Reversibility of deleterious effects of the pisciculture byproduct nitrite on cultured Nile tilapia (Oreochromis niloticus)

Abstract

The effects of nitrite (NO₂^–) on the Nile tilapia (Oreochromis niloticus) were investigated, due to their relevance to worldwide aquaculture. Hematological parameters – functional haemoglobin (oxy+deoxy-Hb, %), methaemoglobin (MetHb%), and hematocrit (Hct) – and – plasma osmolality, plasma chloride ([Cl⁻]) and magnesium ([Mg²⁺]) concentrations – were analysed. Two experiments were conducted, each with its own non-contaminated control, 8 tilapias for each group. In experiment 1 (48 hours), and the first 48 hours of experiment 2, fish were contaminated with 0.4 mM NO₂^– (added as NaNO₂). In experiment 2, ambient NaNO₂^– was removed for the second 48 hours. NaNO₂^– exposure increased plasma [NO₂^–] to 0.4 mM, which increased MetHb from 10.8 ± 2.5% to 46.8 ± 8.0%, and consequently decreased functional Hb from 89.2 ± 2.5% to 53.2 ± 8.0%. NaNO₂ removal led to recovery of both parameters. Both Hct and plasma Mg²⁺ were lowest in fish recovering from nitrite exposure. Thus, these parameters did not show recovery. Plasma osmolality and [Cl⁻] were not affected by NaNO₂^–, probably due to the moderate rise in plasma [NO₂^–]. O. niloticus was thus sensitive to 48 hours of exposure to 0.4 mM NaNO₂, and partially recovered from its effects after 48 hours in sodium nitrite-free water. Cultivators of Nile tilapia should thus consider the toxicity of nitrite, especially in aquaculture systems using recirculating water.