Submitted to: Journal of Applied Aquaculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/3/2006
Publication Date: 12/20/2006
Citation: Lim, C.E., Aksoy, M., Welker, T.L., Veverica, K. 2006. Effect of feeding duration of sodium chloride containing diets on growth performance and some osmoregulatory parameters of nile tilapia (Oreochromis niloticus) after transfer to water of different salinities. Journal of Applied Aquaculture 18: 1-17. Interpretive Summary: Tilapia are euryhaline fish and some species can survive direct transfer from freshwater (FW) to salt water (SW). Among tilapia species, the Nile tilapia, Oreochromis niloticus, is one of the most important species in aquaculture due to its rapid growth. However, it is less salt-tolerant than Oreochromis aureus, O. mossambicus, and Tilapia zillii. It has been reported Nile tilapia survived only for brief periods in full-strength SW. Pre-acclimation also has a crucial physiological significance for some euryhaline teleosts during SW adaptation. Early salinity exposure through spawning and hatching under elevated salinities enhances salinity tolerance of young tilapia fry and may facilitate acclimation to SW. Feeding with diets supplemented with NaCl is another acclimation method that has been used with some success for adapting rainbow trout and chinook salmon to SW. Thus, this study encompassing two separate feeding trials was conducted to determine the effect of feeding duration of a sodium chloride (NaCl)-supplemented diet on growth, hematocrit, blood glucose, and serum osmolarity and cortisol levels of Nile tilapia after exposure to waters of different salinities. Our results show that juvenile Nile tilapia receiving dietary NaCl feeding regimens, even for a two-week period, exhibited consistently better growth and feed efficiency. Feeding NaCl-supplemented may have modest benefit to the osmoregulatory ability of Nile tilapia transferred sea water. Tilapia can be directly transferred from freshwater to 10 ppt SW with minimum stress. Transferring to higher salinity water may require gradual acclimation to reduce osmotic stress. Nile tilapia cannot be directly transferred from freshwater to 30 ppt SW as death occurred after 4 h. It is also suggested that osmotic parameters evaluated should be monitored over a longer period (more than 96 h) to determine the duration at which osmotic stresses subside.
Technical Abstract: Two feeding experiments were conducted to evaluate the effect of feeding duration of dietary salt (NaCl or S) on hematocrit, blood glucose, and serum osmolarity and cortisol of Nile tilapia acclimated for various time periods to salt water (SW) of different salinities (three-factor experiment). Quadruplicate groups of fish averaging 5.52 ± 0.13 g (experiment I) and 10.04 ± 0.19 g (experiment II) were fed to apparent satiation twice daily with the following four feeding regimens: feeding the control diet (C) for 6 weeks (6-wk C), feeding the 6% NaCl (S) diet for 6 weeks (6-wk S) feeding the C diet for 2 weeks and the S diet for 4 weeks (2-wk C + 4-wk S), and feeding the C diet for 4 weeks and S diet for 2 weeks (4-wk C + 2-wk S). At the end of week 6, fish in each aquarium were weighed for growth measurement. Fish from each replicate aquarium in experiment I were transferred to SW at 0, 15 and 30 ppt whereas those from experiment II were transferred to SW at 0, 10 and 20 ppt. Hematocrit (experiment II only), blood glucose, and serum osmolarity and cortisol were determined at 48 and 96 h, and 0, 6, 12, 24 and 48 h for experiments I and II, respectively, after transfer to SW. In both experiments, weight gain after 6 weeks of feeding did not differ (P>0.05) among treatments, although all fish in the treatment receiving the NaCl-containing diet had consistently higher weight gain than those fed the C diet. Dry matter feed intake and survival were similar in both studies. Feed efficiency, although was significantly different only in experiment I, was consistently better for the groups that fed the NaCl-containing diet. All fish transferred to 30 ppt salinity died within 8 h. No mortality occurred in fish transferred to 0, 10, 15 or 20 ppt salinity. Feeding dietary salt had no effect on blood glucose and hematocrit levels in either study. Serum osmolarity of fish in experiment I decreased in fish fed dietary salt, but the differences were not always significant. This value was similar among fish fed dietary salt in experiment II. In both experiments, blood glucose and serum osmolarity significantly increased, whereas hematocrit decreased with increasing water salinity. Duration of exposure to salt water also significantly increased blood glucose levels but decreased hematocrit values. Duration of salt water exposure had no effect on serum osmolarity. The interaction between dietary salt and water salinity, water salinity and exposure time, dietary salt and exposure time had no effect on hematological and serological values in both experiments, except blood glucose and plasma osmolarity and cortisol in experiment II were significantly affected by water salinity and exposure time. The interaction between the three main factors had no effect on measured hematological parameters.