (<220 mm) Walleye in predator diets. In East Okoboji, highest proportions of age-0 Walleye in predator diets generally occurred 14 days after the most recent stocking event with the proportion of age-0 Walleye in predator diets decreasing thereafter. However, in West Okoboji, Northern Pike, adult Walleye, and Largemouth Bass had increasing proportions of age-0 Walleye in diets between stocking events. Collectively, results suggest that local piscivores have the potential to consume more than 27% of stocked age-0 Walleye within the first two months of stocking and that predation risk generally decreases with Walleye size. Thus, post-stocking predation can negatively influence Walleye stocking success, especially if the sizes of Walleye stocked have high predation risk.

Hatchery propagation techniques, such as pellet-rearing, can result in impaired post-stocking feeding behavior of fishes. My third objective was to evaluate diet composition and diet shifts of Walleye. Specifically, my objective was to compare fall diets of wild and stocked Walleye by evaluating the proportion of empty stomachs as well as proportions of zooplankton, benthic invertebrates, and fish in diets. Percent similarity index values were also used to assess the percent similarity of wild and fingerling diets. In total, 590 Walleye were gastrically lavaged, with wild Walleye making up 9.7% and stocked Walleye making up 90.3% of the samples. The average proportion of empty stomachs differed between wild and stocked Walleye, with stocked Walleye having higher average (± 95% CI) proportions of empty stomachs (0.40 ± 0.10) compared to wild fish (0.15 ± 0.09). Proportion of empty stomachs and proportion of Walleye with zooplankton in diets decreased with days post-stocking. Stocked Walleye had a greater proportion of benthic invertebrates in their diet than wild Walleye whereas wild Walleye consumed more fish relative to stocked Walleye. However, the presence of zooplankton, benthic invertebrates, and fish in wild (TL = 129-215 mm) and fingerling (TL = 94-228 mm) Walleye diets were not related to Walleye total length. Finally, percent similarity index values for wild and stocked Walleye diets were highly variable and ranged from 0.0 to 67.9%. Collectively, my results suggest that fall stocked fingerling Walleye consume different prey items compared to their wild counterparts for up to 49 days post-stocking. Results pertaining to post-stocking Walleye diet composition may influence post-stocking survival.

Numerous factors can independently and dependently influence the survival of stocked Walleye, including environmental conditions, predation, and Walleye total length, condition, and post-stocking diet composition. However, the relative effects of these factors on survival of stocked Walleye are challenging to evaluate and rarely assessed simultaneously. Thus, my fourth objective used mark-recapture techniques to evaluate the influence of predation, stocking environment, and Walleye physical characteristics (total length and condition at time of stocking) on apparent weekly survival from stocking until ice cover. Each fall (2015, 2016, and 2017), roughly 4,000 stocked Walleye were implanted with passive integrated transponder tags, stocked, and recaptured via boat electrofishing. Cormack-Jolly-Seber recapture models estimated that Walleye weekly apparent survival was negatively related to the weekly average proportion of Walleye recovered from predator diets (β = -2.86; 95% credibility interval = -5.69 to -0.17) as well as weekly average water temperatures (β = 0.09; 95% credibility interval = -0.08 to 0.20). At an average fall water temperature, mean Walleye apparent survival was 0.83 (95% credibility interval = 0.75 to 0.91) when no Walleye were recovered from predator diets and declined to 0.54 (95% credibility interval = 0.28 to 0.84) when Walleye comprised 60% of predator diets. Between 41-97% of stocked Walleye were lost to mortality from stocking through ice-up among the three years, indicating that the two months post-stocking is a critical period for survival. Our results suggest that post-stocking Walleye survival is most influenced by post-stocking predation rather than individual Walleye or environmental factors. Thus, strategies that minimize predation following stocking may result in the greatest improvements in Walleye post-stocking survival.


Copyright Owner

Emily E Grausgruber



File Format


File Size

244 pages