ABUNDANCE ESTIMATION

Gulf of Oman

The south part of the Gulf of Oman was covered during the period 4 July - 3 August. The northern part was not surveyed. The cruise tracks are shown in Fig. 1.

The integrated echo abundance was converted to biomass per n. mile² using the formula

P = MC = M · 0.1 l

where M is integrator deflection and l is mean length of the scatters. This formula corresponds to that used by Gjøsæter (1978) with compensation for the change in performance of the equipment. Benthosema pterotum with a mean length of about 35 mm was the dominant fish in the area, and this length is therefore used for the transformations.

In the part of the Gulf covered by the cruise, a biomass of about 6 million tonnes was found. If the densities recorded is representative for the whole Gulf, a total mesopelagic fish biomass of about 8 million tonnes is estimated.

The density of fish per m² surface area was calculated for one degree squares. It ranged from 20 to 165 g/m² with a mean of 87 g/m².

Gulf of Aden

The Gulf of Aden was covered during the period 8-29 August. The cruise tracks are shown in Fig. 2. The integrated echo abundance was highest in the inner part of the Gulf. The integrated echo abundance was converted to biomass per square nautical mile using the formula

P = MC = M · 0.1 l

where M is integrator deflection and l is mean length of the scatterers.

In the western part of the Gulf of Aden B. pterotum with mean length about 4 cm was an important component of the scattering layers, and this length is used for the calculation. In the eastern part of the Gulf there was much variation in species and size composition, and fishes with no swim bladder (eg. Chauliodus) had an important position in the fauna. It is therefore difficult to estimate an equivalent mean size. Four centimeters was used, however, although it probably gives an underestimate of the true biomass.

The total biomass of mesopelagic fish was estimated to 2.6 x 10⁶ tonnes in the Gulf of Aden west of 47° E and 1.6 x 10⁶ tonnes between 47° and 51° E. According to the acoustical surveys the total biomass was therefore about 4 million tonnes.

The density of fish per m² surface area was calculated for one degree squares. It varied from 50 g/m² and 7 g/m² within the Gulf of Aden. The mean density was 17 g/m².

Five one degree squares in the inner part of the Gulf of Aden were covered three times during the survey with the following results:

Period	Fish density
8 - 19 August	23 g/m2
20 - 25 August	41 g/m2
26 - 29 August	42 g/m2

As a total of 14 one degree squares were covered more than once and the estimated biomass increased in 13 of these, the increase seems to be significant. The performance of the equipment was followed carefully throughout the survey, and no change was recorded. During the last part of the cruise a different setting of the equipment was used (see Materials and Methods). On very scattered recordings in deep water this can account for a 30% increase in estimated abundance due to a lower threshold effect. For dense recordings, however, there is little difference between the settings. This change can therefore only partly explain the increase in estimated abundance.

A few squares in the central and eastern part of the Gulf were also covered more than once. These too, indicated a rise in biomass, although less pronounced. Therefore, migration can not-account for the increase. Individual growth can explain at least part of the increase.

Recruitment of larva, which according to plankton samples were abundant during the first part of the cruise, may also be part of the explanation.