Abstract

Acute static bioassay was employed to assess the toxicity of various ranges of effluent from the National Fertilizer Company of Nigeria (NAFCON) plant to three fish species: Oreochromis niloticus, Clarias gariepinus and hybrid (Heterobranchus bidorsalis â™€ x C. gariepinus â™‚) from the coastal estuaries of the Niger Delta area, Nigeria. The lethal concentration values at 24, 48 and 72 h were 72.05, 30.81 and 15.26% for O. niloticus and 26.18, 10.32 and 19.84% for the hybrid, respectively. No mortality was recorded for C. gariepinus. The median lethal time for O. niloticus at 70% and hybrid at 50% of the different samples was 18.14 and 6.02%, respectively. Ammonia appeared to be the major toxic component. The safe concentrations of the effluents ranged between 1.53% and 77.21% for O. niloticus, and 3.15 and 5.50 % for the hybrid.  Although the ranges of treated effluents discharged from the plant met set standards and can be classified as non-toxic, yet they caused mortalities to exposed species. This underscores the merit of direct toxicity assessment of effluents over the traditional physicochemical method which does not adequately protect the environment.

Key words: Toxicity assessment, fertilizer effluents, Oreochromis niloticus, Clarias gariepinus, catfish hybrid.