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United States Department of Agriculture

Agricultural Research Service

Research Project: Improving Sustainability of Rainbow Trout Production by Integrated Development of Improved Grains, Feeds, and Trout

Location: Small Grains and Potato Germplasm Research

Title: Susceptibility of Nile tilapia (Oreochromis niloticus) fed dietary sodium chloride to nitrite toxicity

Authors
item WELKER, THOMAS
item Lim, Chhorn
item Aksoy, Mediha
item Klesius, Phillip

Submitted to: Aquaculture International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 23, 2011
Publication Date: June 18, 2011
Repository URL: http://riley.nal.usda.gov/nal_web/digi/submission.html
Citation: Welker, T.L., Lim, C.E., Aksoy, M., Klesius, P.H. 2011. Susceptibility of Nile tilapia (Oreochromis niloticus) fed dietary sodium chloride to nitrite toxicity. Aquaculture International. 20:159-176.

Interpretive Summary: Nitrite can be toxic to fish, reaching high levels in intensive fish culture systems. Toxicity of nitrite to fish is primarily attributed to the oxidation of hemoglobin to form methemoglobin (MetHb), which is incapable of binding oxygen. Methemoglobinemia causes blood to turn brown in color and has been termed “brown blood disease”. Previous research has shown that addition of chloride salts to rearing water is effective in alleviating nitrite toxicity in many species of fish, but this study is the first to examine the effect of dietary NaCl on susceptibility of tilapia to nitrite exposure. Juvenile Nile tilapia (Oreochromis niloticus) were fed diets supplemented with 0 or 6% NaCl for 10 weeks. Tilapia fed the NaCl-supplemented diet had superior growth compared to the groups fed the unsupplemented diet. The improved growth was associated with higher feed intake. Dietary NaCl supplementation caused a significant decrease in plasma nitrite levels after nitrite exposure. The dietary reduction in nitrite may be related to the increase in plasma chloride in the 6% NaCl-supplemented fish. Mortality from nitrite exposure was lower in tilapia fed the NaCl-supplemented diet compared to the control group, but the differences were not significant. Dietary supplementation with NaCl only produced a non-significant reduction in MetHb levels as well. Because tilapia were subjected to acute nitrite levels with only minor gains in resistance to nitrite toxicity, dietary sodium chloride may be more effective in protecting against nitrite toxicity at lower levels of nitrite. Feeding NaCl to tilapia did not affect susceptibility to S. iniae or immune function, but nitrite exposure cause a stress-related reduction in non-specific immune function. This is the first study to examine the effects of dietary salt on nitrite toxicity in tilapia.

Technical Abstract: Juvenile Nile tilapia (Oreochromis niloticus) were fed diets supplemented with 0 or 6% NaCl for 10 weeks. Tilapia were exposed to approximately 21 mg/l nitrite-N after five and ten weeks of feeding to determine the effect of dietary NaCl supplementation on resistance to nitrite toxicity. Fish were sampled before (baseline, pre-exposure) and after 24 h nitrite exposure to determine the effects of dietary NaCl on mortality, hematology (hematocrit, hemoglobin, and methemoglobin), and plasma electrolyte dynamics (nitrite, chloride, sodium, and potassium). After 10 weeks of feeding, tilapia were also challenged with Streptococcus iniae to determine the effect of sodium chloride on immunity. Tilapia fed the NaCl-supplemented diet had significantly higher weight gain compared to the control group, which was associated with a significant increase in feed intake in the NaCl group. Mortality from nitrite exposure was lower in tilapia fed the NaCl-supplemented diet compared to the control group at 5 and 10 weeks, but the differences were not significant. However, dietary NaCl supplementation caused a significant decrease in plasma nitrite levels after nitrite exposure. The dietary reduction in nitrite may be related to the increase in plasma chloride in the 6% NaCl-supplemented fish. A direct link between the effects of dietary NaCl supplementation on methemoglobin (MetHb) could not be established. Tilapia in this study were subjected to acute nitrite toxicity. Dietary sodium chloride may be more effective in protecting against nitrite toxicity at lower levels of nitrite, but the conditions at which it proves to be effective may be limited and requires further investigation. Feeding NaCl to tilapia did not affect susceptibility to S. iniae or immune function, but nitrite exposure cause a stress-related reduction in non-specific immune function. This is the first study to examine the effects of dietary salt on nitrite toxicity in tilapia.

Last Modified: 7/28/2014
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