The six unrelated Nile tilapia strains that were imported for the study is presented in Table 4.1 Table 4.1 The origin of the unrelated Nile tilapia strains used in the study.
Strain Origin Location
Strain 1 Hatchery University of Arizona, USA
Strain 2 Hatchery University of Arizona, USA
Strain3 (Red Tilapia) Hatchery University of Arizona, USA
Strain 4 (GIFT tilapia) Hatchery Thailand
Strain 5 Hatchery Ghana
Strain 6 Hatchery Mozambique
From the imported strains, animals were raised to broodstock age, which were then allowed to spawn to obtain fry for both Experiment 1 and Experiment 2. For Experiment 1, Strain 5 and 6 failed to spawn, therefore only fry obtained from strains S1, S2, S3 and S4 were used in this part of the study (Table 4.2)
Table 4.2 The unrelated Nile tilapia strains used in Experiment 1.
Strain Origin Location
Strain 1 Hatchery University of Arizona, USA
Strain 2 Hatchery University of Arizona, USA
Strain3 (Red Tilapia) Hatchery University of Arizona, USA
Strain 4 (GIFT tilapia) Hatchery Thailand
For Experiment 2, Strain 2 failed to spawn, therefore only fry obtained from strains S1, S3, S4, S5 and S6 were used in this part of the study (Table 4.3)
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Table 4.3 The unrelated Nile tilapia strains used in Experiment 2.
Strain Origin Location
Strain 1 Hatchery University of Arizona, USA Strain3 (Red Tilapia) Hatchery University of Arizona, USA Strain 4 (GIFT tilapia) Hatchery Thailand
Strain 5 Hatchery Ghana
Strain 6 Hatchery Mozambique
For a more detailed information on the hatcheries, please refer to Chapter 3.
On arrival of the strains at the research section, fry were quarantined and acclimatized in the quarantine unit for four weeks. The animals were maintained for a period of seven months and grown out to obtain broodstock. To obtain fish for the experiments, breeding males and females were randomly allocated to a breeding combination for a specific strain, and allowed to reproduce. Care was taken to ensure that strains did not hybridize. Juveniles originating from this broodstock population were then randomly selected at a synchronized age to be allocated to the treatment groups.
The experimental design for Experiment 1 and Experiment 2 is presented in Table 4.4 and Table 4.5, respectively.
Table 4.4. The demographic information of the fish used in Experiment 1. Strain Total number of fish Age
(days)
Average initial weight (g)
Average initial length (mm)
Strain 1 200 56 5.23g 65.26mm
Strain 2 200 56 4.67g 62.85mm
Strain 3 200 56 3.95g 58.72mm
Strain 4 200 56 4.57g 61.77mm
Table 4.5. The demographic information of the animals used in Experiment 2. Strain Total number of fish Age
(days)
Average initial Weight (g)
Average initial Length (mm) Strain 1 200 56 5.55g 66.12mm Strain 3 200 56 5.61g 63.49mm Strain 4 200 56 5.61g 68.21mm Strain 5 200 56 5.74g 68.05mm Strain 6 200 56 5.17g 63.32mm
The experimental design for Experiment 1 was a 4X2 factorial, with four replicates per treatment. The experimental design for Experiment 2 was a 5X2 factorial, with four replicates per treatment. The two nutritional treatments that the experimental animals were subjected to consisted of a high energy (HE) feed consisting of a commercially available tilapia feed (Aquafeeds (Pty) Ltd, South Africa), and a Low energy (LE) feed that consisted of duckweed (Lemna minor) which was propagated at the Aquaculture Research Section at the Welgevallen Experimental Farm.
63 4.2.3 Feeding and maintenance check numbering
The HE treatment groups were fed to satiation on a commercial tilapia feed twice daily between 09:00 and 17:00. The LE treatment group were fed ad libitum twice daily between 09:00 and 17:00, ensuring that enough duckweed was available in the tanks to ensure continuous feeding. The duckweed was initially sourced from the University of Stellenbosch Botanical Garden and then cultured in outdoor concrete tanks at Welgevallen Experimental Farm, from where it was cropped on a daily basis to feed the fish.
Faecal material and food waste were siphoned from the tanks on a daily basis. Physicochemical parameters such as water temperature, dissolved oxygen, conductivity and pH were monitored daily alongside with the ammonia, nitrate, and nitrite levels.
4.2.4 Sampling and recording of data
Complete sampling of all fish per tank were conducted on a fortnightly basis during the experimental period of seventy (70) days for Experiment 1 and ninety-eight (98) days for Experiment 2. The sampled fish were anesthetized through the use of clove oil according to a protocol described by Rapsin (2015) to minimize stress and injuries during sampling and measurement.
The recorded traits included the following:
Body weight (BW) measured in grams using a UWE electronic balance (model HGS-300, capacity: 300 × 0.01g)
Standard length (SL) as measured (in mm) from the most anterior region to the caudal peduncle, and by using a calibrated measuring board.
Total length (TL) as measured (in mm) from the tip of the mouth to the tip of the caudal fin, and by using a calibrated measuring board.
At the end of each experiment a blood sample was collected from each of nine randomly sampled fish per replicate, whilst fish yield (Y) were determined through the recording of the live weight as well the carcass weight after removal of the viscera.
Mean weight gain (MWG) as a percentage, and specific growth rate (SGR) as a mean percentage per day were calculated according to the formulas below the calculation of the various growth parameters based on the measured traits is summarized as:
Mean Weight Gain MWG) as average percentage of body weight gain
=
(Mean Final Body Weight – Mean Initial Body Weight) × 100
64
Specific Growth Rate (SGR) as average percentage of body weight gain per day
=
100(Loge Final Body Weight – Loge Initial Body Weight)
Duration (days) (2)
Average Daily Weight Gain (ADWG) in gram per day
=
Mean final weight – Mean initial weightDuration (days) (3)Average Daily Length Gain (ADLG) in mm/day
=
Mean final length – Mean initial lengthDuration (days) (4) Condition Factor (K)=
100×W 𝐿3 (Fulton, 1904) (5) where: W =Body weight (g)L =Total Length (cm)
Fish yield (Y) as a percentage of Body weight (BW)
=
Gutted weight (g) x 100 Body weight (g) (6)Fish yield describes the edible part of the fish after the viscera and gill has been discarded. A total of nine fish per replicate were randomly selected at the end of each experiment. Fish were killed by firm knock on the head and the collective BW were recorded per replicate. Thereafter the viscera and gills were removed and the collective Gutted weight (GW) was recorded per replicate and the yield calculated as above
Survival rate was expressed as an overall percentage, according to the formula below.
f) Survival rate (%)
=
Final number of fish × 100Initial number of fish (7)4.2.5
Statistical
analysisData were analyzed using SAS Enterprise 5.1 (2015) Guide System version 12 and XLSTAT version 2015.
Body weight, standard length, total length were calculated by means of linear regression after which these were subjected to analysis of covariance where the intercept was included in the model so as to compare the growth performance among the various strains and feeding regimes. Yields and condition factor measured were all subjected to a one-way analysis of variance (ANOVA) on the basis of individual values as well as averages. The Bonferroni adjustment t-test was used to separate differences between the means at a 95% (p< 0.05) significance level. Phenotypic correlations for strains over treatments were calculated as biserial correlations to quantify the G×E among the different strains over the treatments.
4.3
Results
The key objectives of Experiments 1 and 2 were to quantify the growth performance of unrelated strains of Nile tilapia (O. niloticus) under high and low energy feeding regimes, and to elucidate on a potential GxE interaction. Due to differences in the number of strains used in Experiment 1 and Experiment 2, respectively, the results of the two experiments will be presented separately.
65
Experiment 1
The influence of the HE and LE diet on the body weight (BW), standard length (SL) and total length (TL) is presented in Table 4.6. The strains receiving the HE diet outperformed the strains that received the LE diet (p<0.05). Strains receiving the LE diet weighed on average 8.24±0.93g, compared to the strains that were fed the HE diet, that weighed 72.2±0.93g (Table 4.6). The differential ratio between the HE and LE treatments were roughly of 9:1 for BW and 2:1 for SL and TL (Table 4.6). The similarities observed in all the tables for standard error of the mean on HE and LE diets are group standard error of the mean and not for the individual traits or strains.
Table 4.6. The influence of diet on the body weight (mean±SE), standard length (mean±SE), and total length
(mean±SE) of four genetically distinct Nile tilapia strains during a 70-day treatment period
Treatment Body Weight (g) Standard Length (mm) Total Length (mm)
High energy diet 72.22a±0.93 119.32a±0.56 149.89a±0.67
Low energy diet 8.24b±0.93 61.60b±0.56 76.71b±0.67
a, b Different superscripts in columns denote significant differences (p<0.05)
When growth rate of the strains is expressed as a main effect over treatments, similar results in terms of significant differences were observed for BW, SL and TL as for the ammonia, nitrate and nitrite and suspended solids maintained within the optimal range for the species HE E diet, with S4 being superior in terms of BW, SL and TL (Table 4.7).
66 Table 4.7. The
influence of diet main effect on the
body weight (mean±SE), standard
length (mean±SE), and total length (mean±SE) of four genetically distinct Nile tilapia strains
during a 70-day period.Strain
Body Weight (g) Standard Length (mm) Total Length (mm)
1 32.99c±1.32 88.57c±0.79 109.25c±0.96
2 41.34b±1.32 91.80b±0.79 114.54b±0.96
3 36.92bc±1.32 85.82c±0.79 108.51c±0.96
4 49.66a±1.32 95.64a±0.79 120.90a±0.96
a, b, c Different superscripts in columns denote significant differences (p<0.05)
Table 4.8 The influence of both diet on the body weight (mean±SE), standard length (mean±SE), and total
length (mean±SE) of four genetically distinct Nile tilapia strains during a 70-day period.
Strain Body Weight (g) Standard Length (mm) Total Length (mm)
High E Low E High E Low E High E Low E
S1 57.20c±1.87 8.78d±1.87 113.10c±1.11 64.05d±1.11 140.69c±1.36 77.83de±1.36
S2 74.33b±1.87 8.35d±1.87 120.82b±1.11 62.78d±1.11 151.15b±1.36 77.93de±1.36
S3 66.58b±1.87 7.26d±1.87 114.07c±1.11 57.56d±1.11 145.09bc±1.36 71.92e±1.36
S4 90.76a±1.87 8.57d±1.87 129.28a±1.11 62.00d±1.11 162.64a±1.36 79.16d±1.36
a, b,c,d,e Different superscripts in columns denote significant differences (p<0.05)
The results confirm significant differences in the BW of strains on the HE diet with the GIFT (S4) superior to the other strains, whilst S2 and S3 was significantly better than S1. For SL and TL strain S4 performed significantly better than the other strains, with S2 significantly better than both S1 and S3. No significant differences were however detected between the strains on the low energy diet for any of the measured traits.