|
Departure: |
Dubai |
4 July |
|
Arrival: |
Aden |
30 August |
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Ports of call: |
Muscat |
7-10 July |
|
|
Muscat |
12 July |
|
|
Muscat: |
20 - 29 July |
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|
Muscat: |
31 July - 2 August |
|
|
Aden |
11 August |
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|
Djibouti |
12 - 13 August |
Material and methods
R/V "Dr. Fridtjof Nansen" worked in the Gulf of Oman from 4 July to 3 August, and in the Gulf of Aden between 8 August and 29 August.
The vessel is a 150 foot stern trawler with a main engine of 1500 Hp. A description of the vessel is given in Final Report Survey Results of R/V "Dr. Fridtjof Nansen" (Dev. Rep, Indian Ocean Programme No 43 Vol. 2:13-61).
Acoustical equipment consists of three scientific sounders (120, 50 and 38 kHz) and two echo integrators, each of two channels. There are also a sonar (18 kHz) and a netsonde (50 kHz). In addition to hull monted transducers there is one towed transducer.
The echo integration was done by two echo integrators connected with the 38 kHz echo sounder. The basic range applied was 0 - 250 m. An additional echo recorder, coupled with the 38 kHz sounder covered the depth interval 250 - 500 m. Integration was done in four different depth intervals within the total depth interval 10 - 500 m. A ceramic transducer and a 10 KW external transmitter were applied. This gave a system voltage response of 12.5 dB//1 volt pr. µ Bar and a source level of 133.5 dB//1 µ Bar ref. 1 m, when the standard settings of pulse length (0.6 m.sek.) and bandwidth (1 kHz) were applied.
Some settings were changed during the cruise, as follows:
|
1. July - 21. August: |
20 log R - 20 dB |
recorder gain |
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|
20 dB or 30 dB |
integrator gain |
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|
0 |
integrator threshold |
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21. August - 27. August |
20 log R - 0 dB |
recorder gain |
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|
0 dB or 10 dB |
integrator gain |
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|
1 or 2 |
integrator threshold |
The values were referred to 30 dB integrator gain when the first setting was applied and to 10 dB when the last was applied.
The reason for this change was that the first setting was found to underestimate the echo abundance, due to threshold problems, when the fish was distributed in very scattered layers. Comparisons made on such layers showed that the first setting gave 30 per cent less than the last setting. In dence layers the difference was very small.
A 120 kHz scientific sounder was used to get better resolution of the recordings in the upper 100 meters. The additional information from these recordings was utilized during the daily scrutinizing of the acoustic data. Separate integrator values were obtained from each of the three categories plankton, mesopelagic layer and deep-pelagic layer at daytime. At night it was possible to separate only two categories: an upper mixed layer and a lower deep-pelagic layer. The values from the upper layer were splitted up according to the mean integrator values obtained from plankton in the same area during daytime.
Hydrography
Standard hydrographic stations were worked out along transacts during the survey (Fig. 1, 2).
In each station Nansen bottles were used in standard depths down to the bottom or to 500 m depth. Temperature and salinity were observed at all standard depths: 0-10-20-30-50-75-100-125-150-200-250-300-400-500 m. Oxygen was observed at the same depths except 0 m. The salinity and oxygen samples were analysed on board.
Trawls
Two pelagic trawls were used during the survey, one modified Krilltrawl with four equal panels (Fig. 3), and one pelagic trawl (Harstadtrawl), with larger meshes and four equal panels (Fig. 4). The filtering area of the two trawls was calculated to be 250 and 400 m2 for the Krilltrawl and Harstadtrawl respectively.
The Krilltrawl was used with 75 m sweeps and 3 m extensions to the lower sweeps. The Harstadtrawl was used with 120 m sweeps. For both trawls 61 m2 WACO doors were used.
On the trawls net bags were attached to the outside of the panels to get an estimate of fish escaping through the meshes.
