Laboratory hazard characterization of acute and sub-lethal urea exposure in Nile Tilapia (Oreochromis niloticus)

Urea is one of the most widely used nitrogen-based fertilizers worldwide. Although environmental concerns frequently focus on secondary ammonia formation, the intrinsic toxic potential of direct urea exposure under controlled laboratory conditions in freshwater.

This study aimed to determine the acute and sub-lethal toxicity profile of urea in juvenile Nile tilapia (Oreochromis niloticus) and characterize concentration-dependent physiological, oxidative, and histopathological responses.

Acute toxicity was evaluated using a 96-hour semi-static bioassay, in accordance with the OECD Test Guideline 203. The median lethal concentration (96-hour LC₅₀) was calculated using Probit regression analysis. Sub-lethal exposures corresponding to 1/10 (1.18 g/l) and 1/2 (5.90 g/l) of the LC₅₀ were applied for 14 days. We assessed behavioral responses, hematological parameters, antioxidant enzyme activities [superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)], lipid peroxidation Malondialdehyde (MDA), and histopathological alterations in gill, liver, and kidney tissues. Data were analyzed using analysis of variance (ANOVA) or Kruskal-Wallis tests as appropriate.

The 96-hour LC₅₀ of urea was 11.8 g/l (95% CI: 11.1-12.6 g/l), indicating moderate acute intrinsic toxicity. Sub-lethal exposure produced significant concentration-dependent reductions in erythrocyte count (F(2,12) = 36.42, p < 0.001, η² = 0.86) and hemoglobin (F(2,12) = 22.41, p < 0.001, η² = 0.79), with increased leukocyte counts (F(2,12) = 29.68, p < 0.001, η² = 0.83). SOD, CAT, and GPx activities were elevated at 1/10 LC₅₀ but significantly suppressed at 1/2 LC₅₀ (p < 0.001), while MDA levels increased markedly (F(2,12)=58.36, p < 0.001, η² = 0.91). The histopathological lesion scores differed significantly among groups (p < 0.01; ε² range: 0.66-0.75).

In juvenile O. niloticus, direct laboratory exposure to urea induces measurable acute toxicity and robust concentration-dependent hematological, oxidative, and tissue-level alterations. These findings define the urea intrinsic hazard profile under controlled conditions and provide foundational toxicodynamic data without constituting an environmental risk assessment.