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.

An 8-week feeding trial was conducted to determine the dietary copper (Cu) requirement of juvenile hybrid tilapia, Oreochromis niloticus 5 O. aureus. Purified diets with eight levels (0, 1, 2, 3, 4, 8, 12 or 20 mg Cu per kg diet) of supplemental Cu were given to tilapia (mean initial weight 0·79±0·03 g). Each diet was given to three replicate groups of fish. The rearing water contained 0·98 µg Cu per l. Weight gain was highest in fish given diets supplemented with 2 mg Cu per kg diet, followed by the group given 1 mg Cu per kg diet, then the unsupplemented control group, and was lowest in the 20 mg Cu per kg diet group. The differences between each of these four groups were significant (P < 0·05). Food efficiency (FE) of fish showed a similar trend to that of weight gain. Blood haemoglobin (Hb) and haematocrit (Hct) concentrations were lowest in fish given ≥ 12 mg Cu per kg diet, followed by fish given the unsupplemented control diet and highest in fish given 2 mg Cu per kg diet (Hb) and 2 and 3 mg Cu per kg diet (Hct). Plasma ceruloplasmin activity was higher in fish given 1 and 2 mg Cu per kg diet, followed by 3 to 12 mg Cu per kg diet and the control group, and lowest in fish given 20 mg Cu per kg diet. The body Cu content in fish generally increased as the dietary Cu supplementation level increased. Analysis by broken-line regression of weight gain percent and by linear regression of the whole-body Cu retention of fish indicated that the adequate dietary Cu concentration in growing tilapia is about 4 mg Cu per kg diet.