北西部太平洋水域におけるDSLについて-I

Abstract

It is said that deep scattering layer (DSL) in the sea is caused by the reflection of sonic waves propagated into the sea from pycnocline or layers of small organisms. This study deals with the topics mentioned in the title as a step in the study of ecology of salmon in the high-sea region of the northern North Pacific. The observations were carried out on board the Hokkomaru, research boat of Fisheries Agency, Ministry of Agriculture, in the northwestern area of the North Pacific during the period from July 11 to Aug. 5, 1964. Two kinds of ultrasonic finders, 200 KC and 28 KC respectivety, were used by attaching them outside the hull of the ship. The results of the observations are summarized as follow: 1) It was overcast throughout the period of observations. The daylight intensity at the sea surface averaged (4-6)×102 lux at the time of sunrise as well as at sunset, while it was (8-10)×104 lux at noon. 2) The transparency of water was 15m. The depth at which the submarine illumination was as much as one lux corresponded to 62-64m at the time of sunrise and sunset and 123-125m at noon. 3) One or two layers of DSL were usually observed. These layers (1st and 2nd layers) changed their depth diurnally. Somtimes a third layer was found, but it did not shift vertically. 4) When two ultrasonic finders were operated at the same time, both the 1st and 2nd layers of DSL were recorded by the 200 KC finder. While only the 1st one was recorded by the 28 KC finder. 5) In order to observe difference in reflection loss of 200 KC and 28 KC sonic waves with the form of plankters, a test was made using a water tank. It was found that difference in reflection loss of Euphausia pacifica between 200 KC and 28 KC was ca. 5 db, and that of copepods such as Calanus plumchrus and Calanus cristatus was ca. 10 db. Such a difference may have been caused by difference in the size of organisms. The specific gravity of Euphausia pacifica, Calanus plumchrus, Calanus cristaus and Themisto sp. is almost same, i. e. ca. 1.06. 6) Organisms collected from the 1st-3rd layers of DSL by horizontal towing of a plankton net, 60cm diameter, 32 mesh/inch, mesh opening 0.53mm, were mainly euphausiids, copepods, amphipods, etc.