The use of small enclosures to assess the toxic effects of cypermethrin on fish under field conditions

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Abstract
An emulsion concentrate formulation of cypermethrin (‘Ripcord’) was applied at seven different dose rates (between 5 and 500 g a. i. ha−1) to the water surface of 1‐m3 stainless steel enclosures, located in a small outdoor pond, to which were introduced small rainbow trout and common carp. A range of water quality parameters were monitored frequently in all of the enclosures and in the open pond. At daily intervals, the concentration of cypermethrin was determined in water samples, collected from the enclosures treated with 5, 50 and 500 g a. i. ha‐−1. Two further water samples were collected from these enclosures, and their toxicity was assessed by bioassay, using Gammarus pulex as the test organism. The experiment was terminated 96 h after application, when all the fish were removed and examined for mortality or adverse toxic effects. Throughout the study, the water quality parameters in all the enclosures remained similar to those recorded in the open pond. Cypermethrin residues, in the enclosures treated with 5 and 50 g a. i. ha−1, attained peak concentrations 24 h after application, and thereafter declined to about 50% of this level. Maximum cypermethrin concentrations were not attained until about 72 h after application in the enclosure treated with 500 g a. i. ha−1. Highly consistent responses, closely correlated with the applied dose rates, were obtained with G. pulex and both species of fish. Rainbow trout appeared to be about three times more susceptible to cyper‐methrin than common carp, with calculated LD50 values of 92 and 300 g a. i. ha−1, respectively. It is therefore most unlikely that fish mortality would result from the use of cypermethrin in normal agricultural practice.