In the tilapia as well as in the carp, gill acetylcholinesterase (AChE) was more sensitive than brain AChE to organophosphorous (OP) and carbamate (CB) compounds, and the threshold levels for residue detection by the biochemical method were lower in the gills, compared to the brain. The sensitivity of tilapia gill AChE to paraoxon was extremely high, and enabled detection of paraoxon residues in the gills at the ppb level.

The carp showed higher resistance than the tilapia for poisoning by both the OP compound parathion and the carbamate methomyl.

The residual post-mortem brain and gill AChE activities from tilapia killed by either parathion or methomyl poisoning were low, and could easily be differentiated from brain AChE activity in the control ish, which remained normal until 18 hours after death, providing that fish were held at temperature up to 10°C.

Detection of poisoning with OP and CB compounds, in fish that are being marketed, is of extreme importance, especially in Lake Kinneret, where illegal fishing with insecticides occurs occasionally.

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