GULF OF OMAN

BEHAVIOUR AND SPECIES COMPOSITION

During day time two sound scattering layers were usually observed in addition to a weak plankton layer in the surface. The upper layer (DI) was usually found at about 150 m depth and had a vertical extention of 20–40 m (Fig.2). It consisted of small schools and dense concentrations. Below this layer and usually centered about 250 m depth a second scattering layer (DII) appeared. This usually had a vertical extention of 70–100 m. Although it sometimes gave very high echo recordings, it had a more “smoky” appearance on the echograms than D I. About one hour before sunset the upper layer started to migrate slowly upwards. About half an hour before sunset it usually reached about 100 m. At this time both this layer and parts of the lower layer moved rapidly towards the surface where they mixed with the plankton layer. Parts of the deep day layer stayed at depth also during night time (Fig.3).

The shallow day layer (D I) consisted almost exclusively of Benthosema pterotum (Table 1). The salps observed in the trawl catches from this layer may be contamination from the surface layer where they were very numerous. In the deep day layer (D II) B. pterotum was also the dominant form, although salps too made up a large proportion of the catches. In tows from the lower part of the D II layer shrimps were sometimes numerous, and in contrast to the salps, these were probably partly responsible for the echoes received from these layers.

During night time B. pterotum was second to salps in abundance, but among those animals which are expected to give sound scattering, B. pterotum was dominant (Table 1). A towed transducer sounding upwards showed that parts of the night layer (N I) extended upwards beyond 10 m depth which is the upper limit for the echo integration. This experiment was carried out at new moon, and probably most of the fish will stay deeper during the other parts of the moon cycle.

In catches from the deep night layer (N II) B. pterotum made up 55% of the weight, or 86% if the salps are excluded.

Tows taken between the depths of the scattering layers or in the surface during day time gave large numbers of salps, but no fishes. Tows with Juday net showed that small krill and other plankton organisms, too small to be caught in the trawl, could be numerous. It seems, however, safe to conclude that fish, and mainly B. pterotum were responsible for most of the sound scattering. This view is supported by the pictures taken in thes layers, showing mainly fish and salps (Fig. 4,5).

ACOUSTICAL ABUNDANCE ESTIMATION

The abundance of mesopelagic fish in the Gulf of Oman was stu during three surveys (Fig.6):

The period 1 – 9 February was used for a detailed survey with fishing experiments in the area south of 24°50'N and west of 57°40'E.

The upper and lower day layers and the upper and lower night layers were integrated separately (Table 2). The integrated echo was converted to estimates of fish density using the formula:

P = MC

where P is density of the scattering organisms, M is integrat echo intensity (mm/n.mils) and C is a convertion factor (tonnes/n.mile(n.Mile)²). The convertion factor depends on t size of the fish

C = C ' L

where L is fish length and C'depends on the performance of t 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 = 0.62 · L

where L is fish length in cm. This corresponds to an average target strength of -10 log L - 21 dB per. kg. , and shoul therefore be compareable to the convertion factor used on R/V “Dr. Fridtjof Nansen” during the surveys in 1975 – 79; a on R/V “Lemuru” with compensation for the difference in the performance of the equipment.

In the Gulf of Oman the mean length of the fish was 39 mm an the variation between areas and between the different layers were too small to warrant the use of different lengths.

During night time parts of the fish seemed to be found in the upper 10 m and were therefore lost. During day time parts of the organisms responsible for the echo obtained from the lower layer were not fish. No compensation is made for these biases in the following estimates. It is also supposed that those parts of the Gulf covered (about 2/3) are representative for the whole area. Based on these assumptions the following abundance of mesopelagic fish was estimated:

	Survey	I		8 mill.tonnes
		II		11  "      "
		III		13  "      "

The variances are rather high and the differences between the estimates may not be significant. The mean of the three estimates,

11 million tonnes

therefore seems to be the best estimate of the biomass of mesopelagic fish in the Gulf of Oman in January - February 1981

DETAIL SURVEYS

An area in the Gulf of Oman south of 24°50'N and west of 57°40'E was covered six times during the period 1 – 9 February. The coverage differed so the six surveys are not directly comparable. Several trends are, however, evident (Table 3). The upper day layer usually had the lowest echo abundance and high variance. The lower day layer always showed the highest echo abundance and usually high variance. The two night layers had similar abundance. The upper layer had low variance, the lower higher. In general these trends can also be seen in the data from the main surveys.

Totally the day recordings were higher than the night recordings. No significant increase or decrease in abundance should be traced during this period.

Converted to biomass of mesopelagic fish in the area (2120 n.i) the following results are obtained:

CATCH RATES

In the Gulf of Oman 80 trawl hauls were made. Out of these 76 were made in the mesopelagic fish layers. The result of these are shown in Table 4. The best catch rates were always obtained in the upper day layer where the mean catch rate was about 2 tonnes/hour. A large part of the hauls were made to identify the fish and to collect biological data. Those hauls made in D I with the purpose of experimental fishing gave consistently catch rates higher than 2 tonnes/hour. A total of 15 hauls gave catch rates higher than this value, 8 hauls gave more than 4 t/h and 5 more than 5 t/h. The best catch rate obtained was about 9 t/h, and the best catch taken in one single haul was about 6.5 t.

Catches from the upper night layer were generally low. In one haul only a catch rate of about 2 tonnes/hour was obtained. The low catch rates seem to be caused by the scattered vertical distribution of the fish, with an even distribution in the upper 70–100 m. On previous cruises, when rather high catch rates have been obtained from the surface waters, the fish usually formed bands with high density during night time too.

No high catch rates were obtained from the deepest layers (D II and N II).

Most hauls had a duration of 30 minutes, but hauls from 20 min. to 2 hours were also tried. The speed was usually around 2 knots, but both higher and lower speed was tried. No correlation between catch rates and the speed or duration of the hauls could be observed. The material is, however, too small to draw final conclutions.

Size distribution of the fish caught in the different scattering layers are shown in Fig. 10.

CATCH RATES IN RELATION TO DENSITY ESTIMATES OBTAINED BY ECHO-INTEGRATION.

During trawling echo integration was run in a depth interval corresponding to the vertical opening of the trawl. From thes values the average density (g/m³) at the trawl opening was es mated. Table 5 shows estimated densities compared to catch pe m³ filtered water for the various scattering layers. The tabl also shows the estimated “trawl efficiency” (the ratio betwee catch/m³ and density). The average “trawl efficiency” seems t be considerably higher for the D I layer than for any of the other layers. This difference is significant, while the difference between D II, N I and N II are not significant.

The low “trawl efficiency” in the N I, N II and D II-layers may give reasons to suspect that the fish in these layers was mixed up with plankton giving high contribution to the integrator values. The N I-layer was in fact mixed up with some plankton which remained as a separate layer in the upper 100 m during day time. However, the echo-contribution from this plankton layer usually represented just one to ten percent of the total integrator values during day time. Juday net hauls did not give much plankton in any of the layers.

In six cases underwater photos were taken in connection with trawl hauls or light experiments. Fish was recognised on most of the pictures in all layers, while plankton organisms were rather scarce. Salps and jellyfish were the only recognisable plankton on the pictures (Some faint traces could be small shrimps). Table 6 shows the average number of fishes and salps per picture. About 30 pictures were taken at each station.

A cage calibration of salps indicated that the average target strengh is in the order of -50dB per kg. salps, which means that some hundred kilos of salps is needed to give the same echo as one kg. of fish.

The conclution is that the echo contribution from plankton was small compared to the contribution from fish in all layers. Therefore the plankton is not responsible for the observed differences in “trawl efficiency”. There has to be a real difference in catchability.

The D I layer was composed of schools staying in a presumably uplight zone, while the other layers, staying in darkness, had a rather homogeneous density. This difference in light conditi may cause a difference in the fish reaction towards the trawl. Probably the fish in the D I layer are able to see the trawl. The fish has not enough mobility to avoid the trawl opening, but they may be able to avoid coming in contact with the walls of the foremost part of the trawl. That means that they may be scared towards the centre of the trawl, thus being easily caugh The fish in the other layers are probably not able to see th trawl and may therefore have a less directional reaction, so that much of the fish may be filtered through the meshes in th foremost part of the trawl. The average “trawl efficiency” for the D II, N I and N II layers together is 0.11, which seems to be reasonable for small, passive fishes, when the shape and mesh size of the trawl are taken into consideration (Fig.1).

REACTION TO LIGHT

The reaction of mesopelagic fishes to light was studied twice in the Gulf of Oman. A 1000 w high pressure sodium lamp was used as a light source.

The first experiment was carried out at new moon. The light was mounted at the side of the vessel and the echo sounder was used to monitore the reaction of the fish. Underwater photography was used to identify the organisms aggregating. The light made the scattering layer sink and concentrate at a depth of about 50 m (Fig.7). Photographs show that myctophids were the dominant organisms in this layer (Fig.8).

In the second experiment the light was mounted on a small boat. This experiment was carried out about 10 days after new moon. This time the fish concentrated in a cone around the light (Fig.9); but the concentrations were not as dense as those observed during the first experiment.

During both experiments most of the myctophids present were adult B. pterotum with length between 35 and 43 mm. Most of them had mature gonads.

DISTRIBUTION OF EGGS AND LARVAE

For sampling of eggs and larvae a 80 cm Juday net was used. For sampling larvae hauls from 50 – 0 m were usually applied, while eggs were sampled using hauls from 100 – 50 and 50 – 0 m. In some areas hauls were made in 300 – 200 m, 200 – 100 m and in 100 – 0 m. To collect larger larvae and to study the abundance of the organisms the larvae feed on, a 60 cm Bongo net was used. The Bongo had one 90 um net and one 180 um net.

During the two first surveys, 25 – 30 January, larvae were found over most of the Gulf, but the highest concentrations were observed in three different areas (Fig.11 A). The first of these areas was observed in the inner part of the Gulf, stretching from the Oman side and northwards. The second area with high concentrations was found in the middle of the Gulf probably extending from the Iran side. The third area with high concentrations was found in the outer part of the Gulf, just west of 58°E. All these areas showed concentrations higher than 200 larvae/m² while they were separated by belts with fewer than 100 larvae/m². The densest concentration found was 1328 larvae/m².

During the last survey, 9 – 11 February, the abundance of lar was generally lower. The highest concentrations were, however, still found in the same areas as during the first surveys (Fig.11 B), although there was only a slight increase in density of larvae in the middle area.

Eggs were usually found below 50 m, and during night time only. Eggs found during the first part of the night were always new spawned, while those taken during the late part of the night were close to hatching.

ARTIFICIAL FERTILISATION

The eggs and the newly hatched larvae of B. pterotum are not previously described. Therefore it was necessary to carry out fertilisation and hatching in the laboratory. Ripe eggs from the gonads had a yolk globule, segmented yolk, and were slightly elliptical, measuring 0.7 – 0.8 mm. When transferred to sea water a periviteline space developed and the total diameter became 1.0 – 1.1 mm. The chorion was very fragile, a ruptured easily.

Eggs were incubated at 20.5 and 25.0°C and the hatching time was 16 and 10 hours respectively.