In 1979 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 and 29 August. In 1981 she worked in the Gulf of Oman 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 description of the vessel is given by Anon. (1978). The participating scientific and technical staff is listed in Annex 1.
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 transducers, there is one towed body with two transducers.
The 38 kHz sounder used for integration, is coupled to a ceramic transducer and a 2 KW external transmitter was applied. A 120 kHz echo sounder was used to get better resolution in the upper 100 m.
Details about the acoustical equipment and the settings used are given in Annex 2.
During daytime the echo recordings were divided into three groups:
|
Plankton |
|
(Usually upper 100 m) |
|
Upper mesopelagic |
(D I) |
(Usually 100-200, usually absent in Gulf of Aden) |
|
Deep mesopelagic |
(D II) |
(Usually below 200 m) |
|
Upper mixed layer |
(N I, plankton) |
(Upper 100 m) |
|
Deep mesopelagic |
(N II) |
(Below 200 m) |
The integrated echo was converted to estimates of fish biomass using the formula:
where B is biomass of the scattering organisms, M is integrated echo intensity (mm/n.miles). A is the corresponding area. The conversion factor depends on the size of the fish.
C = C'L
where L is fish length and C' is a constant depending on the performance of the acoustical equipment and the fish species in question. As no reliable estimate of C' for myctophids are available, a C' estimated for other small pelagic fishes was used:
C = 0.1 L for 1979 and
C = 0.6 L for 1981
where L is fish length in cm. This corresponds to an average target strength of -10 log L -22 dB ref. 1 kg/m², and should therefore be comparable to the conversion factor used on R/V "Dr. Fridtjof Nansen" during the surveys in 1975-76, and on R/V "Lemuru" with compensation for the difference in the performance of the equipment.
Fishing experiments and sampling to identify the sound scatterers were carried out using a modified Krill trawl and a Harstad trawl. The effective trawl opening was estimated to cover about 250 m². Records of fishing operations are given in Annex 3. Eggs and larvae were sampled with an 80 cm Juday net (180 µm mesh size) in vertical hauls from 50-0 m. Broken hauls were also made from 300-200 m, 200-100 m and 100-0 m. Zoo-plankton were sampled by a 60 cm Bongo with one 180 µm and one 90 µm mesh sized nets. The Bongo samples were made in double oblique hauls from 0-50-0 meters.
Eggs of Benthosema pterotum were artificially fertilized in the laboratory on board the R/V "Dr. Fridtjof Nansen". The eggs were incubated at 18 and 25°C.
Underwater photographs were taken using a camera system consisting of an aluminium casing which housed a motor-driven 35- mm camera, strobe light and an electronic shutter release taking pictures every 40 sec. When used in the upper 70 m an echo transducer could be mounted on the camera casing and the camera could be operated from the surface when fish was observed on the echosounder.
Standard hydrographic stations were worked out along transects during the surveys (Fig. 1-4).
Fig. 1. Cruise tracks in the Gulf of Oman, 4 July - 3 August 1979.
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; 0-10-20-30-50-75-100-125-150-300-250-300-400-500 m. Oxygen was observed at the same depths except 0 m. The salinity and oxygen samples were analysed on board.
Fig. 2. Cruise tracks in the Gulf of Aden, 8-29 August 1979.
Fig. 3A. Cruise tracks in the Gulf of Oman, 1981. 24 - 28 January
Fig. 3B. Cruise tracks in the Gulf of Oman, 1981. 28 January - 1 February
Fig. 3C. Cruise tracks in the Gulf of Oman, 1981. C: 9 - 13 February
1. Pelagic trawl, 2. Bottom trawl, 3. Plankton station, 4. Hydrographical station.
