Use of Suspended Plastic Conduit Arrays during Brown Trout and Rainbow Trout Rearing in Circular Tanks

North American Journal of Aquaculture 81:101 106, 2019 2018 American Fisheries Society ISSN: 1522-2055 print / 1548-8454 online DOI: 10.1002/naaq.10076 COMMUNICATION Use of Suspended Plastic Conduit Arrays during Brown Trout and Rainbow Trout Rearing in Circular Tanks Sarah C. White Benedictine College, 1020 North 2nd Street, Atchison, Kansas 66002, USA Eric Krebs, Nathan Huysman, Jill M. Voorhees, and Michael E. Barnes* South Dakota Department of Game, Fish, and Parks, McNenny State Fish Hatchery, 19619 Trout Loop, Spearfish, South Dakota 57783, USA Abstract Environmental enrichment is the modification of otherwise sterile hatchery rearing units to provide structural complexity. We investigated the use of an array of suspended plastic conduit sections as enrichment in large circular tanks during two experiments. Brown Trout Salmo trutta and Rainbow Trout Oncorhynchus mykiss were reared for 126 and 61 d, respectively, in covered, 3.63-m-diameter circular tanks that were either void of any structure or enriched with a suspended array of twenty 0.94-m-long sections of plastic conduit. Total tank weight gain and feed conversion ratio were significantly improved for both Brown Trout and Rainbow Trout reared with suspended conduit as enrichment compared to unenriched tanks. Enrichment did not significantly affect individual fish length, weight, or condition factor in either experiment, likely because of small sample sizes. The suspended array did not interfere with tank hydraulic self-cleaning. Based on the results of this study, the use of vertically suspended enrichment structures in circular tanks is recommended to improve trout rearing efficiencies. Environmental enrichment is the deliberate addition of materials or structures to otherwise sterile hatchery rearing units to more closely mimic the natural environment of fish (N aslund and Johnsson 2016). Such enrichment has been shown to influence fish behavior and physiology (Berejikian et al. 2001; Millidine et al. 2006; Rodewald et al. 2011) and has been used in an attempt to increase the survival of fish released from conservation hatcheries (Berejikian et al. 1999, 2000; Berejikian and Tezak 2005; Brockmark et al. 2007; Fast et al. 2008; Bergendahl et al. 2017). Some enrichment techniques that have previously been investigated include the addition of materials such as woody or stony debris, plant or root material, plastic plants, or concrete blocks (Berejikian et al. 1999, 2000; Brown et al. 2003; Berejikian and Tezak 2005; Brockmark et al. 2007; Fast et al. 2008; Bergendahl et al. 2017; Krebs et al. 2017). Placing these types of structures in hatchery tanks can be problematic because they may trap food and feces, increase disease risk, or interfere with the hydraulic self-cleaning that is inherent to circular tanks (Tvinnereim and Skybakmoen 1989; Baynes and Howell 1993; Timmons et al. 1998; Tuckey and Smith 2001; Oca and Masalo 2007, 2012; Krebs et al. 2017). Thus, most hatcheries typically keep rearing units that are devoid of internal structures. Kientz and Barnes (2016), Kientz et al. (2018), and Crank et al. (2019) described novel enrichment techniques consisting of arrays of vertical metal rods or colored balls suspended from overhead tank covers. When added to small, circular rearing tanks, these arrays did not interfere with the self-cleaning nature of the tanks or increase hatchery labor requirements compared to sterile rearing tanks. The arrays were also shown to improve growth rates and feed conversion ratios (FCRs) of Rainbow Trout Oncorhynchus mykiss (Kientz and Barnes 2016; Kientz et al. 2018; Crank et al. 2019). Previous studies evaluating vertically suspended structure have been conducted only with Rainbow Trout and *Corresponding author: mike.barnes@state.sd.us Received September 6, 2018; accepted November 6, 2018 101

102 WHITE ET AL. only in relatively small, 1.8-m-diameter circular tanks (Kientz and Barnes 2016; Kientz et al. 2018; Crank et al. 2019). Therefore, the present study had two objectives. The first was to examine the effects of vertically suspended structural enrichment on a fish species other than Rainbow Trout. The second was to examine the effects of vertically suspended structural enrichment in larger circular tanks. We hypothesized that the previously reported benefits obtained from suspended enrichment in relatively small circular tanks would occur in larger units as well as extend to other salmonid species beyond Rainbow Trout. METHODS Methods common to both experiments. Two experiments were conducted at McNenny State Fish Hatchery (rural Spearfish, South Dakota) using well water at a constant temperature of 11 C (water hardness as CaCO 3 = 360 mg/l; alkalinity as CaCO 3 = 210 mg/l; ph 7.6; total dissolved solids = 390 mg/l). Tanks, rather than individual fish, were used as the experimental units. In each experiment, six tanks were used, with two different treatments (n = 3) assigned: barren control tanks and tanks with suspended structure as enrichment. Circular tanks (diameter = 3.63 m; height = 1.09 m; water depth = 0.71 m) were used. All tanks were nearly fully covered by corrugated plastic overhead covers (Barnes and Durben 2003; Barnes et al. 2005; Walker et al. 2016). Control tanks were devoid of any in-tank environmental enrichment structures, while experimental tanks included an array of vertically suspended linear enrichment structures that were modeled in part after the structures described by Kientz and Barnes (2016). The linear enrichment structures for the experiments in this study consisted of 20 hollow-plastic, polyvinyl chloride electrical conduit pipes (diameter = 4.34 cm; length = 0.94 m) in each tank; the pipes protruded downward from attachment points on the overhead covers. Pipes were evenly spaced, approximately 16.5 cm from each other, and arranged in an array within a 43-117-cm area. The array was situated approximately 58 cm from the edge of the tank, a quarter turn of the tank away from the spray bar through which water entered the tank (Figure 1A, B). For each experiment, fish from a common pool were evenly divided by weight into the six tanks. All fish were fed daily over an 8-h period during daylight hours using automatic belt feeders (Pentair Aquatic Ecosystems, Apopka, Florida). Feeding rates were determined for each experiment by the hatchery constant method (Buterbaugh and Willoughby 1967). Experiment 1: Brown Trout. The first experiment used juvenile Plymouth Rock-strain Brown Trout Salmo trutta (TL [mean SE] = 85 2 mm; weight = 6.9 0.5 g; N = 20) and was conducted from May 18 to September 21, 2017 (126 d). Brown Trout were obtained as eyed eggs from Saratoga National Fish Hatchery (Saratoga, Wyoming). Prior to the study, these fish were reared in 1.8-mdiameter, unenriched circular tanks from initial feeding on January 3, 2017. Each of the six tanks was initially stocked with 19.98 kg of fish (~2,900 fish/tank). Brown Trout were fed a diet consisting of 1.5-mm Skretting extruded floating Classic Trout feed (Skretting, Tooele, Utah). They were fed amounts based on a projected growth rate of 0.0550 cm/d and a planned feed conversion of 1.1. Final total tank weight, obtained by weighing nets containing approximately 15 kg of fish with a Chatillon C1110 hanging scale (Ametek, Berwyn, Pennsylvania) until the tank was empty, was recorded to the nearest 0.2 kg at the conclusion of the experiment. Additionally, at the end of the study, five randomly sampled fish from each tank were weighed to the nearest 0.1 g and measured for TL to the nearest 1.0 mm. Experiment 2: Rainbow Trout. The second experiment used juvenile Shasta-strain Rainbow Trout (TL [mean SE] = 242 4 mm; weight = 143.9 7.7 g; N = 20) and extended from February 16 to April 18, 2018 (61 d). Rainbow Trout were obtained as eyed eggs from Ennis National Fish Hatchery (Ennis, Montana). Prior to the study, the fish were reared in 1.8-m-diameter, unenriched circular tanks from initial feeding on March 18, 2017. Each of the six tanks was initially stocked with approximately 1,700 fish (mean weight = 261.54 kg). Rainbow Trout were fed a diet of 4.5-mm Skretting extruded floating Classic Trout feed at amounts slightly above satiation and based on a projected growth rate of 0.0800 cm/d and a planned feed conversion of 1.1. At the conclusion of the experiment, the final total tank weight was obtained by weighing nets containing approximately 15 kg of fish with a Chatillon C1110 hanging scale until the tank was empty; the final total weight was recorded to the nearest 0.2 kg. In addition, at the end of the study, five randomly sampled fish from each tank were weighed to the nearest 0.1 g, and their TLs were measured to the nearest 1.0 mm. Calculations and statistics. Total weight gain was calculated for each tank using the following equation: total weight gain = (final tank weight) (initial tank weight). Feed conversion ratio was determined as FCR = (total feed administered to the tank)/(total tank weight gain). Condition factor (K) was calculated for individual fish as K = 10 5 [weight/(body length) 3 ]. Data for both experiments were analyzed using one-way ANOVA in SPSS version 24.0 (Systat Software, Inc., Chicago). Because the tanks (not individual fish) were the experimental units, nested ANOVA was conducted on the individual fish data. Significance was predetermined at P < 0.05.

COMMUNICATION 103 FIGURE 1. (A) Top view of a 3.63-m-diameter circular tank, indicating the location of the suspended plastic conduit array used as environmental enrichment; and (B) side view of the tank containing the array of suspended plastic conduit sections. RESULTS Experiment 1: Brown Trout Final total tank weights, weight gain, and FCR were significantly improved (Table 1; total tank weight: F 1, 4 = 11.787, P = 0.026; weight gain: F 1, 4 = 11.787, P = 0.026; FCR: F 1, 4 = 9.765, P = 0.035) for Brown Trout that were reared in circular tanks with vertical enrichment compared to fish reared in sterile tanks. Specifically, total tank weight gain was over 18% greater and the FCR was over 16% lower in the tanks containing suspended arrays. However, no significant differences were observed between the treatments in individual fish weight (F 1, 4 = 4.003, P = 0.116) or K (F 1, 4 = 0.001, P = 0.976), although individual body length at the end of the experiment for fish reared with environmental enrichment was nearly significantly different from that of control fish (F 1, 4 = 9.765, P = 0.054; Table 2). Mortality was minimal and less than 0.5% in all tanks. Experiment 2: Rainbow Trout The total tank weight gain and FCR for Rainbow Trout were significantly improved with the addition of vertical enrichment structures to rearing units (Table 1;

104 WHITE ET AL. TABLE 1. Mean ( SE) tank total weight, weight gain, amount of feed administered, and feed conversion ratio (FCR = [total feed administered to tank]/[total tank weight gain]) for Brown Trout and Rainbow Trout reared in circular tanks with or without enrichment structures. For a given species, means with different letters within the same row differ significantly (P < 0.05; n = 3). Variable Control Structures Brown Trout Rearing days 126 126 Initial weight (kg) 20.0 0.0 20.0 0.0 Final weight (kg) 124.6 5.5 z 143.6 0.9 y Weight gain (kg) 104.6 5.5 z 123.6 0.9 y Feed administered (kg) 109.1 0.00 109.1 0.00 FCR 1.05 0.05 z 0.88 0.01 y Rainbow Trout Rearing days 61 61 Initial weight (kg) 269.1 0.7 z 252.5 1.9 y Final weight (kg) 395.6 8.1 z 415.9 6.5 z Weight gain (kg) 126.5 7.4 z 163.4 5.4 y Feed administered (kg) 213.2 0.0 213.2 0.0 FCR 1.70 0.10 z 1.31 0.04 y TABLE 2. Mean ( SE) individual fish TL, weight, and condition factor (K = 10 5 {[individual weight]/[body length 3 ]}) for Brown Trout and Rainbow Trout reared in circular tanks with and without enrichment structures (n = 3). Variable Control Structures Brown Trout TL (mm) 152 2 163 4 Weight (g) 45.4 2.8 56.0 4.5 K 1.28 0.03 1.29 0.04 Rainbow Trout TL (mm) 286 2 280 8 Weight (g) 252.6 10.3 238.6 21.0 K 1.08 0.02 1.09 0.02 total tank weight gain: F 1, 4 = 16.124, P = 0.016; FCR: F 1, 4 = 12.290, P = 0.025). Specifically, in the tanks containing arrays, there was a 29.2% increase in weight gain and a 22.9% decrease in FCR. Ending tank weight was not significantly different between the treatments (F 1, 4 = 3.840, P = 0.122), but initial tank weights were significantly lower in the enriched tanks (F 1, 4 = 64.713, P = 0.001). No significant differences were observed between the treatments in individual fish growth, including fish length (F 1, 4 = 0.708, P = 0.448), weight (F 1, 4 = 0.360, P = 0.581), or K (F 1, 4 = 0.128, P = 0.739; Table 2). Mortality was minimal and less than 0.5% in all tanks. DISCUSSION Despite occupying only a relatively small area of the 3.63-m-diameter circular tanks, the vertically suspended conduit structures produced a relatively large improvement in weight gain and FCR during Brown Trout and Rainbow Trout rearing. This is consistent with the findings of Kientz and Barnes (2016) and Kientz et al. (2018), who showed that vertically suspended environmental enrichment structures improved trout rearing performance in much-smaller diameter circular tanks. The results of this study suggest that the benefits of vertical rod arrays as an enrichment technique can be applied to salmonids other than Rainbow Trout and in different sizes of circular tanks. The improvement in rearing performance identified in this study may be partially due to an alteration in tank water velocity profiles with the addition of the suspended conduit. Vertical rods in smaller tanks have been shown to decrease water velocity behind the structure arrays, thereby creating lower-velocity microhabitats (Moine et al. 2016). Swimming speed, as dictated by water velocity, can influence salmonid feed efficiency (Kiessling et al. 1994). Thus, the refuge areas created by the suspended conduit array may explain the improvement in FCR in the enriched tanks, as such microhabitats would positively influence fish energy expenditure (Fausch 1984; Kientz and Barnes 2016). Although growth rate and FCR for both species in this study were significantly improved by enrichment, measures of individual fish condition were not significantly different. This is likely due to the small sample sizes used in these experiments (Kuehl 2000). Further experimentation with additional replication would allow for a more robust evaluation. However, the positive effects of vertically suspended arrays as enrichment may be fairly strong, given the fact that significant differences in total tank variables were found between the treatments despite the small sample size. Another indication of a relatively strong effect from the use of suspended arrays is the relatively short time required to see positive effects. The experiments in this study lasted 61 and 126 d, while Kientz and Barnes (2016) noted a pronounced positive effect from suspended enrichment after only 49 d of rearing. Hatchery staff labor requirements were not affected by the use of the suspended conduit arrays in this study because the tanks maintained their self-cleaning function. Kientz and Barnes (2016) and Kientz et al. (2018) reported similar observations using suspended structures in much-smaller circular tanks. Maintaining the self-cleaning nature of circular tanks during fish rearing is a key consideration in hatchery tank design, and its importance cannot be overstated (Burrows and Chenoweth 1955; Tvinnereim and Skybakmoen 1989; Timmons et al. 1998; Masalo 2008). Future research may focus on the effects of vertically suspended enrichment on other measures of fish condition

COMMUNICATION 105 besides growth rate and FCR. For instance, enrichment has been shown to decrease fin damage in steelhead (anadromous Rainbow Trout) and Atlantic Salmon Salmo salar under some circumstances (Berejikian and Tezak 2005; Persson and Alanara 2014). Lowered resting heart rates and basal metabolic rates have also been observed in Atlantic Salmon reared in enriched tanks (Millidine et al. 2006). Poststocking impacts of suspended structure should also be investigated, similar to studies conducted for other forms of enrichment (Berejikian et al. 1999, 2000; Brockmark et al. 2007; Fast et al. 2008; Hyv arinen and Rodewald 2013). Comparing these suspended arrays with other forms of enrichment may be worthy of future study. Additionally, further research on the use of vertically suspended structures as rearing tank enrichment will be necessary to observe whether this effect extends to other fish species, particularly nonsalmonids. 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