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Title: Susceptibility of channel catfish (Ictalurus punctatus) fed dietary sodium chloride to nitrite toxicity

Author
item Welker, Thomas
item Lim, Chhorn
item Aksoy, Mediha
item Klesius, Phillip

Submitted to: Aquaculture Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2011
Publication Date: 9/1/2011
Citation: Welker, T.L., Lim, C.E., Aksoy, M., Klesius, P.H. 2011. Susceptibility of channel catfish (Ictalurus punctatus) fed dietary sodium chloride to nitrite toxicity. Aquaculture Nutrition. 17(4):e892-e901.

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 (Hb) 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 fish to nitrite exposure. Juvenile channel catfish (Ictalurus punctatus) were fed diets supplemented with NaCl at 0, 1, 2, or 4 % of diet for 10 weeks. Catfish were exposed to nitrite after six and ten weeks of feeding to determine the effect of dietary NaCl supplementation on resistance to nitrite toxicity. Mortality from nitrite exposure tended to decrease with increasing NaCl in the diet at 6 weeks and was significantly lower in the 4% NaCl group (12.5%) compared to the control group (57.5%). A similar trend in mortality occurred at 10 weeks as well, however, the differences among dietary treatments were not significant. A direct link between the effect of dietary NaCl supplementation on measured physiological parameters, such as MetHb or plasma nitrite levels, and mortality from nitrite exposure could not be established. The acute levels of nitrite used in this study resulted in MetHb levels that were approximately 90% or greater, which is considerably higher than the levels (is equal to or greater than 50% MetHb) deemed detrimental to fish health, and the effects of dietary salt supplementation on lower levels of nitrite toxicity should be further explored.

Technical Abstract: Juvenile channel catfish (Ictalurus punctatus) were fed nutritionally complete, practical basal diets supplemented with NaCl at 0, 1, 2, or 4 % of diet to apparent satiation twice daily for 10 weeks. Catfish were exposed to nitrite after six (7.70 mg/l nitrite-N) and ten (7.18 mg/l nitrite-N) 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 hematology (hematocrit, Hb, and MetHb) and plasma electrolyte dynamics (nitrite, chloride, sodium, and potassium). Mortality from nitrite toxicity was also determined. Mortality from nitrite exposure tended to decrease with increasing NaCl in the diet at 6 weeks and was significantly lower in the 4% NaCl group (12.5%) compared to the control group (57.5%). A similar trend in mortality occurred at 10 weeks as well, however, the differences among dietary treatments were not significant. A direct link between the effect of dietary NaCl supplementation on measured physiological parameters, such as MetHb or plasma nitrite levels, and mortality from nitrite exposure could not be established. The acute levels of nitrite used in this study resulted in MetHb levels that were approximately 90% or greater, which is considerably higher than the levels (is equal to or greater than 50% MetHb) deemed detrimental to fish health, and the effects of dietary salt supplementation on lower levels of nitrite toxicity should be further explored. 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 fish to nitrite exposure.