Although an important part of the natural environment of fish, overhead cover is usually absent during hatchery rearing. To evaluate the possible influence of overhead cover on juvenile brown trout Salmo trutta hatchery rearing performance, this study compared three different cover treatments: near-full (98%) cover, partial (65%) cover, and no cover (completely open). After 12 weeks of rearing in 1.8 m-diameter circular tanks, total tank weight gain was significantly greater and feed conversion ratios were significantly less in tanks of brown trout that were either partially or near-totally covered, in comparison to those tanks that were completely open. The viscerosomatic index, hepatosomatic index, and splenosomatic index values were not significantly different among any of the treatments. Fin condition indices were also not significantly different. The use of either partial or full covers is recommended to maximize brown trout rearing efficiencies, with full covers providing the additional benefit of preventing fish from jumping out of the tanks.
Tanks used during hatchery rearing are typically uncovered to facilitate regular cleaning, allow for observations on fish growth, and discover possible fish health issues [
Because overhead cover is essential in natural habitats for numerous salmonids [
The concept of overhead cover was recently expanded to include covering nearly the entire top of the tank during the rearing of rainbow trout [
There have been no published studies examining the use of near-full covers during the hatchery rearing of brown trout. Thus, the objective of this study was to investigate the use of near-total tank covers on the growth and feeding efficiency of brown trout in comparison to those reared with partial covers or no overhead cover at all.
Experimentation occurred at McNenny State Fish Hatchery, rural Spearfish, South Dakota, USA using twelve circular fiberglass tanks (1.8 m in diameter, 0.8 m deep, 0.6 m operating depth). Each tank received approximately 45 L/min of aerated and degassed well water (11˚C; water hardness as CaCO3-360 mg/L; alkalinity as CaCO3-210 mg/L; pH-7.6; total dissolved solids-390 mg/L). Three overhead tank cover treatments were used (N = 4): open (no cover), partial cover over 65% of the tank, and near-full cover over 98% of the tank (with a small opening for the automatic feeder). Partial and near-total covers were made by riveting 6.35 mm corrugated black plastic sheeting (Coroplast, Vanceburg, Kentucky, USA), which was cut into a semi-circular shape to match the diameter of the open tops of the rearing tank, onto square aluminum tubing at the radius (
The study began on April 29, 2015 and lasted for 12 weeks. At the start of the experiment, each of the twelve tanks received 10.23 kg (approximately 1650 fish) of juvenile Plymouth Rock strain brown trout [Mean (SE) weight and length of 6.17 (0.18) g and 80.8 (0.8) mm, respectively]. Fish were fed 1.5 mm extruded, floating pellets (Skretting, Tooele, Utah, USA) to satiation daily, which was based on a hatchery constant of 7.26 (0.065 mm/day) with an anticipated 1.1 feed conversion [
the experiment. Pellets were dispensed from EWOS 505 (Norco-last AS, Sweden) automatic feeders between 08:00 to 18:00 for 1 minute at 20-minute intervals. Feed rations, along with the number and weight (g) of mortalities, were recorded daily for each tank.
Total body weights to the nearest 0.1 g, and viscera, liver, and spleen weights to the nearest 0.001 g, were recorded at the beginning of the experiment from a common pool (n = 30) and at the end of the experiment from each tank using a model ER-120A A&D electronic balance (Tokyo, Japan). Digital calipers were used to record total lengths, and the lengths of the dorsal fin and one pectoral and pelvic fin to the nearest 0.01 mm. Additionally, total tank biomass (to the nearest 5 g) was measured at the beginning and end of the experiment for each tank using a Ohaus model T1XW scale (Parsippany, New Jersey, USA). At the end of the experiment, total body, liver, spleen and visceral weights were recorded from five randomly-selected individuals from each tank. Total, dorsal, pectoral and pelvic fin lengths were also recorded for the same individual fish.
The following equations were used:
1) Condition Factor (K) = [weight (g)/total length (cm)3] × 100.
2) Fin index (%) = {[fin length (mm) ÷ total length (mm)] ×100}.
3) Viscerosomatic index (VSI) = {[weight of viscera (g) ÷ total fish weight (g)] × 100}.
4) Hepatosomatic index (HSI) = {[liver weight (g) ÷ total fish weight (g)] × 100}.
5) Splenosomatic index (SSI) = {[spleen weight (g) ÷ total fish weight (g)] × 100}.
6) Food conversion ratio (FCR) = feed fed (g)/weight gain (g).
Data were analyzed using the SPSS (9.0) statistical analysis program (SPSS, Chicago, Illinois, USA) with significance predetermined at P < 0.05. Individual tanks were the replicates used for statistical analysis because they were the experimental unit, not the individual fish. At the end of the experiment when multiple fish were sampled from one tank, the mean of that sample was considered a replicate and used for analysis. One way analysis of variance (ANOVA) was conducted, and if the treatments were significantly different, pairwise mean comparisons were performed using the Tukey HSD test.
Total tank weight gain and food conversion ratio were significantly improved in tanks that were either near-completely covered or partially covered, compared to uncovered tanks (
No significant differences were observed among the treatments in individual fish lengths (
Rearing Tank Cover | ||||
---|---|---|---|---|
None | Partial | Near-Full | P value | |
Start weight (kg) | 10.23 ± 0.00 | 10.23 ± 0.00 | 10.23 ± 0.00 | |
Food fed (kg) | 23.35 ± 0.00 | 23.35 ± 0.00 | 23.35 ± 0.00 | |
End weight (kg) | 27.00 ± 0.43 z | 28.54 ± 0.19 y | 28.83 ± 0.09 y | 0.002 |
Gain (kg) | 16.77 ± 0.43 z | 18.31 ± 0.19 y | 18.61 ± 0.09 y | 0.002 |
Food Conversion Ratio | 1.40 ± 0.04 z | 1.28 ± 0.13 y | 1.26 ± 0.06 y | 0.004 |
Mortality (%) | 0.31 ± 0.07 | 0.16 ± 0.04 | 0.22 ± 0.07 | 0.258 |
Rearing Tank Cover | ||||
---|---|---|---|---|
None | Partial | Near-Full | P value | |
Length (mm) | 115 ± 2 | 113 ± 3 | 118 ± 4 | 0.550 |
Weight (g) | 16.3 ± 0.62 | 16.3 ± 1.3 | 18.3 ± 1.7 | 0.476 |
Ka | 1.05 ± 0.03 | 1.08 ± 0.02 | 1.07 ± 0.02 | 0.691 |
HSIb | 1.79 ± 0.18 | 1.68 ± 0.07 | 1.89 ± 0.05 | 0.483 |
VSIc | 8.90 ± 0.10 | 8.42 ± 0.77 | 8.65 ± 0.54 | 0.829 |
SSId | 0.19 ± 0.09 | 0.10 ± 0.01 | 0.09 ± 0.05 | 0.346 |
Pectoral fin indexe | 12.44 ± 0.15 | 13.10 ± 0.24 | 12.56 ± 0.30 | 0.174 |
Dorsal fin indexe | 11.62 ± 0.22 | 11.32 ± 0.92 | 10.52 ± 0.31 | 0.300 |
Pelvic fin indexe | 11.12 ± 0.38 | 11.49 ± 0.50 | 11.45 ± 0.40 | 0.807 |
aK = 105 × (weight)/(length3); bHSI = 100 × (liver weight/body weight); cVSI = 100 × (viscera weight/body weight); dSSI = 100 × (spleen weight/body weight); eFin index = 100 × (fin length/total length).
The significant improvements with the use of either partial or near-total overhead cover observed in this study with brown trout are similar to those reported for rainbow trout [
In the wild, brown trout have been observed to spend a large amount of time under cover, and especially use overhead cover to avoid disturbances [
It is also possible that the location of the tanks may have influenced the results. Pickering et al. reared brown trout and other salmonids in outdoor tanks and noticed little positive effect from overhead cover [
The feeding rate used for this study was based on historical data to achieve satiation and maximum growth at McNenny Hatchery. The relatively high HSI values, similar to those of similarly-sized rainbow trout fed to satiation [
All of the SSI values were within normal limits as reported by Shimma et al., Uyan et al., and Wiens et al. [
The fin index values were generally greater than that reported previously for hatchery-reared brown trout and slightly less than that observed in wild brown trout [
The plastic covers with aluminum supports used in this study have several ad-vantages over the wooden covers used in other studies by Barnes et al. [
In conclusion, the use of either partial or near-total overhead cover is recommended during the indoor rearing of brown trout in circular tanks. The use of near-total covers provides an additional benefit of keeping the trout from jumping out of the tanks, but makes tank cleaning and fish observations more difficult. In situations where frequent tank cleaning or fish observations are required, partial tank covers may be more practical.
We thank Kelby Torgerson, Jeremy Kientz, and Kati Krebs for their assistance with this study.
Krebs, E., Barnes, M.E. and Nero, P.A. (2016) Covering Rearing Tanks Improves Brown Trout Growth and Feed Conversion. Agricultural Scien- ces, 7, 869-878. http://dx.doi.org/10.4236/as.2016.712079