An Evaluation of Vertically Suspended Environmental Enrichment Structures during Rainbow Trout Rearing

Transcription

1 North American Journal of Aquaculture 81:94 100, American Fisheries Society ISSN: print / online DOI: /naaq ARTICLE An Evaluation of Vertically Suspended Environmental Enrichment Structures during Rainbow Trout Rearing Kathleen M. Crank Benedictine College, 1020 North 2nd Street, Atchison, Kansas 66002, USA Jeremy L. Kientz and Michael E. Barnes* South Dakota Department of Game, Fish and Parks, McNenny State Fish Hatchery, Trout Loop, Spearfish, South Dakota 57783, USA Abstract Environmental enrichment is the addition of substrate or structural complexity to typically sterile culture environments, but the effects of enrichment on fish rearing performance are relatively unknown. This 127-d study examined the growth and condition of juvenile Rainbow Trout Oncorhynchus mykiss reared in near-fully covered circular tanks (1.8 m diameter) with three novel environmental treatments: (1) eight vertically suspended nylon strings with seven colored (6.4 cm diameter) plastic spheres (high density), (2) five vertically suspended strings with seven colored plastic spheres (low density), and (3) a mixture of five vertically suspended colored plastic spheres and nine vertically suspended aluminum rods. Near-fully covered tanks without enrichment were used as a control. Weight gain, feed conversion ratio, individual fish length, and fish weight were significantly improved in environmentally enriched tanks compared with those features in the unenriched control. Among all of the treatments, the tanks of fish with highdensity spheres had significantly higher ending tank weights (i.e., total weight of fish in tank) and weight gain, both of which were nearly 65% and 75% greater, respectively, than in the control tanks. However, high density spheres also interfered with the self-cleaning nature of the circular tanks. No significant differences in splenosomatic indices, viscerosomatic indices, hepatosomatic indices, and relative fin lengths were observed among the three enrichment treatments. Based on the results of this study, the use of either low-density spheres or a mix of rods and spheres is recommended during the rearing of juvenile Rainbow Trout in circular tanks. Many hatchery rearing environments are designed for production efficiency. Tanks are uncovered to easily observe fish and to allow for routine husbandry chores to be easily performed. In addition, rearing units are frequently devoid of any internal structure to facilitate the removal of fish waste. While these relatively sterile rearing techniques may be appropriate when rearing fish for food (e.g., for commercial aquaculture), a more natural approach to fish rearing may be beneficial when fish are reared for stocking to support recreational angling or wild stock improvement. Numerous studies have examined the incorporation of environmental enrichment into hatchery rearing tanks in attempts to increase fish poststocking survival (Berejikian et al. 1999, 2000; Berejikian and Tezak 2005; Brockmark et al. 2007; Fast et al. 2008; Bergendahl et al. 2017). Intank environmental enrichment can alter fish physiology and change behavior (Berejikian et al. 2001; Millidine et al. 2006; Rodewald et al. 2011), as well as increase fish growth and decrease the feed conversion ratio (Kientz and Barnes 2016; Kientz et al. 2018). While environmental enrichment may be beneficial, many hatcheries are 94 *Corresponding author: mike.barnes@state.sd.us Received March 7, 2018; accepted August 31, 2018

2 ENRICHMENT STRUCTURES FOR RAINBOW TROUT REARING 95 hesitant to implement these changes. Baynes and Howell (1993) and Krebs et al. (2017) found that feces and food can be trapped in enrichment, requiring tanks to be cleaned more thoroughly and more frequently. Fish reared with environmental enrichment may be at a higher risk of developing disease because may harbor pathogens (Tuckey and Smith 2001). Kientz and Barnes (2016) reported a significant increase in weight gain and a decrease in the feed conversion ratio in Rainbow Trout Oncorhynchus mykiss with the use of an array of five vertically suspended metal rods in circular tanks. Kientz et al. (2018) further expanded this technique by observing positive results with the use of five vertically suspended strings of colored spheres. The objective of this study was to expand on the work of Kientz et al. (2018) by increasing the number of spherical suspended in rearing tanks and also combining both spherical and rod, and observing any potential effects on fish growth or feed conversion ratio. METHODS Experiments were conducted at McNenny State Fish Hatchery, rural Spearfish, South Dakota, using well water at a constant temperature of 11 C (total hardness as CaCO 3, 360 mg/l; alkalinity as CaCO 3, 210 mg/l; ph, 7.6; total dissolved solids, 390 mg/l). On January 13, 2017, juvenile Shasta strain Rainbow Trout (initial weight, g [mean SD]; length, cm; n = 25) were combined into a common pool and then divided evenly into sixteen, 2,000-L, green-colored tanks (1.8 m diameter and 0.8 m deep); each tank initially received 8.41 kg of fish (approximately 1,400 fish). All of these trout had been reared previously in unenriched circular tanks. Tanks were the experimental unit. This experiment used the same tanks, tank covers, and metal rods as described by Kientz and Barnes (2016), but it also expanded the type of structure with the addition of nylon strings threaded with 6.4-cm-diameter colored spheres suspended from the tank cover. Four treatments were used in this study: no tank enrichment (control), low-density spherical enrichment (five strings; illustrated in Kientz et al. 2018), high-density spherical enrichment (eight strings; Figure 1), and a mix of linear and lowdensity spherical (Figure 2). All tanks were near-fully covered as described by Walker et al. (2016). Linear enrichments were created by adding nine vertically oriented aluminum rods (1 cm diameter 57 cm long) into a corrugated plastic cover as described in Kientz and Barnes (2016). Linear aluminum rods were inserted through the corrugated plastic covers on top of the tanks and equally spaced 11 cm apart, arranged into an array within a cm area, and oriented in the tanks directly opposite of the spray bar (Kientz and Barnes 2016). Spherical enrichments were created by cutting two opposing holes in 6.4-cm-diameter plastic spheres (hardplastic, PVC-free pit balls; Kiddy Up brand; Imperial Toy, North Hill, California) and then sliding seven spheres onto FIGURE 1. Schematic of structural complexity involving eight strings of spherical (seven, colored, 6.4-cm spheres threaded onto each nylon string) suspended at equal distances apart in a circular tank (1.8 m diameter 0.8 m deep); not to scale.

3 96 CRANK ET AL. FIGURE 2. Schematic of structural complexity involving a mixture of nine aluminum rods (1 cm diameter 57 cm long, equally spaced 11 cm apart in a cm area) and two strings of spherical (seven, colored, 6.4-cm spheres threaded onto each nylon string) suspended in a circular rearing tank (1.8 m diameter 0.8 m deep); not to scale. a nylon string attached to the corrugated plastic cover. Individual spheres were in solid colors of red, blue, green, yellow, pink, or purple. A 32-g weight was added to the top and bottom of the nylon strings to ensure that the spheres, which filled with water because of the holes that were punched to allow the string to pass through, remained submerged in the tanks. The terminal sphere on each string was approximately 15 cm from the bottom of the tank. To account for positional color bias, the spheres were randomly selected from a common pool during stacking onto a string, with no colors repeated on each string. For the low-density and high-density sphere treatments, five strings and eight strings, respectively, were suspended at equal distances around the tank. In the mixed sphere and rod treatment, two strings of spheres were suspended directly across the tank from the rod array. Fish were fed with 2.5-mm, extruded, floating trout diet (Classic Trout, Skretting USA, Tooele, Utah) every 15 min during daylight hours by means of automatic feeders. Feeding rates were determined by the hatchery constant method (Buterbaugh and Willoughby 1967) based on an expected feed conversion ratio of 1.1 and a projected growth rate of 0.08 cm/d, a rate slightly above satiation. At the end of 127 d of feeding, total weight of the fish in each tank (tank weight) was quantified by weighing all the fish together from the tank to the nearest 0.01 kg. Along with tank biomass, length to the nearest millimeter, weight to the nearest 0.01 g, fin lengths to the nearest 0.01 mm, and weights of the spleen, liver, and viscera to the nearest g were recorded from five fish randomly selected from each tank. The following equations were used: Total tank weight gain ¼ final tank weight initial tank weight Feed conversion ratio (FCR) ¼ feed fed=weight gain Condition factorðkþ ¼100 ½weight (g)=length (cm) 3 Splenosomatic indexðssi; 100%Þ ¼ 100 ½spleen weight (g)=whole fish weight (g) Hepatosomatic indexðhsi; %Þ ¼ 100 ½liver weight (g)=whole fish weight (g) Viscerosomatic indexðvsi; %Þ ¼ 100 ½Viscera weight (g)=whole fish weight (g) Fin index ¼ 100 ½fin length (mm)=total body length (mm) (Kindschi 1987)

4 ENRICHMENT STRUCTURES FOR RAINBOW TROUT REARING 97 Data were analyzed by one-way ANOVA using the SPSS (24.0) statistical analysis program (Systat Software, Chicago). Tukey s post hoc multiple comparison procedure was conducted if the ANOVA indicated significant differences. Because tanks served as the experimental units, nested ANOVA was conducted for the data from individual fish (e.g., length, weight, relative fin length, etc.). Data were considered significantly different at P < RESULTS Tank weight gain (i.e., total weight of all fish in a tank) was significantly improved by the addition of any form of environmental enrichment (F 3, 12, P < 0.001) and was nearly 75% greater in the tanks of fish with high-density spheres than in the control tanks (Table 1). Among the environmentally enriched tanks, total weight gain of all fish per tank was significantly higher in the tanks with high-density spherical than in those with the mixture treatment (P = 0.016). Feed conversion ratio in trout was significantly less in the tanks receiving environmental enrichment, ranging from 0.92 to 1.14 compared with the FCR of 1.63 observed in the unenriched control tanks (P = 0.002). Compared with fish in the unenriched control tanks, individual fish lengths and weights were significantly greater in the tanks with environmental enrichment (F 3, 12, P = and F 3, 12, P = 0.015, respectively; Table 2). Viscerosomatic index was only significantly greater in fish reared in tanks containing a high density of spherical than in those reared in unenriched control tanks (F 3, 12, P = 0.025). There were no significant differences in HSI, SSI, or any of the fin indices among the treatments. A confirmed outbreak of bacterial coldwater disease occurred during the trial, but there was no significant difference in mortality among the treatments. DISCUSSION The suspended used in this study greatly improved Rainbow Trout growth and feed conversion TABLE 1. Mean SE rearing data values, including FCR, for Rainbow Trout raised in tanks containing different environmental enrichment (see Methods). Means with different letters in the same row differ significantly (P < 0.05, n = 4). Variable Control Mixture of Treatment High-density spherical Low-density spherical Final tank weight (kg) z y x yx Tank gain (kg) z y x yx Feed fed (kg) FCR z y y y Mortality (%) TABLE 2. Mean SE individual fish lengths, weights, condition factors (K), VSI, HSI, and SSI values, and fin indices of Rainbow Trout raised in tanks containing different environmental enrichment (see Methods). Means with different letters in the same row differ significantly (P < 0.05, n = 4). Variable Control Mixture of Treatment High-density spherical Low-density spherical Length (mm) z y y 188 2y Weight (g) 58 5z 78 5zy 81 6y 81 3y K VSI z zy y zy HSI SSI Dorsal fin index Pectoral fin index Pelvic fin index

5 98 CRANK ET AL. ratio, supporting the findings of Kientz and Barnes (2016) and Kientz et al. (2018). The magnitude of increase in weight gain and feed conversion ratio with the spherical was similar to that reported by Kientz et al. (2018), who evaluated suspended strings of spheres at a lower density. The results of this study with the suspended rods also are similar in magnitude to those reported by Kientz and Barnes (2016) and Kientz et al. (2018). Compared with other in-tank environmental enrichment methods, these vertically suspended are reasonable to use in production aquaculture because they do not interfere with tank self-cleaning or increase routine husbandry demands (Zydlewski et al. 2003; Hyv arinen and Rodewald 2013; N aslund et al. 2013; N aslund and Johnsson 2016; Krebs et al. 2017). As suggested by Kientz et al. (2018) and described by Moine et al. (2016), the flow dynamics of the circular tanks were likely altered by the suspended used in this study. These enrichment techniques likely produce favorable bioenergetic conditions by creating lower water velocity microhabitats (Fausch 1984), thereby allowing the fish to decrease energy expenditures during nonfeeding times. In addition, changes in overall water velocities can influence trout growth (Leon 1986; Kiessling et al. 1994; Parker and Barnes 2014, 2015). Although the spheres were colored in this study, the possible effect of color on trout growth and behavior is unknown. Other studies have indicated tank wall color can influence fish growth (Tamazouzt et al. 2000; Karakatsouli et al. 2007; Eslamloo et al. 2015), but color preferences may be species-specific (Volpato and Barreto 2001; Luchiari and Pirhonen 2008; Batzina et al. 2014). The higher VSI observed in fish reared in tanks with a high density of spheres in comparison with fish reared with the other treatments may indicate there are lower energy demands on the trout in the high-density sphere tanks, which may allow them to increase fat deposition (Jobling et al. 1998; Company et al. 1999; Yildiz et al. 2006; Ighwela et al. 2014). The lack of differences in HSI indicates that energy partitioning was not affected by the environmental enrichment treatments (Daniels and Robinson 1986; Kim and Kaushik 1992). Similar SSI values also indicate no overt effects on the immune response (Hadidi et al. 2008; Wiens et al. 2014) resulting from spheres or rods in the tanks. The relatively low SSI values reported in this study, compared with those in Barnes et al. (2014) and Parker and Barnes (2015), are surprising because fish in all of the tanks showed symptoms of bacterial coldwater disease (Neiger et al. 2016), and the causative agent of the disease, Flavobacterium psychrophilum, was isolated. It is possible that relative spleen sizes were low because the trout had ample time to recover after the bacterial coldwater disease infection before being sampled at the end of the study. The similar relative fin lengths observed in Rainbow Trout in this study may indicate that the presence of vertical rods and strings of spheres may not be affecting aggressive behavior (Latremouille 2003), increasing the likelihood of abrasions (Bosakowski and Wagner 1994), or negatively affecting fish health (Devesa et al. 1989) or other identified causes of fin erosion. Because fin erosion values were similar among the treatments even though ending weights were significantly greater in the enriched tanks, the presence of environmental enrichment may have mitigated the possible negative effects of higher densities on relative fin length (Miller et al. 1995; Wagner et al. 1997; North et al. 2006). Other studies have reported less fin erosion in fish reared under environmental enrichment regimes (Berejikian and Tezak 2005; Rosengren et al. 2016; Kientz et al. 2018). The results of this study show that adding vertically suspended forms of environmental enrichment can improve growth and feed conversion of Rainbow Trout in hatcheries. While this study did not examine poststocking effects, the addition of environmental enrichment may produce fish more adaptable to their poststocking environment (Bergendahl et al. 2017), possibly by expressing social dominance over fish reared in typically barren hatchery rearing units (Berejikian et al. 2000, 2001). 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