The mesocarp of pawpaw fruits (Carica papaya) is a delicacy in the Tropics but the seeds are known to contain toxic substances (Carpine, Papain). However, there have been very few studies on the toxicity of these substances to organisms. First part of the present study determines the toxicity of an aqueous extract of pawpaw (Carica papaya) seed powder to catfish (Clarias gariepinus) fingerlings using static bioassay. The second part investigates the haematological effects. 400 healthy catfish (Clarias gariepinus) fingerlings 10.5 ± 3.2 cm (total length), 15.5 ± 6.4 g (body weight) were exposed to triplicate concentrations of 10, 11, 12, 13, 14 and 15 mg/l in eighteen 30 L glass tanks, each with 22 fish in 20 L aerated rain water. 24, 48, 72 and 96 h LC₅₀ values were 19.0 ± 3.3, 13.0 ± 2.8, 12.8 ± 1.6 and 12.9 ± 2.1 mg/L, respectively. There was significant reduction (p < 0.05) in the value of blood parameters [blood cell count (erythrocytes and leucocytes) and basic erythrocyte indices (mean cell haemoglobin concentration (MCHC), mean corpuscular volume (MCV), and mean corpuscular haemoglobin (MCH)] of C. gariepinus fingerlings after exposure to 96 h in aqueous extract of C. papaya seed powder. Toxic reactions include; erratic movement, air gulping, loss of reflex, molting and discoloration. Maximum admissible toxicant concentrations ranged from 0.1 to 1.29 mg/L (confidence level: 22 - 78.8%). The results showed that concentrations of pawpaw seed powder in excess of 1.29 mg/L can be potentially harmful to C. gariepinus fingerlings.

Key words: Toxicity, pawpaw seed, catfish, fingerlings, haematology, water quality.