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

Background: 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 remains incompletely characterized in freshwater fish. Aim: This study aimed to determine the acute and sub-lethal toxicity profile of urea in juvenile Nile tilapia (Oreochromis niloticus) and to characterize concentration-dependent physiological, oxidative, and histopathological responses. Methods: Acute toxicity was evaluated using a 96-hour semi-static bioassay in accordance with OECD Test Guideline 203. The median lethal concentration (96-h 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. Behavioral responses, hematological parameters, antioxidant enzyme activities (SOD, CAT, GPx), lipid peroxidation (MDA), and histopathological alterations in gill, liver, and kidney tissues were assessed. Data were analyzed using ANOVA or Kruskal–Wallis tests as appropriate. Results: The 96-h LC50 of urea was 11.8 g/L (95% CI: 11.1–12.6 g/L), indicating moderate intrinsic acute toxicity. Sub-lethal exposure produced significant concentration-dependent reductions in erythrocyte count (F(2,12)=36.42, P