Direct toxic assessment of treated fertilizer effluents to Oreochromis niloticus, Clarias gariepinus and catfish hybrid (Heterobranchus bidorsalis x Clarias gariepinus)

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 x C. gariepinus ) 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 6.02hrs, 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.