Studies on Potential Use of Salinity to Increase Growth of Tilapia in Aquaculture in Malawi

In a series of studies conducted in Malawi to determine the effects of different salinity concentrations on survival, growth, feed conversion, reproduction, and whole-body composition of five taxonomic groups of tilapia—Oreochromis shiranus chilwae (Lake Chilwa strain), O. shiranus chilwae (Bunda College strain), O. karongae, O. shiranus shiranus, and Tilapia rendalli—it was observed that the first three species grew faster in 10‰ salinity and would be recommended as potential candidates for brackishwater aquaculture in Malawi. T. rendalli and O. shiranus shiranus grew faster in fresh water (0‰ salinity) and are unsuitable for brackishwater aquaculture. With the exception of O. shiranus chilwae (Lake Chilwa strain) and O. shiranus chilwae (Bunda College strain), all species had lost carcass protein at the end of the study, suggesting that they used tissue protein as an additional energy source for osmoregulation and homeostasis. Salinity tolerance varied ontogenetically in almost all the above taxonomic groups, with younger individuals tolerating salinity longer than larger individuals. This study has also shown that the range of T. rendalli and O. shiranus shiranus would effectively be limited by salinity. The interactive effect of salinity and water temperature was not investigated in this study since all experiments were conducted at room temperature and ambient photoperiod. Temperature, however, has an influence on salinity tolerance, and in that light, we strongly recommend further investigations on the combined influence of the two abiotic factors (salinity and temperature) since they fluctuate together in nature, and their fluctuations may positively or negatively influence growth and reproductive performance of the above cichlids. NINETEENTH ANNUAL TECHNICAL REPORT 168 The fish were fed at 5% body weight per day (BWD). Fishmeal and soybeans were the main sources of dietary protein, while wheat offal and rice bran were the main carbohydrate energy sources in the formulated diet. During the experimental periods the fish were checked every day for mortalities. Salinity, temperature, and pH were also checked daily. Dissolved oxygen (DO), ammonia, conductivity, and turbidity were checked three times a week (Monday, Wednesday, and Friday). Fish were sampled every two weeks to monitor changes in weight. All species ranged in average weight from 5.5 to 10.0 g. In each experiment only one statistically uniform size of fish was used. The effect of salinity on whole-body composition of fish (dry matter, crude protein, fat, and ash) was determined by taking initial fish samples before starting the experiments. Final fish samples were taken at the end of the experiments to determine salinity effects on whole-body composition. There was an overlap in the experimental periods for the five experiments, but each had different stocking and sampling dates. In the next series of five supporting experiments, selected individuals of each species were introduced into 30‰ salinity water to determine their response to salinity stress. Specific growth rate (SGR), expressed as percent body weight per day, was calculated from: SGR = 100 (ln Wf – ln Wi)/t where Wf = final mean weight, Wi = initial mean weight, and t = experimental time in days. Feed conversion efficiency (FCE) was expressed as the ratio of growth (weight gain) to total feed consumed (dry weight). The mean concentration of salinity across treatments ranged from 0.10 to 20.6‰. DO ranged from 7.89 to 9.21 mg l-1, water temperature ranged from 22.9 to 27.0°C, and pH ranged from 6.18 to 7.99. Ammonia concentration ranged from 0.70 to 2.60 mg l-1, conductivity ranged from 0.292 to 38.40 mS cm-1, and turbidity ranged from 2.0 to 48.0 mg l-1. Experiment 1: Growth and Whole-body Composition of Oreochromis shiranus chilwae (Lake Chilwa Strain) Cultured in Four Salinity Concentrations After a week of acclimation in a fiberglass tank, juveniles of O. shiranus chilwae (Lake Chilwa strain) were transferred into twelve 50-l rectangular plastic tanks. Four salinity levels (0, 10, 20, and 30‰) were assigned to these tanks in triplicate. Each tank was filled with well water and was stocked with ten fingerlings (average weight = 8.7 to 9.6 g) at 1.58 fish cm l-1. Fish were acclimated to their respective experimental salinity concentrations by adding salt at 2.5‰ d-1. Fish were fed twice a day on a diet formulated from fish meal, soybean meal, wheat offal, and rice bran and containing 30% protein. Fish were fed at about 5% BWD. Fish mortality, salinity, and temperature were checked daily, while DO, ammonia, pH, conductivity, and turbidity were monitored three times weekly. Fish started dying within two weeks in the highest (30‰) salinity treatment. Between 20 January and 17 February 2001 (28 d), all fish in 30‰ salinity died, and that treatment was withdrawn from the study. We then continued with three treatments (0, 10, and 20‰ salinity). The effect of salinity on reproduction was evaluated by determining gonadosomal indices (GSI) of the experimental fish. Experiment 2: Response of O. shiranus chilwae (Lake Chilwa Strain) to Salinity Stress This experiment was conducted in two parts. In the first part of the experiment, smaller fish (average weight = 2.36 g) were subjected to a sudden transfer into an equivalent of 86% seawater (30‰ salinity). In the second part, larger individuals of the same species (average weight = 15.22 g) were also transferred to the same environment (30‰ salinity). The objective of this experiment was to determine differences in the response of O. shiranus chilwae to acute stress following direct transfer to 30‰ salinity. Thirty-seven fingerlings (average weight = 2.36 g) were collected from a fiberglass tank in the hatchery where they were maintained. Ten of these individuals were transferred into a 30-l circular tank half-filled with well water containing 30‰ salinity. The experiment was started at 1030 h and was timed. The water and fish in the tank were adequately aerated with a blower while the experiment was in progress. Cessation of opercular movement and the fish’s failure to respond to physical touch or gentle prodding were used as the criteria for death. The opercular and jaw movements in the fish were therefore closely monitored. Dead individuals were instantly removed from the tank. The experiment was conducted under ambient photoperiod. The second part of this experiment started at 1045 h when ten individuals of the above species (average weight = 15.22 g) were transferred into another 30-l tank half-filled with well water containing 30‰ salinity. Experiment 3: Growth and Whole-body Composition of O. shiranus chilwae (Bunda College Strain) Cultured under Laboratory Tank Conditions in Three Salinity Concentrations. The cichlid O. shiranus chilwae (Bunda College strain) was collected locally from a breeding pond at Bunda College of Agriculture. After a week of acclimation to their respective experimental salinity concentrations in tanks, the fish were stocked in nine 100-l plastic tanks in a wet laboratory. Three salinity levels (0, 10, and 20‰) were assigned to the tanks in triplicate. The highest salinity concentration (30‰) was not included in the experiment due to a shortage of experimental tanks. Fish were acclimated to their respective salinity concentrations by adding salt at 2.5‰ d-1 as in the first experiment. Each tank was stocked with ten fingerlings (average weight = 5.42 to 5.60 g) at 0.66 fish cm l-1. Fish were fed a formulated diet containing 30% protein, two times a day at 5% body weight. Fish mortality, salinity, pH, and temperature were checked every day, while DO, ammonia, conductivity, and turbidity were monitored three times weekly. Experiment 4: Response of O. shiranus chilwae (Bunda College Strain) to Salinity Stress At 1535 h, ten juveniles of O. shiranus chilwae (Bunda College strain) (average weight = 6.00 g) were placed in a 50-l plastic tank half-filled with well water of 30‰ salinity. The experiment was timed and the tank was aerated. The pH and the DO concentration of the water were both normal (refer to range above). In the second part of this experiment, we used slightly larger fish (average weight = 10.0 g). At 1515 h, ten of these individuals were again placed in a 50-l plastic tank containing well water at 30‰ salinity, and the experiment was again timed. ADOPTION/DIFFUSION RESEARCH 169 Experiment 5: Growth and Whole-body Composition of Tilapia rendalli Reared under Laboratory Tank Conditions at Two Salinity Concentrations Tilapia rendalli fingerlings (average weight = 6.9 to 9.0 g) were collected from a breeding pond at Bunda College and introduced into nine 200-l circular plastic tanks filled with well water. Each tank was stocked with 14 fingerlings at 0.53 fish cm l-1. Initially, three treatments (0, 10, and 20‰ salinity) were assigned in triplicate (the highest salinity concentration (30‰) proposed earlier in this study was not included). Fish succumbed to 20‰ very early (within two to three weeks), and this treatment had to be withdrawn from the study. The treatment of 20‰ salinity may not have been necessary since T. rendalli has been reported to tolerate a maximum salinity level of 19‰. We therefore continued the study using 0 and 10‰ salinity as treatments. The general maintenance of the experimental animals (in relation to water quality, feeding, and sampling) was the same as that outlined above for other experiments. Experiment 6: Response of T. rendalli to Salinity Stress In this experiment only juveniles of one size were used. It was not possible to get another size for this study. Ten juveniles of T. rendalli (average weight = 11.4 g) were introduced into a 50-l plastic tank containing well water at 30‰ and pH 7.46. The tank was adequately aerated. The experiment started at 1510 h and was also timed. Experiment 7: Growth and Whole-body Composition of O. karongae Cultured under Laboratory Tank Conditions in Two Salinity Concentrations Fingerlings of O. karongae (average weight = 6.9 to 9.0 g) were collected from a breeding pond at Bunda College and introduced into nine 200-l circular plastic tanks filled with well water. For this experiment each tank was stocked with 14 fingerlings at 0.53 fish cm l-1. Initially, three treatments (0, 10, and 20‰ salinity) were assigned, but lack of adequate tanks forced us to investigate two salinity levels, 0 and 10‰. Salinity was increased in the experimental tanks by adding salt at 2.5‰ d-1. The general maintenance of the experimental animals (in relation to water quality, feeding, and sampling) was carried out as outlined in the above experiments. Experiment 8: Response of O. karongae to Salinity Stress The method used was similar to that used for the other species above. The tank was stocked using O. karongae (average weight = 6.89 g). The experiment was started at 1540 h and was timed. The experiment was carried out using one size range of fish, as larger sizes were not available for the experiment. Experiment 9: Growth and Whole-body Composition of O. shiranus shiranus Cultured under Laboratory Tank Conditions in Three Salinity Concentrations Juvenile O. shiranus shiranus (average weight = 7.03 to 7.09 g) were collected from a nursery pond at Bunda College of Agriculture and placed in a concrete tank for five days acclimation. Twelve 30-l plastic tanks were cleaned and set up in the wet laboratory. Three treatments (0, 10, and 20‰ salinity) were assigned to the plastic tanks in triplicate. Each tank contained aerated well water. The fish were collected from the concrete tank and transferred into the experimental tanks at a stocking density of 8 fish tank-1 (1.88 fish cm l-1). Salinity was increased in the experimental tanks by adding salt at 2.5‰ d-1. The fish were fed on 30% protein diet, twice a day at 5% body weight. Fish were sampled every two weeks to monitor changes in weight. Fish mortality, salinity, pH, and temperature were monitored every day, while DO, ammonia, conductivity, and turbidity were checked three times a week. Experiment 10: Response of O. shiranus shiranus to Salinity Stress At 1428 h, ten juveniles (average weight = 6.52 g) and ten others (average weight = 11.0 g) were simultaneously introduced into two separate 30-l plastic tanks containing water at 30‰ salinity.