| Departure | Muscat | 24 January 1981 |
| Call | " | 31 " " |
| " | 13–14 February 1981 | |
| Aden | 21–22 " " | |
| Arrival | Djibuti | 26 " " |
Scientific staff:
A. Aglen, A. Al-Amry (31 Jan.-13 Feb.),
M. Bouhlel (22 – 26 Feb.),
J. Gjøsæer (from 31 Jan.), K. Hansen,
J.P. Maude, I. Svellingen,
S. Tilseth (till 13 Feb.),
R/V “Dr. Fridtjof Nansen” worked in the Gulf of Oman from 24 January to 13 February, and in the Gulf of Aden from 17 February to 26 February.
The vessel is a 150 foot stern trawler with a main engine of 1500 Hp. A discription of the vessel is given in the Final report Survey results of R/V “Dr. Fridtjof Nansen” (Dev. Rep. Indian Ocean Programme. No 43 Vol.2: 13 – 61).
The acoustical equipment consisted of three scientific sounders (120, 50 and 38 kHz) and two echo integrators, each with two channels. The vessel is also equipped with a sonar (24 kHz) and a net sonde (50 kHz). In addition to the hull mounted transdusers, there is one towed body with two transdusers.
The 38 kHz sounder used for integration, is coupled to a ceramic transducer and a 2 KW external transmitter was applied. This gave a performance (system voltage response plus source level) of 138 dB. Calibrations were done two times (20 January and 8 February), with a -29.4 dB steel sphere and a -34.9 dB cuprous sphere, both giving the same results.
The basic range applied was 0 –250 m, and an additional echo recorder covered the depth range 250 – 500 m. Integration was carried out in four depth intervals selected to separate the different scattering layers. The following settings were used:
| Receiver gain | 20 log R - 20 dB |
| Integrator gain | 20 dB |
| Integrator treshold | I (0.01 Volt peak) |
| Pulse length | 0,6 m.sek. |
| Bandwith | 3 kHz |
All values were referred to 30 dB integrator gain.
A 120 kHz echo sounder was used to get better resolution in the upper 100 m.
During daytime the echo recordings were divided into three groups:
| Plankton | (Usually upper 100 m) |
| Upper mesopelagic (DI) | (Usually 100–200, usu absent in Gulf of Ade |
| Deep mesopelagic (D II) | (Usually below 200 m) |
During night usually two groups could be recognized:
| Upper mixed layer (N I+plankton) | (Upper 100 m) |
| Deep mesopelagic (N II) | (Below 200 m) |
To estimate the amount of mesopelagic fish in the upper mixed layer, the recordings of plankton during daytime in the same in a near by area were subtracted.
Fishing experiments and sampling to identify the sound scatte were carried out using a Modified Krilltrawl (Fig.1). The tra opening was estimated to cover about 250 m². Larvae were samp using a Juday net and a Bongo 60.
Underwater photographs were taken using a camera system consi of an aluminium casing which housed a motor-driven 35-mm came strobe light and an electronic shutter release taking picture every 40 sec.
When used in the upper 70 m a echo transducer could be mounte on the camera casing and the camera could be operated from th surface when fish was observed on the echosounder.
