# Game over for gamefishes after zebra and quagga mussel invasion: not in prairie lakes?

Caroline B Ofosu, Anas M. Usoof, Erin Hillis, Zoraida J Quiñones-Rivera, Björn Wissel

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Caroline B Ofosu, Anas M. Usoof, Erin Hillis, Zoraida J Quiñones-Rivera, Björn Wissel. Game over for gamefishes after zebra and quagga mussel invasion: not in prairie lakes?. Canadian Journal of Fisheries and Aquatic Sciences, 2026, 83, pp.1-14. 10.1139/cjfas-2025-0028. hal-05498245

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Game Over for Gamefishes after Zebra and Quagga Mussel
Invasion: Not in Prairie Lakes?

<table><tr><td>Journal:</td><td>Canadian Journal of Fisheries and Aquatic Sciences</td></tr><tr><td>Manuscript ID</td><td>cjfas-2025-0028.R1</td></tr><tr><td>Manuscript Type:</td><td>Research Article</td></tr><tr><td>Date Submitted by the Author:</td><td>n/a</td></tr><tr><td>Complete List of Authors:</td><td>Ofosu, Caroline; University of Regina, Biology; Memorial University of Newfoundland, Earth Sciences; Memorial University of Newfoundland, Ocean Sciences<br/>Usoof, Anas; University of Regina, Biology; University of Toronto Scarborough, Biological Sciences; The University of Winnipeg, Biology<br/>Hillis, Erin; University of Regina, Biology<br/>Quiñones-Rivera, Zoraida; University of Regina, Biology; Université Claude Bernard Lyon 1, aboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés (LEHNA)<br/>Wissel, Björn; University of Regina, Biology; Université Claude Bernard Lyon 1, Laboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés (LEHNA)</td></tr><tr><td>Is the manuscript for consideration in a Special Issue or Collection?:</td><td>Not applicable (regular submission)</td></tr><tr><td>Keyword:</td><td>Zebra and Quagga mussels, Invasion, Hardwater lakes, Gamefishes, Walleye</td></tr></table>
Game Over for Gamefishes in Prairie Lakes?

Authors: Caroline B. Ofosu¹,²*, Mohamed U. M. Anas¹,³, Erin Hillis¹, Zoraida J.

Quiñones-Rivera¹,⁴ and Björn Wissel¹,⁴

¹University of Regina | U of R · Department of Biology, Regina, Regina, SK, Canada.

²Memorial University of Newfoundland | MUN · Department of Earth and Ocean

Sciences, NL, Canada.

³University of Toronto | U of T · Department of Biological Sciences at Scarborough, ON,
Canada.

⁴Université Claude Bernard Lyon 1 | UCBL · Laboratoire d'Ecologie des Hydrosystèmes
Naturels et Anthropisés (LEHNA), Villeurbanne, France.

Correspondence: [cbofosu@mun.ca](mailto:cbofosu@mun.ca)
25 **Abstract**

26 Mitigating threats of aquatic invasive species in lakes requires holistic insights into the
27 biological, physical, and chemical conditions sustaining fish communities. This study
28 establishes the ecological baseline conditions for three key gamefishes—walleye,
29 northern pike, and yellow perch—across 15 hardwater prairie lakes in south-central
30 Saskatchewan, Canada, that are threatened by zebra mussel invasion. We employed
31 stomach content analysis to evaluate diet composition and ontogenetic nutritional shifts
32 in adult fishes and assessed how fish size and environmental factors influence body
33 condition. Adult walleye transition from a diet of amphipods to fishes as they grow,
34 demonstrating greater dietary flexibility compared to perch and pike, which maintain a
35 stable diet of chironomids and fish, respectively. While walleye and pike decline in body
36 condition with size, perch maintain a consistently high body condition as they grow.
37 Lake productivity and alkalinity are important predictors of body conditions of adult
38 walleye and perch in hardwater prairie lakes. Given the combined threats of zebra
39 mussel invasions, climate change, and potential local forage fish scarcity, novel
40 management strategies are essential.

41

42 **Keywords:** Zebra and Quagga mussels, Invasion, Hardwater lakes, Gamefishes,
43 Walleye.

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## 1.0 INTRODUCTION

Despite the growing interest in invasion ecology (Simberloff, 2004), there is limited understanding on how pre-existing regional stressors and Aquatic Invasive Species (AIS) interact to impact the survival of native fish populations (Arrington et al., 2016).

Zebra and quagga mussels (*Dreissena polymorpha*, Pallas, 1771, and *D. bugensis*, Andrusov, 1897, respectively; hereafter referred to as ZQM) are two aquatic invaders that have significantly modified food webs across continents (CABI, 2020). These bivalves now pose threats to aquatic ecosystems in the prairie region of Western Canada, with its many hardwater lakes and a unique hydrology (shallow depth, high alkalinity, absence of thermal stratification, varying salinity, and suitable calcium concentrations (> 25 mg/L, see Table S1), (Therriault et al., 2013). These physicochemical attributes are expected to create conditions that support successful ZQM invasion and, in the event of their introduction, increase the susceptibility of fish populations in this region (Starks et al., 2014; Nanayakkara et al., 2021).

Once introduced, ZQM exhibit robust tolerance to prevailing conditions, especially temperature and salinity, increasing their likelihood to thrive in lakes across south-central Saskatchewan (SK) (Therriault & Orlova, 2010), which represents the western invasion front. The proximity of SK to invasion corridors, notably Manitoba (LWF, 2020) and three US states (Montana, North Dakota, and Minnesota), heightens the risk of ZQM introduction into western prairie lakes. Ecological impacts of these invaders reverberate through food-webs (Higgins & Vander Zanden, 2010). As efficient filter feeders, ZQM enhance water clarity while reducing pelagic primary productivity. This causes disruptions in phytoplankton and zooplankton populations, which form the basis
71 of the food web upon which most native juvenile fishes depend (Pace et al., 1998;

72 Higgins & Vander Zanden, 2010). Such disturbances invariably affect the productivity of

73 higher trophic-level gamefishes like walleye (*Sander vitreus*, Mitchill, 1818), northern

74 pike (*Esox Lucius*, Linnaeus, 1758; hereafter referred to as pike), and yellow perch

75 (*Perca flavescens*, Mitchill, 1814; hereafter referred to as perch), and may dramatically

76 alter trophic relationships (Karatayev et al., 2012; Ozersky et al., 2012).

77 The impacts of ZQM invasion vary across regions, systems, and species and often

78 through time (Hansen et al., 2020), making predictions and management efforts

79 challenging. In north-temperate lakes, ZQM invasions increased water clarity, especially

80 in stratified lakes, with an average increase in Secchi depth of 0.68 m and a 0.31 m

81 increase for mixed lakes (Giesler, 2015). This enhanced water clarity adversely affected

82 walleye, a light-sensitive species (Scott & Crossman, 1973; Ryder, 1977), increasing

83 predation pressure and competition, and ultimately reducing suitable habitat and yields

84 (decline of 2.36 kg/ha/yr post-invasion; Giesler, 2015). In contrast, Manitoba prairie

85 lakes exhibited a positive response, with predicted walleye yield increases of 2.63

86 kg/ha/yr.

87 In the Great Lakes region, dreissenids negatively impacted phytoplankton, zooplankton

88 (Pace et al., 1998; Mavrin & Strel'nikova, 2011), and benthic invertebrates (e.g.,

89 *Diporeia*) that are crucial to the diet of apex predators including whitefish (*Coregonus*

90 *clupeaformis*) (Hoyle et al., 2008). Following the collapse of *Diporeia*, whitefish were

91 compelled to incorporate larger quantities of lower-energy food items, particularly

92 dreissenids (Pothoven et al., 2001; Lumb et al., 2007), leading to reduced body

93 condition, growth rates, and reproductive success (Hoyle et al., 1999; Hoyle et al.,
94 2008). Similar nutritional consequences were observed in alewife (*Alosa pseudoharengus*), where energy density declined due to reduced *Diporeia* availability in their diet, from 29% pre-invasion to 12% post-invasion (Madenjian et al., 2006).

95 96 Interestingly, adult walleye and smallmouth bass in southern Ontario boreal lakes successfully navigated resource limitations imposed by ZQM, switching to advantageous prey like crayfish and fish (Nienhuis et al., 2014).

100 Recent assessments revealed that gamefish populations in SK hardwater lakes are already stressed, particularly walleye, which recorded high parasite loads and poor body condition (Nanayakkara et al., 2021). Hence, comprehending how current ecological conditions support (or limit) fish communities is vital to develop management efforts that aim to minimize the impacts of future ZQM invasion. Accordingly, we conducted a baseline assessment of the ecological status (diet and body conditions) of the three recreational and commercially targeted gamefish species—walleye, pike, and perch—across south-central SK lakes. Walleye, the focal species, is primarily sustained by provincial stocking programs to replenish populations stressed by angling pressure and low recruitment (SSWG 2017, 2019), unlike perch and pike, which occur naturally in prairie lakes.

101 102 103 104 105 106 107 108 109 110 For gape-limited animals like fishes, growth is maintained by size-dependent shifts in food or habitat use (Werner & Gilliam, 1984). Piscivorous gamefishes typically set out foraging zooplankton, transitioning to macroinvertebrates and eventually may become piscivores (Liao, 2001; Uphoff et al., 2019). To ensure robust growth, condition, and survival, fish must have access to suitable-sized prey at all life stages. Using 15 fish-bearing lakes as model systems, we tested the hypotheses that:
117 1. Diet preferences will be species-specific, where "early switching piscivores," pike and walleye, will show marked feeding specialization. However, perch, a "late switching piscivores", may demonstrate a more flexible feeding strategy.

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2. Changes in diet composition for these gape-limited gamefish species will be evident as they grow.

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4. Physicochemical parameters (e.g., water temperature, dissolved oxygen, pH, and nutrient concentrations) will significantly influence the body condition of gamefishes.

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For this study we focused on adult size classes only to focus on diet shifts that are associated with prey preference or availability, rather than those related to anatomic mouth-gape limitations.

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## 2.0 METHODS

### 2.1 STUDY AREA

Initially, eighteen fish-bearing lakes in the Northern Great Plains (NGP) of Western Canada were sampled to provide a baseline on the ecological status and dynamics of gamefish populations across south-central SK (Figure 1). All study lakes were within the prairie and boreal transitional ecozones. Of the 18 lakes, twelve are in hydrologically closed basins (endorheic) that lack channelized surface inflow or outflow, and are mainly sustained through winter precipitation. The remaining six lakes are exorheic (open basins) draining into downstream systems. Surface area of study lakes ranged from 0.35 to 33.0 km², with maximum depth ranging from 4.4 to 38.0 m (Table S1). Nine of the study lakes are part of a 21-lake Long-Term-Ecological-Research program conducted in the majority of drainage basins across southern SK since 2002 (Wissel et al. 2011, Starks et al. 2014, Nanayakkara et al., 2021). The Ministry of Environment, Government of Saskatchewan, uses the remaining nine lakes for fisheries assessment in the Saskatoon and Prince Albert areas.

Five lakes lie within the southwestern semi-arid ecoregion, four within Canada's moist mixed grassland interior plains, eight in the Aspen Parkland, and one within the boreal transitional zone (Figure 1, Table S1). Fifteen lakes had high fish complexity (forage fishes, benthivores, and gamefishes), (Table S1). Of these, 10 were predominantly sustained by provincial stocking programs to replenish walleye populations stressed by angling pressure and low recruitment (SSWG 2017, 2019). The remaining three lakes were excluded from further analyses due to their low fish complexity. These lakes contained only forage fishes, which are potential prey for gamefishes in lakes with
higher complexity, along with other fishes commonly found in low-complexity lakes (Essington et al., 2015, (Table S1). Of the three low-complexity lakes, From and Grenfell were suspected to have experienced recent summer or winterkills (SFF, 2019), while Shannon Lake was anoxic at sampling. The selected 15 lakes are exposed to large seasonal and inter-annual variations (Sauchyn & Kulshreshtha, 2007), and their small size makes them particularly susceptible to the impacts of climate change and the potential invasion by ZQM. Hence, they are excellent models for monitoring the possible effects of environmental stressors, representing early-warning systems.

## 2.2 LAKE SAMPLING

### 2.2.1 Integrated water samples

The methods described by Cooper and Wissel (2012a, 2012b) and Starks et al. (2014) were adopted for collecting water samples from May to August of 2019. Samples for water chemistry were collected at the location of maximum depth. Lake depths were recorded using an electronic depth meter, model 71624. Using a YSI model 556, vertical profiles of temperature (°C), specific conductivity (µS cm⁻¹), total dissolved solids (mg TDS L⁻¹), salinity (ppt), dissolved oxygen (mg O₂ L⁻¹, % saturation) and pH were recorded at 0.5 m intervals for shallow lakes (< 10 m) and 1 m intervals for deep lakes (> 10 m). Water transparency was measured with a 20 cm diameter black and white Secchi disk.

### 2.2.2 Gamefish samples

Gamefishes were captured overnight using sets of multi-panel gill nets (24.7 m long x 1.5 m deep; mesh sizes; 1½, 2, 2½, 3, 3½, 4, 4½, and 5 inches) placed perpendicular to
shore, between May and August of 2019. Multi-panel gill nets were used to obtain representative samples of the different sizes of gamefish species. Fishes were retrieved after 16-20 hours, identified to species (Scott & Crossman, 1973) and their sexes determined. This was followed by in-situ total length (cm) and weight (g) measurements. Fish samples were then placed immediately on ice to reduce the decomposition of stomach content and stored at -15 °C (Zacharia et al., 2004) for a few weeks.

## 2.3 STUDY SPECIES

Walleye, perch, and pike generally recurred across study lakes. Walleye occurred in all 15 high-complexity lakes, perch in 9, and pike in 6 (Table S2). Walleye is the focal species because it is the most preferred by anglers in study lakes (Nanayakkara & Wissel, 2017). However, unlike native perch and pike, walleye populations lack genetic diversity (Starks et al., 2014), which is related to the stocking programs.

## 2.4 ETHICS STATEMENT

Animal sampling in this study was conducted under approved animal use protocols from the Animal Care Committee of the University of Regina, and in accordance with special collection research permits provided to B; Wissel by the Government of Saskatchewan, and the Ministry of Environment (SCP2019/AR08/FWLB).

## 2.4 LABORATORY ANALYSIS

### 2.4.1 Water chemistry

Depth-integrated water samples were pre-screened using a 80-µm mesh before filtration through 0.45-µm sterile membrane filters. A subset of the filtered water was
frozen and later analyzed for nitrate ($NO_3$), nitrite ($NO_2$), ammonium ($NH_4^+$), total nitrogen (TN), soluble reactive phosphorous (SRP), and total phosphorous (TP) (µg L$^{-1}$) following standard methods described by Pham et al. (2008). The remaining filtrate was refrigerated and analyzed for alkalinity and Calcium (Ca) (mg L$^{-1}$) following Pham et al. (2008). Dissolved inorganic and organic carbon (DIC/DOC) were quantified using refrigerated filtrate on an OI Analytical Aurora 1030W TOC analyzer. Algal biomass (measured as chlorophyll a; chl a) was quantified by filtering 1 L through GF/C filters, stored at -15 °C until extraction with acetone: methanol: water (80:15:5).
Concentrations were quantified using a spectrophotometer (Hewlett-Packard model 84452A) (Wetzel & Likens, 1996; Cooper & Wissel, 2012a). All laboratory analyses were conducted at the Institute of Environmental Change and Society at the University of Regina, Saskatchewan, Canada.

### 2.4.2 Stomach content analysis

Fish stomachs were removed and stored in 70% ethanol (Jobling et al., 2001) until analysis. Two-hundred-and-eighty-three preserved gamefish stomachs comprising 188 walleye, 65 perch, and 30 pike were examined under a dissecting microscope (SZX12 Olympus model) to ascertain important prey contributing to their diets. Prey items were identified to the lowest practical taxonomic resolution, mostly species-level for fishes (SWA, 2010) and family-level for macroinvertebrates (Thorp & Rogers, 2010) and quantified by counting individuals of each taxon and determining their volumes using the displacement method (Pinkas et al., 1971; Hyslop, 1980; Manko, 2016). For objective interpretation of results, the *Index of Relative Importance (IRI)* was used to express the
227 importance of food items (Gelwick & Matthews, 2006; Chipps & Garvey, 2007). *IRIi* of
228 specific food category is:

$$
229 \qquad \mathit{IRIi} = (\mathit{\%N}_i + \mathit{\%V}_i) \times \mathit{\%F} \qquad (1)
$$

230 Where %Ni = Numerical percentage of specific food category, %Vi = Volumetric
231 percentage of specific food category and %Fi = Frequency of occurrence.

232 The single index for extrapolating *IRIi* was calculated as follows:

$$
233 \qquad \%N = N_i/N_t \times 100 \qquad (2)
$$

$$
234 \qquad \%V = V_i/V_t \times 100 \qquad (3)
$$

$$
235 \qquad \%F = F_i/F_t \times 100 \qquad (4)
$$

236 Where Nᵢ = the number of food categoryᵢ and Nₜ = the total number of individuals in all
237 food categories, Vᵢ = the volume of food categoryᵢ and Vₜ = the total volume for all food
238 categories in all individuals, and Fᵢ = number of fish containing food categoryᵢ and Fₜ =
239 the number of fish with food in their stomachs.

240 IRI values were then expressed in percentages for possible comparison among food
241 items based on Desmond et al. (2002) summing total IRI values across all prey items:

$$
242 \qquad \%IRIi = 100 \times \frac{IRIi}{\sum_{i=1}^{n} IRIi} \qquad (5)
$$

243 Where %IRIi is the percent index of relative importance of food item i, IRIi is the index
244 of relative importance of food items i, and n is the total number of food categories
245 calculated at a given taxonomic level.
To calculate %IRI, the three gamefish species were grouped into two size groups based on their total lengths as follows: (≤ 40cm and > 40cm for walleye), (≤ 60cm and > 60cm for pike), and (≤ 22.5cm and > 22.5cm for perch), respectively, and their %IRI compared.

Existing literature largely focuses on the feeding ecology of early life stages (Yang, 2022). We focused on adult sizes to assess to what extent the study species realized a switch to piscivory when their mouthgapes where already sufficiently large. For all three species, the size cut-offs represented age-classes of 4-5 years. While all captured fish were sufficiently large for piscivory, its likelihood is expected to increase with size class. Prey were grouped according to dominant taxa: fish, amphipods, chironomids, other invertebrates, plant matter, and unidentified components.

### 2.4.3 Fish condition

Relative weight (Wr) condition indices were computed for 514 gamefishes comprising 317 walleye, 41 pike, and 156 perch (Table 1) and reported as a whole number using equations described by Willis (1989), Willis et al., (1991), Murphy et al. (1991) and Blackwell et al., (2000).

Wr of gamefish populations was estimated using the following equation:

$$Wr = (W/Ws) \times 100 \quad (6)$$

Where Wr = relative weight, W = actual weight, and Ws = standard weight for fish of the same length. Ws values were calculated for each fish exceeding the required minimum total length (walleye > 150 mm, pike > 100 mm, perch > 100 mm). Parameters for Ws were derived from the regression equations of Blackwell et al. (2000). Values of Wr =
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95-105 indicate good condition, near 80 suggest poor health due to unfavorable

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environmental conditions, and extremely high scores (> 115) indicate excess energy

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reserves (Murphy et al., 1991; Blackwell et al., 2000).

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## 2.5 STATISTICAL ANALYSIS

273

### 2.5.1 Relative importance of prey items to gamefishes

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We used MANOVA (Multivariate analysis of variance) based on redundancy analysis (RDA) to test significant differences in diet composition between small and large groups of three gamefish species. RDA models for gamefish diet composition were fitted using the 'rda' function (vegan package) with 'size group' as the main factor and 'lake' as a covariate. Diet data were square root transformed for normality. Before analyses, the PERMDISP2 test (Anderson, 2006) was used to evaluate multivariate homogeneity of size groups using betadisper (vegan package).

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RDA based on MANOVA provides an elegant alternative over traditional MANOVA because conditions of application are less stringent for RDA-based MANOVA compared to traditional MANOVA (e.g., RDA-based MANOVA does not need to meet the condition of multivariate normality of groups). Besides its versatility of permutation tests, it includes a triplot representation of results (Borcard et al., 2018).

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### 2.5.2 Effects of body size on fish condition

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Linear mixed-effects (lme) models, fitted by restricted maximum likelihood, were used to assess the impact of body size (total length) on gamefish body condition while controlling for lake (site) effects. lme analysis was performed using the lmer function

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290 from the R package *lme4* with total length set as a fixed effect and site (lake) as a
291 random effect. Three models per gamefish species were evaluated (Table S3). To
292 facilitate model convergence, we used the 'bobyqa' optimizer (optimix R version 2020-
293 4.2). We assumed a Gaussian distribution, checked model residuals for normality, and
294 conducted standard diagnostic tests to confirm regression assumptions.

295 Model selection was established by the most parsimonious model, informed by the
296 Akaike information criterion corrected for small sample sizes (AICc). The significance of
297 the model coefficient (total length) was tested with t-tests using Satterthwaite
298 approximations (Luke, 2017) via the *anova* function (lmerTest R package). Conditional
299 and marginal R-squared estimates (Nakagawa and Schielzeth, 2013) were calculated
300 with the r-squared GLMM function (MuMln package, R-version 1.43.17), (Barton,
301 2020).

### 2.5.3 *Environmental controls of fish condition*

303 Considering the practical difficulty in including all 18 environmental variables in the
304 analyses, we first performed principal component analysis (PCA) using standardized
305 (scaled to unit variance) environmental variables to evaluate the correlations among
306 them. Surface area, maximum depth, elevation, and temperature were included as
307 supplementary variables in the PCA.

308 Based on literature (Katzenmeyer, 2010; Rueda et al., 2015) and results of the PCA
309 (correlation among variables, Figure S1), five main variables (surface area, maximum
310 depth, alkalinity, chlorophyll a, and total phosphorus) known to influence fish body
311 condition were selected as explanatory variables for the regression models. To quantify
312 the relative importance of selected regressors, the “lmg” metric of the R package
“relaimpo” was used (Groemping & Matthias, 2018). The “lmg” metric decomposes R² into non-negative contributions attributable to each regressor based on averaging over all orderings of regressors in a multiple regression setting (Grömping, 2007).

Bootstrapping (1000 runs) of models was done using functions boot and boot.ci from R package boot (Davison & Hinkley, 1997). Due to the small sample size for pike, both small and large groups were combined for analysis. All analyses were performed using R version 4.0.3. (R Core Team, 2020).

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# 3 RESULTS

322

## 3.1 Diet composition and ontogenetic dietary shifts

Across 15 lakes and 283 individual fish stomachs, we identified 23 prey taxa: two Amphipoda genera (*Gammarus* and *Hyallela*); seven fish species, Brook stickleback (*Culaea inconstans*), Ninespine stickleback (*Pungitius pungitius*), Fathead minnow (*Pimephales promelas*), Johnny darter (*Etheostoma nigrum*), walleye fry, perch fry, and pike fry; 13 taxa of benthic macroinvertebrates, (*Chironomidae*, *Corixidae*, *Physidae*, *Planorbidae*, *Asilidae*, *Lestidae*, *Cambaridae*, *Diptera*, *Odonata*, *Hirudinea*, *Oligochaeta*, *Nematoda* and *Notonecta*); and one predatory zooplankton species, *Chaoborus punctipennis*. Plant matter, stones and unidentified items were also found.

Approximately 40% of the sampled fishes had empty stomachs. Of the 82 small- and 106 large-sized walleye, 26 (32%) and 31 (29%) stomachs were empty respectively (Table S4). Similarly, 5 (33%) of the 15 small-sized pike had empty stomachs, but the proportion of empty stomachs increased to 53% (8 of 15) in large pike (Table S5). Small

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perch exhibited a 48% incidence of empty stomachs (14 of 29), relative to 28% (10 of 36) for large perch (Tables S6).

The importance of prey items varied among size groups of walleye, with the small size group (≤ 40 cm) demonstrating greater reliance on amphipods (41.91±10.54% IRI), while larger walleye (> 40 cm) targeted more fish (38.07±11.20% IRI), (Table S4). In contrast, pike and perch showed less size-dependent shifts in diet. Vertebrate (fish) prey was the most common diet for both small (≤60 cm) and large (>60 cm) pike (41.50±20.82% IRI and 51.26±21.32% IRI) respectively, while a considerable portion of stomach content was unidentifiable (Table S5). Small perch heavily relied on invertebrates, particularly chironomids (61.21%), whereas amphipods (27.81±17.49% IRI) and chironomids (25.28±16.51% IRI) formed appreciable proportions of the diet of larger perch (Table S6).

MANOVA revealed a significant difference between the diet of small and large walleye (p = 0.028; Figure S3). In contrast, no size-related distinctions were observed in the diet of pike (p = 1.00; Figure S4) and perch (p = 1.00; Figure S5). Walleye diet shifted from feeding on amphipods by the smaller size group (41.91±10.54% IRI) to a more fish-based diet in the larger size group (38.07±11.20% IRI) (Table S4, Figure S3). Despite no statistically significant ontogenetic shift in diet of perch and pike (Tables S5 and S6, Figure S4 and S5), visual observation of the dietary plots revealed clear patterns in diet variability between small and large size groups (Figure 2). Specifically, there was a decrease in reliance on chironomids from small to large perch and an increase in foraging on fish from small to large pike (Figure 2). Yet, independent of gamefish species and size class, piscivory never contributed more than half of the diet.
358 **3.2 Effects of body size on fish condition**

359 We conducted condition assessments for 514 individual fish, (317 walleyes, 41 pike, 156 perch), with sizes from 14 to 86 cm in total length and weights from 28 to 3760 g (Table 1). The average body condition (Wr) for walleye, pike, and perch populations were 92, 87 and 105, respectively (Table 1).

363 After accounting for the variation in lakes (sites), *lme* models showed a significant negative correlation between total length (body size) and the condition of walleye (p = 0.04; Figures 3A, Table S7), and pike across lakes (p = 0.02; Figure 3B, Table S8). This translated to a decline of approximately 0.24% Wr ± 0.10 SE for walleye and 0.31% Wr ± 0.13 SE for pike (Figures 3A and 3B; Tables S7 and S8). In contrast, no significant link existed between body size and condition of perch (p = 0.26; Figure 3C and Table S9).
364
365
366
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368
369 The relatively large unexplained variance in the perch model (>80%), suggests sources of variability beyond the model's capacity to predict.

371 Respective estimated Marginal R² and Conditional R² values were 0.05 and 0.55 for walleye (Table S7; model 3), 0.11 and 0.43 for pike (Table S8; model 4), and 0.01 and 0.19 for perch (Table S9; model 7).

374 **3.3 Relative importance of environmental regressors to fish body-condition**

375 We concentrated on five key environmental variables—surface area, maximum depth, alkalinity, chl a (measure of phytoplankton biomass), and total phosphorus—as controls 376 of gamefish body condition (Rudstam et al., 1996; Katzenmeyer, 2010; Rueda et al., 377 2015). Assessment of relative importance of the five regressors indicated that phytoplankton biomass and alkalinity were two major factors impacting the conditions 378
379
of large walleye and perch. These parameters accounted for 50% and 16% of the variation in body condition for large walleye (Figure 4A; Adj R² = 0.68). Similarly, they explained 37% and 51% of the variability, respectively in large perch (Figure 4B; Adj R² = 0.91). However, for their smaller counterparts no strong associations existed between body condition and environmental parameters (Figures S5 and S6). In addition, in large walleye and perch we observed a positive relationship between body condition and chl a (Figures 2A and 2B). However, alkalinity had nuanced effects; while it positively correlated with large walleye condition (Figure 4A), it negatively impacted large perch (Figure 4B). In contrast, the model for pike explained only 7% of the variability in their condition (Figure S8).

## 4.0 DISCUSSION

### 4.1 Dietary importance

Understanding the dietary needs of co-existing gamefish species — walleye, pike, and perch — in SK prairie lakes is crucial for predicting their responses to imminent aquatic stressors such as the invasion of ZQM and the growing threat of climate change. In line with our hypothesis, this study recorded remarkable variations in diet composition and foraging strategies among species. Gamefishes relied on a broad range of prey taxa (Figure S2, Tables S4-6), suggesting their capacity to thrive amidst changing prey resources. While both invertebrates and fish constituted substantial components of walleye diets, pike slightly favored fish prey, while perch leaned towards an invertebrate-based diet, especially chironomids and amphipods. Surprisingly, even as walleye grew and transitioned towards more piscivorous diets, invertebrates remained a
large part of their diet (Figure S4, Table S4). This finding challenges the classic notion that “early switching piscivores” or “specialists” rapidly shift to energetically beneficial vertebrate prey with increasing size/gape width (Graeb et al., 2005).

In this study, the invertivory levels observed in walleye and pike were higher than their typical trophic-specialist feeding patterns in other prairie lakes (Sammons et al., 1994).

This suggests an adaptive shift toward generalist feeding or a response to limited forage fish in these lakes. For example, the prevalence of *Gammarus* (abundant taxon in study lakes, data not shown) in adult walleye diets may reflect opportunistic feeding when preferred forage fishes are scarce (Kaufman et al., 2009). This warrants further investigation into the prevailing status of forage fishes in prairie lakes.

As expected, small and large perch showed a clear preference for invertebrates. While piscivory can occur in perch, it is less prominent as perch favors invertebrates over fish when both are available (Liao, 2001; Graeb et al., 2005). In an experimental study where perch were offered more fish prey than invertebrates, perch displayed a neutral selection for fish across all size groups. In contrast, walleye maintained a positive preference for fish even at lower densities of fish relative to alternate prey. This suggests that morphology, feeding behavior, and life history contribute to the differences in dietary ontogenies in these two percid predators (Graeb et al., 2005).

Additionally, diel movements of forage fishes (moving inshore during the day and offshore at night) might impact their accessibility to adult walleye that forages in shallow zones at night. Observed cannibalism in large walleye (42.6 – 50 cm) supports the hypothesis of limited availability of forage fishes, like scenarios reported in Lake Oneida, New York, where low prey densities resulted in cannibalism (Forney, 1974). To
that end, Forney (1976) proposed augmenting the densities of young perch to mitigate
the scarcity of forage fish and sustain walleye populations.

Overall, this study demonstrates dietary plasticity of gamefish communities in prairie
lakes, and their ability to adapt to shifting prey resources by supplementing their diets
with invertebrates when preferred fish prey is scarce. However, it also warns of trophic
bottlenecks and stunted growth (Venturelli & Tonn, 2006; VanDeValk et al., 2008). The
absence of forage fishes may impact higher trophic levels and increase vulnerability to
invasive species. At the same time, cannibalism among walleye may further constrain
population size.

## 4.2 Ontogenetic dietary shift

Zooplanktivory is a precursor to invertivory and benthivory during the early life stages
of pike, walleye, and perch (Frost, 1954; Mathias & Li, 1982; Slipke & Duffy, 1997). In
walleye, the transition from zooplankton to macroinvertebrates and later to fish prey
occurs between 30-50 mm total length (TL) and 60-117 mm TL, respectively
(Galarowicz & Wahl, 2005; Uphoff et al., 2019; Yang, 2022). Perch shifts from
zooplankton to macroinvertebrates at 20 to 60 mm TL (Wu & Culver, 1992; Graeb et al.,
2006). However, the timing of piscivory varies, ranging from 80 mm TL (Graeb et al.,
2006) to over 150 mm TL (Keast, 1985; Fullhart et al., 2002). At 50 mm TL, pike begins
foraging on fish while continuing to supplement their diet with invertebrates as they
grow; however, this behavior remains partly understood (Hook & Hackle Company,
2013).

This study presents evidence of ontogenetic diet shifts within adult walleye populations.
Smaller individuals transition from invertebrates (amphipods) to fish as they mature. In
contrast, pike and perch populations showed less pronounced dietary shifts, maintaining piscivorous and invertivorous foraging strategies, respectively. Despite no statistical differences in perch diets, visual observations of dietary patterns reveal nuanced shifts: larger perch favor amphipods and “other invertebrates,” while smaller perch avoid these items. These results, though statistically insignificant, may hold ecological consequences.

The hypothesis that shifts in diet would be detectable as gamefish species grew was accepted only for walleye. Piscivory levels were higher in larger individuals, while smaller ones showed a prevalence of invertivory. Fish transitioning early to piscivory potentially benefit from higher somatic growth, earlier maturation, and increased fitness (Sánchez-Hernández et al., 2019), which might be the case for pike and walleye in this study. Such a shift may directly impact fish communities (e.g., fish abundance, biomass, and population or community structure). Several factors, including ontogeny, vulnerable prey sizes, competition, community structure or fish species richness (Sánchez-Hernández et al., 2017), and temperature (Amundsen et al., 1999) influence the prevalence of piscivory within fish populations. Although the onset of piscivorous behavior has been associated with high water temperatures, which promote growth during early development (Mehner et al., 1996), it is likely triggered also by resource competition and the availability of profitable small prey fish (Sánchez-Hernández et al., 2017).

This study affirms the influence of size structure on gamefish foraging habits, particularly in walleye. Such dietary variations may reduce competition and promote their ecological success (Graeb et al., 2005). In contrast, the shared diet across size
groups in pike and perch suggests intra-species competition (Venturelli & Tonn, 2006), potentially affecting energy reserves, growth, and overwinter survival (Sogard & Olla, 2000; Grausgruber & Weber, 2021). These findings underscore the ecological significance of size-related feeding dynamics and provide valuable insights for aquatic ecologists. As revealed in this research, predator-prey interactions carry critical ecological implications for food-web structure and the long-term survival of fish populations.

### 4.3 Body condition of gamefishes

Our findings unveil stark contrasts in the physiological condition of gamefish populations within the studied lakes. Remarkably, perch were the only species exhibiting optimal body condition, boasting an average Wr of 105 (Table 1). In contrast, walleye and pike populations fell below the target range for healthy populations, Wr of 95-105 (Blackwell et al., 2000). These findings echo previous reports of poor conditions in walleye and pike compared to perch (Nanayakkara et al., 2021). Similar patterns were observed in other prairie (Willis, 1989) and boreal lakes (VanDeValk et al., 2008).

However, our study reports substantially lower Wr values for walleye and pike and higher values for perch than those in other North American systems (Porath & Peters, 1997; Flinder & Bonar, 2008; D'Arco et al., 2015), suggesting further deterioration in these gamefishes' body condition over time.

Dietary analyses suggest a scarcity of forage fish or prevailing trophic bottlenecks, potentially contributing to the diminished body condition in adult walleye and pike.

This decline in walleye Wr aligns with previous reports across three length classes over 23 years (1961–1983) in Oneida Lake, New York, where walleye reliance on small, age-0
prey fish was linked to decreased condition (VanDeValk et al., 2008). In contrast, walleye Wr increased in Lake McConaughy, Nebraska, due to the consumption of larger prey fish (160–200 mm long) and a lower abundance of age-0 fish (Porath & Peters, 1997). In our study, the forage fish consumed were relatively smaller and perhaps more energetically costly to capture (22–183 mm TL, data not shown) than those promoting walleye well-being in Lake McConaughy (Graeb et al., 2005).

For northern pike, the negative regressions between Wr and total length observed in Upper Lake Mary, Arizona (Flinder & Bonar, 2008), contrast with positive relationships reported in Long and Parker Canyon Lakes, Arizona, where catchable-sized rainbow trout (>200 mm TL) were available. Our results mirror the challenges faced by adult pike in boreal lakes of northeast Alberta, experiencing slow growth possibly due to a shift from piscivory to invertivory following winterkills. Adult pike (> 45 cm) that relied on leeches experienced retarded growth, whereas juveniles (<30 cm) preying on invertebrates (amphipods, zygopterans, glossiphoniid leeches) demonstrated exceptional growth (Venturelli & Tonn, 2006). The reliance on less-preferred invertebrates may contribute to the declining body condition of pike, compounded by the lack of thermal refugia in the study lakes during summer (Diana, 1987).

Our findings affirm that larger piscivorous fishes (i.e., pike and walleye) optimize net energy by targeting larger prey as they age. In contrast, perch does not require larger vertebrate prey to maintain a high Wr; rather, the availability/abundance of macroinvertebrates (Lott et al., 1996) influences their body condition. Beyond reflecting changes in prey-fish availability, size-specific patterns in Wr are crucial for inferring the prey-fish population characteristics and benefits to large and small-sized predators.
## 4.4 Environmental controls of fish condition

Widespread evidence highlights the impact of environmental factors on fish body condition (Katzenmeyer, 2010; Rueda et al., 2015). We observed distinct species- and size-specific responses of gamefish body conditions to environmental variables.

Specifically, chl a positively correlated with the body condition of adult walleye and perch (Figures 4A and 4B), aligning with a North American study (Rudstam et al., 1996). However, the declining condition of walleye, despite high productivity in our study lakes, may be attributed to elevated alkalinity levels. Optimal alkalinity levels between 75 and 200 mg/L is crucial for healthy fish populations. Outside this range, primary productivity declines, which negatively impacts aquatic ecosystems (Wurts, 2002). Our investigation revealed significant differences in alkalinity levels among sampled lakes: walleye lakes exhibited higher alkalinity (110–485 mg/L; mean = 239) compared to perch lakes (110–368 mg/L; mean = 208) (Table S1). Further analysis of 65 prairie lakes confirmed a substantial negative correlation between alkalinity and chl a (data not shown). This suggests that elevated alkalinity may hinder primary productivity in walleye lakes, and in turn affect the body condition of adult walleye.

In contrast, lower alkalinity in perch lakes may enhance productivity, promoting better body conditions in adult perch. However, the relationship between alkalinity and adult gamefishes condition —positive for walleye (Figure 4A) and negative for perch (Figure 4B)— is complex and warrants further investigation. Future studies could benefit from examining how alkalinity impacts gamefish feeding behaviors and metabolic responses.

Contrary to what was hypothesized, no discernible relationship existed between the five environmental parameters (chl a, alkalinity, maximum lake depth, surface area, and total phosphorus) and body condition of younger walleye and perch (Figure S6-S8). This
might suggest that the well-being of these cohorts may be more sensitive to other
variables, thermal stress, parasite loads, or predation (Sammons et al., 1994; Flinders &
Bonar, 2008; Nanayakkara et al., 2021). Turbidity affects visual predators like pike,
which has been linked to their poor growth due to impediments in feeding (Sammons et
al., 1994; Flinders & Bonar, 2008), though, it was not the case for gamefishes in this
study, data not shown).

Our findings underscore the sensitivity of different life stages of fish species to the
biochemical properties of lakes, which interact synergistically to impact gamefish
health. We anticipate that any future reduction in phytoplankton biomass associated
with ZQM will detrimentally affect adult gamefishes, especially walleye, due to the
strong influence of chl-a on walleye adult body condition (Figure 4A). Excessively high
alkalinity poses additional risks, such as respiratory difficulties and the dissolution of
the protective mucus layer, rendering gamefishes vulnerable to bacterial attacks and
infections (Quarters, 2017). These factors can increase gamefish vulnerability to aquatic
stressors, including invasive species.

## 5.1 Potential ecological impacts of ZQM

ZQM can exert strong direct or indirect effects on fish populations. However, the
magnitude of these impacts varies widely among ecosystems and fish species,
complicating efforts to draw simple generalizations (Strayer et al., 2004; Hansen et al.,
2020). Overall impacts of ZQM on fish may depend on several factors, including fish
foraging habits, morphology, and the ecological status of invaded water bodies
(Karatayev et al., 2002; Strayer et al., 2004; Geisler, 2015).
565 ZQM are famous for disrupting planktonic food webs while benefiting littoral food webs.

566 For example, Strayer et al. (2004) reported a 28% decline in planktivorous fish

567 populations and a 97% increase in benthivorous fish populations following ZQM

568 invasion. Consequently, enhanced water clarity exposes planktivorous fish larvae to
569 higher predation, and light-sensitive walleye could face suboptimal feeding conditions
570 and reduced reproductive success in much clearer waters (Ryder, 1977).

571 Nevertheless, walleye have responded differently to water clarity changes post-invasion

572 in other systems. In Manitoba prairie lakes, walleye yields increased by 2.63 kg/ha/yr,

573 while in Boreal shield lakes they declined by 2.36 kg/ha/yr. Lakes with lower pre-

574 invasion Secchi depths (1.24 ± 0.57 m) showed yield increases after invasions. In

575 comparison, lakes with higher pre-invasion Secchi depths (1.88 ± 0.89 m) experienced

576 declines, especially when post-invasion clarity exceeded 2 meters (Geisler, 2015). About

577 20% of our study lakes have high pre-invasion Secchi depths of (3.53 ± 1.38), making

578 them susceptible to walleye declines after ZQM invasion. Effective management of these
579 lakes will be vital for walleye conservation.

580 The rise in benthivorous fish populations following ZQM invasion has been linked to

581 increased biomass of native benthic invertebrates in the littoral zone (Strayer et al.,

582 2004; Karatayev et al., 2007; Kelley et al., 2010). By acting as ecosystem engineers,

583 ZQM alter the benthic community by depositing organic matter in the benthos and

584 forming extensive mussel shell beds (druses). These druses provide shelter and food (via

585 feces and pseudofeces) for native epifaunal invertebrates such as amphipods

586 (*Gammarus*) and chironomids (Karatayev et al., 2007; Burlakova et al., 2012). While
587 gamefishes may benefit from the increased availability of invertebrate prey, the
structural complexity of ZQM beds can hinder efficient foraging, potentially limiting access to these resources.

Potential anoxic conditions caused by ZQM feces and pseudofeces will also threaten invertebrate populations, such as chironomids, which support the nutrition of perch.

That may seriously impact pike and walleye, which depend largely on perch as a food source in the NGP. This situation echoes cautionary tales from the Great Lakes, where ZQM significantly reduced *Diporeia spp.*, a crucial prey for lake whitefish. The reduction in *Diporeia* led to declines in whitefish condition, growth, abundance and a shift in whitefish diet from *Diporeia spp.* to quagga mussels. Alewife, sculpin, and bloater populations also decreased, affecting larger piscivores, including salmon and trout (Nalepa, 2010).

In boreal ecosystems (e.g., eastern Lake Ontario), lake whitefish and walleye populations have shown drastic, negative responses to ZQM invasion, establishing new equilibriums driven by lower growth rates for lake whitefish and decreased early life history survival for walleye (Hoyle et al., 2008). Similar scenarios could unfold in prairie lakes, where filter-feeding activities caused by ZQM may impact both micro and macroinvertebrate populations, worsen the scarcity of forage fish, and affect gamefishes, leading to broader ecosystem implications. Considering the potential trophic bottleneck in hardwater prairie lakes, the growth, reproduction, and recruitment of fish populations post-ZQM invasion will likely be negatively impacted.

In light of these findings, fishery managers, biologists, and policymakers must prioritize the protection of shallow littoral feeding habitats where key invertebrates thrive while assessing the prevailing state of forage-fish populations. Ensuring the continued existence of favorable feeding conditions will help maintain optimal habitats for adult
fish populations (life stage of interest) whose body conditions particularly decline with age. By doing so, we can better safeguard the ecological health of these gamefish populations and ensure the sustainability of these valuable yet vulnerable aquatic ecosystems.

## 6.0 CONCLUSION

The findings of this study support previous research on prairie gamefishes and deepen our knowledge about species-specific diets and size-related body condition trends on a spatial scale. Assessment of three gamefish species unveil unique strategies for adapting to changing environmental conditions. Particularly, walleye exhibits notable ontogenetic dietary shifts and a unique ability to adjust their diets in response to changing prey availability compared to native pike and perch. Starting with macroinvertebrates as young adults, they progressively incorporate more fish as they mature. Despite this shift, walleye persistently rely heavily on macroinvertebrates. This may be a response to the limited availability of preferred forage fish in hardwater prairie lakes. Adult pike and walleye populations that depend heavily on macroinvertebrates experience declining body conditions and perhaps an increased vulnerability to parasites. Additional strains on body conditions such as high alkalinity, and eutrophication in these lakes will further jeopardize fish health and increase the risk of winterkills. These challenges will likely intensify with the dual threats of climate change and the invasion of ZQM and may directly or indirectly affect fish populations by impeding dominant processes like nutrient cycling and the depletion of deepwater dissolved oxygen in uninvaded systems. Changes to the chemical, biophysical, and deepwater habitat quality can constrain or
635 distort the habitability of these environments, making survival increasingly challenging
636 for fish.

637 Addressing these challenges requires urgent and coordinated action. By proactively
638 mitigating ZQM impacts, we can safeguard the vital role these gamefishes play in our
639 aquatic environment and local economies. For instance, implementing strategies to
640 enhance habitat for forage fish and closely monitoring water quality (temperature,
641 oxygen, alkalinity) can help sustain these ecosystems for future generations.

642

## AUTHOR STATEMENTS

## ACKNOWLEDGEMENTS

645 This research was conducted on Treaty 4 and Treaty 6 lands, which consist of the
646 traditional territories of the Cree, Dene, Blackfoot, Saulteaux, Nakoda, Dakota, and
647 Lakota peoples and the homeland of the Métis Nation. We are grateful to Gord
648 Sedgewick, James Villeneuve, Jennifer Merkowsky, and Rebecca Eberts from the
649 Saskatchewan Ministry of Environment for their partnership and collaboration in this
650 project. Field support was provided by Buddhine Meegahage, Afruza Begum, Xingzi
651 Zhou, and Alison Walker.

652

## COMPETING INTERESTS STATEMENT

654 The authors declare there are no competing interests.

655

## AUTHOR CONTRIBUTION STATEMENT

657 BW and CBO co-designed the study, developed the conceptual framework, and secured
658 funding for the project. EH and CBO conducted field sampling, coordinated logistics,
and managed all technical details. ZQR assisted with water chemistry analysis and numerical computations.

CBO performed fish stomach content analyses, related laboratory work, numerical calculations, and data analyses. BW, MUM, EH, and CBO contributed to the interpretation of the results. MUM and BW supervised the project and validated the findings. CBO drafted the original manuscript. All authors reviewed, edited, and discussed the final manuscript, providing constructive feedback that shaped its content.

**FUNDING STATEMENT**

Funding was provided through NSERC Discovery Grant # RGPIN-06475-2018 to Björn Wissel and the SK Fish and Wildlife Development Fund (no grant number) to Caroline B. Ofosu and Björn Wissel.

**DATA AVAILABILITY STATEMENT**

Data generated and/or analyzed during this study are available in the DYRAD repository, [confidential Reviewer URL, http://datadryad.org/stash/share/AqWBdUoeyGmQhOHZinmXCb889RSOL5LhgO7PQqkTrNs].

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869 Sammons, S.M., Scalet, C.G. and Neumann, R.M., 1994. Seasonal and size-related changes in the diet of northern pike from a shallow prairie lake. *Journal of Freshwater Ecology, 9*(4), pp.321-329.

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950 Fisheries-Building Mass Balance Trophic and Simulation Models.

951
<table><caption>**Table 1:** Number (N), total length (TL) and weight (W) of walleye, northern pike and yellow perch sampled from May – August 2019, from 15 Prairie lakes for the calculation of gamefishes’ body condition, Wr. Perch exhibited the highest body condition while walleye and pike fell below target range of healthy populations, Wr < 95-105.</caption><thead><tr><th rowspan="2">Fish species</th><th colspan="2">Walleye</th><th colspan="2">Northern pike</th><th colspan="2">Yellow perch</th></tr><tr><th>TL (cm)</th><th>W (g)</th><th>TL (cm)</th><th>W (g)</th><th>TL (cm)</th><th>W (g)</th></tr></thead><tbody><tr><td>Metrics</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mean</td><td>41.3</td><td>790.9</td><td>60.3</td><td>1433.6</td><td>21.1</td><td>161.9</td></tr><tr><td>Range</td><td>16.4-75.2</td><td>40.0-3760.0</td><td>30.9-86.2</td><td>180.0-3910.0</td><td>14.1-32.5</td><td>28.0-666.0</td></tr><tr><td>Average Wr</td><td colspan="2">92</td><td colspan="2">87</td><td colspan="2">105</td></tr><tr><td>N</td><td colspan="2">317</td><td colspan="2">41</td><td colspan="2">156</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>**Figure 1:** A) Map of Canada highlighting the proximity of Saskatchewan to zebra and quagga mussels (ZQM) infested provinces particularly, Manitoba to the east. B) Location of the 18 study lakes in south-central Saskatchewan. Lakes are coded by ecoregions in stars, Red = moist mixed grassland, White = mixed grassland. Green = boreal transitional zone, Blue = Aspen Parkland. Black circles indicate the major urban cities.</figcaption></figure>
<figure><img src="image_3.png" /></figure>

**Figure 2:** Diet composition of three gamefish species (May-August 2019). Significant diet shifts were observed in walleye (p = 0.028, Figure S3) but not in pike or perch (Figures S4–S5). The colored legend depicts prey items.
<figure><img src="image_4.png" /></figure>

A

<figure><img src="image_5.png" /></figure>

B
<figure><img src="image_6.png" /></figure>

**Figure 3:** Linear mixed-effects model (Tables S7-9) of predicted fit of the relationship between A) walleye, B) pike and C) perch body condition (Wr) and total length (size). Each point represents the Wr of individuals and the corresponding length measurements in each lake. The light grey area around the line depicts standard error, and the colored dots represent lakes. There was a significant negative relationship between the size and condition of walleye and pike but not perch.
<figure><img src="image_7.png" /></figure>

**Figure4:** Relative contribution of selected environmental variables to A) large walleye, R² = 82.5% and B) large perch R² = 97.4% body condition. Vertical lines represent 90% bootstrap confidence intervals. Chl *a* and Alkalinity positively correlated (p < 0.05) with walleye condition. However, alkalinity was negatively correlated with condition of large perch with Chl *a* positively correlated (p < 0.05). Metrics were not normalized.
1 **Game Over for Gamefishes** *after Zebra and Quagga Mussel Invasion?: Not*

2 **in Prairie Lakes.**?

3

4 **Authors:** Caroline B. Ofosu¹,²,³*, Anas M. Usoof¹,⁴,⁵, Erin Hillis¹, Zoraida J. Quiñones-

5 Rivera¹,⁶ and Björn Wissel¹,⁶

6

7 ¹University of Regina | U of R · Department of Biology, Regina, Regina, SK, Canada.

8 ²Memorial University of Newfoundland | MUN · Department of Earth and Ocean

9 Sciences, NL, Canada.

10 ³Memorial University of Newfoundland | MUN · Department of Ocean Sciences, NL,

11 Canada.

12 ⁴University of Toronto Scarborough | U of T · Department of Biological Sciences, ON,

13 Canada

14 ⁵University of Winnipeg | U of W · Department of Biology, MB, Canada.

15 ⁶Université Claude Bernard Lyon 1 | UCBL · Laboratoire d'Ecologie des Hydrosystèmes

16 Naturels et Anthropisés (LEHNA), Villeurbanne, France.

17

18 Correspondence: [cbofosu@mun.ca](mailto:cbofosu@mun.ca)

19

20

21

22

23

24
25 **Abstract**

26 Mitigating threats of aquatic invasive species in lakes requires holistic insights into the
27 biological, physical, and chemical conditions sustaining fish communities. This study
28 establishes important *some of the ecological trophic* baseline conditions for three key
29 gamefish species—walleye, northern pike, and yellow perch—across 15 hardwater prairie
30 lakes in south-central Saskatchewan, Canada, that are threatened by zebra mussel
31 invasion. We employed stomach content analysis to evaluate diet and ontogenetic
32 nutritional shifts in adult fishes and assessed how fish size and environmental factors
33 influence body condition. Adult walleye transition from a diet of amphipods to fish as
34 they grow, whereas perch and pike maintain a stable diet of *ehironomids invertebrates*
35 and fish, respectively. While walleye and pike decline in body condition with size, perch
36 maintain consistently high condition. Lake productivity and alkalinity are key predictors
37 of adult walleye and perch condition. *Yet, zebra mussel invasion* in hardwater lakes *may*
38 *not have the detrimental impacts observed elsewhere, assuming that management*
39 *strategies will address the combined threats of invasion and climate change. These*
40 findings *provide a foundation for assessing* shifts in foraging ecology *in yet-to-be-*
41 invaded prairie lakes. *Given the combined threats of zebra mussel invasions, climate*
42 *change, and potential local forage fish scarcity, novel management strategies are*
43 *essential.*

44

45

46

47 **Keywords:** Zebra and Quagga mussels, Invasion, Hardwater lakes, Gamefishes,
48 Walleye.
49 **1.0 INTRODUCTION**

50 Despite the growing interest in invasion ecology (Simberloff and Gibbons 2004), there is
51 limited understanding on how pre-existing regional stressors and Aquatic Invasive
52 Species (AIS) interact to impact the survival of *native resident* fish populations
53 (Arrington et al. 2006).

54 Zebra and quagga mussels (*Dreissena polymorpha*, [Pallas, 1771], and *D. bugensis*,
55 [Andrusov, 1897], respectively; hereafter referred to as ZQM) are two aquatic invaders
56 that have significantly modified food webs across continents (Bukontaite and Zaiko
57 2008). These bivalves now pose threats to aquatic ecosystems in the prairie region of
58 *Western western* Canada, with its many hardwater lakes *and characterized by* a unique
59 *combination of* hydrology *and other limnological parameters* (e.g., shallow depth, high
60 alkalinity, absence of thermal stratification, varying salinity, and suitable calcium
61 concentrations (→ 25 mg/L, see Table S1), (Therriault et al. 2013). These
62 physicochemical attributes are expected to create conditions that support successful
63 ZQM invasion and, in the event of their introduction, increase the susceptibility of fish
64 populations in this region *to ZQM invasionimpacts* (Starks et al. 2014; Nanayakkara et
65 al. 2021).

66 Once introduced, ZQM exhibit robust tolerance to prevailing conditions, especially
67 temperature and salinity, increasing their likelihood to thrive in lakes across south-
68 central Saskatchewan (SK; Therriault and Orlova 2010), which represents the western
69 invasion front. The proximity of SK to invasion corridors, notably Manitoba (LWF,
70 2020) and three US states (Montana, North Dakota, and Minnesota), heightens the risk
71 of ZQM introduction into western prairie lakes. Ecological impacts of these invaders
reverberate through food-webs (Higgins and Vander Zanden 2010). As efficient filter-feeders, ZQM enhance water clarity while reducing pelagic primary productivity. This causes disruptions in phytoplankton and zooplankton populations, which form the basis of the food web upon which most native juvenile fishes depend (Pace et al. 1998; Higgins and Vander Zanden 2010). Such disturbances invariably affect the productivity of higher trophic-level gamefishes like walleye (*Sander vitreus* [Mitchill, 1818]), northern pike (*Esox lucius* [Linnaeus, 1758]; hereafter referred to as pike), and yellow perch (*Perca flavescens* [Mitchill, 1814]; hereafter referred to as perch), and may dramatically alter trophic relationships (Karatayev et al. 2012; Ozersky et al. 2012).

The impacts of ZQM invasion vary across regions, systems, and species and often through time (Hansen et al. 2020), making predictions and management efforts challenging. In north-temperate lakes, ZQM invasions increased water clarity, especially in stratified lakes, with an average increase in Secchi depth of 0.68 m, and a 0.31 m increase for mixed-polymictic lakes (Geisler 2015). This enhanced water clarity adversely affected walleye, a light-sensitive species (Scott and Crossman 1973; Ryder 1977), increasing predation pressure and competition, and ultimately reducing suitable habitat and yields (decline of 2.36 kg/ha/yr post-invasion in boreal shield lakes; Geisler 2015). In contrast, Manitoba prairie lakes exhibited a positive response, with predicted walleye yield increases of 2.63 kg/ha/yr (Geisler 2015).

In the Great Lakes region, dreissenids negatively impacted phytoplankton, zooplankton (Pace et al. 1998; Mavrin and Strel'nikova 2011), and benthic invertebrates (e.g., *Diporeia*) that are crucial to the diet of apex predators including whitefish (*Coregonus clupeaformis* [Mitchill, 1818]) (Hoyle et al. 2008). Following the collapse of *Diporeia*,
95 whitefish *were compelled to* incorporated larger quantities of lower-energy food items,
96 particularly dreissenids (Pothoven et al. 2001; Lumb et al. 2007), leading to reduced
97 body condition, growth rates, and reproductive success (Hoyle et al. 1999, 2008).

98 Similar nutritional consequences were observed in alewife (*Alosa pseudoharengus* 99 [Wilson, 1811]), where energy density declined due to reduced *Diporeia* availability in
100 their diet, from 29% pre-invasion to 12% post-invasion (Madenjian et al. 2006).

101 Interestingly, adult walleye and smallmouth bass (*Micropterus dolomieu* [Lacepède, 1802]) in southern Ontario boreal lakes successfully navigated resource limitations
102 imposed by ZQM, switching to advantageous prey like crayfish and fish (Nienhuis et al.
103 2014).
104

105 Recent assessments revealed that gamefish populations in SK hardwater lakes are
106 already stressed, particularly walleye, which recorded high parasite loads and poor body
107 condition (Nanayakkara et al. 2021). Hence, comprehending how current ecological
108 conditions support (or limit) fish communities is vital to develop management efforts
109 that aim to minimize the impacts of future ZQM invasion. Accordingly, we conducted
110 *somedetailed a* baseline assessments of the ecological status (diet and body conditions)
111 of the three recreational and commercially targeted gamefish species—walleye, pike, and
112 perch—across south-central SK lakes. Walleye, the focal species, is primarily sustained
113 by provincial stocking programs (*stocked at ages 3-5 days, Gord Sedgewick, pers. comm.*) to replenish populations stressed by angling pressure and low recruitment
114 (Saskatchewan Ministry of Environment 2017, 2019), unlike perch and pike, which
115 occur naturally in prairie lakes.
116
For gape-limited animals like fishes, growth is maintained by size-dependent shifts in food or habitat use (Werner and Gilliam 1984). Pike, walleye and perch, like other piscivorous gamefishes, typically undergo ontogenetic diet shifts, starting with zooplankton, progressing to macroinvertebrates, and ultimately to piscivory feeding on fishes. Piscivorous gamefishes typically set out foraging zooplankton, transitioning to macroinvertebrates and eventually may become piscivores (Liao et al. 2001; Uphoff et al. 2019). To ensure robust growth, condition, and survival, fish must have access to suitable-sized prey at all life stages. Using 15 fish-bearing lakes as model systems, we tested the hypotheses that:

In this study, we focus on diet shifts in adult gamefish that may be likely due to reflected prey preference or availability rather than anatomical mouth-gape limitations. Previous studies have largely focused on juveniles, showing that they are primarily affected by ZQM through suspension feeding, which reduces their planktonic food base (Nalepa 2010; Hansen et al. 2020; Hoyle et al. 2008). Here, we focus on adult size classes to help project their responses to changes in prey composition and availability following ZQM invasion, using baseline measures of diet and body condition in uninvaded prairie lakes. Using 15 fish-bearing lakes as model systems, we tested the hypotheses that:

134

135

1. Diet preferences will be species-specific, where "early switching piscivores," pike and walleye, will show marked feeding specialization. However, perch, "late switching piscivores", may demonstrate a more flexible feeding strategy.

136

137

138
139

2. Changes in diet composition for these gape-limited gamefish species will be evident as they grow.

140

141

3. Positive or negative trends in gamefish body conditions will be closely associated with their diet.

142

143

4. Physicochemical parameters (e.g., water temperature, dissolved oxygen, pH, and nutrient concentrations) will significantly influence the body condition of gamefishes.

144

145

146

4. In this study, diet shifts in adult gamefish may reflect prey preference or availability rather than anatomical mouth-gape limitations. Previous studies have largely focused on juveniles, showing that they are primarily affected by ZQM through suspension feeding, which reduces their planktonic food base (Hoyle et al., 2008; Nalepa, 2010; Hansen et al., 2020). Here, we focus on adult size classes to help project their responses to changes in prey composition and availability following ZQM invasion, using baseline measures of diet and body condition in uninvaded prairie lakes.

147

148

149

150

151

152

153

Future invasion by ZQM may have indirect negative or stabilizing effects on gamefish populations. The former is plausible if gamefishes heavily rely on low-trophic levels (i.e., macroinvertebrates). However, if ZQM improve feeding conditions to support populations of native invertebrates, their impacts may be muted.

154

155

156

157

158

For this study we focused on adult size classes only to focus on diet shifts that are associated with prey preference or availability, rather than those related to anatomical mouth-gape limitations.

159

160
161 **2.0 METHODS**

162 **2.1 STUDY AREA**

163 Initially, eighteen fish-bearing lakes in the Northern Great Plains (NGP) of Western
164 Canada were sampled to provide <u>someimportant a baselines</u> on the ecological status and
165 dynamics of gamefish populations across south-central SK (Figure 1). All study lakes
166 were within the prairie and boreal transitional ecozones. Of the 18 lakes, twelve are in
167 hydrologically closed basins (endorheic) that lack channelized surface inflow or outflow,
168 and are mainly sustained through winter precipitation. The remaining six lakes are
169 exorheic (open basins) draining into downstream systems (<u>Table S1</u>). Surface area of
170 study lakes ranged from 0.35 to 33.0 km², with maximum depth ranging from 4.4 to
171 38.0 m (Table S1). Nine of the study lakes are part of a 21-lake Long-Term-Ecological-
172 Research program conducted in the majority of drainage basins across southern SK
173 since 2002 (Wissel et al. 2011; Starks et al. 2014; Nanayakkara et al. 2021). The Ministry
174 of Environment, Government of Saskatchewan, uses the remaining nine lakes for
175 fisheries assessment in the Saskatoon and Prince Albert areas (<u>Saskatchewan Ministry
176 of Environment 2017, 2018, 2019</u>).

177 Five lakes lie within the southwestern semi-arid ecoregion, four within Canada's moist
178 mixed grassland interior plains, eight in the Aspen Parkland, and one within the boreal
179 transitional zone (Figure 1, Table S1). Fifteen lakes had high fish complexity (forage
180 fishes, benthivores, and gamefishes), (Table S1). Of these, 10 were predominantly
181 sustained by provincial stocking programs to replenish walleye populations stressed by
182 angling pressure and low recruitment (Saskatchewan Ministry of Environment 2017,
183 2019). The remaining three lakes were excluded from further analyses due to their low
184 fish complexity. These lakes contained only forage fishes, which are potential prey for 185 gamefishes in lakes with higher complexity, along with other fishes commonly found in 186 low-complexity lakes (Essington et al. 2015) (Table S1). Of the three low-complexity 187 lakes, From and Grenfell were suspected to have experienced recent summer or 188 winterkills (Saskatchewan Fish Facts 2019), while Shannon Lake was anoxic at 189 sampling. The selected 15 lakes are exposed to large seasonal and inter-annual 190 variations (Sauchyn and Kulshreshtha 2008), and their small size makes them 191 particularly susceptible to the impacts of climate change and the potential invasion by 192 ZQM. Hence, they are excellent models for monitoring the possible effects of 193 environmental stressors, representing early-warning systems.

194 **2.2 LAKE SAMPLING**

195 **2.2.1 Integrated water samples**

196 The methods described by Cooper and Wissel (2012a, 2012b) and Starks et al. (2014) 197 were adopted for collecting water samples from May to August of 2019. Samples for 198 water chemistry were collected at the location of maximum depth. Lake depths were 199 recorded using an electronic depth meter, model 71624. Using a YSI model 556, vertical 200 profiles of temperature (°C), specific conductivity (µS cm⁻¹), total dissolved solids (mg 201 TDS L⁻¹), salinity (ppt), dissolved oxygen (mg O₂ L⁻¹, % saturation) and pH were 202 recorded at 0.5 m intervals for shallow lakes (< 10 m) and 1 m intervals for deep lakes (> 203 10 m). Water transparency was measured with a 20 cm diameter black and white Secchi 204 disk.

205 **2.2.2 Gamefish samples**
206 Gamefishes were captured overnight using sets of multi-panel gill nets (24.7 m long x 1.5 m deep; mesh sizes; 38.1, 50.8, 63.5, 76.2, 88.9, 101.6, 114.3, 127 mm$1^{1/2}$, 2, 2$^{1/2}$, 3, 3$^{1/2}$, 4, 4$^{1/2}$, and 5 inches) placed perpendicular to shore, between May and August of 2019.

207
208

209 Multi-panel gill nets were used to obtain representative samples of the different sizes of
gamefish species. Fishes were retrieved after 16-20 hours, identified to species (Scott
and Crossman 1973) and their sexes determined. This was followed by in-situ total
length (cm) and weight (g) measurements. Fish samples were then placed immediately
on ice to reduce the decomposition of stomach content and stored at -15 °C (Zacharia
and Abdurahiman 2004) for a few weeks.
214

210
211

212
213

215 **2.3 STUDY SPECIES**

216 Walleye, perch, and pike were observed generally recurred across study lakes. Walleye
217 occurred in all 15 high-complexity lakes, perch in 9, and pike in 6 (Table S2). Walleye is
218 the focal species because it is the most preferred by anglers in study lakes (Nanayakkara
219 and Wissel 2017). However, unlike native perch and pike, walleye populations lack
220 genetic diversity (Starks et al. 2014), which is related to the stocking programs.
221

221 **2.4 ETHICS STATEMENT**

222 Animal sampling in this study was conducted under approved animal use protocols
223 from the Animal Care Committee of the University of Regina, and in accordance with
224 special collection research permits provided to B; Wissel by the Government of
225 Saskatchewan, and the Ministry of Environment (SCP2019/AR08/FWLB).
226

226 **2.4 LABORATORY ANALYSIS**

227 **2.4.1 Water chemistry**
228 Depth-integrated water samples were pre-screened using a 80-µm mesh before
229 filtration through 0.45-µm sterile membrane filters. A subset of the filtered water was
230 frozen and later analyzed for nitrate (NO3), nitrite (NO2), ammonium (NH4+), total
231 nitrogen (TN), soluble reactive phosphorous (SRP), and total phosphorous (TP) (µg L-1)
232 following standard methods described by Pham et al. (2008). The remaining filtrate was
233 refrigerated and analyzed for alkalinity and Calcium (Ca) (mg L-1) following Pham et al.
234 (2008). Dissolved inorganic and organic carbon (DIC/DOC) were quantified using
235 refrigerated filtrate on an OI Analytical Aurora 1030W TOC analyzer. Algal biomass
236 (measured as chlorophyll a; chl a) was quantified by filtering 1 L through GF/C filters,
237 stored at -15 °C until extraction with acetone: methanol: water (80:15:5).
238 Concentrations were quantified using a spectrophotometer (Hewlett-Packard model
239 84452A) (Wetzel and Likens 1991; Cooper and Wissel 2012a). All laboratory analyses
240 were conducted at the Institute of Environmental Change and Society at the University
241 of Regina, Saskatchewan, Canada.

242 **2.4.2 Stomach content analysis**

243 Fish stomachs were removed and stored in 70% ethanol (Jobling et al. 2001) until
244 analysis. Two-hundred-and-eighty-three preserved gamefish stomachs comprising 188
245 walleye, 65 perch, and 30 pike were examined under a dissecting microscope (SZX12
246 Olympus model) to ascertain important prey contributing to their diets. Prey items were
247 identified to the lowest practical taxonomic resolution, mostly species-level for fishes
248 (Saskatchewan Watershed Authority 2010) and family-level for macroinvertebrates
249 (Thorp and Rogers 2011) and quantified by counting individuals of each taxon and
250 determining their volumes using the displacement method (Pinkas et al. 1970; Hyslop
1980; Manko 2016). We used the percent Index of Relative Importance (%IRI), a composite diet index integrating three individual diet measures (%N, %F, and %V or %M), to provide a robust estimate of diet composition (Chipps and Garvey 2007; Gelwick and McIntyre 2017). Although %IRI may be more time-consuming and require larger sample sizes for robust estimates (Ahlbeck et al. 2012), it offers a more representative measure of diet and enables unambiguous interpretation (Cortés 1997; Manko 2016). It also overcomes some of the inherent biases of single-component metrics. For example, %N and %F can overemphasize small prey, while bulk measures (%M/%V) may underestimate rarely consumed prey (but see Ahlbeck et al. 2012; Baker et al. 2014).

For objective interpretation of results, the Index of Relative Importance (IRI) was used to express the importance of food items (Gelwick & Matthews, 2006; Chipps & Garvey, 2007). IRIi of specific food category is given as:

$$ IRIi = (\%N_i + \%V_i) \times \%F \quad (1) $$

Where %Ni = Numerical percentage of specific food category, %Vi = Volumetric percentage of specific food category and %Fi = Frequency of occurrence.

The single index for extrapolating IRIi was calculated as follows:

$$ \%N = N_i/N_t \times 100 \quad (2) $$

$$ \%V = V_i/V_t \times 100 \quad (3) $$

$$ \%F = F_i/F_t \times 100 \quad (4) $$
Where $N_i$ = the number of food category$_i$ and $N_t$ = the total number of individuals in all food categories, $V_i$ = the volume of food category$_i$ and $V_t$ = the total volume for all food categories in all individuals, and $F_i$ = number of fish containing food category$_i$ and $F_t$ = the number of fish with food in their stomachs.

IRI values were then expressed in percentages for possible comparison among food items based on Cortés (1997) and Desmond et al. (2002) summing total IRI values across all prey items:

$$ \%IRI_i = 100 \times \frac{IRI_i}{\sum_{i=1}^{n} IRI_i} \quad (5) $$

Where %IRI$_i$ is the percent index of relative importance of food item$_i$, IRI$_i$ is the index of relative importance of food items$_i$, and n is the total number of food categories calculated at a given taxonomic level.

To calculate %IRI, the three gamefish species were grouped the three gamefish species into two size groups categories based on their total lengths as follows: walleye (≤ 40cm and > 40cm for walleye), pike (≤ 60cm and > 60cm for pike), and perch (≤ 22.5cm and > 22.5cm for perch)., respectively We then compared %IRI between the size groups for each species, and their %IRI compared.

We focused on adult sizes to assess to the what extent to which the study game species realized a switch to piscivory when once their mouth gapes were here already sufficiently large, as .Existing literature largely focuses on the feeding ecology of early life stages (Yang 2022). For all three species, the size cut-offs represented age-classes of 4-5 years. AWhile all captured fish were sufficiently large for piscivory, though its likelihood is expected to increases with size-class. Prey were grouped by according to
dominant taxa: fish, amphipods, chironomids, other invertebrates, plant matter, and
unidentified components.

### 2.4.3 Fish condition

Relative weight (Wr) condition indices were computed for 514 gamefishes comprising 317 walleye, 41 pike, and 156 perch (Table 1) and reported as a whole number using equations described by Willis (1989), Willis et al. (1991), Murphy et al. (1991) and Blackwell et al. (2000).

Wr of gamefish populations was estimated using the following equation:

$$Wr = (W/Ws) \times 100 \quad (6)$$

Where Wr = relative weight, W = actual weight, and Ws = standard weight for fish of the same length. Ws values were calculated for each fish exceeding the required minimum total length (walleye > 150 mm, pike > 100 mm, perch > 100 mm). Parameters for Ws were derived from the regression equations of Blackwell et al. (2000). Values of Wr = 95–105 indicate good condition, near 80 suggest poor health due to unfavorable environmental conditions, and extremely high scores (> 115) indicate excess energy reserves (Murphy et al. 1991; Blackwell et al. 2000).

## 2.5 STATISTICAL ANALYSIS

### 2.5.1 Relative importance of prey items to gamefishes

We used MANOVA (Multivariate analysis of variance (MANOVA) based on redundancy analysis (RDA) to test significant differences in diet composition between small and
large groups of three gamefish species. RDA models for gamefish diet composition were fitted using the 'rda' function (vegan package, Oksanen et al. 2013) with 'size group' as the main factor and 'lake' as a covariate. Diet data were square root transformed for normality. Before analyses, the PERMDISP2 test (Anderson, 2006) was used to evaluate multivariate homogeneity of size groups using betadisper (vegan package).

Prior to analyses, the PERMDISP2 test (Anderson 2006) was used to assess the assumption of homogeneous multivariate dispersion between size groups via the betadisper function (vegan package, Oksanen et al. 2013). This step was necessary because multivariate analyses, such as PERMANOVA, assume similar group dispersions (variances). Differences in dispersion can confound PERMANOVA results, making it unclear whether significant effects arise from differences in group centroids (true differences in composition) or differences in within-group variability. Therefore, we validated this assumption ensuring that subsequent tests of diet composition differences reflect genuine ecological patterns rather than artifacts of unequal variability.

RDA based on MANOVA provides a robust alternative to traditional MANOVA, with less stringent assumptions. Unlike classical MANOVA, RDA does not require multivariate normality and is well-suited for our non-normally distributed diet data. Besides it supports s versatility of permutation-based testings and offers, it includes a triplot representation of results (Borcard et al. 2011). To complement this analysis, we used Wilcoxon rank-sum tests to assess whether the contributions of individual diet items differed between fish size groups.

### 2.5.2 Effects of total length *body size* on fish condition
Linear mixed-effects (lme) models, fitted by restricted maximum likelihood, were used to assess the impact of **body size** (total length) on gamefish body condition while controlling for lake (site) effects. lme analysis was performed using the *lmer* function from the R package *lme4* (Bates et al. 2015) with total length set as a fixed effect and site (lake) as a random effect. Three models per gamefish species were evaluated (Table S3).

To facilitate model convergence, we used the 'bobyqa' optimizer (optimix R version 2020-4.2). We assumed a Gaussian distribution, checked model residuals for normality, and conducted standard diagnostic tests to confirm regression assumptions.

Model selection was established by the most parsimonious model, informed by the Akaike information criterion corrected for small sample sizes (AICc). The significance of the model coefficient (total length) was tested with t-tests using Satterthwaite approximations (Luke 2017) via the *anova* function ([lmerTest R package, Kuznetsova et al. 2017]). Conditional and marginal R-squared estimates (Nakagawa and Schielzeth 2013) were calculated with the r-squared GLMM function via R-version 1.43.17; MuMln package (Barton 2009).

### 2.5.3 Environmental controls of fish condition

Considering the practical difficulty in including all 18 environmental variables in the analyses, we first performed principal component analysis (PCA) using standardized (scaled to unit variance) environmental variables to evaluate the correlations among them. Surface area, maximum depth, elevation, and temperature were included as supplementary variables in the PCA.

Based on literature (Katzenmeyer 2010; Rueda et al. 2015) and results of the PCA (correlation among variables, Figure S1), five main variables (surface area, maximum
depth, alkalinity, chlorophyll a, and total phosphorus) known to influence fish body
condition were selected as explanatory variables for the regression models. To quantify
the relative importance of selected regressors, the “lmg” metric of the R package
“relaimpo” was used (Grömping and Matthias 2013). The “lmg” metric decomposes R²
into non-negative contributions attributable to each regressor based on averaging over
all orderings of regressors in a multiple regression setting (Grömping 2006).
Bootstrapping (1000 runs) of models was done using functions boot and boot.ci from R
package boot (Davison and Hinkley 1997). Due to the small sample size for pike, both
small and large groups were combined for analysis. All analyses were performed using R
version 4.0.3. (R Core Team 2020).

3 RESULTS

3.1 Diet composition and ontogenetic dietary shifts

Across 15 lakes and 283 individual fish stomachs, we identified 23 prey taxa comprising: two *aAmphipoda genera species* (*Gammarus lacustris* [Sars, 1863] and *Hyallela azteca* [Saussure, 1858]); seven fish species; *bBrook stickleback* (*Culaea inconstans* [Kirtland, 1840]), *nNinespine stickleback* (*Pungitius pungitius* [(Linnaeus, 1758)], *fFathead minnow* (*Pimephales promelas* [(Rafinesque, 1820)]), *jJohnny darter* (*Etheostoma nigrum* [Rafinesque, 1820]), walleye fry, perch fry, and pike fry; 13 taxa of benthic macroinvertebrates, (Chironomidae, Corixidae, Physidae, Planorbidae, Asilidae, Lestidae, Cambaridae, Diptera, Odonata, Hirudinea, Oligochaeta, Nematoda and Notonecta); and one predatory zooplankton species, (*Chaoborus punctipennis* [Say, 1823]). Plant matter, stones and unidentified items were also found.
Approximately 40% of the sampled fishes had empty stomachs. Of the 82 small- and 106 large-sized walleye, 26 (32%) and 31 (29%) stomachs were empty respectively (Table S4). Similarly, 5 (33%) of the 15 small-sized pike had empty stomachs, but the proportion of empty stomachs increased to 53% (8 of 15) in large pike (Table S5). Small perch exhibited a 48% incidence of empty stomachs (14 of 29), relative to 28% (10 of 36) for large perch (Tables S6).

Prey importance varied between size groups of walleye, and to a lesser extent for pike and perch. Smaller walleye (≤ 40 cm) relied heavily on amphipods (41.91 ± 10.54 %IRI), whereas larger walleye (> 40 cm) consumed more fish (38.07 ± 11.20 %IRI; Table S4). Vertebrate prey dominated whereas the most important diets of both small (≤ 60 cm) and large (> 60 cm) pike (41.50 ± 20.82 %IRI and 51.26 ± 21.32 %IRI, respectively), although a considerable portion of their diet was unidentifiable (Table S5). Small pPerch primarily consumed invertebrates, particularly chironomids for small individuals (61.21 %IRI), while larger perch consumed appreciable proportions of both amphipods (27.81 ± 17.49 %IRI) and chironomids (25.28 ± 16.51 %IRI; Table S6).

MANOVA revealed significant differences between the diets of small and large walleye (*p* = 0.028, Figure S3, Table S4), but no differences in diet composition of pike (*p* = 1.00; Figure S4, Table S5) or perch (*p* = 1.00; Figure S5, Table S6). Walleye exhibited an ontogenetic dietary shift, transitioning from amphipod-based diets in smaller individuals (4-5 yrs.) to more fish-based diets in larger individuals (> 5 yrs.) (Table S4, Figure S3). By contrast, pike and perch showed less pronounced shifts in diet.
The proportions of individual dietary items did not differ substantially among gamefish groups (Wilcoxon rank-sum test: *p* > 0.05), except in perch, where amphipod contributions varied significantly (*W* = 9, *p* = 0.044). Amphipods accounted for 27.81 ± 17.49 %IRI in the diet of large perch, but were entirely absent from that of small perch (Table S6, Figure 2). The importance of prey items varied among size groups of walleye, with the small size group (≤ 40 cm) demonstrating greater reliance on amphipods (41.91±10.54% IRI), while larger walleye (> 40 cm) targeted more fish (38.07±11.20% IRI), (Table S4). In contrast, pike and perch showed less size-dependent shifts in diet. Vertebrate (fish) prey was the most common diet for both small (≤60 cm) and large (>60 cm) pike (41.50±20.82% IRI and 51.26±21.32% IRI) respectively, while a considerable portion of stomach content was unidentifiable (Table S5). Small perch heavily relied on invertebrates, particularly chironomids (61.21%), whereas amphipods (27.81±17.49% IRI) and chironomids (25.28±16.51% IRI) formed appreciable proportions of the diet of larger perch (Table S6). In contrast, pike and perch showed less size-dependent shifts in diet.

MANOVA revealed a significant difference between the diet of small and large walleye (*p* = 0.028; Figure S3). In contrast, no size-related distinctions were observed in the diet of pike (*p* = 1.00; Figure S4) and perch (*p* = 1.00; Figure S5). Walleye diet shifted from feeding on amphipods by the smaller size group (41.91±10.54% IRI) to a more fish-based diet in the larger size group (38.07±11.20% IRI) (Table S4, Figure S3). Despite no statistically significant ontogenetic shift in diet of perch and pike (Tables S5 and S6, Figure S4 and S5), visual observation of the dietary plots revealed clear patterns in diet variability between small and large size groups (Figure 2). Specifically, there was a
427

decrease in reliance on chironomids from small to large perch and an increase in
foraging on fish from small to large pike (Figure 2). Yet, independent of gamefish
species and size class, piscivory never contributed more than half of the diet.

428

429

430

431

432

**3.2 Effects of **total length body size**-on fish condition**

433

We conducted condition assessments for 514 individual fish, (317 walleyes, 41 pike, 156 perch), with sizes from 14 to 86 cm in total length and weights from 28 to 3760 g (Table 1). The average body condition (Wr) for walleye, pike, and perch populations were 92, 87 and 105, respectively (Table 1).

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435

436

437

After accounting for the variation in lakes (sites), *lme* models showed a significant negative correlation between total length (**body size**) and the condition of walleye (*p* = 0.04; Figures 3A, Table S7), and pike across lakes (*p* = 0.02; Figure 3B, TableS8). This translated to a decline of approximately 0.24% Wr ± 0.10 SE for walleye and 0.31% Wr ± 0.13 SE for pike (Figures 3A and 3B; Tables S7 and S8). In contrast, no significant link existed between **total length body size** and condition of perch (*p* = 0.26; Figure 3C and Table S9). The relatively large unexplained variance in the perch model (>80%), suggests sources of variability beyond the model's capacity to predict.

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443

444

445

Respective estimated Marginal R² and Conditional R² values were 0.05 and 0.55 for walleye (Table S7; model 3), 0.11 and 0.43 for pike (Table S8; model 4), and 0.01 and 0.19 for perch (Table S9; model 7).

446

447

448
449 **3.3 Relative importance of environmental regressors to fish body-condition**

450 We concentrated on five key environmental variables—surface area, maximum depth,  
451 alkalinity, chl *a* (measure of phytoplankton biomass), and total phosphorus—as controls  
452 of gamefish body condition (Rudstam et al. 1996; Katzenmeyer 2010; Rueda et al. 2015).

453 Assessment of relative importance of the five regressors indicated that phytoplankton
454 biomass and alkalinity were two major factors impacting the conditions of large walleye
455 and perch. These parameters accounted for 50% and 16% of the variation in body
456 condition for large walleye (Figure 4A; Adj R² = 0.68). Similarly, they explained 37%
457 and 51% of the variability, respectively in large perch (Figure 4B; Adj R² = 0.91).

458 However, for their smaller counterparts no strong associations existed between body
459 condition and environmental parameters (Figures S5 and S6). In addition, in large
460 walleye and perch we observed a positive relationship between body condition and chl *a*
461 (Figures 2A and 2B). However, alkalinity had nuanced effects; w. W while it positively
462 correlated with large walleye condition (Figure 4A), it negatively impacted large perch
463 (Figure 4B). In contrast, the model for pike explained only 7% of the variability in their
464 condition (Figure S8).

465
466
467 **4.0 DISCUSSION**

467 **4.1 Dietary importance**

468 Understanding the dietary needs of co-existing gamefish species —walleye, pike, and
469 perch— in SK prairie lakes is crucial for predicting their responses to imminent aquatic
470 stressors such as the invasion of ZQM and the growing threat of climate change. In line
with our hypothesis, this study recorded remarkable variations in diet composition and foraging strategies among species. Gamefishes relied on a broad range of prey taxa (Figure S2, Tables S4-6), suggesting their capacity to thrive amidst changing prey resources. While both invertebrates and fish constituted substantial components of walleye diets, pike slightly favored fish prey, while perch leaned towards an invertebrate-based diet, especially chironomids and amphipods. Surprisingly, even as walleye grew and transitioned towards more piscivorous diets, invertebrates remained a large part of their diet (Figure S4, Table S4). This finding challenges the classic notion that “early switching piscivores” or “specialists” rapidly shift to energetically beneficial vertebrate prey with increasing size/gape width (Graeb et al. 2005).

In this study, the reliance on invertivory levels observed in walleye and pike were higher than their typical trophic-specialist feeding patterns in other prairie lakes (Sammons et al. 1994). This suggests an adaptive shift toward generalist feeding, or a response to limited forage fish in these lakes. For example, the prevalence of *Gammarus* (abundant taxon in study lakes, data not shown) in adult walleye diets may reflect opportunistic feeding when preferred forage fishes are scarce (Kaufman et al. 2009). This warrants further investigation into the prevailing status of forage fishes in prairie lakes.

As expected, small and large perch showed a clear preference for invertebrates. While piscivory can occur in perch, it is less prominent as perch favors invertebrates over fish when both are available (Liao et al. 2001; Graeb et al. 2005). In an experimental study where perch were offered more fish prey than invertebrates, perch displayed a neutral selection for fish across all size groups. In contrast, walleye maintained a positive
494 preference for fish even at lower densities of fish relative to alternate prey (Graeb et al. 2005). This suggests that morphology, feeding behavior, and life history contribute to the differences in dietary ontogenies in these two percid predators (Graeb et al. 2005).
495
496

Additionally, diel movements of forage fishes (moving inshore during the day and offshore at night) might impact their accessibility to adult walleye that forages in shallow zones at night. Observed cannibalism in large walleye (42.6 – 50 cm) supports the hypothesis of limited availability of forage fishes, like scenarios reported in Lake Oneida, New York, where low prey densities resulted in cannibalism (Forney 1974). To that end, Forney (1976) proposed augmenting the densities of young perch to mitigate the scarcity of forage fish and sustain walleye populations.
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498
499
500
501
502
503

Overall, this study demonstrates dietary plasticity of gamefish communities in prairie lakes, and their ability to adapt to shifting prey resources by supplementing their diets with invertebrates when preferred fish prey is scarce. However, it also warns of trophic bottlenecks and stunted growth (Venturelli and Tonn 2006; VanDeValk et al. 2008).
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505
506
507
508
509
510
The absence of forage fishes may impact higher trophic levels and increase vulnerability to invasive species. At the same time, cannibalism among walleye may further constrain population size.
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512

## 4.2 Ontogenetic dietary shift

Zooplanktivory is a precursor to invertivory and benthivory during the early life stages of pike, walleye, and perch (Frost, 1954; Mathias & Li, 1982; Slipke & Duffy, 1997). In walleye, the transition from zooplankton to macroinvertebrates and later to fish prey
occurs between 30-50 mm total length (TL) and 60-117 mm TL, respectively (Galarowicz & Wahl, 2005; Uphoff et al., 2019; Yang, 2022). Perch shifts from zooplankton to macroinvertebrates at 20 to 60 mm TL (Wu & Culver, 1992; Graeb et al., 2006). However, the timing of piscivory varies, ranging from 80 mm TL (Graeb et al., 2006) to over 150 mm TL (Keast, 1985; Fullhart et al., 2002). At 50 mm TL, pike begins foraging on fish while continuing to supplement their diet with invertebrates as they grow; however, this behavior remains partly understood (Hook & Hackle Company, 2013).

This study presents evidence of ontogenetic diet shifts within adult walleye populations. Smaller individuals transition from invertebrates (amphipods) to fish as they mature. In contrast, pike and perch populations showed less pronounced dietary shifts, maintaining piscivorous and invertivorous foraging strategies, respectively. Despite broadly similar diets among gamefish groups, amphipods dominated the diets of large perch and also contributed notably to those of walleye and pike, whereas small perch largely avoided them. This suggests large perch predominantly forage in sediment (benthic) habitats, whereas small perch avoid shallow areas where amphipods are abundant. This strong reliance on amphipods has important implications for juvenile gamefish, particularly if ZQM create conditions that enhance amphipod availability. Despite no statistical differences in perch diets, visual observations of dietary patterns reveal nuanced shifts: larger perch favor amphipods and "other invertebrates," while smaller perch avoid these items. These results, though statistically insignificant, may hold ecological consequences.
539 Zooplanktivory is a precursor to invertivory and benthivory during the early life stages of pike, walleye, and perch (Frost 1954; Mathias and Li 1982; Slipke and Duffy 1997). In walleye, the transition from zooplankton to macroinvertebrates and later to fish prey occurs between 30-50 mm TL and 60-117 mm TL, respectively (Galarowicz and Wahl 2005; Uphoff et al. 2019; Yang 2022). Perch shifts from zooplankton to macroinvertebrates at 20 to 60 mm TL (Wu and Culver 1992; Graeb et al. 2006). However, piscivory varies in perch, ranging from 80 mm TL (Graeb et al. 2006) to over 150 mm TL (Keast 1985; Fullhart et al. 2002). From 100-200 mm TL, pike begins foraging on fish while continuing to supplement their diet with macroinvertebrates as they grow (Nolan and Britton 2025).

549 The hypothesis that shifts in diet would be detectable as gamefish species grew was accepted only for walleye. Piscivory levels were higher in larger individuals, while smaller ones showed a prevalence of invertivory. Fish that transitioning early to piscivory potentially benefit from higher somatic growth, earlier maturation, and increased fitness (Sánchez-Hernández et al. 2019), which might be the casewas an option for pike and walleye in this study. Such a shift may directly impact fish communities (e.g., fish abundance, biomass, and population or community structure).
550
551
552
553
554
555 Several factors, including ontogeny, vulnerable prey sizes, competition, community structure or fish species richness (Sánchez-Hernández et al. 2017), and temperature (Amundsen et al. 1999) influence the prevalence of piscivory within fish populations.
556
557
558
559 Although the onset of piscivorous behavior has been associated with high water temperatures, which promote growth during early development (Mehner et al. 1996), it
560
561 is likely triggered also by resource competition and the availability of profitable small
562 prey fish (Sánchez-Hernández et al. 2017).

563 This study affirms the influence of size structure on gamefish foraging habits,
564 particularly in walleye. Such dietary variations may reduce competition and promote
565 their ecological success (Graeb et al. 2005). In contrast, the shared diet across size
566 groups within pike and perch suggests intra-species competition (Venturelli and Tonn
567 2006), potentially affecting energy reserves, growth, and overwinter survival (Sogard
568 and Olla 2000; Grausgruber and Weber 2021). These findings underscore the ecological
569 significance of size-related feeding dynamics and provide valuable insights for aquatic
570 ecologists. As revealed in this research, predator-prey interactions carry critical
571 ecological implications for food-web structure and the long-term survival of fish
572 populations.

573 **4.3 Body condition of gamefishes**

574 Our findings unveil stark contrasts in the physiological condition of gamefish
575 populations within the studied lakes. Remarkably, perch were the only species
576 exhibiting optimal body condition, boasting an average Wr of 105 (Table 1). In contrast,
577 walleye and pike populations fell well below the target range for healthy populations
578 (Wr of 95-105; Blackwell et al. 2000). These findings echo previous reports of poor
579 conditions in walleye and pike compared to perch (Nanayakkara et al. 2021). Similar
580 patterns were observed in other prairie (Willis 1989) and boreal lakes (VanDeValk et al.
581 2008). However, our study reports substantially lower Wr values for walleye and pike
582 and higher values for perch than those in other North American systems (Porath and
Peters 1997; Flinders and Bonar 2008; D'Arco et al. 2015), suggesting further deterioration in these gamefishes' body condition over time.

Dietary analyses suggest a scarcity of forage fish or prevailing trophic bottlenecks, potentially contributing to the diminished body condition in adult walleye and pike.

This decline in walleye Wr aligns with previous reports across three length classes over 23 years (1961–1983) in Oneida Lake, New York, where walleye reliance on small, age-0 prey fish was linked to decreased condition (VanDeValk et al. 2008). In contrast, walleye Wr increased in Lake McConaughy, Nebraska, due to the consumption of larger prey fish (160–200 mm long) and a lower abundance of age-0 fish in Lake McConaughy, Nebraska (Porath and Peters 1997). In our study, those forage fish that were consumed were relatively smaller and perhaps more energetically costly to capture (22–183 mm TL, data not shown) than those promoting walleye well-being in Lake McConaughy (Graeb et al. 2005).
606 contribute to the declining body condition of pike, compounded by the lack of thermal
607 refugia in the study lakes during summer (Diana 1987).

608 Our findings affirm that larger piscivorous fishes (i.e., pike and walleye) **should** optimize
609 net energy by targeting larger prey (i.e., forage fish) as they age. In contrast, perch does
610 not require larger vertebrate prey to maintain a high Wr; rather, the availability/
611 abundance of macroinvertebrates (Lott et al. 1996) influences their body condition.
612 **Beyond In addition to** reflecting **changes shifts** in prey-fish availability, size-specific
613 patterns in Wr **are offer insights** crucial for inferring the **into** prey-fish population
614 **characteristics structure** and **the energetic benefits** they provide to a large and small-
615 **sized** predators **of different sizes**.

## 4.4 Environmental controls of fish condition

617 Widespread evidence highlights the impact of environmental factors on fish body
618 condition (Katzenmeyer 2010; Rueda et al. 2015). We observed distinct species- and
619 size-specific responses of gamefish body conditions to environmental variables.

620 Specifically, chl *a* positively correlated with the body condition of adult walleye and
621 perch (Figures 4A and 4B), aligning with a North American study (Rudstam et al. 1996).

622 However, the declining condition of walleye, despite high productivity in our study
623 lakes, may be attributed to elevated alkalinity levels. Optimal alkalinity levels between
624 75 and 200 mg/L **is are** crucial for healthy fish populations. Outside this range, primary
625 productivity declines, which negatively impacts aquatic ecosystems (Wurts 2002). Our
626 investigation revealed significant differences in alkalinity levels among sampled lakes, *p*
627 = 0.015: walleye lakes exhibited higher alkalinity (110–485 mg/L; mean = 239)
628 compared to perch lakes (110–368 mg/L; mean = 208) (Table S1). Further analysis of
629 65 prairie lakes confirmed a substantial negative correlation between alkalinity and chl
630 a (Figure S9data not shown). This suggests that elevated alkalinity may hinder primary
631 productivity in walleye lakes, and in turn affect the body condition of adult walleye.

632 In contrast, lower alkalinity in perch lakes may enhance productivity, promoting better
633 body conditions in adult perch. However, the relationship between alkalinity and adult
634 gamefishes condition —positive for walleye (Figure 4A) and negative for perch (Figure
635 4B)— is complex and warrants further investigation. Future studies could benefit from
636 examining how alkalinity impacts gamefish feeding behaviors and metabolic responses.

637 Contrary to what was hypothesized, no discernible relationship existed between the five
638 environmental parameters (chl a, alkalinity, maximum lake depth, surface area, and
639 total phosphorus) and body condition of younger walleye and perch (Figure S6-S8). This
640 might suggest that the well-being of these cohorts may be more sensitive to other
641 variables, thermal stress, parasite loads, or predation (Sammons et al. 1994; Flinders
642 and Bonar 2008; Nanayakkara et al. 2021). Turbidity (Secchi depth) affects visual
643 predators like pike, which has been linked to their poor growth due to impediments in
644 feeding (Sammons et al. 1994; Flinders and Bonar 2008), though, it was not the case for
645 pike in this study; (F (1, 8) = [0.30], p = 0.601data not shown).

646 Our findings underscore the sensitivity of different life stages of fish species to the
647 biochemical properties of lakes, which interact synergistically to impact gamefish
648 health. We anticipate that any future reduction in phytoplankton biomass associated
649 with ZQM *will may* detrimentally affect adult gamefishes, especially walleye, due to the
650 strong influence of chl-a on walleye adult body condition (Figure 4A). Excessively high
651 alkalinity poses additional risks, such as respiratory difficulties and the dissolution of
652 the protective mucus layer, rendering gamefishes vulnerable to bacterial attacks and
infections (Quarters 2017). These factors can increase gamefish vulnerability to aquatic stressors, including invasive species.

### 4.5.1 Potential ecological impacts of ZQM

Potential anoxic conditions caused by ZQM feces and pseudofeces will also threaten invertebrate populations, such as chironomids, which support the nutrition of perch.

That may seriously impact pike and walleye, which depend largely on perch as a food source in the NGP. This situation echoes cautionary tales from the Great Lakes, where ZQM significantly reduced *Diporeia* spp., a crucial prey for lake whitefish. The reduction in *Diporeia* led to declines in whitefish condition, growth, abundance and a shift in whitefish diet from *Diporeia* spp. to quagga mussels. Alewife, sculpin, and bloater populations also decreased, affecting larger piscivores, including salmon and trout (Nalepa, 2010).

In boreal ecosystems (e.g., eastern Lake Ontario), lake whitefish and walleye populations have shown drastic, negative responses to ZQM invasion, establishing new equilibriums driven by lower growth rates for lake whitefish and decreased early life history survival for walleye (Hoyle et al., 2008). Similar scenarios could unfold in prairie lakes, where filter-feeding activities caused by ZQM may impact both micro and macroinvertebrate populations, worsen the scarcity of forage fish, and affect gamefishes,
677 leading to broader ecosystem implications. Considering the potential trophic bottleneck
678 in hardwater prairie lakes, the growth, reproduction, and recruitment of fish
679 populations post-ZQM invasion will likely be negatively impacted.

680 In light of these findings, fishery managers, biologists, and policymakers must prioritize
681 the protection of shallow littoral feeding habitats where key invertebrates thrive while
682 assessing the prevailing state of forage-fish populations. Ensuring the continued
683 existence of favorable feeding conditions will help maintain optimal habitats for adult
684 fish populations (life stage of interest) whose body conditions particularly decline with
685 age. By doing so, we can better safeguard the ecological health of these gamefish
686 populations and ensure the sustainability of these valuable yet vulnerable aquatic
687 ecosystems.

688 Future invasions by ZQM may exert indirect negative or stabilizing effects on gamefish,
689 depending on species-specific foraging habits, lake morphology, and the ecological
690 status of invaded systems (Strayer et al. 2004; Karatayev et al. 2012; Geisler 2015).

691 However, the magnitude of these impacts varies widely, complicating efforts to draw
692 simple generalizations (Strayer et al. 2004; Hansen et al. 2020). ZQM are well known
693 for disrupting planktonic food webs while supporting littoral pathways. Acting as
694 ecosystem engineers, they filter pelagic biomass (phytoplankton, zooplankton) and
695 redirect nutrients to the benthos. This has fueled a 97% increase in benthivorous fishes
696 and a 28% decline in planktivorous fish populations (Strayer et al. 2004; Karatayev et
697 al. 2007; Kelley et al. 2010).

698
699 Walleye may be disproportionately impacted in prairie lakes, as juveniles rely heavily on
700 pelagic prey (E. Hillis & S. Davies, pers. comm., 2020). ZQM-driven prey declines could
constrain walleye growth and recruitment, although perch may remain largely unaffected (Hansen et al. 2020). Increased water clarity may heighten predation risk and impair feeding for light-sensitive walleye that best forage under dim light conditions (Ryder 1977). Varied outcomes have been observed across systems. In Manitoba prairie lakes, walleye yields increased by 2.63 kg/ha/yr, whereas boreal shield lakes experienced declines of 2.36 kg/ha/yr. Increases mainly occurred in lakes with lower pre-invasion Secchi depths (1.24 ± 0.57 m), while declines were observed in clearer lakes (1.88 ± 0.89 m), especially when post-invasion clarity exceeded 2 m (Geisler 2015). About 20% of our study lakes already exhibit high water clarity (3.53 ± 1.38 m), making them particularly vulnerable to post-invasion declines.

Ambush predators such as northern pike may respond variably to ZQM-induced changes in environmental conditions. Increased water clarity and macrophyte growth can enhance macroinvertebrate abundance and boost foraging opportunities, as well as improve spawning habitat and reproductive success for these predators (Walker et al. 2013; Søndergaard et al. 2025). However, long-term reliance on invertebrates will be more energetically costly than piscivory (Venturelli and Tonn 2006). Persistent forage fish trophic bottlenecks in these systems may further compromise the body condition of pike and walleye.

In contrast to pike and walleye, yellow perch may benefit more from shifts in prey assemblages toward benthic pathways. Invasive mussel beds (druses) provide food, via feces and pseudofeces, and shelter for littoral benthic invertebrates. These include amphipods, isopods, leeches, turbellarians, hydrozoans, and some oligochaetes and
725 chironomids (Cobb and Watzin 2002; Beekey et al. 2004; Karatayev et al. 2007;

726 Burlakova et al. 2012). Enhanced benthic production may improve perch condition and

727 growth, as perch rely predominantly on invertebrates in prairie lakes, indirectly

728 supporting pike and walleye populations where perch are abundant.

729 Nonetheless, invasive mussel biodeposits from feces and pseudofaeces can also induce

730 anoxia, threatening key invertebrates such as oligochaetes, chironomids, and other

731 burrowing organisms (Caraco et al. 2000). This can compromise perch nutrition and, in

732 turn, affect predators dependent on perch. In the Great Lakes, similar trophic

733 bottlenecks occurred when the loss of the benthic amphipod *Diporeia* spp.—a crucial

734 prey for whitefish—cascaded to forage fish (e.g., alewife, sculpin *Cottus* spp. [Linnaeus,

735 *1758*], *bloater* *Coregonus hoyi* [Milner, 1874]) and salmonids, ultimately reducing

736 whitefish condition and abundance (Nalepa 2010; Hoyle et al. 2008). Comparable

737 outcomes may occur in prairie lakes if declines in both invertebrate and vertebrate prey

738 assemblages disrupt the growth, reproduction, and recruitment of upper-trophic-level

739 species, including pike and walleye.

740 Unlike the Great Lakes, where *Diporeia* declined, benthic amphipods such as

741 *Gammarus lacustris* and *Hyalella azteca* in prairie lakes may persist or even increase

742 post-invasion (Burlakova et al. 2012). These amphipods serve as key prey for gamefish

743 occupying a similar trophic niche as *Diporeia*. At the same time, structural complexity

744 of ZQM beds can limit prey accessibility, lowering foraging success and leading to

745 poorer feeding outcomes. Foraging barriers have been documented in lake sturgeon

746 (*Acipenser fulvescens* [Rafinesque, 1817]), bluegill (*Lepomis macrochirus* [Rafinesque,

747 *1819*]), and perch, where feeding on amphipods and chironomids was inhibited under

748 ZQM-dominated beds (Cobb and Watzin 2002; Beekey et al. 2004; McCabe et al. 2006).
In other systems, however, amphipod densities increased and perch predation on them remained unaffected, but perch required greater foraging effort (González and Downing 1999). These divergent outcomes highlight that ZQM impacts on gamefish–prey interactions, particularly with amphipods, are species- and system-specific. Ultimately, the benefits of increased benthic production may depend on the trade-off between higher prey availability and reduced foraging efficiency in these structurally complex mussel habitats (Beekey et al. 2004).

In a nutshell, foraging conditions may move closer to optimal for yellow perch through enhanced benthic prey. For walleye and pike, conditions will improve only if forage fish increase, but may become less ideal where forage resources remain limited in prairie lakes post-ZQM invasion. To sustain these valuable yet vulnerable ecosystems, fisheries managers should prioritize protecting shallow littoral habitats that support key invertebrates (e.g., amphipods, chironomids, corixids) and monitor forage-fish availability. Long-term monitoring of gamefish diets and body condition—especially in aging adults will be critical to safeguarding population health.

ZQM can exert strong direct or indirect effects on fish populations. However, the magnitude of these impacts varies widely among ecosystems and fish species, complicating efforts to draw simple generalizations (Strayer et al., 2004; Hansen et al., 2020). Overall impacts of ZQM on fish may depend on several factors, including fish foraging habits, morphology, and the ecological status of invaded water bodies (Karatayev et al., 2002; Strayer et al., 2004; Geisler, 2015).
773 ZQM are famous for disrupting planktonic food webs while benefiting littoral food webs.

774 For example, Strayer et al. (2004) reported a 28% decline in planktivorous fish

775 populations and a 97% increase in benthivorous fish populations following ZQM

776 invasion. Consequently, enhanced water clarity exposes planktivorous fish larvae to

777 higher predation, and light-sensitive walleye could face suboptimal feeding conditions

778 and reduced reproductive success in much clearer waters (Ryder, 1977).

779 Nevertheless, walleye have responded differently to water clarity changes post-invasion

780 in other systems. In Manitoba prairie lakes, walleye yields increased by 2.63 kg/ha/yr,

781 while in Boreal shield lakes they declined by 2.36 kg/ha/yr. Lakes with lower pre-

782 invasion Secchi depths (1.24 ± 0.57 m) showed yield increases after invasions. In

783 comparison, lakes with higher pre-invasion Secchi depths (1.88 ± 0.89 m) experienced

784 declines, especially when post-invasion clarity exceeded 2 meters (Geisler, 2015). About

785 20% of our study lakes have high pre-invasion Secchi depths of (3.53 ± 1.38), making

786 them susceptible to walleye declines after ZQM invasion. Effective management of these

787 lakes will be vital for walleye conservation.

788 The rise in benthivorous fish populations following ZQM invasion has been linked to

789 increased biomass of native benthic invertebrates in the littoral zone (Strayer et al.,

790 2004; Karatayev et al., 2007; Kelley et al., 2010). By acting as ecosystem engineers,

791 ZQM alter the benthic community by depositing organic matter in the benthos and

792 forming extensive mussel shell beds (druses). These druses provide shelter and food (via

793 feces and pseudofeces) for native epifaunal invertebrates such as amphipods

794 (Gammarus) and chironomids (Karatayev et al., 2007; Burlakova et al., 2012). While

795 gamefishes may benefit from the increased availability of invertebrate prey, the
796 structural complexity of ZQM beds can hinder efficient foraging, potentially limiting access to these resources.
797
798 Potential anoxic conditions caused by ZQM feces and pseudofeces will also threaten invertebrate populations, such as chironomids, which support the nutrition of perch.
799
800 That may seriously impact pike and walleye, which depend largely on perch as a food source in the NGP. This situation echoes cautionary tales from the Great Lakes, where ZQM significantly reduced *Diporeia* spp., a crucial prey for lake whitefish. The reduction in *Diporeia* led to declines in whitefish condition, growth, abundance and a shift in whitefish diet from *Diporeia* spp. to quagga mussels. Alewife, sculpin, and bloater populations also decreased, affecting larger piscivores, including salmon and trout (Nalepa, 2010).
801
802
803
804
805
806
807 In boreal ecosystems (e.g., eastern Lake Ontario), lake whitefish and walleye populations have shown drastic, negative responses to ZQM invasion, establishing new equilibriums driven by lower growth rates for lake whitefish and decreased early life history survival for walleye (Hoyle et al., 2008). Similar scenarios could unfold in prairie lakes, where filter-feeding activities caused by ZQM may impact both micro and macroinvertebrate populations, worsen the scarcity of forage fish, and affect gamefishes, leading to broader ecosystem implications. Considering the potential trophic bottleneck in hardwater prairie lakes, the growth, reproduction, and recruitment of fish populations post-ZQM invasion will likely be negatively impacted.
808
809
810
811
812
813
814
815
816 In light of these findings, fishery managers, biologists, and policymakers must prioritize the protection of shallow littoral feeding habitats where key invertebrates thrive while assessing the prevailing state of forage-fish populations. Ensuring the continued existence of favorable feeding conditions will help maintain optimal habitats for adult
817
818
819
820 fish populations (life stage of interest) whose body conditions particularly decline with age. By doing so, we can better safeguard the ecological health of these gamefish populations and ensure the sustainability of these valuable yet vulnerable aquatic ecosystems.

824

825

## 56.0 CONCLUSION

826 The findings of this study support previous research on prairie lake gamefishes and deepen our knowledge about species-specific diets and size-related body condition trends on a spatial scale. Assessment of three gamefish species unveil unique strategies for adapting to changing environmental conditions. Particularly, walleye exhibits notable ontogenetic dietary shifts and a unique ability to adjust their diets in response to changing prey availability compared to native pike and perch. Starting with macroinvertebrates as young adults, they progressively incorporate more fish as they mature. Despite this shift, walleye persistently rely heavily on macroinvertebrates. This, which may be a response to the limited availability of preferred forage fish in hardwater prairie lakes. Adult pike and walleye populations that depend heavily on macroinvertebrates experience declining body conditions and perhaps an increased vulnerability to parasites. Additional strains on body conditions such as high alkalinity, and eutrophication in these lakes will further jeopardize fish health and increase the risk of winterkills. These challenges will likely intensify with the dual threats of climate change and the invasion of ZQM and may directly or indirectly affect fish populations by impeding dominant processes like nutrient cycling and the depletion of deepwater dissolved oxygen in uninvaded systems. Changes to the chemical,

827

828

829

830

831

832

833

834

835

836

837

838

839

840

841

842
843 biophysical, and deepwater habitat quality can constrain or distort the habitability of
844 these environments, making survival increasingly challenging for fish.

845 Addressing these challenges requires urgent and coordinated action. By proactively
846 mitigating ZQM impacts, we can safeguard the vital role these gamefishes play in our
847 aquatic environment and local economies. For instance, implementing strategies to
848 enhance habitat for forage fish and closely monitoring water quality (temperature,
849 oxygen, alkalinity) will be vital for walleye conservation and can help sustain these
850 ecosystems for future generations.

851
852
853 AUTHOR STATEMENTS
854 ACKNOWLEDGEMENTS
855 This research was conducted on Treaty 4 and Treaty 6 lands, which consist of the
856 traditional territories of the Cree, Dene, Blackfoot, Saulteaux, Nakoda, Dakota, and
857 Lakota peoples and the homeland of the Métis Nation. We are grateful to Special thanks
858 to Gord Sedgewick, James Villeneuve, Jennifer Merkowsky, and Rebecca Eberts, from
859 the Saskatchewan Ministry of Environment and the Saskatchewan Ministry of
860 Environment for their collaboration on this project. Field support was provided by
861 Meegahage, Afruza Begum, Xingzi Zhou, and Alison Walker.

862
863 COMPETING INTERESTS STATEMENT

864 The authors declare there are no competing interests.

865
866
867

868

## AUTHOR CONTRIBUTION STATEMENT

869

BW and CBO co-designed the study, developed the conceptual framework, and secured

870

funding for the project. EH and CBO conducted field sampling, coordinated logistics,

871

and managed all technical details. ZQR assisted with water chemistry analysis and

872

numerical computations.

873

CBO performed fish stomach content analyses, related laboratory work, numerical

874

calculations, and data analyses. BW, MUM, EH, and CBO contributed to the

875

interpretation of the results. MUM and BW supervised the project and validated the

876

findings. CBO drafted the original manuscript. All authors reviewed, edited, and

877

discussed the final manuscript, providing constructive feedback that shaped its content.

878

879

## FUNDING STATEMENT

880

Funding was provided through NSERC Discovery Grant # RGPIN-06475-2018 to Björn

881

Wissel and the SK Fish and Wildlife Development Fund (no grant number) to Caroline

882

B. Ofosu and Björn Wissel.

883

## DATA AVAILABILITY STATEMENT

884

Data generated and/or analyzed during this study are available in the DYRAD

885

repository, [confidential Reviewer URL,

886

http://datadryad.org/stash/share/AqWBdUoeyGmQhOHZinmXCb889RSOL5LhgO7P

887

QqkTrNs].

888

889
890

891

892

893

894

895

896

897

898

899

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1579

Draft
1 **Game Over for Gamefishes after Zebra and Quagga Mussel Invasion: Not in**
2 Prairie Lakes?

3

4 **Authors:** Caroline B. Ofosu<sup>1,2,3*</sup>, Anas M. Usoof<sup>1,4,5</sup>, Erin Hillis<sup>1</sup>, Zoraida J. Quiñones-

5

Rivera<sup>1,6</sup> and Björn Wissel<sup>1,6</sup>

6

7

<sup>1</sup>University of Regina | U of R · Department of Biology, Regina, Regina, SK, Canada.

8

<sup>2</sup> Memorial University of Newfoundland | MUN · Department of Earth Sciences, NL,

9

Canada.

10

<sup>3</sup> Memorial University of Newfoundland | MUN · Department of Ocean Sciences, NL,

11

Canada.

12

<sup>4</sup> University of Toronto Scarborough | U of T · Department of Biological Sciences, ON,

13

Canada.

14

<sup>5</sup> University of Winnipeg | U of W · Department of Biology, MB, Canada.

15

<sup>6</sup> Université Claude Bernard Lyon 1 | UCBL · Laboratoire d'Ecologie des Hydrosystèmes

16

Naturels et Anthropisés (LEHNA), Villeurbanne, France.

17

18

Correspondence: [cbofosu@mun.ca](mailto:cbofosu@mun.ca)

19

20

21

22

23

24

25
26 **Abstract**

27 Mitigating threats of aquatic invasive species in lakes requires holistic insights into the
28 biological, physical, and chemical conditions sustaining fish communities. This study
29 establishes important trophic baseline conditions for three key gamefish species—
30 walleye, northern pike, and yellow perch—across 15 hardwater prairie lakes in south-
31 central Saskatchewan, Canada, that are threatened by zebra mussel invasion. We
32 employed stomach content analysis to evaluate diet and ontogenetic nutritional shifts in
33 adult fishes and assessed how fish size and environmental factors influence body
34 condition. Adult walleye transition from a diet of amphipods to fish as they grow,
35 whereas perch and pike maintain a stable diet of invertebrates and fish, respectively.
36 While walleye and pike decline in body condition with size, perch maintain consistently
37 high condition. Lake productivity and alkalinity are key predictors of adult walleye and
38 perch condition. Yet, zebra mussel invasion in hardwater lakes may not have the
39 detrimental impacts observed elsewhere, assuming management strategies address the
40 combined threats of invasion and climate change. These findings provide a foundation
41 for assessing shifts in foraging ecology in yet-to-be-invaded prairie lakes.
42

42

43 **Keywords:** Zebra and Quagga mussels, Invasion, Hardwater lakes, Gamefishes,
44 Walleye.

45

46

47

48
49 **1.0 INTRODUCTION**

Despite the growing interest in invasion ecology (Simberloff and Gibbons 2004), there is
limited understanding on how pre-existing regional stressors and Aquatic Invasive
Species (AIS) interact to impact the survival of resident fish populations (Arrington et
al. 2006).

Zebra and quagga mussels (Dreissena polymorpha [Pallas, 1771], and D. bugensis
[Andrusov, 1897], respectively; hereafter referred to as ZQM) are two aquatic invaders
that have significantly modified food webs across continents (Bukontaite and Zaiko
2008). These bivalves now pose threats to aquatic ecosystems in the prairie region of
western Canada, with its many hardwater lakes characterized by a unique combination
of hydrology and other limnological parameters (e.g., shallow depth, high alkalinity,
absence of thermal stratification, varying salinity, and suitable calcium concentrations,
see Table S1 (Therriault et al. 2013). These physicochemical attributes are expected to
create conditions that support successful ZQM invasion and, in the event of their
introduction, increase the susceptibility of fish populations in this region to ZQM
impacts (Starks et al. 2014; Nanayakkara et al. 2021).

Once introduced, ZQM exhibit robust tolerance to prevailing conditions, especially
temperature and salinity, increasing their likelihood to thrive in lakes across south-
central Saskatchewan (SK; Therriault and Orlova 2010), which represents the western
invasion front. The proximity of SK to invasion corridors, notably Manitoba (LWF,
2020) and three US states (Montana, North Dakota, and Minnesota), heightens the risk
of ZQM introduction into western prairie lakes. Ecological impacts of these invaders
reverberate through food-webs (Higgins and Vander Zanden 2010). As efficient filter-
72 feeders, ZQM enhance water clarity while reducing pelagic primary productivity. This
73 causes disruptions in phytoplankton and zooplankton populations, which form the basis
74 of the food web upon which most native juvenile fishes depend (Pace et al. 1998;
75 Higgins and Vander Zanden 2010). Such disturbances invariably affect the productivity
76 of higher trophic-level gamefishes like walleye (*Sander vitreus* [Mitchill, 1818]),
77 northern pike (*Esox lucius* [Linnaeus, 1758]; hereafter referred to as pike), and yellow
78 perch (*Perca flavescens* [Mitchill, 1814]; hereafter referred to as perch), and may
79 dramatically alter trophic relationships (Karatayev et al. 2012; Ozersky et al. 2012).

80 The impacts of ZQM invasion vary across regions, systems, and species and often
81 through time (Hansen et al. 2020), making predictions and management efforts
82 challenging. In north-temperate lakes, ZQM invasions increased water clarity, especially
83 in stratified lakes, with an average increase in Secchi depth of 0.68 m, and a 0.31 m
84 increase for polymictic lakes (Geisler 2015). This enhanced water clarity adversely
85 affected walleye, a light-sensitive species (Scott and Crossman 1973; Ryder 1977),
86 increasing predation pressure and competition, and ultimately reducing suitable habitat
87 and yields (decline of 2.36 kg/ha/yr post-invasion in boreal shield lakes; Geisler 2015).
88 In contrast, Manitoba prairie lakes exhibited a positive response, with predicted
89 walleye yield increases of 2.63 kg/ha/yr (Geisler 2015).

90 In the Great Lakes region, dreissenids negatively impacted phytoplankton, zooplankton
91 (Pace et al. 1998; Mavrin and Strel'nikova 2011), and benthic invertebrates (e.g.,
92 *Diporeia*) that are crucial to the diet of apex predators including whitefish (*Coregonus
93 clupeaformis* [Mitchill, 1818]) (Hoyle et al. 2008). Following the collapse of *Diporeia*,
94 whitefish incorporated larger quantities of lower-energy food items, particularly
dreissenids (Pothoven et al. 2001; Lumb et al. 2007), leading to reduced body condition, growth rates, and reproductive success (Hoyle et al. 1999, 2008). Similar nutritional consequences were observed in alewife (*Alosa pseudoharengus* [Wilson, 1811]), where energy density declined due to reduced *Diporeia* availability in their diet, from 29% pre-invasion to 12% post-invasion (Madenjian et al. 2006). Interestingly, adult walleye and smallmouth bass (*Micropterus dolomieu* [Lacepède, 1802]) in southern Ontario boreal lakes successfully navigated resource limitations imposed by ZQM, switching to advantageous prey like crayfish and fish (Nienhuis et al. 2014).

Recent assessments revealed that gamefish populations in SK hardwater lakes are already stressed, particularly walleye, which recorded high parasite loads and poor body condition (Nanayakkara et al. 2021). Hence, comprehending how current ecological conditions support (or limit) fish communities is vital to develop management efforts that aim to minimize the impacts of future ZQM invasion. Accordingly, we conducted detailed baseline assessments of the ecological status (diet and body conditions) of the three recreational and commercially targeted gamefish species—walleye, pike, and perch—across south-central SK lakes. Walleye, the focal species, is primarily sustained by provincial stocking programs (stocked at ages 3-5 days, Gord Sedgewick, pers. comm.) to replenish populations stressed by angling pressure and low recruitment (Saskatchewan Ministry of Environment 2017, 2019), unlike perch and pike, which occur naturally in prairie lakes.

For gape-limited animals like fishes, growth is maintained by size-dependent shifts in food or habitat use (Werner and Gilliam 1984). Pike, walleye and perch, like other piscivorous gamefishes, typically undergo ontogenetic diet shifts, starting with