Accumulation of endosulfan residues in fish and their predators after aerial spraying for the control of tsetse fly in Botswana

Abstract

Residues of endosulfan insecticide (α‐ and β‐isomers, and ‘endosulfan sulphate’) in fish and their predators were measured during and after operations to control tsetse fly in the Okavango Delta, Botswana. Six ultra‐low‐volume doses of endosulfan 35% e.c. (6–12 g a.i. ha⁻¹) were applied from the air in a period of 12 weeks over 2500 km². The concentration of residues found in living fish was up to 0.19 mg kg⁻¹ wet wt in caudal muscle, and usually < 0.8 mg kg⁻¹ wet wt in pooled viscera (maximum 2.8 mg kg⁻¹). These values returned to near‐normal within 3 months after cessation of spraying, but residues were still detectable after 12 months. By comparison, fish killed by spraying contained a maximum residue level (whole‐body) of 1.5 mg kg⁻¹ wet wt. The residue level in fish was approximately proportional to their fat content. Lean fish were more susceptible to poisoning than fat fish. The proportion of the ‘endosulfan sulphate’ metabolite in fish increased at least six times with respect to the parent isomers (α+β) during the period of spraying, but more advanced stages of metabolic breakdown were not monitored. Residue levels in fish predators (fish‐eating birds and crocodiles) were similar to those in their prey, and the risk to them was consequently low.