Threshold toxicity and repeated exposure studies of dichlorvos to the larvae of the common lobster (Homarus gammarus L)

A series of tests were conducted on larvae of the common lobster, Homarus gammarus, to determine their sensitivity to dichlorvos (0,0-dimethyl-2,2-dichlorovinyl phosphate, DDVP), the active ingredient of Aquagard(R) (Ciba-Geigy Agrochemicals, Cambridge, UK) a product administered to farmed fish as a treatment for sea lice (Lepeophtheirus salmonis Kroyer and Caligus elongatus Nordmann) infestation. The acute toxicity threshold (NOEC after 23 days exposure) of dichlorvos to stage 4 larvae was 0.63 mu g l(-1) DDVP. Acetylcholinesterase activity in exposed larvae was not significantly different from that in controls at concentrations up to 0.16 mu g l(-1) DDVP. Lobster larvae were able to tolerate and recover from extended exposure to sub-lethal concentrations. Exposure of stage 4 larvae to 50 mu g l(-1) dichlorvos for 6 h resulted in significant inhibition of acetylcholinesterase. Whilst AChE levels recovered after such exposure, repeated dosing at this level resulted in further inhibition and mortalities. Exposures to 25 mu g l(-1) DDVP for two 6-h periods resulted in AChE inhibition, although no significant mortalities were observed. Five exposures to 50 mu g l(-1) DDVP for 1-h periods also resulted in AChE inhibition, but no mortalities. The data are discussed in relation to DDVP dispersion simulation field experiments, and patterns of DDVP use.