The substance 2-methylisoborneol (MIB) often affects aquatic organisms and causes problem of off-flavours in fish. In order to know how fish become tainted with MIB, response and chemical sensitivity of three freshwater fishes to MIB were examined behaviorally and electrophysiologically. The electrocardiographic tests showed that the threshold of detection for this compound was 4.8 × 10−5 ngl−1 for the Nile tilapia and 2.1 × 10−5 ngl−1 for the rainbow trout. Fish whose olfactory rosettes were removed also showed the cardiac response to MIB, but the threshold increased by 3 and 5 orders of magnitude in the Nile tilapia and rainbow trout respectively. The recordings of neural response of the olfactory tract of the carp to MIB solutions showed the olfactory threshold at a concentration of 4 × 10−8 ngl−1. In behavioral tests in tanks, MIB solutions did not attract these fishes and evoked no avoidance in these fishes. It was concluded that, while freshwater fishes are extremely sensitive to MIB, they would not escape from waters contaminated with MIB and thus easily be tainted with MIB.

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