THRESHOLD TOXICITY AND REPEATED EXPOSURE STUDIES OF DICHLORVOS TO THELARVAE OF THE COMMON LOBSTER (HOMARUS-GAMMARUS L)

A series of tests were conducted on larvae of the common lobster, Homa rus gammarus, to determine their sensitivity to dichlorvos (0,0-dimeth yl-2,2-dichlorovinyl phosphate, DDVP), the active ingredient of Aquaga rd(R) (Ciba-Geigy Agrochemicals, Cambridge, UK) a product administered to farmed fish as a treatment for sea lice (Lepeophtheirus salmonis K royer and Caligus elongatus Nordmann) infestation. The acute toxicity threshold (NOEC after 23 days exposure) of dichlorvos to stage 4 larva e was 0.63 mu g l(-1) DDVP. Acetylcholinesterase activity in exposed l arvae was not significantly different from that in controls at concent rations up to 0.16 mu g l(-1) DDVP. Lobster larvae were able to tolera te and recover from extended exposure to sub-lethal concentrations. Ex posure 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 signific ant mortalities were observed. Five exposures to 50 mu g l(-1) DDVP fo r 1-h periods also resulted in AChE inhibition, but no mortalities. Th e data are discussed in relation to DDVP dispersion simulation field e xperiments, and patterns of DDVP use.