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Keywords:

  • NH 3 ;
  • FAN ;
  • dietary protein;
  • intensive mariculture;
  • pH

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

The farming of abalone, Haliotis midae L., can be intensified in serial-pass systems, but water re-use increases the concentration of NH3 (free ammonia nitrogen, FAN) and reduces water pH. Changing the percentage dietary protein from 33% to 26% reduced the concentration of FAN (F42, 252 = 2.79; P < 0.0001) in a serial-pass system and did not reduce weight gain (F1, 12 = 1.09; P = 0.31) or length gain (F1, 12 = 1.08; P = 0.31). Low water pH was the most important variable to contribute to a reduction in abalone growth (weight gain: F1, 19 = 64.5; P < 0.0001; r2 = 0.76; length gain: F1, 19 = 41.9; P < 0.0001; r2 = 0.67). In addition, supplemental oxygen (103% saturation) improved length gain (t = 3.45, P = 0.026) in abalone exposed to an average FAN concentration of 2.43 ± 1.1 μg L−1) and an average pH value of 7.6 ± 0.13, relative to a treatment with no oxygen supplementation. Thus, in an abalone serial-use raceway with three passes, FAN was not the first growth-limiting variable. It is suggested that future studies should examine the major causes of reduced water pH in serial-use systems and their effect on the growth and health of H. midae.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

The South African abalone, Halitois midae, is a slow-growing marine gastropod that fetches high prices in East-Asian markets. Due to South Africa's high-energy coastline, abalone are farmed intensively in land-based facilities using flow-through systems that require efficient utilization of pumped water (Siikavuopio, Dale, Foss & Mortensen 2004). Thus, under such conditions, identifying potentially limiting water quality variables and species-specific tolerance limits is essential (Siikavuopio et al. 2004; Colt 2006). This important aspect is not well documented in the culture of H. midae.

In high-intensity culture and in systems operating on water re-use, an increase in the concentration of the metabolic waste products, carbon dioxide and ammonia, may occur (Sanni & Forsberg 1996; Huchette, Koh & Day 2003; Piedrahita 2003; Colt, Watten & Rust 2009; Naylor, Kaiser & Jones 2011). In water, total carbon dioxide and ammonia exist in both the ionized form, i.e. as bicarbonate, HCO or carbonate, CO3 2− and ammonium, NH4 + and in the un-ionized form, i.e. as aqueous carbon dioxide, CO2 aq, and free ammonia nitrogen, FAN or NH3. The equilibrium reactions are controlled primarily by pH, whereas temperature and ionic strength of the water play a minor role (Bower & Bidwell 1978; Smith 1988; Randall & Tsui 2002). The sum of the ionized and un-ionized forms of ammonia is total ammonia nitrogen, TAN. When carbon dioxide hydrates in water, hydrogen ions (H+) are released and the resultant changes in water pH allow an estimation of the concentration of dissolved carbon dioxide (Covington, Bates & Durst 1985), provided factors such as alkalinity and hardness remain constant.

The toxic effects of carbon dioxide and ammonia are mostly related to the concentration of the un-ionized forms (Thurston, Russo & Vinogradov 1981; Sanni & Forsberg 1996). In H. rubra (Leach) and H. laevigata (Donovan), FAN concentrations of 4 and 41 μg L−1, respectively, were estimated to cause growth reductions and an increase in oxygen consumption (Harris, Maguire & Hindrum 1998; Huchette et al. 2003). In H. midae of 10–20, 50–80 and 100–150 mm shell length, the 36-h LC50 for FAN was estimated at 9.3, 12.7 and 16.2 μg L−1 respectively (Reddy-Lopata, Auerswald & Cook 2006). The authors suggested that exposure to concentrations as low as 4 μg L−1 may reduce growth in H. midae. However, in addition to an increase in FAN concentration, intensive water use can also reduce water pH as a result of the accumulation of carbon dioxide (Harris, Maguire, Edwards & Hindrum 1999; Colt et al. 2009). The effect that water pH has on growth and health has only been studied in H. laevigata and H. rubra. Harris, Maguire, Edwards & Hindrum 1999) estimated that in these species, a 50% reduction in growth (EC50) could be expected at pH values of 7.39 and 7.37 respectively. A decrease in oxygen consumption was also noted at low pH. Exposure of the marine mussel, Mytilus galloprovincialis (Lamarck), to pH 7.3, using CO2-gassed water, resulted in a reduction in growth, haemolymph pH and metabolic rate, and an increase in haemolymph CO2 concentration (Michaelidis, Ouzounis, Paleras & Pörtner 2005). A decrease in pH can also lower the saturation state of essential carbonate minerals, calcite and aragonite, which are important for calcifying organisms (Kleypas et al. 2006; Lopez, Chen, McKittrick & Meyers 2011).

In this study, medium-size abalone were exposed to a range of FAN concentrations and variations in water quality at three positions in a serial-use raceway on an abalone farm. As the dosing of ammonia can be costly and technically demanding under farm conditions, the percentage protein in the artificial pelleted diet fed to the abalone was changed to create a range of FAN concentrations. As ammonia is the primary metabolite of nitrogen catabolism in H. midae (Barkai & Griffiths 1987), it was hypothesized that increasing the amount of dietary protein will result in increased ammonia excretion. This has been observed in both red drum, Sciaenops ocellatus L. (Webb & Gatlin 2003) and silver perch, Bidyanus bidyanus (Mitchell) (Yang, Liou & Liu 2002). Growth in H. midae had been positively correlated to percentage dietary protein (Britz 1996a; Britz & Hecht 1997). However, recent studies showed that reducing the dietary protein content to ≤26% did not reduce growth of H. midae of >50 mm shell length (Jones & Britz 2006; Green, Jones & Britz 2011) and has been attributed to the maintenance of dietary energy levels above 13.5 MJ kg−1 (Green et al. 2011).

This study tested the extent to which FAN and changes in water quality influenced abalone growth in a serial-use raceway. In addition, the effect that supplementing dissolved oxygen at high FAN and low pH had on abalone growth was tested.

Methods and materials

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

The study was conducted at HIK Abalone Farm Pty Ltd in Hermanus, on the south-west coast of South Africa (34°26′04.35″S; 19°13′12.51″E) from July to October 2009. The abalone, Haliotis midae, had an average length of 65.9 ± 2.6 mm and a mass of 49.3 ± 3.3 g (= 3080) at the beginning of the experiment. They were spawned in the farm hatchery and reared in well aerated, flow-through tanks at ambient water temperatures (12–20°C). They were weaned from diatom coated plates onto an artificial diet (Abfeed®; Marifeed, Hermanus, South Africa) at a young age.

Experimental design

Experiment 1 – Effect of serial re-use intensity on abalone growth

To test the effect of FAN concentration on abalone growth, a diet containing 26% protein (P26) and a diet containing 33% protein (P33) were used to create a low FAN and a high FAN treatment in triplicate serial-use raceways. Dietary protein level was used to manipulate FAN concentrations in the raceways without confounding growth rates, as dietary protein level, in the range used here, does not affect the growth of H. midae in the size class used in this experiment (Green et al. 2011). Each diet was fed to all tanks in three raceways, thereby creating a range of water quality conditions.

Experiment 2 – Effect of oxygen supplementation at high FAN and low pH

Supplementation with oxygen allowed for a comparison of abalone growth at high FAN (2.4 ± 1.1 μg L−1), low pH (7.6 ± 0.1) and high dissolved oxygen concentrations (103 ± 8%) (P33 with O2 added) with a treatment in which the average FAN concentration was high, and where pH and dissolved oxygen concentration was low (92 ± 6%). The objective of this trial was to determine the effect of dissolved oxygen concentration on abalone growth at conditions of low flow indices. Treatments were done in triplicate.

Experimental system

The experiment was conducted in six serial-use raceways each with three tanks in series. Each tank (0.9 × 0.6 × 0.6 m; 300 L water volume) contained one abalone basket with a surface area of 3.2 m2, provided by a plastic rack and feeder plate. Each tank was aerated using polyvinylchloride (PVC) airlines placed horizontally beneath the abalone basket, 100 mm above the tank bottom. Filtered seawater (100 μM) entered the first tank in series (position 1) and flowed by gravity to tank positions 2 and 3. Water pH in the header tank averaged pH 8.0 ranging from 7.72 to 8.17 (n = 26).

Water flow between the second and third tank in series was halved by means of a valve to simulate conditions of low flow indices (L h−1 kg−1). These flow rates were checked and maintained each time water samples were taken. Where abalone were fed the low-protein diet, half of the water from tank position 2 entered tank position 3, with the other half going to waste, i.e. being returned to the ocean. To create the conditions for an additional experimental design, effluent water from abalone fed the high-protein diet was also split equally to flow into two tanks with one tank supplemented with oxygen (P33–O2) while the other tank remained part of the experiment designed to test the effect of FAN on abalone growth. Oxygen supplementation allowed oxygen saturation in these tanks to be maintained at approximately 100%, and thus a comparison between two treatments was made possible, i.e. a treatment with high FAN concentrations, reduced pH and reduced oxygen levels, and a treatment with high FAN concentrations, reduced pH and oxygen supplementation. All tanks were cleaned every 14 days.

Experimental diets and feeding

The diets were iso-energetic (15.6 MJ kg−1) and contained a low percentage of lipid (3%). Fish meal and soya meal were used as the protein source. Protein to energy ratio was therefore 2.13–1.67 in the P33 and P26 diets respectively. The abalone were fed to apparent satiation at 16:00 hour daily with feed from a bucket assigned to each tank. They were fed according to farm protocol to obtain results representative for these conditions. The buckets were weighed and refilled at 2-week intervals, with the amounts fed added up to calculate the total mass of feed given per basket.

Growth analysis

At the start of the trial, each tank was stocked with 7.5 ± 0.01 kg (= 21) of size graded H. midae (45–55 g abalone−1). Fifty abalone per tank were weighed individually to the nearest 0.01 g using an electronic balance (Snowrex BBA-600; Snowrex International, Taipei, Taiwan) and measured to the nearest 0.1 mm from photographs using the software, SigmaScan® Pro 5 (Systat Software, San Jose, CA, USA). Photographs were taken from directly above the abalone using a digital camera on a tripod, and calibrated using length measurements taken with Vernier callipers. After 105 days, i.e. the duration of the farm management's quarterly grading schedule, the abalone were weighed and measured again to determine average wet weight gain, length gain and condition factor (CF) of abalone in each tank. The CF was determined using the equation: CF = 5575 (weight × length−2.99) (Britz 1996b), where weight is in g abalone−1 and length in mm abalone−1. Feed conversion ratio (FCR) was calculated as FCR = dry weight feed given/biomass gain, where biomass gain is the difference in total wet weight of abalone within a tank between the start and end of the growth period.

Water quality analysis

The concentration of total ammonia nitrogen (TAN), nitrite-N, dissolved oxygen, temperature, pH and oxygen saturation were monitored twice a week. Measurements or samples were taken at the outflow of each tank during mid-morning. This sampling time was chosen as suggested by Yearsley (2008) for diurnal water quality patterns in H. midae tanks. Hand-held metres were used to measure temperature, pH (Model # 60/10 FT; YSI, Yellow Springs, OH, USA), dissolved oxygen and oxygen saturation (Model # 55D; YSI). The pH and oxygen metres were calibrated weekly using a three-point calibration with buffer solutions of pH 4, 7 and 10, and air-saturated seawater respectively. The oxygen metre membrane was replaced at intervals prescribed by the manufacturer. The TAN concentrations were measured using the method described by Solorzano (1969), whereas Merck Nitrite Test Kits (Cat. no. 1.14776.0001; Merck, Modderfontein, South Africa) were used to measure the concentration of nitrite-N. Colour absorbance for TAN and nitrite were read using a spectrophotometer (Prim Light; Secomam, 30319 Ales, France) and converted to concentration (μg L−1) using the coefficients derived from least-square linear regression standard curves (TAN, ammonium chloride, n = 15, r2 = 0.989; nitrite-N, sodium nitrite, n = 10, r2 = 0.975).

The concentration of FAN was calculated using the values for TAN, temperature, pH and salinity (35 ppt) of the water sample (Bower & Bidwell 1978). The flow rate of clean seawater entering the first tank in series of each serial-use raceway was measured in duplicate at each sampling time using a 2-L graduated container and a stop watch. Adjustments to the flow rate were made when necessary to maintain flow rates at 155 L h−1.

Statistical analysis

As there were two diets and three serial-use positions per diet, data were analysed using multi-factorial analysis of variance including a test for interactions between the two main effects, diet and serial-use. Differences between means of the dependent variables were compared between tank positions using Tukey's HSD test. A Student's t-test for independent data was used to compare means of the dependent variables for experiment 2, which was designed to test the effect of oxygen supplementation on growth. The assumptions of equality of variance and normality of residuals were checked with Levene's test (Levene 1960) and the Shapiro–Wilk test (Shapiro & Wilk 1965) respectively. An α-error level of 5% was used for all tests. Values presented in the text are mean ± standard error, unless otherwise stated.

Repeated Measures Analysis of Variance was used to estimate the within-subject variance of repeated measurements of water quality to avoid the bias of pseudoreplication.

Least-square regression analysis and correlation tests were used to model weight gain and length gain as a function of water pH, and the concentrations of dissolved oxygen and FAN. Stepwise forward multiple regression analysis (Fstop = 1) was used to estimate which of the water quality variables was the best predictor of weight and length gain. For this, weight and length gain were regressed to pH, dissolved oxygen and FAN concentration, and the product of these independent variables.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

Effect of diet and serial-use on abalone growth

At the start of the experiment, average length (66.04 ± 0.14 mm) and weight (49.4 ± 0.21 g abalone−1) did not differ between dietary treatments (P = 0.99; P = 0.39) and the three serial-use positions (P = 0.64; P = 0.83). For weight gain (g abalone−1), there was a significant interaction between diet and serial-use position (F2,12 = 10.4; P = 0.0024), and mean values for all serial-use positions differed significantly from each other (F2,12 = 200.5; P < 0.0001; Fig. 1). Diet did not have a significant effect on weight gain (F1,12 = 1.09; P = 0.31). Thus, the significant drop in weight gain in both dietary treatments with increasing water re-use was more pronounced in abalone fed P33 (Fig. 1). Serial-use reduced weight gain by 60.2% from 12.3 g abalone−1 in position 1–4.9 g abalone−1 in position 3.

image

Figure 1. Weight gain (g abalone−1), length gain (mm abalone−1), final CF and FCR values of H. midae grown in a serial-use raceway with three tanks in series. Abalone were fed a diet containing either 26% or 33% protein for 105 days. Values are averages and 95% confidence intervals. Different superscripts represent significant differences.

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Length gain (mm abalone−1) differed significantly between tank positions (F2,12 = 53.0; P < 0.0001), but not between diets (F1,12 = 1.08; P = 0.31) with no significant interaction between these two main effects (F2,12 = 3.05; P = 0.09). The average length gain of both dietary treatments was 49% lower in position 3 than in position 1.

Although there were no significant differences in CF between dietary treatment and serial-use position (P = 0.47; P = 0.31), there was a significant interaction between these two effects at the end of the experiment (F2,12 = 5.5; P = 0.02) as CF had dropped between position 2 and 3 in abalone fed P33 (Fig. 1).

Although there were no differences in the total amount of food fed during the 105-day experimental period (2446 g ± 18 g tank−1) between dietary treatments and serial-use (P = 0.86 and P = 0.99, respectively), average FCR differed between all tank positions (F2,12 = 41.4; P < 0.0001), but not between diets (F1,12 = 1.77; P = 0.21). There was a significant interaction between the two main effects (F2,12 = 4.64; P = 0.03; Fig. 1), showing a relatively higher increase, i.e. reduced feed conversion rate, from position 2–3 in abalone fed the high-protein diet.

The difference in the dietary protein level of the feed resulted in significantly different FAN concentrations between treatments at each position within the raceways (within-subjects analysis of variance, F42,252 = 2.79; P < 0.0001) (Fig. 2).The FAN concentrations in tanks fed P26 were on average 51% lower than in tanks fed P33 (Table 1). Variations in water quality were due to fluctuations in environmental conditions and management (Figs 2 and 3; Table 1).

image

Figure 2. Mean (± SE) free ammonia nitrogen (FAN) concentration at three positions in an abalone serial-use raceway for two diets differing in percentage protein, 26% (P26) and 33% (P33).

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image

Figure 3. Mean (± SE) total ammonia nitrogen (TAN) concentration at three positions in an abalone serial-use raceway for two diets differing in percentage protein, 26% (P26) and 33% (P33).

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Table 1. Mean values (± SD) and range from three replicates of temperature, total ammonia nitrogen (TAN), free ammonia nitrogen (FAN), water pH, nitrite nitrogen (Nitrite-N), dissolved oxygen and oxygen saturation for abalone fed either a 26% (P26) or 33. 3% (P33) protein diet, in each of three tanks in series
Dietary treatmentPosition 1Position 2Position 3
Temperature (°C)P2615.5 ± 0.615.3 ± 0.715.1 ± 0.9
13.9–16.813.4–16.712.8–16.6
P3315.5 ± 0.615.3 ± 0.715.0 ± 0.9
14.0–16.813.5–16.812.9–16.7
TAN (μg L−1)P2647.6 ± 24.180.3 ± 51.7118.9 ± 99.1
12–117 12–2060.3–339
P3386.3 ± 32.7163.7 ± 63.2256.8 ± 134.5
24–164 66–308 27–521
FAN (μg L−1)P260.7 ± 0.40.9 ± 0.61.0 ± 0.9
0.1–2.10.1–2.40.01–3.8
P331.2 ± 0.51.8 ± 0.72.4 ± 1.3
0.3–2.60.7–3.70.3–5.7
pHP267.79 ± 0.147.67 ± 0.157.60 ± 0.14
7.5–8.07.3–7.97.2–7.9
P337.79 ± 0.137.67 ± 0.157.62 ± 0.14
7.4–8.07.4–7.97.3–7.9
Nitrite–N (μg L−1)P267.9 ± 1.310.3 ± 2.117.4 ± 9.0
6–10 7–14 7–40
P339.1 ± 1.813.5 ± 4.831.5 ± 20.4
6–14 7–27 8–77
Dissolved oxygen (Mg L−1)P267.5 ± 0.67.3 ± 0.67.4 ± 0.6
6.1–8.46.2–8.26.1–8.3
P337.4 ± 0.67.3 ± 0.67.5 ± 0.5
6.0–8.26.1–8.26.3–8.4
Oxygen saturation (%)P2692.8 ± 8.090.5 ± 7.490.7 ± 6.9
75–104 76–101 73–102
P3392.3 ± 7.590.6 ± 7.591.8 ± 6.0
75–102 69–100 80–100

Weight gain, Wg, (g abalone−1) and length gain, Lg, (mm) could be predicted from water pH using least-square regression analysis (Wg: F1,19 = 64.5; P < 0.0001; r2 = 0.76; Lg: F1,19 = 41.9; P < 0.0001; r2 = 0.67) (Fig. 4). The relationship between weight gain and length gain and dissolved oxygen concentration was not significant (Wg: F1,19 = 0.86; P = 0.36; Lg: F1,19 = 0.004; P = 0.95). Weight gain was significantly correlated to FAN concentration (Wg: F1,19 = 7.4; P = 0.01), whereas there was no significant relationship between this variable and length gain (F1, 19 = 3.5; P = 0.07) (Fig. 4).

image

Figure 4. Correlation plots between weight gain (g abalone−1) and length gain (mm abalone−1) with least-square regression lines and 95% confidence intervals and (a) pH, (b) the concentrations of dissolved oxygen (mg L−1) and (c) FAN (μg L−1). There was a significant correlation between weight and length gain and pH. Dissolved oxygen did have an effect on weight and length gain, whereas weight gain but not length gain was significantly correlated to the concentration of FAN.

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Stepwise forward regression analysis with pH, FAN, pH × FAN, DO × FAN, pH × DO as independent variables and weight gain or length gain as the dependent variables suggested pH as the best predictor as the other variables did not contribute significantly to the multiple regression model.

Effect of oxygen supplementation at low flow indices

The comparison of production variables (Table 2) in tanks in the third position of the serial-use system in which one treatment received additional oxygen showed that there were no significant differences in mean wet weight gain (t = 2.61; P = 0.059) of abalone between the control and the tanks supplemented with oxygen. Length gain differed significantly (t = 3.44; P = 0.026) between treatments. Total amount of feed given was significantly lower (t = 3.97; P = 0.017) in the oxygen supplemented tanks (2262 ± 68 g) than in the control tanks (2436 ± 35 g). Abalone in tanks supplemented with oxygen had a significantly better FCR (t = 3.04; P = 0.04) than those from the control tanks (Table 2). There were no differences in condition factor between treatments. Due to oxygen supplementation, the P33–O2 treatment had significantly higher (within-subjects analysis of variance: O2 (mg L−1), F14,56 = 15.8, P < 0.0001; Oxygen saturation, F14,56 = 35.4, P < 0.0001) dissolved oxygen concentrations and saturation percentages (8.3 ± 0.7 mg L−1; 103 ± 8%), when compared with control tanks (7.5 ± 0.5 mg L−1 O2; 92 ± 6%). Mean FAN concentrations in the P33 and P33–O2 treatments were 2.4 ± 1.3 and 2.4 ± 1.1 μg L−1 respectively. Mean pH for both treatments ranged between 7.60 and 7.62. Mean flow indices in the P33–O2 and P33 treatments were 3.34 ± 0.13 and 3.35 ± 0.04 L h−1 kg−1 respectively.

Table 2. Mean (±SD) for production variables of abalone in the last tanks of a serial-use system using three passes without (control) and with supplementation of oxygen (treatment). Abalone were reared for 105 days on a 33% protein diet. T-statistics and P-values compare means between control and treatment. The column on the right shows values from the first tank in series of abalone fed the same diet. W0 and We are average initial and final weight (g abalone−1), L0 and Le are average initial and final length (mm abalone−1), Wg and Lg are average weight gain and length gain, CF0 and CFe are average initial and final condition factor, Feed is total amount of feed fed (g) and FCR is feed conversion ratio (Wg feed fed−1)
ControlTreatmentt-statistics/P-valuePosition 1
W0 49.5 ± 1.3348.7 ± 0.55t = 0.93; P = 0.4050.2 ± 0.49
We 53.6 ± 0.7655.6 ± 2.06t = 1.54; P = 0.1963.1 ± 1.27
Wg4.1 ± 0.876.8 ± 1.59t = 2.61; P = 0.0612.9 ± 0.80
L066.5 ± 1.1965.3 ± 1.20t = 1.23; P = 0.2965.9 ± 0.20
Le 69.5 ± 0.5270.0 ± 1.29t = 0.56; P = 0.6072.5 ± 0.25
Lg3.00 ± 0.734.66 ± 0.39t = 3.44; P = 0.0266.65 ± 0.35
CF0 0.97 ± 0.031.02 ± 0.05t = 1.29; P = 0.271.02 ± 0.02
CFe 0.93 ± 0.010.94 ± 0.02t = 1.08; P = 0.340.96 ± 0.01
Feed2436 ± 34.832261 ± 67.79t = 3.97; P = 0.0172427 ± 122.98
FCR4.23 ± 0.992.32 ± 0.45t = 3.05; P = 0.041.29 ± 0.05

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

FAN concentration in a serial-use raceway

The percentage dietary protein had a significant effect on the concentration of FAN at each tank position. Thus, within the range of protein levels tested an increase in dietary protein level of 21% resulted in 50% higher FAN concentrations. In H. midae, growth has been shown to be independent of dietary protein, provided that dietary energy is maintained above 13.5 MJ kg−1 and that it is provided mostly in the form of carbohydrates (Green et al. 2011). The diets used here conformed with these requirements, in that energy levels were 15.6 MJ kg−1 and lipid content was low (3%). There was also no difference in growth between the two diets for position 1 where the flow indices were above those shown to cause reductions in growth as a function of water quality (Naylor et al. 2011). Thus, it is hypothesized that differences in growth were the result of water quality or environmental conditions.

Despite being exposed to lower FAN concentrations, abalone in the P26 treatment did not gain weight or length faster at any of the positions within the serial-use raceways. However, the reduction in growth as a result of increasing water use for both dietary treatments was more pronounced in abalone fed the high-protein diet from the second to the third tank in the series. It is hypothesized that a low-protein diet may become more beneficial as the intensity of water use increases. Abalone are sensitive to FAN, and have demonstrated a large reduction in growth rate (Harris et al. 1998; Basuyaux & Mathieu 1999; Huchette et al. 2003; Reddy-Lopata et al. 2006) well below the 50–200 μg L−1 safe limit described for many marine finfish species (Russo & Thurston 1991; Person-Le Ruyet, Galland, Le Roux & Chartois 1997; Lemarie, Dosdat, Coves, Dutto, Gasset & Person-Le Ruyet 2004). Although no information is available regarding the EC5 for FAN in H. midae, Reddy-Lopata et al. (2006) determined that the EC50 for FAN in juvenile H. midae was <7 μg L−1 and speculated that the EC5 would be lower than the 4 μg L−1 as was reported by Huchette et al. (2003) for H. rubra. As there was no difference in growth between the P26 and P33 treatments at position 3, where FAN concentrations were 1.0 ± 0.9 and 2.4 ± 1.3 μg L−1, respectively, suggests that FAN concentration in the P33 treatment was below the level that can significantly reduce growth. This may be due to the larger size class used (69.5 ± 2.5 mm versus 10–25 mm used by Reddy-Lopata et al. 2006), as larger abalone are less sensitive to high concentrations of FAN (Fallu 1991; Reddy-Lopata et al. 2006). Other environmental conditions, especially pH (Thurston et al. 1981), may also influence the estimation of EC5. As the average FAN concentration could not explain differences in growth between tank positions, other water quality variables may have influenced growth.

Using predictions by Britz, Hecht and Mangold (1997), the decrease in temperature between position 1 and 3 accounts for only 11–12% of the difference in weight gain observed and therefore may have influenced growth, but this was not likely the most limiting variable. Similarly, mean dissolved oxygen concentration (7.3–7.5 mg L−1 O2) and percentage oxygen saturation (90.5–92.8%) were similar between positions and treatments and are unlikely to have caused the differences in abalone growth. This was supported by the low correlation between oxygen concentration and growth, and as oxygen concentration, within the range tested here, did not contribute to a predictive model. The EC50 for dissolved oxygen in H. laevigata has been estimated at 5.91 mg L−1 and 77% saturation (Harris, Maguire, Edwards & Johns 1999). Although nitrite–N concentrations increased with increasing serial-use, reaching a maximum of 34.1 μg L−1, these values were below those shown to affect the growth in other abalone species. Harris, Maguire, Edwards and Hindrum (1997) recorded growth reductions in H. laevigata at nitrite–N concentrations above 540 μg L−1, whereas Basuyaux and Mathieu (1999) estimated safe concentrations at <5000 μg L−1 in H. tuberculata L. There was, however, a significant correlation between the reduction in growth and the decrease in water pH. In addition, water pH contributed the most to a multiple regression model to predict growth as a function of water quality. The hypothesis that low pH caused a reduction in growth is supported by the negative effects of chronic hypercapnia on growth rate and shell dissolution shown in echinoderms, gastropods and bivalves (Shirayama 2002; Michaelidis et al. 2005). In this study, the decrease in pH was most likely caused by the respiration of the abalone and subsequent release of carbon dioxide. As carbon dioxide hydrates in water, it dissociates into aqueous carbon dioxide (CO2 aq), carbonic acid (H2CO3), bicarbonate (HCO3 ), carbonate (CO3 −2) and hydrogen ions (H+), in a pH-equilibrated reaction (Smith 1988). The lower the pH, the greater the proportion of CO2 aq, which could lead to reduced CO2 diffusion at the gills and subsequent blood acidosis and altered oxygen-haemocyanin affinity (Sanni & Forsberg 1996; Harris, Maguire, Edwards & Hindrum 1999). In H. corrugata (Wood), Burnett, Scholnick and Mangum (1988) showed a decrease in haemocyanin oxygen affinity with a decrease in pH. In the marine mussel, Mytilus galloprovincialis, exposure to pH 7.3 caused a significant reduction in haemolymph pH and metabolic rate and increased haemolymph CO2-concentrations, when compared to controls held at pH 8.05 (Michaelidis et al. 2005). A decrease in pH also reduces the availability of essential CO3 −2 ions (Kleypas et al. 2006; Lopez et al. 2011) and it lowers the saturation state of calcite and aragonite, which are calcium carbonate minerals essential for the formation of skeletal structures and shell in many marine organisms (Feely, Sabine, Lee, Berelson, Kleypas, Fabry & Millero 2004). In the greenlip abalone, Haliotis laevigata, the EC5 and EC50 values for pH have been estimated as 7.78 and 7.39, whereas in the blacklip abalone, Haliotis rubra, EC5 and EC50 values were 7.93 and 7.37 (Harris, Maguire, Edwards & Hindrum 1999). The pH values recorded in this study were therefore within the range known to affect other abalone species. They were also well below the mean pH recorded by Yearsley (2008) for morning (pH 8.06) and afternoon (pH 8.13) samples of influent water quality for this specific farm during 2007, representing the pH levels of this species' natural environment.

Effect of oxygen supplementation at low flow indices

Supplementation of pure oxygen to saturation levels of 103 ± 8% resulted in a significantly higher length gain in abalone exposed to high FAN concentrations and low pH. It is hypothesized that a significant statistical difference in wet weight gain could have emerged as these conditions continue over a longer growth period. The growth was, however, not as good as that observed in position 1 where oxygen saturation percentages averaged only 92 ± 8. It is suggested that the oxygen available in the supplemented tanks alleviated some of the negative effects of either low pH or high FAN concentrations, or a combination of both. As abalone reared under conditions of low dissolved oxygen levels and low pH were less efficient at converting feed into biomass, it is suggested that the water quality conditions reduced growth by influencing FCR.

However, a low water pH as a result of a high CO2 concentration in the water can lead to reduced CO2 excretion at the gills and subsequent blood acidosis. In H. diversicolor supertaxa (Reeve), Cheng, Liu, Cheng and Chen (2004) reported that the partial pressure of haemolymph CO2 was inversely related to environmental dissolved oxygen concentrations. As a result of this metabolic imbalance, the increased CO2 concentration of the haemolymph may affect the oxygen carrying capacity of haemocyanin (Burnett et al. 1988; Harris, Maguire, Edwards & Hindrum 1999). Under natural sea conditions, the energetic cost of respiration is estimated at 32% of absorbed energy (Barkai & Griffiths 1988). The increased dissolved oxygen concentration in the supplemented tanks may have reduced the energetic cost of respiration when compared with non-supplemented tanks, thereby allowing a greater proportion of energy to be used for somatic growth, and is supported by the lower FCR values recorded in this treatment.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

Dietary protein level had a significant effect on mean FAN concentrations within the serial-use raceways. The resultant high FAN and low FAN conditions did not cause significant differences in growth and indicates that the mean FAN concentrations (≤2.4 μg L−1) were within a range that did not affect growth. However, significant differences in growth were observed between positions, and thus under the conditions prevalent in this serial-use system, the concentration of FAN was not the first limiting water quality variable. Weight gain and length gain were significantly correlated to water pH. Supplemental oxygen improved the length gain of abalone held at low pH and high FAN concentrations. Future studies should examine the effect of pH on abalone growth through its effect on the carbonate system in seawater.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References

The financial support of HIK Abalone Farm, Aquafarm Development, Roman Bay Sea Farm and the National Research Foundation (NRF) towards this research is gratefully acknowledged. The opinions expressed and the conclusions arrived at are those of the authors and are not necessarily to be attributed to the NRF. We thank Morena Khasane for assistance during the trial period.

References

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  2. Abstract
  3. Introduction
  4. Methods and materials
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgments
  9. References
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