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A Comparison of Extensively and Intensively Raised Early-Life Stage Walleye fed
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Minnows
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Samuel D. Hempel*,1
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Northern Aquaculture Demonstration Facility
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University of Wisconsin-Stevens Point
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2100 Main Street
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Stevens Point, WI 54481-3897
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* Corresponding author: Samuel.D.Hempel@uwsp.edu
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167 TNR Building, 800 Reserve Street, Stevens Point, WI 54481-3897
Present address: Biology Department, University of Wisconsin-Stevens Point,
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Abstract
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Two commonly used techniques for propagating Walleye are intensive and
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extensive rearing treatments; fish reared using both methods can be fed commercialized
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feed or natural feed or a combination of the two. The two treatments have differing
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control levels with their own advantages. The objective of this research was to compare
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growth of Walleye reared using intensive and extensive methods. Walleye fry were
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reared in either an intensive system (commercial feed) or extensive system (zooplankton)
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for 54 to 70 days. At that time juvenile Walleye were brought indoors, separated by
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treatment, and evenly distributed among three flow-through tanks at a density of 20 fish
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per tank. All fish were then fed fathead minnows for the duration of the experiment. Total
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length (mm) and weight (g) of all fish were recorded every four days. Mean total length
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and mean weight gained and specific growth rates were calculated and compared between
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the treatments. The intensive treatment exhibited significantly higher sizes at the
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beginning and end of the experiment, along with total length and total weight gained over
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the course of the experiment. However, specific growth rates were higher in the extensive
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treatment. The results suggest that the extensive treatment was deficient in essential
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nutrients and when provided ample feed, growth rates will increase.
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Introduction
Walleye Sander vitreus are a cool-water species native to a substantial portion of
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North America and are among the most sought after fishes by recreational anglers
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(NEED CITATION). Due to their popularity, Walleye have been stocked far outside
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their original distribution (Summerfelt 2000). They are a large predatory fish that feed
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exclusively on living organisms, such as zooplankton, insects, and other fish
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(Kapuscinski 1986), and exhibit certain characteristics (including intractable feeding
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habits and a tendency toward cannibalism when young), walleye are very difficult to
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culture (Kapuscinski 1986). The quality and taste is desired by fishermen and consumers
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throughout the nation. They are described as one of the “best eating of all freshwater
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fishes” (Carmichael et al. 1991), and “one of the most delicious of fresh-water fishes”
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(Cameron and Jones 1983). Their popularity as a food fish continues to grow for and it is
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one of the most targeted species in Wisconsin for recreational fishermen.
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Walleye has been recognized as a species with substantial aquaculture potential in
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the National Aquaculture Plan (Joint Subcommitte on Aquaculture 1983). The Wisconsin
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Walleye Initiative, started in May 2013, has provided funding to produce large fingerling
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walleye in aquaculture systems for Wisconsin waters. Public hatcheries have spawned,
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incubated, and hatched walleye for more than 100 years (Summerfelt 2000).
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Two commonly used techniques for propagating walleye are intensive and
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extensive rearing treatments. Intensively reared walleye are hatched and raised in a
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controlled indoor environment, or tank, that has less vulnerability to biohazards and
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diseases, and permits-control of environmental factors such as temperature, light,
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turbidity, and effluents. Intensive culture has many factors in its favor: It is not subjected
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to variable environmental conditions, so cultural conditions are fairly stable, temperature
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can be controlled to lengthen growth and the growing season lengthened, nuisance
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aquatic organisms are eliminated, and the quality and quantity of the feed is controlled
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(Summerfelt 1996). Extensively reared walleye are hatched indoors and transferred to
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outdoor ponds with less control over their environmental factors. Variability in water
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quality and uncontrollable parameters, such as the weather, cause problems when trying
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to maintain adequate zooplankton populations (Summerfelt 1996). Feed is naturally
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available in the ponds (e.g., zooplankton and macroinvertebrates) but the fish must search
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out the prey to eat. When zooplankton populations are lacking, fish may starve (McIntyre
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et al. 1987).
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Walleye are predators and feed primarily on other fishes and feeding them
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commercial pellets in a cultured system can be difficult. Two of the primary techniques
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used to feed walleye in cultured settings are commercialized feed or natural feed such as
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minnows. Walleyes start to feed on fish when they reach 30-50 mm total length (Smith
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and Pycha 1960; Forney 1966; Bulkley et al. 1976). More specifically, fathead minnows
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Pimephales promelas, because they are highly cultured as baitfish and are relatively easy
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to propagate, making them economically practical. Development of formulated feeds and
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modified rearing techniques for the intensive production of walleyes would bring several
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advantages over the use of live feed: labor costs associated with the collection or culture
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of live feeds would be eliminated; and introductions of disease and contaminants would
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be much less likely (Barrows et al. 1988). Commercialized or formulated feed (pellets)
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can come from all over the world and in comparison, commercialized feeds are cheaper
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because the resources are easier to obtain, packaging is simplified, and transportation is
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less complicated.
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Knowledge of the two feeding techniques varies between treatments. Strategies
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consist of using either commercialized or natural feed for the entire rearing process, or a
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mix of both techniques, depending on what works best for a given producers’ situation.
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Feeding a predator something natural (e.g. minnows) can have setbacks, reports on
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converting (habituating) pond-reared fingerlings to commercial feeds (Cheshire and
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Steele 1972), confirm that walleye can be domesticated on commercial feed to reduce or
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eliminate setbacks such as late arrival of minnows or gape limitations of Walleye.
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The objective of this study was to determine if intensively reared early life-stage
walleye exhibited faster growth than extensively reared walleye fed minnows.
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Methods
Experimental Design and Study Environments
Eyed-eggs were obtained from the Genoa National Fish Hatchery on April 29th,
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2014 (Mississippi River Strain) and hatched on May 5th, 2014. Three days after hatch, fry
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were transferred to a 240 L fiberglass tank and given a commercially available feed. They
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remained on commercial feed until the start of the experiment but were transferred to
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1000 L tanks on May 25th, 2014. These fish will be referred to as the intensive treatment.
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Eyed-eggs were also obtained from Hayward Bait (Upper Chippewa River Strain) and
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hatched on May 21th, 2014. These eggs were collected by the Wisconsin Department of
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Natural Resources Governor Thompson Fish Hatchery. Three days after hatch, 80,000 fry
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were transferred to a 0.16 ha (0.4 acre), clay-lined pond and were allowed to feed on
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zooplankton and aquatic invertebrates until July 14th, 2014, the start of the experiment.
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These fish will be referred to as the extensive treatment.
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On July 14th, 2014(day 1 of the experiment), 60 intensive age-0 walleye were
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randomly selected and transferred from a 1000 L holding tank into three-240L round
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fiberglass tanks at a density of 20 fish per tank (Figure 1). At the same time, 60 extensive
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age-0 walleye were randomly selected and transferred indoors into three-240 L round
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fiberglass tanks at a density of 20 fish per tank. Weight (g), and total length (TL; mm),
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was recorded for all individual fish.
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All tanks were on a flow-through system with flow rates at 4L/min. In-flow water
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was degassed, aerated, and held at a temperature of 20°C. Low light conditions were
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accomplished by enclosing the experimental area under black tarps and overhead lights at
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a low setting. Dissolved oxygen, temperature, and pH were recorded daily. Turbidity of
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the water was zero.
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Sampling Design
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Total fish biomass (g) was calculated for each tank and 20% of total biomass was
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given as feed in fathead minnows the next day. Fathead minnows were purchased as feed
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for both intensive and extensive treatments. They were kept in a separate holding tank
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away from the experimental tanks. Once tempered to approximately 20°C, minnows were
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weighed to 20% total fish biomass per tank and added to the tanks in the afternoon.
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Feeding activities of walleye were recorded for observational data. Left over minnows
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were removed every morning and wet weight of the total minnow biomass left in the
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tanks were recorded daily.
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Every four days Walleye were sedated with MS-222 (Tricaine Methanesulfonate)
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and removed from the tanks for cleaning. At this time we measured and recorded total
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length and wet weight (g) for all fish. Mortalities were measured, recorded, and removed.
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Total fish biomass (g) was calculated for each tank after each sampling day and 20% of
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that was given as feed. The experiment lasted for six weeks and all fish were sampled for
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final measurements (i.e. weight and length).
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Statistical Analysis
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Weight gain, length gain, and specific growth rate (SGR) of weight and length
were calculated using the following formulas
Weight Gain = wt2-wt1
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Length Gain = TL2-TL1
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SGR = [loge(xf) – loge(xi)]/ET ×100
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where, wt1 = mean weight (g) at initial time point, wt2 = mean weight (g) at the final time
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point, TL1 = total length (mm) at initial time point, TL2 = total length (mm) at final time
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point, ET = elapsed time (d) between sample days, xi = initial weight or length, and xf =
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final weight or length.
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A two sample t-test: assuming unequal variances was used to determine if length and
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weight differed between treatments at the beginning of the experiment. Analysis of
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covariance (ANCOVA) was used to determine if growth (weight and length gained)
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differed between treatments over time (∝= 0.05). Linear regressions were used to
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level. Average weight and total length were plotted against days post hatch.
determine if specific growth rates differed between treatments at the 95% confidence
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Results
Mean Total Length Comparisons
Mean total length was significantly different between the two treatments at the
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beginning of the experiment and there was no overlap of the upper and lower 95%
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confidence intervals (t Stat=34.753, df=80) (Table 1). Standardized data showed Walleye
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in the intensive treatment grew at a rate of 1.30 mm, while Walleye from the extensive
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treatment grew at a rate of 0.86 mm (Figure 2). Mean total length gain was significantly
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faster in the intensive treatment than the extensive treatment (F=8.659, df=5, P=0.0604).
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The intensive treatment grew at a rate of -0.09 mm/d, while Walleye from the extensive
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treatment grew at a rate of 0.15 mm/d (Figure 3). When the data from sample day 86 was
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removed, the intensive treatment grew at a rate of 0.03 mm/d, while Walleye from the
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extensive treatment grew at a rate of 0.31 mm/d (Figure 4). Mean total length and mean
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total length gain exhibited faster growth for the intensive treatment than the extensive
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treatment.
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Specific growth rate increased significantly faster in the extensive treatment than
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the intensive treatment. The extensive treatment had a slope that grew at a rate of 0.04
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mm/mm/d, while Walleye from the intensive treatment slope grew at a rate of -0.03
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mm/mm/d (Figure 5).
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Mean Weight Comparisons
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Mean weight was significantly different between the two treatments at the
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beginning of the experiment and there was no overlap of the upper and lower 95%
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confidence intervals (t Stat=24.653, df=61). The intensive treatment slope grew at a rate
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of 0.3987 g which was over three times greater than the extensive rate of 0.1325 g
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(Figure 6). Mean weight gain was significantly faster in the intensive treatment than the
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extensive treatment (F=15.9246, df=7, P=0.0104). The intensive treatment grew at a rate
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of 0.0954 g/d, while Walleye from the extensive treatment grew at a rate of 0.0388 g/d
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(Figure 7). Mean weight and mean weight gain exhibited faster growth for the intensive
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treatment than the extensive treatment.
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Specific growth rate increased significantly faster in the extensive treatment than
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the intensive treatment. The extensive treatment grew at a rate of 0.125 g/g/d, while
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Walleye from the intensive treatment grew at a rate of 0.1027 g/g/d (Figure 8).
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Discussion
We found that, on average, Walleye from the extensive treatment exhibited higher
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specific growth than Walleye from the intensive treatment. A similar relationship has
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been seen between intensive and wild populations as the growth rate of intensively
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cultured walleyes is usually less than that of fish in the wild (Malison et al. 1990).
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Specific growth rate of both length and weight of the extensive treatment increased at a
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higher rate than the intensive treatment. However, total weight gained and total length
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gained were both higher for the intensive treatment compared to the extensive treatment.
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Total weight and length gained comparisons may not be appropriate because the two
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treatments differed significantly in both length and weight at the beginning of the
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experiment; the intensive treatment was overall larger in size than the extensive treatment
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and remained higher throughout the experiment. In intensive culture, growth rates can be
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increased or slowed by temperature manipulations (Summerfelt 2000,) which could have
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contributed to the overall larger size of the intensive treatment throughout the experiment
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because they were reared at higher temperatures before the experiment began.
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Two different strains were used in the experiment and were derived from two
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different broodstocks. One was from Genoa National Fish Hatchery and consisted of the
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Mississippi strain (Intensive Treatment), while the other broodstock were collected by
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Wisconsin Department of Natural Resources Governor Thompson Fish Hatchery and
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consisted of the Hayward Bait strain (Extensive Treatment). Hatch dates differed between
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the two strains and the data was standardized to make accurate comparisons between the
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two treatments. Three replicates helped to eliminate error and determine significance.
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Our SGR results suggest that the extensive treatment outperformed the intensive
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treatment. A factor that could have influenced this was the extensive treatment moving
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from a less nutrient rich pond to a higher nutrient environment of minnows being
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provided in limited space. The more nutrient rich environment of the experiment is
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supported by the SGR results and suggests greater assimilation of feed than the intensive
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treatment. Due to their fast growth rate, fish larvae have a high requirement for amino
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acids, the building blocks for the protein synthesis (Ronnestad et al. 2000). These nutrient
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requirements may have been limiting for the extensive treatment compared to the
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complete early-life stage diet that the intensive treatment received. The decreasing rate of
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SGR (total length) for the intensive treatment may be correlated with observational data
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that fin nipping was occurring and in fact had an effect on growth in length. This could
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not be determined as 24 hour surveillance did not occur but we suggest future studies to
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focus on such behaviors. Although the intensive treatment was larger in size overall, we
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refute our hypothesis that intensively reared walleye grew at a faster rate than extensively
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reared walleye.
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The results of weight and length gain suggest that the intensive treatment grew at
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a faster rate. This was possibly because the extensive treatment needed more than the
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20% total fish biomass provided in minnow feed to grow at the same rate of the intensive
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treatment. The number of minnows provided per walleye for the extensive treatment was
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less than a 2:1 ratio while the intensive treatment had a ratio above 11:1 minnows to
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walleye. This occurred at the start of the experiment and may have had an additive effect
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throughout the experiment as there was not sufficient forage for optimal growth of the
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extensive treatment but as the number of minnows provided increased, so did their
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growth rates. Walleye studies show similarities to several studies that have been
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conducted on the effects of environmental temperature on growth, digestion, and
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metabolism in channel catfish (Shrable et al., 1969; West, 1966;Asadi, 1967; Kilambi et
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al., 1970), but few quantitative data are available on the interactions of feeding rates and
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temperature and their effects on growth, food conversion, and body composition of
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catfish (Andrews and Stickney 1972).
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A complete early-life stage diet may have also contributed to the overall higher
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growth of the intensive treatment as we propose that this treatment assimilated feed better
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than the extensive treatment. Stress may have limited feeding and assimilation of the
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extensive treatment when they were moved from a natural pond to indoor tanks with
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increased activities in and around the tanks. Wild broodstock were used; these walleye
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are skittish and more easily stressed by overhead movement and hatchery activities than a
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domesticated aquaculture species such as rainbow trout (Summerfelt 2000).
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Conclusion
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Rearing techniques and feed types will continue to change to optimize Walleye
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growth for both conservation and aquaculture purposes. To improve the experiment,
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Walleye strains should be the same as well as hatch dates to make a more accurate
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comparison of growth between the intensively and extensively reared treatments. The
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amount of spending a culturist is allowed can have a limiting effect on feed types to
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provide the best chances of optimal growth.
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Acknowledgments
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This research was funded by the University of Wisconsin Stevens Point, College
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of Letters and Sciences. I would like to thank everyone involved at the UWSP-Northern
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Aquaculture Demonstration Facility and the University of Wisconsin-Stevens Point. I
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would also like to thank Dr. Hartleb and Greg Fischer for a great learning experience. I
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would also like to thank the Genoa National Fish Hatchery, Wisconsin DNR Governor
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Thompson Fish Hatchery, and Hayward Bait & Bottle Shoppe.
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Table 1. Summary of significance for mean total length and weight and mean total length
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and weight gained of Walleye between treatments from the UWSP-Northern Aquaculture
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Demonstration Facility 2014 intern project.
df
t-Stat
F-Value
P-Value
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34.753
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-
61
24.653
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-
5
-
8.659
0.0604
7
-
15.9246
0.0104
Mean Total
Length
Mean Weight
Mean Total
Length Gained
Mean Weight
Gained
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temperature on growth, food conversion, and body composition of Channel
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Kilambi, R.V., J. Noble, and C.E. Hoffman. 1970. Influence of temperature and
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Malison, J.A., T.B. Kayes, J.A. Held, and C.H. Amundson. 1990. Comparative survival,
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McIntyre, D.B., F.J. Ward, and G.M. Swanson. 1987. Factors affecting cannibalism by
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Figure Legends
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Figure 1. Schematic outlining the study design of the intensive and extensive walleye
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reared at the Northern Aquaculture Demonstration Facility during the summer of 2014.
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Walleye started out in large circular tanks (intensive) and 0.4 acre outdoor rearing ponds
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(extensive) prior to the experiment starting. At this time the fish were transferred to 240 L
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indoor tanks at a density of 20 fish per tank.
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Figure 2. Mean total length for days post hatch of walleye from the 2014 UWSP-
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Northern Aquaculture Demonstration Facility intern project. Black diamonds and dashed
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line represent the intensive treatment. White squares and a solid line represent the
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extensive treatment.
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Figure 3. Mean total length gained for days post hatch of walleye from the 2014 UWSP-
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Northern Aquaculture Demonstration Facility intern project. Black diamonds and dashed
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line represent the intensive treatment. White squares and a solid line represent the
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extensive treatment.
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Figure 4. Mean total length gained for days post hatch of walleye, excluding day 86, 2014
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UWSP-Northern Aquaculture Demonstration Facility intern project. Black diamonds and
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dashed line represent the intensive treatment. White squares and a solid line represent the
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extensive treatment.
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Figure 5. Specific growth rate (SGR) of total length for days post hatch of walleye from
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the 2014 UWSP-Northern Aquaculture Demonstration Facility intern project. Black
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diamonds and dashed line represent the intensive treatment. White squares and a solid
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line represent the extensive treatment.
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Figure 6. Mean weight for days post hatch of walleye from the 2014 UWSP-Northern
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Aquaculture Demonstration Facility intern project. Black diamonds and dashed line
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represent the intensive treatment. White squares and a solid line represent the extensive
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treatment.
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Figure 7. Mean weight gained for days post hatch of walleye from the 2014 UWSP-
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Northern Aquaculture Demonstration Facility intern project. Black diamonds and dashed
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line represent the intensive treatment. White squares and a solid line represent the
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extensive treatment.
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Figure 8. Specific growth rate (SGR) of weight for days post hatch of walleye from the
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2014 UWSP-Northern Aquaculture Demonstration Facility intern project. Black
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diamonds and dashed line represent the intensive treatment. White squares and a solid
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line represent the extensive treatment.
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Figure 1.
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Figure 2.
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Figure 8.