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Growth and survival of age-0 walleye (Sander vitreus): interactions among walleye size, prey availability, predation, and abiotic factors R. John H. Hoxmeier, David H. Wahl, Ronald C. Brooks, and Roy C. Heidinger
Abstract: We examined the importance of prey availability, predation, and abiotic factors in determining growth and survival of age-0 walleye (Sander vitreus) across 15 Illinois reservoirs during 7 years. Multiple life stages were examined by stocking walleye at three different size groups: larval (6 mm total length (TL)), small (46 mm TL), and large (100 mm TL). Factors affecting growth and survival of walleye varied depending on walleye size. Growth of small and large walleye increased with benthic invertebrate density. Temperature had a positive effect on larval and small walleye growth but a negative effect on large walleye growth. Prey availability was an important factor for walleye survival across all size groups, whereas temperature affected only larval and large walleye. Juvenile centrarchid density had a negative effect on larval walleye survival, presumably caused by predation. Our best predictive models explained substantial variation in survival for larval (97%), small (57%), and large (83%) walleye. We also explained a high proportion of variation in growth of large (98%), small (55%), and larval (52%) walleye. Our study demonstrates the importance of examining multiple life stages to predict growth and survival and leads to a better understanding of walleye recruitment and recommendations for stocking strategies. Résumé : Nous avons examiné l’importance de la disponibilité des proies, de la prédation et des facteurs abiotiques dans la détermination de la croissance et de la survie chez les dorés (Sander vitreus) d’âge 0 dans 15 réservoirs de l’Illinois pendant 7 ans. Nous avons étudié plusieurs stades de vie en empoissonnant avec des dorés de trois classes de taille différentes, des larves (6 mm de longueur totale (TL)), des petits (46 mm TL) et des grands (100 mm TL) dorés. Les facteurs qui affectent la croissance et la survie du doré varient en fonction de la taille des poissons. La croissance des petits et grands dorés augmente en fonction de la densité des invertébrés benthiques. La température a un effet positif sur la croissance des larves et des petits dorés, mais un effet négatif sur la croissance des grands dorés. La disponibilité des proies est un facteur important pour la survie des dorés de toutes tailles, alors que la température n’affecte que les larves et les grands dorés. La densité des jeunes centrarchidés a un effet négatif sur la survie des larves de dorés, probablement à cause de la prédation. Nos meilleurs modèles prédictifs expliquent une partie substantielle de la variation de la survie chez les larves (97 %), les petits (57 %) et les grands (83 %) dorés. Nous pouvons aussi expliquer une forte proportion de la variation de la croissance chez les grands (98 %) et les petits (55 %) dorés et chez les larves (52 %). Notre étude démontre l’importance d’examiner plusieurs stades de vie pour prédire la croissance et la survie; elle apporte une meilleure compréhension du recrutement chez le doré et elle propose des recommandations sur les stratégies d’empoissonnement. [Traduit par la Rédaction]
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Introduction Growth and survival of young fish can be dependent on predation, prey availability, and abiotic factors. These factors can play an important role in both natural recruitment and stocking success and are often dependent on fish size. For example, abiotic factors often have the greatest effect on
growth and survival during initial life stages (Houde 1987; Claramunt and Wahl 2000). Similarly, smaller fish are more susceptible to predation and starvation than larger fish (Rice et al. 1987; Miller et al. 1988). Resident predators can be an important source of mortality for a number of stocked fish species (Wahl and Stein 1989; Hoxmeier and Wahl 2002; Henderson and Letcher 2003). These variables can affect
Received 11 October 2005. Accepted 23 May 2006. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 13 September 2006. J18929 R.J.H. Hoxmeier1,2 and D.H. Wahl. Kaskaskia Biological Station, Illinois Natural History Survey, RR 1, Box 157, Sullivan, IL 61951, USA. R.C. Brooks and R.C. Heidinger. Fisheries and Illinois Aquaculture Center, Southern Illinois University, Carbondale, IL 62901, USA. 1 2
Corresponding author (e-mail:
[email protected]). Present address: Minnesota Department of Natural Resources, 1801 South Oak Street, Lake City, MN 55041, USA.
Can. J. Fish. Aquat. Sci. 63: 2173–2182 (2006)
doi:10.1139/F06-087
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Can. J. Fish. Aquat. Sci. Vol. 63, 2006 Table 1. Surface area, latitude, and number of stockings for three size classesa of walleye (Sander vitreus) stocked into 15 Illinois lakes from 1991–1997. Number of stockings Lake Bloomington Dutchman East Fork Fox Chain George Kinkaid Leaquana Pierce Randolph Ridge Sam Dale Sara Shelbyville Springfield Sterling
Area (ha) 250 53 379 2663 68 1114 16 66 26 6 78 237 4455 1715 53
Latitude (N) 39°69 ′98′′ 37°30 ′00′′ 38°45 ′43′′ 42°28 ′26′′ 41°25 ′58′′ 37°46 ′25′′ 42°25 ′08′′ 42°20 ′43′′ 37°58 ′25′′ 39°27 ′08′′ 39°00 ′52′′ 39°20 ′01′′ 39°25 ′07′′ 39°42 ′43′′ 42°28 ′26′′
Total
Larval 3 5 3
5 5 4 1 6 5 4 41
Small 5 1 2 2 4 4 5 7 5 2 5 4 5
Large
2 7 5 3
4
3 1 5
55
26
a
Size classes: larval (6 mm total length (TL)); small (46.0 ± 0.9 mm TL (mean ± 1 standard error)); large (99.7 ± 2.0 mm TL).
growth and survival at multiple life stages, suggesting that recruitment may not be set at a single stage (Parkos and Wahl 2002). Assessments of which variables are important in determining recruitment during early life stages of walleye (Sander vitreus) are needed for this important sportfish. A number of studies have assessed the importance of individual factors, but few have either examined them in combination to determine their relative effects or assessed their importance across size. Variability in water temperature and spring water levels can affect age-0 walleye survival (Serns 1982; Quist et al. 2003). Younger stocked walleye are more susceptible to change in water temperature than larger individuals (Santucci and Wahl 1993; Clapp et al. 1997). Predation can also be a limiting factor in walleye survival, especially in lakes with dominant centrarchid populations. For example, white crappie (Pomoxis annularis) abundance has been negatively related to walleye recruitment (Quist et al. 2003). In addition, largemouth bass (Micropterus salmoides) consumed up to 17% of a stocked walleye population (Santucci and Wahl 1993). Predation rates on walleye are also related to size, with smaller walleye being more susceptible to predation than larger walleye (Santucci and Wahl 1993). Prey resources important to walleye change during their first year of life through ontogenetic diet shifts. Walleye begin feeding on zooplankton after endogenous feeding at about 8 mm total length (TL) and then switch to macroinvertebrates (35–50 mm TL) and eventually to fish (60–80 mm TL) (Priegel 1969; Mathias and Li 1982; Galarowicz and Wahl 2005). The availability of these resources during the appropriate life stage can influence both growth and survival. An increase in zooplankton density and species composition has been shown to affect both growth and survival of larval walleye (Mayer and Wahl 1997; Hoxmeier et al. 2004), whereas larger walleye have been
shown to rely on larger prey items such as benthic invertebrates and fish (Priegel 1969; Mathias and Li 1982; Kolar et al. 2003). Stocking walleye to create or supplement existing populations is a common practice in North America. Often times, a single size is stocked throughout a region because of its availability or because a particular size has generally been shown to have high survival. Survival varies by lake and year (Brooks et al. 2002), but specific underlying mechanisms are not well understood. Matching the size of stocked walleye to the appropriate lake characteristics could further increase survival. Prey availability, predators, and abiotic factors could all affect growth and survival of stocked walleye. The interaction between walleye size and factors influencing their growth and survival is likely complex. Most studies have examined these relationships in laboratory settings rather than in natural environments. Previous field studies have generally focused on the importance of a single factor on recruitment or stocking success. The objectives of our study were to determine factors influencing the growth and survival of walleye during the first year of life. Specifically, we examined the effects of prey (zooplankton, benthic invertebrates, and fish), predation, and abiotic factors (temperature and transparency) on three sizes of walleye stocked into Illinois reservoirs. Our hypothesis was that the relative importance of these factors would change with walleye size. Based on our results, we also present guidelines to help increase success of walleye stocking programs.
Materials and methods Study area We sampled 15 lakes across Illinois ranging in size from 6 to 4500 ha (Table 1), with depths from 8 to 24 m. Lakes also © 2006 NRC Canada
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varied in latitude, productivity, prey resources, and predator abundances. Largemouth bass were the most abundant predator and were present in all study lakes. Sunfish (Lepomis spp.), white bass (Morone chrysops), yellow bass (Morone mississippiensis), crappie (Pomoxis spp.), and channel catfish (Ictalurus punctatus) were abundant in most lakes. Gizzard shad (Dorosoma cepedianum), sunfish, and brook silversides (Labidesthes sicculus) were the primary prey species. Naturally reproducing walleye were not present at the time of stocking in any of our study lakes and no unmarked fish were recovered. Aquatic vegetation consisted primarily of pondweed (Potomogeton spp.), coontail (Certatophyllum demersum), water milfoil (Myriophyllum spp.), and niad (Najas spp.). Walleye were stocked as larval (6 mm TL), small (46.0 ± 0.9 mm TL, mean ± 1 standard error (SE)), or large (99.7 ± 2.0 mm TL) sizes during 1991 through 1997. We monitored the success of 41 larval, 55 small, and 26 large walleye stockings in the 15 study lakes (Table 1). All three size classes were stocked in five lakes, whereas six lakes were stocked with two size classes, and four lakes were stocked with small walleye only. Walleye were typically stocked in April as larvae, June for the small size class, and August for the large size class. Walleye were obtained from Jake Wolf Memorial Fish Hatchery and LaSalle Fish Hatchery, Illinois. Before stocking, all size classes of walleye were given a distinct mark for future identification. Larval and small walleye were marked by immersing them in 500 mg·L–1 oxytetracycline (OTC) for 6 h before stocking (Brooks et al. 1994). Larval walleye had OTC marks located close to the otolith nucleus, whereas small walleye had OTC marks further out from the otolith center. Large walleye were marked by removing either the right or left pelvic fin, depending on the year stocked. Stocking rates for larval walleye ranged from 1000 to 7000·ha–1 but were most often stocked at around 3000·ha–1. Target numbers for stocking were 90·ha–1 for small and 65·ha–1 for large size classes, but varied with availability in some instances. Larvae were transported to the lake in oxygenated bags in styrofoam coolers, whereas the larger size classes were transported in oxygenated hauling tanks. To reduce mortality from stocking stress (Clapp et al. 1997), walleye larvae were tempered by floating the hatchery bags in the lake until water temperatures in the bag and lake were within 1 °C. Larger size classes were acclimated by transferring lake water into the hauling tanks until water temperatures were equalized. We measured 50 individuals (TL, mm) for each walleye stocking event. To examine success of stocked walleye, we conducted night shoreline electrofishing twice monthly in each lake during fall (September–December). All walleye were measured, weighed, and examined for clips. A subsample of unclipped walleye was frozen for later examination of OTC marks. Within 2 weeks of being removed from individual walleye, otoliths were examined for marks in the laboratory, using a compound microscope equipped with a 100-watt ultraviolet light source, 450–490 nm excitation filter, 515 nm barrier filter, and a 510 nm dichroic mirror. Population estimates were completed with the Schnable multiple census method. Because population estimates were not possible on a number of lakes because of an insufficient number of re-
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captures, survival of stocked walleye was subsequently based on catch-per-unit-effort (CPUE) from electrofishing. CPUE was highly correlated to number of fish per hectare (N·h–1) based on population estimates (log-transformed, r = 0.79, n = 27, P < 0.0001). Stomach contents of potential predators on stocked walleye were examined by gastric flushing (Foster 1977). Predator diets were examined for three consecutive days following each stocking and weekly thereafter until walleye were no longer found. Growth of walleye was calculated as the difference in TL between time of stocking and collection in fall. Data for abiotic and biotic variables that may influence age-0 walleye survival were collected at the time of stocking and at biweekly intervals thereafter through October. Zooplankton were collected at six sites on each sampling date in each lake with a 0.5 m diameter zooplankton net with 64 µm mesh. Samples were taken from the thermocline to the surface during stratification or the entire water column when the lake was not thermally stratified. Zooplankton samples were preserved in a 4% Lugol’s solution and returned to the laboratory for processing. Zooplankton samples were adjusted to a volume of 100 mL and were subsampled by 1 mL aliquots until 200 organisms from the major groups were counted or until 10% of the sample was counted (Welker et al. 1994). Copepods were identified as either cyclopoids or calanoids, and cladacerans were identified to genus. Individuals (N = 30) from each taxonomic group were measured (nearest 0.01 mm) with the aid of a digitizing pad. Copepod nauplii and rotifers were not included in density estimates because they are rarely consumed by age-0 walleye (Houde 1967; Mathias and Li 1982; Hoxmeier et al. 2004). Benthic invertebrates were collected at four sites in each lake with a Ponar bottom sampler. Samples were passed through a 600 µm sieve and preserved in a Rose bengal – ethanol solution. Samples were returned to the laboratory where they were counted and identified to order. To estimate forage fish densities, we sampled both larval and juvenile prey fish in each lake. Larval fish were collected by towing a 0.5 m diameter ichthyoplankton net with 500 µm mesh behind the boat for 5 min at six offshore sites in each lake. Volume of water filtered was estimated from a flowmeter mounted in the opening of the net. Larval fish were preserved in ethanol and identified to genus. Seine hauls were taken with a 9.2 m bag seine (3.2 mm mesh) along the shoreline at four sites to estimate juvenile fish densities. Fish collected in the seine were identified to species, counted, and measured (nearest 1.0 mm). We used the number of fish between 20 and 50 mm TL per seine haul as a relative measure of prey fish abundance based on preferred prey sizes reported for walleye in the literature (Parsons 1971; Einfalt and Wahl 1997). Abiotic variables examined included dissolved oxygen, turbidity, and water temperature. Dissolved oxygen and water temperature readings were taken every 2 weeks at 1 m depth intervals with a YSI oxygen meter (YSI Inc., Yellow Springs, Ohio) at the deepest area in the lake. Water transparency was measured with a secchi disk at the same location. Because we did not have daily water temperature data for all lakes, air temperature data from the Illinois State Water Survey were analyzed as a metric of lake temperature © 2006 NRC Canada
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conditions. Cooling degree-days (CDDs) were obtained for each lake from the nearest weather station and were highly correlated with mean summer water temperatures from a subset of the study lakes (r = 0.86, n = 22, P < 0.0001). Warmer summer air temperatures are indicated by higher number of cooling degree-days. Correlational analysis was used to examine individual relationships between environmental variables and walleye survival and growth. Multiple regression was used to examine the contribution of select variables to walleye growth and survival. Variables were added to the model in a hierarchical approach based on their presumed causal priority and results from individual correlations (Cohen and Cohen 1983). Independent variables were only added to the model if they were significant at α = 0.1. To avoid temporal pseudoreplication, growth rates and CPUE of walleye, along with summer means of biotic and abiotic variables, were averaged across years for each lake when conducting regression analysis. Because some individual lakes were not stocked all of the 7 years, sample sizes for mean environmental variables differed. Data were tested with the UNIVARIATE procedure in SAS (SAS Institute Inc. 2001) and were log-transformed as needed to normalize the distributions before analyses. Because of the high variability associated with field data, we used α = 0.1 to denote significance. Lake Leaquana was removed from regression analysis for larval sizes as the lake was atypical of the rest of the study lakes because of extremely high nutrient loads and hypoxic conditions at the time of stocking.
Can. J. Fish. Aquat. Sci. Vol. 63, 2006 Fig. 1. Survival of (a) larval, (b) small, and (c) large fingerling stocked walleye (Sander vitreus) related to benthic invertebrate densities (N·m–2). Catch-per-unit-effort (CPUE) is the mean number of walleye collected per hour of night electrofishing the following fall after stocking. Each data point represents a mean value of CPUE and invertebrate density across years for each lake.
Results Survival Survival of young-of-year (YOY) walleye varied across lakes and size classes. Small fingerlings had higher survival than both larval and large size classes across most lakes; however, in some lakes, larval or large sizes had the highest survival (Brooks et al. 2002). Although stocking densities sometimes varied, we did not see an effect on survival for any size class (larval, r = –0.03, n = 40, P = 0.86; small, r = –0.16, n = 53, P = 0.27; large, r = –0.02, n = 22, P = 0.96). Prey availability, predators, and latitude were important variables determining the survival of walleye; however, the importance of these variables varied by fish size. Benthic invertebrates and juvenile fish density were the most important prey influencing walleye survival. Higher benthic invertebrate densities resulted in increased survival for both larval (P = 0.01) and large (P = 0.03; Fig. 1) walleye. However, the small walleye size class was not affected by benthic invertebrate density (P = 0.15). Bluegill (Lepomis macrochirus), gizzard shad, and brook silversides were the most common forage fish species found in both larval and seine samples. Mean summer larval fish abundance ranged from 0.01 to 16.4 m–3 across lakes but was not correlated with survival of larval (r = –0.21, n = 9, P = 0.60), small (r = 0.37, n = 14, P = 0.20), or large (r = 0.11, n = 7, P = 0.81) sizes of walleye. However, abundance of forage fish based on shoreline seining was important for small (P = 0.01; Fig. 2) but not larval (r = –0.57, n = 9, P = 0.11) or large (r = –0.27, n = 7, P = 0.55) walleye.
Mean zooplankton densities during summer ranged from 12 to 420 L–1 (excluding copepod nauplii and rotifers) across lakes. Despite the wide range in zooplankton density, we did not detect any affect on survival of larval (r = 0.17, n = 9, P = 0.65), small (r = –0.17, n = 14, P = 0.55), or large (r = 0.53, n = 7, P = 0.22) size classes. Because zooplankton density at the time of stocking could be particularly important for larval walleye, we also examined these relationships. Zooplankton densities at the time of larval walleye stocking also ranged widely from 3 to 396·L–1. However, similar to results for summer densities, there were no significant effects of zooplankton on survival of larval fish (r = 0.16, n = 9, P = 0.68). Predation had more of an effect on survival of larval walleye than on the survival of larger size classes. Although we did not directly assess predation on larval walleye at the time of stocking, survival of larval walleye was negatively correlated with juvenile centrarchid density (P = 0.01; Fig. 3). We were able to directly measure the amount of predation on fingerling walleye because of their larger size. However, the number of predators with walleye in their diet © 2006 NRC Canada
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Fig. 2. Survival of small walleye (Sander vitreus) related to the number (N) of forage fish (20–50 mm) per metre of shoreline. Catch-per-unit-effort (CPUE) is the mean number of walleye collected per hour of night electrofishing the fall following stocking. Each data point represents a mean value of CPUE and forage fish density across years for each lake.
Table 2. Numbers of predator fish containing age-0 walleye (Sander vitreus) collected by electrofishing after stocking. Diets with walleye Predator species Bluegill Largemouth bass Muskellunge Smallmouth bass Walleye Crappie spp. White bass
Stomachs examined (N) 184 12 226 384 248 10 026 240 216
2 200 9 2 43 6 3
% 1.1 1.6 2.3 0.8 0.4 2.6 1.4
Total
23 524
265
1.1
N
Note: Predator diets were examined for three consecutive days following each stocking and weekly thereafter until walleye were no longer found. Data are combined across 15 lakes in Illinois from 1991 to 1997. N, number.
was low for both small and large walleye size classes (1.1%; Table 2). The most abundant predators on small and large walleye in our study lakes were largemouth bass. Similar to results from stomach analyses, adult (>200 mm TL) largemouth bass abundance was not significantly correlated with survival for either small (r = 0.14, n = 14, P = 0.64) or large (r = –0.49, n = 7, P = 0.27) size classes. Larval and large walleye had higher survival in lakes located in cooler latitudes as measured by CDDs (larval, P < 0.01; large, P = 0.04; Fig. 4). CDDs ranged from 456 to
Fig. 3. Survival of larval walleye (Sander vitreus) related to juvenile centrarchid densities (N·m–1 shoreline). Catch-per-uniteffort (CPUE) is the mean number of walleye collected per hour of night electrofishing the fall following stocking. Each data point represents a mean value of CPUE and centrarchid density across years for each lake.
1705 and were directly related to the latitude at which the lake was located. In contrast, small walleye survival was not related to CDDs (P = 0.53). We developed predictive models using multiple regression to further explore relationships with walleye survival. The combination of CDDs, benthic invertebrate densities, and juvenile centrarchid density accounted for 97% of the variability in larval walleye survival (multiple regression, P < 0.01; Table 3). A high proportion of the variability in survival for small walleye (57%) was accounted for by prey fish density and benthic invertebrate densities (P < 0.01; Table 3). The best model for large walleye survival included benthic invertebrate density and CDDs (R2 = 0.83, P = 0.03; Table 3). Growth Factors important in determining growth were different than those for survival across all size classes of walleye. CDDs had a positive effect on larval walleye growth (P = 0.04) but a negative effect on large walleye growth (P = 0.08; Fig. 5). Larval walleye at more northern latitudes were around 180 mm TL when collected in the fall, whereas those collected in southern latitudes were over 220 mm TL. Large walleye grew over 50 mm from the time they were stocked until they were collected at northern latitudes but only grew 36 mm in the same time period at more southern latitudes. Zooplankton density did not affect growth of larval (r = –0.10, n = 8, P = 0.81), small (r = 0.01, n = 14, P = 0.96), or large (r = 0.25, n = 5, P = 0.68) walleye. However, growth of both small and large walleye increased with benthic invertebrate density. Large walleye were influenced by total ben© 2006 NRC Canada
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Can. J. Fish. Aquat. Sci. Vol. 63, 2006 Table 3. Multiple regression analysis of factors affecting survival (fall catch-per-unit-effort) of three size classes of stocked walleye (Sander vitreus) in Illinois lakes from 1991 to 1997. Size
Variable
Coefficient
Partial R2
Model R2
F value
P value
Larval
Log(cooling degree-days) Log(benthic invertebrate density) Log(centrarchid density) Overall model Forage fish density Benthic invertebrate density Overall model Benthic invertebrate density Log(cooling degree-days) Overall model
–1.49 1.13 –1.35 2.73* 0.06 0.00008 –0.05* 0.0007 –13.93 42.8*
0.73 0.13 0.11
0.73 0.86 0.97 0.97 0.44 0.57 0.57 0.63 0.83 0.83
5.66 23.23 16.90 50.52 10.56 3.42 7.37 5.06 4.56 9.51
0.06
