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Characteristics of the Optomotor Reaction of Fishes Acted upon by a Pulsed Electric Field G. P. Danyulite and Yu. M. Maksimov . Klaypeda Branch of the Special Experimental and Design Office of the Commercial Fishing Fleet; Institute of Zoology and Parasitology, Lithuanian Academy of Sciences On entering a trawl fish are subjected to the action of diverse stimuli, to which they respond by various defensive reactions comucive to their escape from the trawl. o A large role is here played by the optomotor reaction, which is an unconditioned reflex, automatic response of the organism, i. e. , movement after reference points changing position in the field of vision (Protasov and Altukhov, 1961; Pavlov, 1970). Study of the characteristic manifestations of the optomotor reaction of fishes under the in- fluence of an electric field and determina- tion of the current parameters disturbing this reaction are of particular ipterest in connection with the. design of an electrified trawl. The results of our first research along these lines are set out in the present article. in diameter at the outer wall. Around the canal there was a screen approximately 6 m long with black and white stripes, each 3 cm wide. The rate of rotation of the screen was varied gently between 10 and 75 cm/sec. The experiments were conducted in the spring and autumn of 1972 on smelt (Osmerus eperlanus eperlanus L. ) measuring 15-20 cm. The fish were caught by trawl in the inshore zone of the Baltic, transported in tanks and kept in aquaria containing running sea water. Water temperature in the spring was 12-14.C and salinity 7. 7q\,,'; the corresponding figures in the autumn were 6-10.C and 5.7 -6.0%0. METHODS Two electrodes, each consisting of four copper wires, were vertically positioned in the canal, dividing It Into two equal parts (Fig. 2). When current was fed to the elec- trodes a practically uniform electric field was created in the canal, except that It was dis- torted in the zones around the electrodes. A unipolar pulsed electric current at a frequency f = 30 Hz was employed. Pulse duration was The experiments were conducted in an optomotor layout (Figs. 1 and 2) consisting of a circular canal O. 27 m wide and 1. 85 m Table 1 Optomotor Reaction of Smelt at Different Rates of Screen -~-j,l. Rotation V screen' em/lee Characterlstics of the optomoter reaction of the fish VF, em/see (mean) \ Number of expo I of fish i Tf3i 5 52 i 9 '0- 12 ao 38 60 75 Weak Distinct . . Irregular. wlth intenuptlcms 28 29 34 33,45 604 r Fig. 1. General appearance of the optomotor layout. rates of rotation of the screen (V ) and screen determined the latent reaction time. RESEARCH RESULTS J A clear optomotor reaction was discovered in the smelt: both the groups and the solitary fish followed the moving screens for 10, 20 and. even 30 min without falling appreciably behind. The latent reaction time of the smelt was 2-10 sec in most experiments. The fish were subjected to reversal: when the direc- tion of motion of the screen was changed they ~rned round and moved in the opposite direc- hon. Fig. 2. Location of electrodes in the canal of the optomotor layout. 1) Electrode; 2) Fish In de- scending current field; 3) fish In ascending current field. The state of the fish had a great effect on the manifestation of the optomotor reaction which was weakly revealed in newly caught' fish and in those which had spent several days in the aquaria. The clearest, 100% optomotor reaction was observed on the 2nd-3rd day after transportation of the fish to the aquaria. The minimum V at which it was pos- screen sible to obtain an optomotor reaction In the smelt was 12 cm/sec. At V equals 30 screen cm/sec the swimming speed of the fish equal~ led the speed of the screen. Thereafter the speed of the fish increased, but lagged behind the rate of movement of the screen. At T = 1. 6 msec, the pulse shape was sinu- soidal and field strength (E) was varied be- tween 0.07 and 27 v/cm. The experiments were conducted on groups and on solitary fish. Observations were made on the manifestation of the optomotor reaction before, during and after the action of the electric current. We mea- sured the swimming speed (Vf) at different 605