Toshiro Kuroki

Basic Experiments on Echo Survey for Estimating Fish Population by R. V. Tansei Maru

In order to furnish quantitative information for population analysis, the develop-ment of a direct method to count the echo patterns of large sized fish individuals such as tuna, using a pattern analysis technique, was undertaken by the Division of Population Dynamics of Marine Organisms, Ocean Research Institute, University of Tokyo. The development of this direct method was started from 1967. For this project, basic experiments were planned by the staff of the Division and the authors conducted the said basic experiments on two cruises (KT-68-5, KT-68-24) of R. V. Tansei Maru (257.69 ton), University of Tokyo, on Sagami Bay in April and November 1968. The results are summarized as follows: 1) The maximum depth in which a single fish (Ts=-26dB) can possibly be detected by the use of the fish detector is about 150m. 2) The beam axis is inclined forward and the angle from the beam axis to the vertical axis is 3.54-4° 3) The range of the fork length of fishes (yellowfin tuna, rainbow runner and little tuna), hung by rope to a drifting buoy, was 50-130cm, and the range of target strength for those targets was -28.5 ?? -14.5dB. 4) The effect of ship speed on attenuation of received signal was measured with the effect of wind direction at three levels of ship speed. 5) Echo signals were recorded on magnetic tapes at several ship speeds when the ship passed near a buoy. The considerable variability of echo patterns was recognized even for the same buoy at the same ship speed.

Biophysical Studies on the Auditory Characteristic of Fish-I

The author considered the auditory characteristic in a lateral line of fish and calculated it on the physiological data about Carassius auratus (Body length; about 20cm.: Absolute refractory periods; 3 milli-sec. in A-neuron and 11 milli-sec. in B-neuron: Transmitting verocities of stimulation; 28.9m/sec. in A-neuron and 6.5m/sec. in B-neuron). The results got are as follow: a) When there is a sound-source in the elongate direction of body-axis, the upper limit of audible frequency may be about 7500 c/sec. and the most sensitive frequency is about 160 c/sec. In this case, the influence of the distance between the sound-source and the fishbody on the range of audible frequency is negligible. b) When there is the sound-source in the vertical direction against body-axis, the influence of the distance between the source and the body on the range of audible frequency is serious. For an example, although the most sensitive frequency is about 710 c/sec, at one meter distance, the most sensitive one is about 1300 c/sec. at a long (1000m.) distance. c) There is not the lower limit of audible frequency. It seem that the audible frequency may be lower as the sound-pressure becomes higher almost independently of the distance between the source and the body. Then, in experiments about the auditory characteristic of fish, we must be careful for the definitions of a sound-source position against fish-body and of the intensity of sound-pressure. Especially, as the sound in a small aquarium causes its reflections, interferences and -transmissions through walls, the devices to suppress them must be adopted so as to gain clear effects or correct results., The author is studying about aquaria with walls isolated from the influences of sound.

A Relation between the Fluctuations of Catches and the Diffusive Movements of Fish-schools

As regards the flactuation of catches, there are few examples of the discussions about the causes of this fluctuation throngh the view of the diffusive movements of fish schools, although many examples of the discussions about the statistic dealing of the fluctuable results. The author attended to explain, this fluctuation by means of the combination of the movements and tha appearances of fish schools. In the result, it is said that soon after beginning of catch the maximum abundant one appears and then successively the gradually scarce ones continue till the last of one period of catching.