Hydrography
Figures 2-5 show the distribution of temperature, salinity, density and oxygen content in the four hydrographical sections (Figure 1). In the upper 100 meters the previous indications of anti-cyclonic movement of the offshore waters is not found here. A decreasing depth of the isolines towards deeper waters in the Soamiani Bay section rather indicate a reversed circulation pattern. In all sections, the water masses on the continental shelf have a high oxygen content with the 3 ml/l isoline below 100 meters depth, except in the last section at the Iranian border. Here, an oxygen content of 6 ml/l was recorded on the shelf, while the 3 ml/l isoline was significantly shallower than in the other sections. This, together with the tendency of coastal rising of temperature and salinity isolines indicates that a local upwelling has occurred in this area. The high oxygen content of the water masses on the shelf was probably caused by a high phytoplankton production, as brownish water masses were seen all along the shelf from Gwadar to the Iranian border. The phytoplankton, preliminary identified as dinoflagellates, were distributed in large patches or belts about 1-2 nautical miles wide, and a pronounced reduction of fish density were seen inside the belts. As the two hydrographical stations on the shelf on this section were taken at sunset, the oxygen content may be lower during the night. The high variance in oxygen content caused by phytoplankton production/respiration, or toxic elements excreted by the algae, is suggested to cause the lowered fish density within the patches.
The freshwater runoff outside Indus river in Section I (Figure 2) and in Sonmiani Bay (Figure 3) could barely be detected. The salinity is high at the shallow stations in both sections.
Pelagic fish
Figure 6 shows the distribution of pelagic fish. Note that the density scalings are not directly comparable with earlier cruises due to changed performance of new echo sounders and integrator. (Ref. Annex III). The fish was distributed over most of the shelf area, and no recordings of pelagic fish were made further offshore. In general the recordings were very scattered, and no area with real pelagic schooling activity was found. Denser concentrations were recorded in the Sonmiani Bay, where a pelagic trawl station. (Figure 1) gave an almost clean catch of ponyfish (Leiognathus sp.). In the inner end of the Indus Swatch the denser pelagic recordings probably contained a mixture of Indian oil sardine (Sardinella longiceps), goldstripe sardinella (S. gibbosa), anchovy (Thryssa dussumieri), Rainbow sardine (Dussumieria acuta), Scad (Decapterus russelli), ponyfish (Leiognathus sp., Gazza minuta) and Spanish mackerel (Scomberomorus koreanus), which all were present in the bottom trawl catches in the area.
In the denser concentrations off Sind and outside Karachi, the indian ilisha (Ilisha melastoma), ponyfish. (Leiognathus sp.) Rainbow sardine (Dussumieria acuta), anchovy (Thryssa sp., T. dussumieria, T. vitrirostris), barracudas (Sphyraena putnamiae), gold-stripe sardinella (S. gibbosa), and indo-pacific Spanish mackerel (Scomberomorus guttatus) were present both in pelagic and bottom trawl catches.
Along the Makran coast the pelagic fish distribution was very scattered. The anchovies Thryssa mystax, T. vitrirostris, T_ dussumieri, rainbow sardine (Dussumieria acuta), blacktip sardinella (S. elanura), ponyfish (Gazza minuta) and Spanish mackerels (Scomberomorus commersoni, S. guttatus) were frequently recorded in the catches.
At the Makran coast small schools or individual tunas were observed jumping in front of the vessel. As no sonar contacts were made, it was concluded that no fishable schools were present. A few sonar contacts of fast swimming fish were also made just outside the Indus Swatch, but no catches were made as the fish strongly avoided the vessel and kept a distance of about 100 meters on approaches.
Demersal fish
The distribution of demersal fish is shown in Figure 7. The recordings all along the Sind coast were generally scattered while limited areas with higher densities were found in the Sonmiani Bay and along the eastern part of the Makran coast.
At the Sind coast, the concentrations of bottom fish were dominated by hairtails (Trichiurus lepturus), threadfin bream (Nemipterus japonicus), catfish (Arius sp.), grunters (Pomadasys maculatus), lizardfish (Saurida tumbil), Indian driftfish (Ariomma indica), croakers (Argyrosomus sp.), snappers (Lutjanus malabaricus) and groupers (Epinephelus diacanthus). Catch rates were generally low, 100-500 kg per hour trawling, with a maximum at St. no. 113, outside Karachi, of 1.6 tonnes pr. hour.
In the Sonmiani bay the bottom trawl catches was dominated by the threadfin bream (Nemipterus japonicus), hairtails (Trichiurus lepturus), croaker (Otolithes ruber), and catfish (Ancharius brevibarbis). Catch rates of 0.5-1.1 tonnes per hour trawling were usual in this area.
Along the eastern part of the Makran coast the catch rates increased, and hauls up to 7.5 tonnes per hour trawling were experienced. The hairtails (Trichurus lepturus), catfish (Ancharius brevibarbis) seabreams (Acanthopagurus sp.) and croakers (Pennahia sp.) dominated the catch. Except for one trawl station, giving 4 tonnes per hour trawling, fish density on the western Makran coast was low. The larger catch in this area contained about 30% grunters (Pomadasys commersoni) with trevallies (Lactarius lactarius), croaker (Otolithes ruber) and catfish (Ancharius brevibarbis) as secondary components.
Mesopelagic fish
Recordings of mesopelagic fish (Figure 8) were made at and off the edge of the continental shelf in the entire area, as well as inside the Indus Swatch. The recordings were mainly scattered with low catch rates. The fish showed a daily migration pattern with two distinct scattering layers both at day and nighttime. The vertical migrations were fast, the downward migration from surface to 150 - 200 meters often completed within 20 minutes. The lanternfish Benthosema pterotum was predominant in the mesopelagic scattering layers, but driftfish (Cubiceps cubiceps) also represented about 10% of the catch at the last pelagic trawl station at the Iranian border.
A peculiar behaviour pattern of mesopelagic fish, identified as Benthosema ptrotum was seen in a limited shelf-edge area off Sind. (Figures 8 and 10).
Large schools at shallow depths, from 0-150 meters, extending from the DAY I-layer, was recorded at highlight day-conditions (hours 0800-1200). The fenomenon seemed to be more a delayed downwards migration than a regular pattern, and schools joining the D-I scattering layer were observed at about 1200 hours. The fish from two pelagic trawl stations of aimed trawling on sonar contacts was larger and significantly lighter in colour than the common Benthosema pterotum. Samples from the catches were taken for further analysis at the Institute of Marine Research.
Plankton
In most of the shelf areas planktonic scatterers contributed the major part of the total echo abundance (integrated echo energy) (Figure 9). Jellyfish, small shrimps, fish larvae and krill were identified as main contributors. Planktonic scattering layers were observed in all depths both during day and night at high densities, which to a great extent made it impossible to obtain reliable integrated values of scattered fish. Any systematic differences in scattering strength on the two systems have to be further analysed later, as the 120 kHz system must be recalibrated due to imperfect weather conditions on the last test.
Surface observations
No systematic record of whales and dolphins were made at this cruise. Four whales were reported to be seen from the bridge on the eastern Makran coast. At long distance they were identified as three bryde whales and one humpback-whale. Flocks of dolphins were frequently observed off the Sind shelf.
Scattered observations of individual or small schools of tuna were observed jumping in front of the vessel along the Makran coast, but no real sonar contacts were made on these.
A phytoplankton bloom was seen as brown belts, 1-2 nautical miles wide, at the western Makran coast, from Gwadar to the Iranian border. The brown water was caused by a high density of dinoflaggelates.
Fig. 10. Echogram recordings of shallow day schools of lantern fish, Benthosema pterotum.
