Evaluation of Potential Overwinter Mortality of Age-0 Walleye and Appropriate Age-1 Sampling Gear

Recruitment of age-0 fishes to recreational fisheries is often influenced by critical periods of high mortality during early life-history stages (Miller et al. 1988), including but not limited to egg incubation (Ware 1975), hatching (Vigliola and Meekan 2002), swim bladder inflation and swim-up (Ware 1975), switching from endogenous to exogenous feeding (Hunter 1981), and ontogenetic shifts in food habits (Post 2003). The first winter period is often thought to be the final critical period after which no further catastrophic mortality occurs. Several factors have been shown to affect the winter survival of age-0 fish, including prey availability (Sogard and Olla 2000), predation (Biro and Booth 2009), limited availability of habitat refugia (Werner and Hall 1988), starvation (Oliver et al. 1979), and thermal stress (Mazeaud et al. 1977). Thus, overwinter mortality is thought to be one of the main drivers in structuring future year-class strength (Post and Evans 1989; McCollum et al. 2003).

The likelihood of surviving the first winter is often size related (Sogard 1997), with smaller individuals usually experiencing higher mortality rates than larger fish (Post and Evans 1989; Huss et al. 2008). Variability in hatch dates and individual growth rates result in a range of body sizes and conditions within a single cohort (Sogard 1997). Thus, vulnerability to overwinter mortality may vary within a cohort. Greater mortality rates of smaller individuals are often thought to occur because of inadequate energy reserves needed to survive the first winter and periods of reduced prey resources (Schultz and Conover 1999). For example, Miranda and Hubbard (1994) found that smaller age-0 Largemouth Bass Micropterus salmoides had fewer energy reserves than larger members of the same cohort and spent those reserves at a faster rate during starvation periods, therefore likely increasing the mortality rate among smaller Largemouth Bass. The potential exists for the first overwinter period to selectively remove a large portion of the age-0 cohort if a substantial proportion of that cohort is not of appropriate size or body condition to withstand the conditions of that year. As a result, eventual year-class strength and recruitment to the fishery may be substantially affected.

Potential recruitment of age-0 Walleye Sander vitreus to adult sizes is often indexed by the relative abundance of age-0 individuals in their first summer or fall (Serns 1982; Hansen et al. 2004). However, previous research has shown weak or nonsignificant relationships between these two life stages (Johnson et al. 1996; Hansen et al. 2004). Overwinter mortality during the first winter has been hypothesized to be a substantial limitation to recruitment in Walleye populations (Johnson et al. 1996), but limited evidence of this exists. Similar to other fish populations, overwinter mortality of Walleye may be related to body length or condition. To date, few studies have examined the influence of body size and condition on first-year survival of age-0 Walleyes. Evidence indicates that size-selective mortality may occur in some waters (Copeland and Carline 1998; Pratt and Fox 2002) but not in others (Copeland and Carline 1998; Lucchesi 2002; Pratt and Fox 2002).

Evaluation of overwinter mortality of Walleye may be limited by the ability to effectively sample fish during the pre- and postwinter periods. Prewinter assessment of age-0 Walleye relative abundance is often indexed from nighttime electrofishing surveys (Serns 1982), but no published study to our knowledge has identified the most efficient gear for sampling age-1 individuals in the spring. Further, most standardized sampling surveys fail to capture age-1 Walleye. Early detection of age-0 overwinter mortality and the ability to better predict eventual recruitment to the adult fisheries are important in making appropriate stocking and regulations decisions to meet management goals (Jennings et al. 2005). Therefore, the objectives of this study were to: 1) compare results from nighttime electrofishing to index relative abundance of age-1 Walleye with relative abundance indices of minifyke nets in four eastern South Dakota lakes; 2) determine whether size-selective mortality was occurring in those four lakes; and 3) if size-selective mortality was occurring in these lakes, determine whether that mortality was attributed to body condition.

Lakes selected for this study are included in the South Dakota Department of Game, Fish, and Parks standardized surveys for fall age-0 Walleye (Lucchesi and Scubelek 2001; Kaufmann et al. 2016). We used annual fall electrofishing sampling reports to select lakes with age-0 Walleye catch rates of at least 60 fish/h to increase the likelihood that age-1 Walleye would be adequately sampled during the following spring (Copeland and Carline 1998). We sampled four eastern South Dakota natural lakes each in one fall and one corresponding spring during this study (Table 1). We sampled South Buffalo Lake in fall 2012 and spring 2013, and the other three lakes each in fall 2013 and spring 2014. Surface areas of lakes varied but mean depths were similar (Table 1). All four study lakes are considered productive, ranging in trophic state from mesotrophic to eutrophic, according to the classification system by Carlson (1977).

We compared minifyke net sampling and nighttime electrofishing to identify which gear would be better to index spring age-1 Walleye. Both gears are commonly used for sampling juvenile and adult Walleye (Bonar et al. 2009). We conducted all age-1 sampling 1 to 2 wk after winter ice-off in the spring of 2013 and 2014. Copeland and Carline (1998) found that this waiting period after ice-off was necessary for increased fish movement. We sampled each lake once a week with each gear within a 24-h period to ensure similar water temperatures and environmental conditions. We sampled lakes for 4 wk in 2013 and 5 wk in 2014. We extended sampling in 2014 because of cooler water temperatures that potentially limited fish movement and to ensure that temperatures sampled were similar to those in the spring of 2013.

We used minifyke nets with 0.9 × 1.5 m frames, 9-mm mesh (bar measure), 9.2-m-long lead line, and a single throat during each sample period. We set six minifyke nets at standardized transect sites to make sure similar depths and habitats were sampled by both gears for an appropriate comparison. We selected six 10-min stations by using a stratified random sampling protocol. We conducted pulsed direct current nighttime electrofishing at each of the 10-min stations with a Smith-Root® boat equipped with a 7.5 generator-powered pulsator electrofisher system that generated 170–500 V and 8–18 A. We used two dip netters at all times for maximum catchability of age-1 Walleye. We completed electrofishing the night before minifyke net sampling.

We sacrificed the first 20 assumed age-1 Walleye sampled from each lake for each gear for age verification using otoliths and for size-selective overwinter condition analyses. We measured all other age-1 Walleye sampled by either gear for total length (TL; mm) and released them. We calculated electrofishing catch per unit effort (CPUE) for each lake as the number of age-1 Walleye caught per hour, and calculated minifyke net CPUE for each lake as the number of age-1 Walleye per net. We compared sampling precision and bias between the two gears. We calculated the coefficient of variation (standard error/mean overall CPUE) for each gear from all lakes pooled together. We determined potential sampling bias by examining length-frequency distribution of age-1 Walleye collected from each gear using a Kolmogorov–Smirnov test. In both analyses, we combined all data for all lakes because of the low sample size of age-1 Walleye in minifyke nets. We determined the gear with greater precision (lowest coefficient of variation) to be the most appropriate gear for age-1 Walleye spring sampling.

To evaluate overwinter mortality of age-0 Walleye, we used Walleye collected from age-0 standard South Dakota Department of Game, Fish, and Parks fall nighttime electrofishing surveys and all spring age-1 Walleye collected during spring age-1 electrofishing sampling. We immediately placed the first 20 Walleye caught during both sampling events on ice and returned them to the laboratory at South Dakota State University for body condition analysis, TL and weight (g) measurements, liver extraction, and aging. We conducted measurements in the laboratory to minimize field measurement errors often associated with smaller fishes (Flammang et al. 1999). We handled all fish collected in this study according to the Guidelines for the Use of Fishes in Research (AFS 2004).

We compared fall age-0 Walleye nighttime electrofishing CPUE and spring age-1 Walleye nighttime electrofishing CPUE using a Wilcoxon ranked-sum test. We evaluated potential size-selective overwinter mortality of smaller age-0 Walleye from age-0 and age-1 length frequency histograms using quantile–quantile regression as described by Post and Evans (1989) and Braaten and Guy (2004). We calculated 1, 5, 10, 25, 50, 75, 90, 95, and 99% quantiles for pre- and postwinter Walleye length frequency distributions for each lake. The first quantile–quantile plot regressed spring quantiles against fall quantiles; a significant positive slope < 1.0 indicated size-selective mortality of small individuals within a given lake (Post and Evans 1989). For the second plot, we calculated incremental growth as the difference between mean TL of fall age-0 Walleye and mean TL of spring age-1 Walleye at each quantile and then regressed them against the fall quantile lengths. Significant negative slopes indicated size-selective mortality of small individuals within a given lake. If one or both of the quantile–quantile regression plot criteria were met, then size selectivity of small Walleyes was occurring (Post and Evans 1989).

We also calculated the several body condition indices of Walleye. We wet weighed (g) extracted livers to calculate liver somatic index (LSI) calculated as

We chose this index because LSI is considered an index of walleye condition, and condition increases with increasing LSI (Bulow et al. 1978; Allen and Wootton 1982; Hoque et al. 1998). We also indexed Walleye condition using relative weight (W_r; Murphy et al. 1990). We calculated relative weight for Walleye >150 mm TL as:

where W is the individual weight of the fish and W_s is a length-specific standard weight for a specific species (Wege and Anderson 1978). For Walleyes < 150 mm TL, we used the W_s equation for juvenile Walleye (Flammang et al. 1999). Relative weight index has previously been suggested as an appropriate tool for assessing juvenile Walleye condition (Piper et al. 1982; Flammang et al. 1999). We calculated mean LSI and W_r for both fall age-0 Walleye and the corresponding spring age-1 Walleye within each lake. We used a t-test to test for differences between fall and spring LSI and W_r values in all four study lakes. In addition, we pooled fall age-0 LSI and W_r among those lakes that demonstrated size-selective mortality. We regressed each index against fall age-0 TL to determine if those lakes that experienced size-selective mortality had smaller age-0 Walleye with lower LSI or W_r indices, which may increase the likelihood of overwinter mortality. A significantly positive linear relationship between age-0 Walleye condition indices (LSI and W_r) and TL would indicate that smaller fish are in poorer condition compared with larger fish. We determined significance in all statistical analyses at α < 0.05. We performed all statistical analyses using the Statistical Analysis System software package (SAS Institute 2010). Data pertinent to this study are provided through the Journal of Fish and Wildlife Management archive (see Data S1 and Data S2, Supplemental Material).

Mean CPUE for spring nighttime electrofishing varied between 4 and 69 Walleye/h, and mean catch rates for spring minifyke net sampling was less than 1 Walleye/net among the four lakes (Table 2). Sampling precision was higher for nighttime electrofishing (coefficient of variation = 216.6) compared with minifyke nets (coefficient of variation = 338.2). There was no difference in the length-frequency distributions of Walleye caught from spring nighttime electrofishing and minifyke nets (P = 0.21; Figure 1). Mean TL for age-1 Walleye caught during night electrofishing and minifyke nets among all four lakes were 152.8 (SE = 6.5) and 153.7 (SE = 0.15) mm, respectively (Table 3).

Fall nighttime electrofishing catch rates were significantly greater than spring nighttime electrofishing catch rates at all four study lakes (P < 0.01 for all lakes; Table 2; Figure 2). Mean TL of collected Walleye increased significantly from fall to spring in Lake Kampeska and South Buffalo Lake (P < 0.01 for both lakes) but not in Roy Lake (P = 0.16) or Lake Madison (P = 0.45; Table 3; Figure 2). We detected size-selective mortality for Lake Kampeska, Roy Lake, and South Buffalo Lake but not in Lake Madison (Figure 3).

Among the three lakes where we detected size-selective mortality, W_r was significantly greater in the fall compared with spring in Roy (t = 2.99, P < 0.01) and South Buffalo (t = 13.15; P < 0.01 ) lakes, but we found no differences in W_r for Lake Kampeska (t = 0.79, P = 0.22; Table 3; Figure 4). Fall and spring LSI values were not significantly different from one another in Lake Kampeska (t = −0.67; P = 0.74) and South Buffalo Lake (t = −0.48; P = 0.68), but LSI values were higher in the spring compared with fall and Roy Lake (t = 3.41, P < 0.01; Table 3; Figure 4). We detected no significant relationships between age-0 Walleye TL and LSI or W_r for lakes that showed size-selective mortality.

We recommend that nighttime electrofishing, rather than minifyke nets, be used to index relative abundance of age-1 Walleyes in the spring. Nighttime electrofishing produced higher total catches of walleye and provided more precise estimates of catches compared with minifyke nets across all four lakes in our study. Further, using nighttime electrofishing to index relative abundance of age-1 Walleye in the spring is beneficial, as the same approach is often used by agencies to index age-0 relative abundance during the previous summer or fall (Serns 1982; Isermann and Parsons 2011). The use of nighttime electrofishing would allow for more direct comparisons of relative abundance and size structure between these two time periods.

We sampled few age-1 Walleye in minifyke nets during our study. One reason may have been due to cool spring water temperatures. The efficiency of passive gears such as minifyke nets is affected by the degree of fish movement during the time of sampling (Hubert 1996). Cooler water temperatures are often associated with decreased fish movement (Fuiman and Batty 1997), which may have reduced the vulnerability of age-1 Walleye to capture by this gear. Additionally, potential escapement may have contributed to low catch rates. Several studies have documented fish escapement as a factor affecting minifyke net catch rates (Hansen 1944; Crowe 1950, Patriarche 1968; Laarman and Ryckman 1982). To determine whether this phenomenon was occurring in this study, we conducted an ad hoc pilot study on Lake Madison during May 2014 using five overnight minifyke nets. Each net included five randomly selected and fin-clipped age-1 Walleye. Escapement rates varied between 0 and 80% (mean = 20%), indicating that escapement may be a factor limiting the effectiveness of the minifyke nets used in our primary study as well. Smith and Simpkins (2017) similarly found that minifyke nets did not collect Walleye in the nearshore zone of Lake Michigan. Further research is needed to evaluate escapement rates and the factors that influence those rates to determine whether corrections can be made to the gear itself to prohibit escapement or to correct estimates of relative abundance from that gear.

Effective sampling of age-1 Walleye directly after the first winter period may provide a clearer understanding of overwinter mortality as well as a better indicator of eventual adult recruitment. To our knowledge, our study is the first to document overwinter mortality of Walleye occurring across several systems. Overwinter mortality of Walleye in natural lakes is believed to be common, but only a limited number of studies has demonstrated its occurrence (Chevalier 1973; Forney 1976; Copeland and Carline 1998). Evaluation of the extent and magnitude of overwinter mortality among Walleye populations across their range and their influence on adult recruitment over time warrants further study.

Additionally, we found evidence of size-selective mortality of smaller age-0 Walleye in three of the four study lakes. Overwinter mortality generally functions to remove smaller individuals from a cohort (Sogard 1997). The mechanism for size-selective mortality is often related to prewinter condition (Oliver et al. 1979; Shuter and Post 1990), presumably due to higher energy storage by larger individuals compared with smaller individuals of the same cohort. For example, Oliver et al. (1979) found that smaller Smallmouth Bass Micropterus dolomieu that experienced higher overwinter mortality rates also had lower energy stores compared with larger individuals of the same cohort. In our study, we found no relationship between body size and condition. Previous research on the survival of stocked Walleye fingerlings in eastern South Dakota lakes found similar results (Lucchesi 1997). In fact, overwinter survival rates were often lowest in lakes where the body condition of Walleye fingerlings was often the highest at the time of stocking. Thus, smaller Walleye in eastern South Dakota lakes are not likely to be vulnerable to mortality because of low energy reserves.

Similar results have been found among other Walleye populations in North America (Copeland and Carline 1998; Pratt and Fox 2002), indicating that other factors likely influence the vulnerability of smaller fish to mortality. One such factor may be predation by other piscivores in the system. Smaller individuals may be more vulnerable to predation compared with larger individuals due to gape size limitations of the predator populations (Hambright 1991). Predation risk may be extended in winter as food resources could be limiting during that time (Pratt and Fox 2002). Post and Evans (1989) found that winter duration coupled with predation-related size-selective overwinter mortality were important factors contributing to overwinter mortality of age-0 Yellow Perch Perca flavescens, and other studies have noted similar patterns for Walleye (Chevalier 1973; Forney 1976; Madenjian et al. 1991). For example, Forney (1976) found that during longer and harsh winters, smaller age-0 Walleye may have been vulnerable to potential predation from adult Walleye longer. We noted the winter of 2013–2014 in South Dakota to be longer and harsher than previous winters (personal observation). Walleye in the three study lakes sampled in fall 2013 and spring 2014 where we noted size-selective mortality did not achieve significant growth over the winter and thus, may have been subjected to longer periods of predation risk relative to the fourth lake (South Buffalo) sampled in fall 2012 and spring 2013 when winter severity and duration were reduced and Walleye grew over winter. Future research is needed to identify the length thresholds needed to reduce vulnerability to predation during various categories of winter severity and duration.

The relative influence of overwinter mortality of age-0 Walleye may reduce the ability of agencies to predict potential adult Walleye recruitment during commonly used fall age-0 assessments (Serns 1982). The potential for overwinter mortality of age-0 Walleye in South Dakota natural lakes shown in this study may be higher than previously believed. Therefore, assessments of age-1 Walleye relative abundance directly after the overwinter period may allow for better insight into future adult Walleye recruitment. Examining correlations between spring age-1 electrofishing CPUE and subsequent adult CPUE would provide further information on whether spring age-1 catch rates would be a more appropriate early index of recruitment. In addition, further study is needed to examine the relationships between winter severity and duration and overwinter predation risk of age-0 Walleye to aid in management of the fish community as a whole.

Data S1. Catch data for age-1 Walleye Sander vitreus sampled by nighttime electrofishing and minifyke nets in spring 2013 (South Buffalo Lake) and 2014 (Lake Kampeska, Lake Madison, and Roy Lake).

Found at DOI: http://dx.doi.org/10.3996/082017-JFWM-062.S1 (26 KB XLSX).

Data S2. Total length (mm) and total weight (g) of fall age-0 and corresponding spring age-1 Walleye Sander vitreus collected from South Buffalo Lake (2012 and 2013) and Lake Kampeska, Lake Madison, and Roy Lake (2013 and 2014).

Found at DOI: http://dx.doi.org/10.3996/082017-JFWM-062.S2 (63 KB XLSX).

Reference S1. Hunter JR. 1981. Feeding ecology and predation of marine fish larvae [California]. Pages 33–77 in Lasker R, editor. Marine fish larvae. Olympia, Washington: Washington Sea Grant.

Found at DOI: http://dx.doi.org/10.3996/082017-JFWM-062.S3; also available at http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.569.9230&rep=rep1&type=pdf (2478 KB PDF).

We thank all those from the South Dakota Department of Game, Fish and Parks who helped collect fall electrofishing data used in this study. Matt Phayvanh, Dalton Benage, Craig Schake, and B.J. Schall provided invaluable assistance in the field and laboratory. Our appreciation is extended to the Associate Editor and three reviewers assigned to this manuscript for their thoughtful and helpful suggestions for improvement. This research was funded by Federal Aid in Sport Fish Restoration Program, project number F-15-R, study number 1524-F administered through the South Dakota Department of Game, Fish and Parks.

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