The material used in this study was collected during the cruises of R/V “Dr. Fridtjof Nansen” in the Arabian Sea between Mogadisco and the Indo-Pakistanean border. The area (Fig. 1) was covered five times:
|
Cruises |
1, 2 |
14 |
January - 3 July |
1975 |
|
Cruise |
3 |
17 |
August - 22 November |
1975 |
|
Cruise |
4 |
9 |
January - 31 March |
1976 |
|
Cruise |
5 |
9 |
April - 23 June |
1976 |
|
Cruise |
6 |
22 |
August - 23 November |
1976 |
The acoustic equipment consisted of three scientific sounders (120, 50 and 38 kHz). Two electronic echo integrators with two channels each were coupled to the 38 kHz echo sounder. The 38 kHz sounder was operated at a basic range of 0-250 m with an extra paper recorder covering the depth 250-500 m. The effect of the transducer was 10 kw, the pulse length 0.6 msec and the bandwidth 1 kHz. The angular aperture between 3 dB points was 7.5 and the source level 130.2 dB/lµ Bar ref. 1 m. The TVG (time varied gain) was 20 lg R + 2 aR, where R is the distance between the transducer and the target and a is the attenuation coefficient. On cruises 1 and 2 the source level was 132.0 dB/lµ Bar ref. 1 m. The integrator readings were therefore divided by 1.5 to make them comparable with those of the other cruises.
The echo integrators (NAKKEN & VESTNES 1970, FORBES & NAKKEN 1972) were integrating the echo intensities in four depth slices between 8 and 450 meters.
The fundamental background for the integration method is: When a time varied gain compensating for one way geometrical spreading and two ways absorption of the sound is applied and the voltage of each echo is squared before integration, the output M of the echo integrator is linearly related to the number of fish per unit area in the integrated depth columns (see FORBES & NAKKEN 1972).
The number of fish per unit area PA, can be written PA = CM + d, where C expresses the number of fish per unit area, which contributes one unit to the integrated echo intensity and d is the lowest density which can be recorded (THORNE & WOODEY 1970, MIDTTUN & NAKKEN 1977). M is measured in millimeter deflection per nautical mile and averaged over five miles. The density coefficient C depends on fish species and size and on the characteristics of the sounder and integrator system used:
C = Ci . Cs . l-b (1)
where Ci is an instrumentation constant, l is fish length and CS and b are constants for a given species (NAKKEN 1975). Usually b is close to two (NAKKEN & OLSEN 1977).
In order to arrive at a density coefficient C in terms of
weight per unit area, we have to multiply equation 1 with the average weight
of the fish
CW = C .
= (C1 .
l-2). (C2 . l3) (2)
where C1 refers to a particular integration system applied on a particular species of fish and C2 is the condition factor in the length weight equation. The weight of fish per unit area CW, which contributes one unit of the integrated echo intensity is then:
CW = C . l (3)
The numerical value of CW applied to the “Dr. Fridtjof Nansen” data was 10.5 tonnes/mm per nautical mile. square nautical mile. This figure was established for a mixture of fish species with an average length of about 17 cm (ANON. 1977a). The density coefficients used in the present study are calculated from equation 3 using
(4)
where l is the mean length (in cm) of the observed mesopelagic fishes. Hence, the estimates of abundance arrived at for mesopelagic fish are based on the assumption that the scattering properties of these fishes at 38 kHz are similar to those of pelagic fish.
Continuous watch was kept on the acoustic instruments, and fishing was carried out whenever the echo sounder recordings changed their characteristics. Every day the acoustic data were scrutinized and compared to data from fishing stations. Integrator contributions from false bottom, wakes etc. were deleated, and the remaining integrator readings were grouped in four categories: small pelagic fish, mesopelagic fish, demersal fish and plankton and 0-group fish.
In addition to identification of the sound scatterers, fishing was also carried out in order to get samples for biological studies. The most commonly used gear was a 1360 mesh pelagic trawl with 1 cm inner net in the cod end. Occasionally, a 1600 mesh pelagic trawl or a bottom trawl was used. Details about the gears are given by ANON (1975). A krill trawl designed by Institute of Fisheries Technology Research (BELTESTAD & BRUNVOLL 1975) was also used at some stations.
During all fishing operations with the pelagic trawls the netsonde was used to monitor the position of the trawl, relative to the fish.
On all cruises the myctophids were sorted out and the volume was measured or estimated. On cruises 1, 3, 5 and 6 random samples were preserved in formaline and brought to the laboratory for examination. On cruise 4 identification and biological studies were carried out on board immediately after the capture, and additional samples were preserved and brought to the laboratory.
Standard length was measured to the nearest millimeter of all the fish studied. To make the measurements from cruise 4 comparable with those made on preserved material; the equation
lpreserved = 0.98lfresh - 0.55
established for Benthosema glaciale was used (GJØSÆTER 1973).
In some samples the sex, maturity stage and the content of food in the stomachs were also studied. Otoliths were taken from a few fishes to count primary growth layers. The otoliths were studied by microscope with 150-300 x magnification.
