Study on echo sounding for estimating fisheries resources

Bases and applications of the echo-sounding method for accurately estimating biomass, size, and other information of fish are studied. Scattering characteristics or the target strength of fish and other marine organisms are clarified by precise measurements and prolate spheroid theoretical models. The transmission losses of acoustic waves caused by the absorption of seawater, sailing of a ship, wind-induced bubbles, and schooling fish are discussed. The measurement of the attenuation by schooling fish revealed that no significant attenuation is caused by ordinary schools. The strict relationship between the echo level and target strength and distribution density are derived by introducing concepts, such as an echo shape function of a single fish, equivalent beam width, averaged target strength, and equivalent pulse width, to give the base for several estimation methods. The estimation methods for single fish echoes are presented. Some traditional methods can be made effectively by displaying the absolute echo levels. Newly developed echo trace analysis can show in situ fish behavior and target strength as a function of the fish tilt angle. The estimation for individual schools of fish is described. The schooling density is accurately measured by applying the concept of a contribution factor for the beam pattern. The echo integration method is discussed and improved. The thresholding error can be minimized by introducing the threshold function based on the echo level of unwanted scatterers and on the noise level. Further, the newly developed two-step echo integration method can lessen the error caused by the above two contributors. The general designing procedure of quantitative echo sounders is established under the principle of minimum error and easy use