Several instances of massive mortality of whales coincident in time with a navy exercise, using pow-erful sonars led to the beginning of studies on the investigation of the effect of intensive audio signals on the behavior and hearing of cetaceans.

An important aspect of these studies is the effect of intensive noises on the temporary thresh-olds shifts, TTS. It is thought that these studies would help estimating the parameters of audio signals causing some hazardous permanent thresholds shifts, PTS. The studies of the effect of intensive noise on the hearing of cetaceans are still at their initial stage as there are too many variables responsible for the auditory sensitivity of the animal and laborious experiments on cetaceans. As a consequence, the bulk of the already obtained data can be hardly compared due to the difference in the experimental conditions. One of the promising approaches to the study of the conditions for TTS is the utilization of electrophysiological methods for the testing of hearing, using non-invasive remote recording of various versions for auditory brain stem responses.

In contrast to behavioral ap-proaches those methods under well controlled condi-tions and in a relatively brief time period permit obtaining a fairly large body of data on changes in the sensitivity of the animal hearing.

The present study addressed the effect of intensive noise on the auditory sensitivity of the beluga whale. The experiments were per-formed at the Utrish Marine Station of the Institute of Ecology and Evolution Research on young (about 2 years old) male and female All the experiments were performed according to the regulations for using animals in biomedical studies of the Ministry if Science and Education, RF. For the experiment, the animal in a special stretcher was placed in a bathtub with sea water (4.5х0.85х1.2 m) so that the dorsal surface of the head with the blowhole was over the water surface. For non-invasive recording of evoked potentials disk electrodes of stainless steel were used, 15 mm, installed into 60 mm sucker cups.

The active electrode was fixed on the head 7 cm caudal of the blowhole; and the indifferent electrode on the back of the animal. Both electrodes were over the head surface. The electrical activity recorded from the head surface amplified in the frequency range from 200 to 5000 Hz, was digitized and recorded in the computer memory. In order to distinguish re-sponse from the noise, the method of coherent aver-aging in relation to the beginning of the stimulus. Used as a test stimulus was a series of 16 ms in du-ration, containing 16 tone bursts at a frequency of 1000/sec. The tone burst carrier frequency ranged from 8 to 128 kHz. That type of stimulus is very effective in evoking a rhythmical potential, which in its nature is similar envelope following response (EFR). Used as the fatiguing signal was the semioctave bandpass noise with central frequencies from 11.2 to 90 kHz. The animal was exposed to the test and fatiguing audio signals with ceramic transducer: ITC-1032 (International Transducer Corporation, USA) for the frequencies 8-45 kHz or B&K 8104 (Bruel & Kjaer, Denmark) for the frequencies of the signal 64-128 kHz.

The determination of the threshold to the test stimulus started directly after the threshold was re-stored completely but it did not exceed one hour even in case when the threshold did not reach the background values.

The maximal decrease in sensitivity was recorded in the frequency areas of 0.5-1 higher than the central frequency of the rejection noise, which coincides with data of other operations.The noise affecting the low-frequency areas of the beluga whale hearing more effectively suppresses the auditory sensitivity.

The study was supported by the Russian Geographical Society.

Popov V.V., Supin A.Ya., Rozhnov V.V., Sysueva E.V., Klishin V.O., Nechaev D.I., Pletenko M.G., Tarakanov M.B.

A.N. Severtsov Institute of Ecology and Evolution RAS, Moscow, Russia

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