| Abstract [eng] |
There are many areas in the world where scarcity of freshwater are threatening the human population. So humans have to look for solutions to replace freshwater when use of it is not necessary. This also has an effect to aquaculture – more and more farmers are farming freshwater fish in brackish water. Many authors have demonstrated the benefits of brackish water for fish growth and other physiological functions. Nile tilapia (Oreochromis niloticus) is valuable fish for its tolerance to high temperatures, salinity, low amount of dissolved oxygen. They are fairly easy to farm and the quality of the meat and the taste is good. But, despite all the qualities, farmers in Lithuania do not choose to farm these fish. The aim of this study was to evaluate the effect of brackish water on the efficiency of Nile tilapia aquaculture technology. To achieve the goal an experiment was conducted in the Laboratory of Fishery and Aquaculture of the Marine Research Institute of Klaipeda University on August 5 December 18, 2020. During it, three separate recirculating aquaculture systems were constructed from system B. One such system consisted of three tanks. Tank volume – 2,83 m3. Total amount of biofilter filler – 130 L, water inflow – 36 L min-1. All tanks were equipped with the same bio and mechanical filters, oxygen diffusers, the room is heated by heaters. In the experiment water of three different salinities were used: fresh water (0‰), water from the Baltic Sea (6‰) and diluted sea water (3‰). The remaining three basins are used as reserve tanks for storing and diluting seawater. Seawater is pumped to the laboratory from the shores of the Baltic Sea, and fresh water is obtained from a well. Nile tilapia were sorted and distributed into tanks. At the beginning of the study, the average fish weight was: 0‰ – 84,4 ± 19,51 g, 3‰ – 81,28 ± 22,31 g, 6‰ – 84,08 ± 16.47 g, stocking density - 0‰ – 8,08 kg m-3, 3‰ ir 6‰ – po 8,22 kg m 3. Fish were acclimatized for three weeks, their health and behavior were monitored. Water quality parameters were measured daily – temperature (°C), dissolved oxygen content (%), pH. Tests of water saples from each system were conducted to determine the amount of ammonium (NH4), nitrite (NO2) and nitrate (NO3) in the water. Dead fish were counted. During the experiment, approximately every three weeks, 30 fishes were randomly caught and measured from tank. Caught individuals were placed in plastic buckets filled with water. If present, juveniles were collected. Eggs and larvae were photographed, the amount was counted from the photo and fry was counted by transferring from one container to another. Counted juveniles were released into individual aquariums filled with water of appropriate salinity. Adult individuals were weighed, the total length (L) and the length to the caudal fin (l) were measured. During the last two measurements, the sex of the fish was determined according to the shape and size of the papilla and the urogenital organs. A toluidine blue solution diluted with distilled water was used for determination of gender. At the end of the experiment, the following were calculated: mean fish weight, specific growth rate (SGR), weight gain (Δw), feed conversion ratio (FCR), initial and final Fulton coefficients (Ki and Kf), final biomass, and survival. A respiration experiment was conducted using Nile tilapia juveniles to determine their metabolic rate (MO2). A PyroScience FireSting O2 optical oxygen meter was used for the respiration experiment. Experiment was conducted in two trials. First one: water of different salinity (0‰, 3‰, 6‰) was added to each respirometry system, the water temperature was 22 °C. Juveniles from 0‰ aquarium were placed in 0‰ system, from 3‰ to 3‰ systems and from 6‰ aquarium to 6‰ system. Second trial: water of different salinity (0‰, 3‰, 6‰) was added to each system, the water temperature was 23 °C., Juveniles from 0‰, 3‰ and 6‰ aquariums were placed in each system. Intermitted respirometry method were used: when the amount of oxygen dropped ~ 2 mg L-1 from the initial amount, the water was renewed – the former water was poured out of the bottle and a new, well-oxygenated water was added. Fry in the bottles were not changed. After the experiment, the fry was weighed (g). A study of physico-chemical properties of fish meat was conducted. After the growth experiment, three females and three males were randomly transferred from each pool to the reservoirs of the system with water of appropriate salinity. A total of 54 tilapias were used in the study. Skinless fish fillets were used for the study, each piece was weighed. A total of 18 samples were taken, 6 from each system (3 females and 3 males). Due to the low weight of the female fillets, the samples were pooled – one sample consisted of three fish using one half of their fillets. Samples were kept on ice until they reached the laboratory. The study of physico-chemical properties of meat was performed at the Veterinary Academy of the Lithuanian University of Health Sciences. The percentage of protein, fat, ash, dry matter, water binding and water content of the sample was determined and the pH was determined. In this study were found that water salinity had a statistically significant effect on dissolved oxygen content, pH, and nitrite content in water. Also, due failed adaptation of freshwater recirculating aquaculture system to the brackish water, the water quality deteriorated during the study, especially in the 6 ‰ system, which affected other results. Growth and feed conversion ratios did not differ statistically significantly under different salinity conditions. The worst survival was in the 6 ‰ system due to deteriorating water quality during the study, and the best in the 0 ‰ system. Gender had a statistically significant effect on fish weight, with the final weight of males being 54% higher than females. Brackish water conditions did not significantly affect the spawning of Nile tilapia females, larval and fry amount. There were also no statistically significant differences in the measurement of pup metabolic rate. When assessing meat quality, there was a statistically significant difference in meat pH and water content between systems. Water content in fish meat also differed statistically significantly between the sexes. |
