Hydrography
Figures 2-5 show the distribution of temperature, salinity, density and oxygen contents in the four hydrographic sections (Figure 1). In the upper 100 m there is a tendency that the isolines are at greater depths offshore than at the coast, indicating an anti-cyclonic movement of the offshore water masses. In all sections, water masses of low oxygen contents, 1-2 ml/l cover substantial parts of the continental shelf. Along the Makran coast the 2 ml/l isoline was observed at depths between 20 and 30 m, while off the Indus delta (Sind coast) waters of oxygen contents less than 2 ml/l were found at 15 m at the innermost station. The effect of the freshwater outflow from the River Indus was observed at the three innermost stations in Section IV (Figure 5). A similar salinity distribution, but much less pronounced, was found at the innermost station in Sonmiany Bay (Figure 4). The transition between the "Indus water" and the oceanic water was sharp and was easily determined from the colour contrasts at the surface; the Indus water being green-yellowish in colour while the oceanic water was deep blue. This transition could be seen at the innermost parts of all the transects off southern Sind.
Pelagic fish
Figure 6 shows the distribution of pelagic fish. The fish was distributed on the shelf areas, and no recordings of pelagic fish were made further offshore. In general the recordings were very scattered. Dense concentrations were observed at five localities, all with very limited extensions. At the Makran coast the predominant species was rainbow sardine (Dussumieria acuta). The fish occurred in scattering layers and small schools in daytime at depths between 20-40 m, just off the bottom. During nighttime it was observed both as scattering layers and small schools at the surface. It ranged in length from 18-20 cm. Catch rates of rainbow sardines up to 3.7 tonnes per hour were obtained in pelagic night hauls.
Other pelagic fish species were scarce in the Makran area, but scads (Decapterus spp.) and hardtail scad (Megalaspis cordyla) were caught in limited numbers at some few trawl stations.
Off Sind concentrations of pelagic fish were found in three localities (Figure 6). At the inner end of the Indus Swatch very dense recordings of anchovy (Stolephorus sp.) were obtained in mixture with juvenile rainbow sardine. The fish was distributed in large schools or scattering layers at depths between surface and 20 m over 30-40 m bottom depth, but the extension of the area was limited to 2-3 square nautical miles. Quantities of bottom fishes - rays, sharks, grunts and croakers - were feeding on these concentrations.
Scattering layers and small schools of rainbow sardine, ranging from 10-20 cm in length, were observed in two localities off Sind at bottom depths between 60 and 80 m. During the day the fish occurred in small schools close to the bottom, at night it dispersed into a scattering layer at 30-40 m depth. In the Sind area the maximum catch rate of pelagic fish was about 500 kg per hour in the pelagic trawl.
Bottom fish
The distribution of bottom fish is shown in Figure 7. Relatively dense patches were observed several places along the Makran coast while the recordings off Sind were more scattered. At the Makran coast the concentrations of bottom fishes were predominated by hairtails (Trichiurus lepturus and Lepturacanthus savala) which made up the bulk of the catches in the area. The hairtails were observed as weak scattering layers both at the bottom and in midwater over bottom depths ranging from 25-30 m to the edge of the shelf. Grunts (Pomadasys sp.) and croakers (Epinephelus spp.) were also frequently caught in the bottom trawl hauls but in significantly less quantities than hairtails. Catch rates up to 6 tonnes per hour trawling were experienced.
Off Sind, both the recordings and the catches of bottom fish were more variable than at the Makran coast. In most of the investigated area the abundance was found Co be low, and dense patches of fish were found only at two localities, in southern Sonmiany Bay and at the inner end of the Indus Swatch. In Sonmiany Bay catfish was the predominant scatterer, while small-sized croakers and grunts together with hairtails and rays made up the bulk of the catches off the Indus delta. Catch rates up to 16 tonnes per hour were obtained in bottom trawl hauls. On the outer banks off Sind, catch rates were low and variable and the threadfin bream (Nemipterus japonicus) was a major constituent.
Mesopelagic fish
Recordings of mesopelagic fish (Figure 8) were made at and off the edge of the continental shelf in the entire area. The fish showed the usual daily migration pattern: small schools and scattering layers at depths of 150 m or more in daytime and a scattering layer in the upper 50 m during the night. The recordings were mainly scattered and the catch rates were low. Lantern fish (Myctophidae) was predominant in the mesopelagic fish layer.
Plankton
In most of the shelf areas planktonic scatterers contributed the major part of the total echo abundance (the integrated echo energy) (Figure 9). Jellyfish and krill (Euphausiids) were probably the main contributors. Planktonic scattering layers were observed in all depths both during day and night at densitites which to a great extent made it impossible to obtain reliable integration values of scattered fish. The planktonic layers showed no systematic differences in back scattering strength at the echo sounder frequencies 38 kHz and 120 kHz.
At the eastern Makran coast and in Sonmiany Bay the trawl hauls indicated that krill was a major constituent of the planktonic biomass, while different kinds of jellyfish were caught in quantities off Sind.
Surface observations
Figure 10 shows the surface observations which were made. Large whales were spotted off the eastern coast of Makran, probably feeding on the quantities of krill in that area. Dolphins were observed at the outer end of the transects off Sind.
Due to bioluminescence, surface schooling fish could easily be spotted also at nighttime. In the offshore part of the southernmost transect a strange occurrence of bioluminescence was observed. At a distance it looked like waves of light moving at the sea surface at a high speed, 10-20 times the speed of the vessel (10 knots). The phenomenon was obviously caused by bioluminescence originating from sources which were situated 1-3 m apart. All these sources within a 10-30 m wide belt were triggered on and off with a short time delay from one side to the belt to the other, thus giving the impression of "waves of light" propagating through the water. The pulsations were quite regular, 95-100 per minute, and observed over a distance of 3-4 nautical miles. The observation was made between 0100 and 0200 hours local time. The sea was almost calm with a small swell of 1-1.5 m height from southwest. It seems unlikely that the regular periodic "behaviour" of the phenomenon could becaused by the observed sea state or other physical factors. More likely the bioluminescence itself enabled the organisms to adjust their flash frequency and phase in order to strengthen the light intensity. Plankton samples from a Juday net hauled in the surface layers were preserved for later analysis.
