Conditions for precise measurement of in situ fish target strength (TS) are empirically studied and two indexes are introduced for this purpose. One is the number of fish in the effective reverberation volume which contributes echo formation at a certain instant and the other is the percentage of the multiple echoes which is derived from a residual of the single echo extraction. With the decrease of both indexes measured target strength approach a certain asymptotic value which is admitted as reliable from the past study. This shows the existence of some threshold values and below these threshold values TS measurement will be successful. The effectiveness of both indexes is confirmed by the data set obtained from one large same fish school in the eastern shelf of Bering sea during the intership calibration between Japanese and U.S. vessels on 15 and 16 August 1991.
Published in J.Mar.Acoust.Soc.
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Abstract:
In 1993 Ukraine has recommenced krill fishing which was terminated in 1993. It is conducted by two vessels BAT GRIGORY KOVTUN and RKTS GENERAL PETROV. Scientific observer M. Speranskaya, YugNIRO scientist is on board the latter one.
The fishery and scientific information given in this paper is based on operative data. That is why some correction of these data is possible after the observer’s return from the cruise.
Abstract:
An acoustic survey at 38 and 120 kHz was carried out from RRS Discovery in the Bellingshausen Sea from 23 November to 7 December 1992 as part of the UK Joint Global Ocean Flux Study (JGOFS) Southern Ocean investigations (TuRNER and OWENS, 1995). A total of 285 targets were identified and described from the chart record of the echo sounder. Mean Volume Backscattering Strength data were collected using an echo integration system. These data are used to describe the spatial and temporal variability of krill (Euphausia superba) distribution and biomass in the marginal ice edge zone. Krill biomass density varied from a mean of 42 g m-2 during the first survey phase to 19.6 g m-2 on the second survey phase. The number of small swarms detected during the second phase was greater than during the first phase. On this first survey a large swarm (2.8 km in extent) had a potential biomass of 3.7 x 104 tonnes. Some calculations are presented to show the potential impact of krill on the flux of carbon in the area of the survey.
