Keywords: environmental enrichment; structure; Oncorhynchus mykiss; salmonids
Enrichment techniques have ranged from placing real or plastic vegetative material in tanks [4,6,7,9,11,17], to cobblebased bottom structures [8.10,17]. While these methods may have been beneficial to the fish, they were typically not appropriate for use in production hatcheries because they impede the hydraulic self-cleaning inherent to circular tanks [18,19]. However, Kientz and Barnes [12] described vertically-suspended environmental enrichment that did not affect tank self-cleaning, but also improved rainbow trout Oncorhynchus mykiss growth during hatchery rearing. Other arrays of vertically-suspended structures have also produced positive results during salmonid rearing [13,14,15,16,20]. However, one study did not observe any hatchery-rearing benefits from using suspended environmental enrichment [21].
In the studies where vertically-suspended arrays have produced growth benefits, some of the largest improvements have been associated with increased water volume displacement by the environmental enrichment (i.e. better growth with bigger structures). The linear rod array successfully used previously consisted of only nine aluminum rods [12,13,16]. The objective of this study was to evaluate an expanded array of 15 rods during rainbow trout rearing in circular tanks.
Two different vertically-suspended enrichment treatments were used, in addition to a control with no suspended structure (n = 4). One treatment used an array of nine aluminum rods (0.95 cm diameter x 57.15 cm long) suspended vertically through a corrugated plastic tank cover as described by Kientz and Barnes [12] (figure 1). The other treatment expanded the nine-rod array to 15 rods to encompass a 50 x 24 cm area. This doubled the rod coverage area from 600 cm2 from the 9-rod array to 1,200 cm2 for the 15 rod array.
At the end of the experiment, total tank weights were recorded to the nearest 0.1 kg. A subsample of five fish per tank were individually weighed to the nearest 0.1 g, and measured (total length) to the nearest 1.0 mm, with fin (dorsal, pectoral, and pelvic) lengths measured to the nearest 0.01 mm. To calculate gain, percent gain, Specific Growth Rate (SGR), Feed Conversion Ratio (FCR), condition factor (K), and relative fin lengths [23], the following equations were used:
aFCR = Food fed / gain
Environmental Enrichment |
|||
None |
9-Rod Array |
15-Rod Array |
|
End tank weight (kg) |
107.7 ± 1.4 |
108.3 ± 1.8 |
106.3 ± 1.7 |
Gain (kg) |
100.5 ± 1.4 |
101.1 ± 1.8 |
99.1 ± 1.7 |
Gain (%) |
1396 ± 19 |
1404 ± 25 |
1376 ± 24 |
Food fed (kg) |
86.7 ± 0.0 |
86.7 ± 0.0 |
86.7 ± 0.0 |
FCR |
0.86 ± 0.01 |
0.86 ± 0.01 |
0.88 ± 0.02 |
Mortality (%) |
0.2 ± 0.1 |
0.2 ± 0.1 |
0.1 ± 0.0 |
a SGR = 100 * [ln(end weight) – ln(start weight)/number of days]
b K = 105 * [fish weight/fish length3]
c Relative fin length = 100 * [fin length/fish length]
Environmental Enrichment |
|||
None |
9-Rod Array |
15-Rod Array |
|
Length (mm) |
179 ± 2 |
175 ± 2 |
181 ± 1 |
Weight (g) |
63.9 ± 3.7 |
60.6 ± 1.6 |
66.2 ± 5.0 |
K |
1.12 ± 0.06 |
1.12 ± 0.02 |
1.12 ± 0.07 |
SGR |
2.33 ± 0.01 |
2.34 ± 0.01 |
2.32 ± 0.01 |
Dorsal length (%) |
6.2 ± 0.3 |
6.6 ± 0.4 |
6.6 ± 0.7 |
Pectoral length (%) |
8.4 ± 0.6 |
8.6 ± 0.1 |
7.6 ± 0.4 |
Pelvic length (%) |
6.7 ± 0.5 |
6.8 ± 0.3 |
6.5 ± 0.3 |
The feed conversion ratios of 0.86 to 0.88 reported in this study were lower than the values previously reported for rainbow trout in similar experiments [2,13,15]. Both [2,13] reported substantially higher feed conversion ratios of 1.25 and 1.31 for enriched tanks using the same feeding rates and regimes as this present study. The smaller starting size of the juvenile fish in this study may partially explain the lower feed conversion ratios [29], but fish size alone likely does not explain the lack of significant differences between treatments. In addition, while the satiation feeding rates used in this study were based on prior studies and numerous observations, it is possible that the fish may have been able to consume additional feed.
The lack of difference in feed conversion ratio and gain among the treatments in this study may not support the hypothesis that environmental enrichment creates an area of reduced velocity that leads to improvements in foraging efficiency compared to un-enriched tanks [12,30,31]. Also, one study did not observe any significant improvement in growth or feed conversion ratio with the use of vertically-suspended structures utilizing brown trout (Salmo trutta), Atlantic salmon (Salmo salar), or Chinook salmon (Oncorhynchus tshawytscha) [21]. However, numerous other studies evaluating vertically suspended environmental enrichment structures have reported positive effects on feed conversion ratio and gain [12,13,14,15].
While the use of environmental enrichment in sterile tank environments for fish rearing is not a novel concept, verticallysuspending structure is a very recent development. Many experiments have used a bottom structure or matrix to mimic natural environments [2,4,5,6,7,8,9,10,32,33,34,35,36]. However, these types of structures can impede the hydraulic self-cleaning inherent to circular tanks [2,18,37]. Just as reported by [15] who used different vertically-suspended enrichment structures, neither of the rod arrays used in this trial affected tank selfcleaning with appropriate tank velocities.
The relative fin lengths in this study were similar to those obtained by [13] and lower than those obtained by [15] using the same strain of rainbow trout and tanks. However, [15] used relatively low initial stocking densities which may have led to less fin abrasion and aggressive behavior. Environmental enrichment has been hypothesized to reduce aggression while providing areas of shelter for subordinate fish to hide, which in turn improves fin conditions [3]. These behaviors have been observed in Atlantic salmon [7] and steelhead trout (Oncorhynchus mykiss) [38,39]. Enriching raceways with cobble-based bottoms has also been shown to improve fin indices of rainbow trout [18]. Fin erosion comparisons across different strains of rainbow trout is difficult as there could possibly be genetic differences for fin sizes across strains [40]. Hatchery fish in general have reduced fin lengths in comparison to their wild counterparts due to “fin-nipping” and aggressive behaviors [41,42], tank or raceway wall abrasion, and density related fin erosional processes [43,44,45]. The verticallysuspended rod arrays used in this study may not have provided enough hiding or refuge areas to impact the social behaviors related to aggression and fin condition [3].
Ethical Approval: NA
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