Organophosphates are a group of chemicals which act by inhibiting the enzyme acetylcholinesterase (AChE) in the nervous system. Organophosphorus pesticides are effective insecticides and are widely used in the UK. However, they can have a harmful effect on non-target organisms by inhibiting AChE activity. Toxic effects of AChE inhibition vary with the degree of exposure and the type and concentration of organophosphate pesticide. Sub-lethal effects are poorly understood, but have shown changes in behaviour, reproduction, ability to cope with food and environmental stress, leading to paralysis, convulsions and ultimately death.

The solubility, sorption, potential to bioaccumulate and extent of toxicity to aquatic life tend to vary according to the organophosphate. In addition, based on their mode of action, effects on aquatic mammals could be expected. However, data on such effects do not appear to be available.

Azinphos-methyl, malathion, fenitrothion and dimethoate have been identified as of potential concern in the marine environment. Profiles of these organophosphates are presented in sections linked below.

Azamethiphos is an organophosphate pesticide used for the treatment of sea lice in cage fish farming. The control of this substance in this application is detailed in SEPA's Fish Farming Manual which includes operational EQSs for azamethiphos (SEPA Policy No. 17). The reader is referred to the Fish Farm Manual for further information on this substance.

Azinphos-methyl

Malathion

Fenitrothion

Dimethoate

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