Background. Animals acquire new skills due to conditioned responses resulting from a sequence of neutral and unconditioned stimuli affecting their bodies; there is a constant time interval between the stimuli. Both stimuli become associated after a number of repetitions and animal bodies start to respond to the initially neutral stimulus as to the unconditioned one. The aim of this study was to determine whether exposure of fish to a pyrethroid—deltamethrin (an active ingredient in Decis 2.5 EC pesticide that disturbs the proper operation of the nervous system) affects the ability to learn in common carp. Materials and Methods. Production of new types of behaviour was triggered in fish by using the unconditioned stimulus (an irritating electric impulse) and the conditioned stimulus (light). The experiment was performed in a tank divided into two compartments where fish could swim freely from one compartment to another. The ability to learn was assessed on the basis of the following types of behaviour evoked in response to the conditioned stimulus: locomotor agitation, preference to occupy or avoid certain areas in the tank, an escape response, etc. Results. In fish exposed to 0.35 μg · L–1 deltamethrin (for common carp concentration 10 times lower than lethal) for 35 min the ability to produce and remember the conditioned defence response was reduced and time interval since presenting the conditioned stimulus till occurring the initial signs of the conditioned response was prolonged. Conclusion. The classical conditioning method revealed that sublethal concentration of deltamethrin restricted ability to learn and retain information in common carp, despite absence of observable intoxication symptoms.
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