0-group fish and plankton
Fig. 2-4 show the horisontal distribution of temperature, salinity and oxygen contents at 10 m depth. The temperature varied between 15°C and 28°C. The highest temperature was observed in the central Gulf of Aden and in the Gulf of Oman and the lowest along the Somali coast around Ras Hafun. The salinity at 10 m depth varied from 35.3 along the eastern Somali coast to 37.3 in the inner parts of the Gulf of Aden and Gulf of Oman. The oxygen content in the area at the same depth was usually between 4.0 and 5.0 ml/l.
Along the eastern Somali coast the upper 100 m of the water masses were fairly homogeneous and only minor vertical gradients were observed. There was a strong current towards north reaching velocities of 5 knots estimated by the drift of the vessel. Around Ras Hafun an upwelling was obvious as shown by the upward bending of the isotherms in Fig. 5 and 6.
The characteristic features of the vertical hydrographic distribution in the Gulf of Aden appear in Fig. 7. The temperature, salinity and oxygen contents decrease with depth. The outflow of warm saline water from the Red Sea follows the Somali coast with a core at about 400 m causing intermediate maxima in the vertical distribution. Along the northern Somali coast and the coast of South-Yemen there was a marked vertical temperature gradient in the upper 50 m reaching values up to 0.8°C/m. In the western Gulf of Aden the oxygen content was below 0.5 ml/l by 30 m, while in the eastern part the upper and oxygen rich layer was deeper. Indications of local upwelling were observed at several places along the South-Yemen and Oman coast eastward to Ras Al Hadd.
Fig. 8 shows the vertical distribution of the hydrographic parameters in the Gulf of Oman. The saline outflow from the Persian Gulf flows along the Oman coast between 200 and 300 m. There is a gradual decrease in the vertical temperature gradient from the Gulf of Aden to the Gulf of Oman and the Pakistani waters.
Fig. 9 shows a hydrographic section off the coast of Pakistan. The surface temperature and salinity are lower than in the Gulf of Oman and the vertical gradients are less pronounced.
The integrator recordings classified as 0-group fish and plankton are shown in Fig. 10. The highest integrator readings were found at the eastern Somali coast from 1°N to 5°N, around Socotra and off Masira Island. The lowest values were observed off the Pakistani coast.
Along the southern Somali coast up to about 8°N this cruise recorded considerably higher integrator values than did the previous cruise. In the area around Socotra the values were approximately the same for the two coverages, but for the rest of the investigated area the integrator recordings were less than during the first cruise.
Fig. 11 shows the wet displacement volume of plankton from the Bongo net samples. The samples obtained by 20 cm diameter Bongo nets have been corrected in order to obtain comparable values to the 60 cm net results. On comparison of plankton volumes obtained during this cruise with results from the previous coverage there seems to be a tendency opposite to that of the integrator recordings. In most of the investigated area the first coverage observed lower values. The difference between the two coverages was most pronounced in the eastern Gulf of Aden and along the Oman coast up to Masira Island. This apparent divergency between the two methods is most likely due to differences in the composition of the layer classified as plankton on the two cruises and the distribution of day and night haul. The splitting of the echo recordings of the upper layers during the night into planktonic and mesopelagic organisms might also introduce an error.
One of the main echo contributor in the layer classified as plankton in the eastern Gulf of Aden was the Mantis prawn Oratosquilla investigatoris. Along the Oman coast around Masira Island there was a significant contribution from jellyfish with catches in the pelagic trawl of up to 15 tons/trawl hour.
Off Mukalla and north of the Kuria Muria Islands large aggregates of phytoplankton were observed floating at the surface. These accumulations seem to be connected with local upwelling.
The samples of larvae and postlarvae remain still to be analysed.
The contribution from pelagic fish to the integrated echo abundance is shown in Fig. 12. The most significant concentrations were observed in the following areas: The Somali coast between 3° and 4°N, in the Mukalla area, around Ras Asir and off Masira Island. At the two latter locations both the abundance and school size were acceptable from a commercial fishing point of view. Compared to the previous coverage our observations showed lesser values off the northern Somali and the Pakistani coasts and approximately the same values off the eastern Somali coast and along the coast of South-Yemen and in the Gulf of Oman. Along the southeastern Oman coast the integrator recordings showed higher values on this cruise.
The main contributior to the echo recordings north of Mogadiscio was round herring (Etrumeus teres) and to a lesser degree spotted herring (Herklotsichthys spp.) and sardinellas (Sardinella spp.). Outside Ras Asir the main contributions came from the Indian Oil sardinella (Sardinella longiceps) and layang scad (Decapterus macrosoma). Smaller quantities of round herring and mackerel (Scomber sp.) were also caught in this area. The porcupine fish (Diodon maculifer) which were the dominant pelagic specie off the eastern Somali coast on the first survey was only occasionally caught this time.
The shelf east of Ras Asir was surveyed twice. During the first period, 20 August - 2 September, only insignificant quantities of pelagic fish were observed. During the second period 2 October - 5 October, however, very good concentrations of pelagic fish were recorded. A migration had obviously taken place to this area during the time between the two surveys. In Fig. 13 is shown in more details the distribution of small pelagic fish during the two periods. Analysis of the maturity stages of the fish caught here showed that both the Indian oil sardinella and the layang scad were almost ripe, and that the round herring had recently spawned. We might therefore conclude that this location is most likely a spawning area for the above species. During the night the Indian oil sardinella was recorded both as small schools and as a scattering layer in the upper 20 m. During daytime these gathered into larger schools up to 50 m in depth. The largest of these were estimated from the sonar trace to be more than 100 tons. The layang scad was recorded during daytime as a layer close to the bottom, while during the night it was apparently scattered in the whole water column.
Along the northern Somali coast only minor amount of small pelagic fish were recorded and the catches consisted of only 0-group CARANGIDAE. Along the entire coast, however, surface schools of larger pelagic fish were observed, with the best concentrations found between Ras Alula and Elayu, around Mait Island and in the inner part of Gulf of Aden. The considerable temperature gradient observed here diminished the usefullness of the sonar to a great extent, and in addition the schools were apparently confined to the very near surface waters. Therefore the schools were not recorded on any of the acoustic instruments. The vast distribution area of these large pelagic species might justify further investigations. These fish are fast swimmers and avoided both the pelagic trawl and purse seine. A few specimens of bullet mackerel (Auxis rochei) and dolfin fish (Coryphaena hippurus) were, however, caught in drift nets.
Off the South-Yemen coast the concentrations of pelagic fish found west of Aden during the first survey was not observed this time, while the concentrations of small pelagic fish recorded in the Mukalla area were of the same magnitude. The dominant species in our catches were shad (Hilsa sp.) and crevalle (Alepes sp.). Only a few specimens of Indian oil sardinella were caught in the Mukalla area. The purse seiner Dhab - Dhab, however, had good catches of this species in shallow water only a week after we had passed the area. There must have been a migration to the Mukalla area of Indian oil sardinella during the first part of October.
Scattered recordings of the Indian oil sardinella and shorthead anchovy (Stolephorus heterolobus) were obtained in shallow water between Mukalla and Ras Fartak. Just east of Ras Fartak some concentrations of rainbow sardine (Dussumieria acuta) and round scad (Decapterus maruadsi) were recorded. Various CARANGIDAE contributed mainly to the very scattered recordings of pelagic fish in the areas west of Mukalla. Fig. 14 shows the distribution of echo integrator readings of the pelagic fish between Mukalla and Ras Fartak.
In addition to the small pelagic species also surface schools of larger pelagic fish were observed along the South-Yemen coast. Off Mukalla frigate mackerel (Auxis thazard), streaked Spanish mackerel (Scomberomorus commerson) and hardtail scad (Megalaspis cordyla) were caught in drift nets.
Along the South-east Oman coast north to Masira Island significantly higher integrator values were recorded on this survey compared to the first coverage. Around Ras Marbat the dominant species was the Indian oil sardinella and to a lesser extent the Japanese anchovy (Engraulis japonicus). The best recordings were obtained within the 40 m depth contour and consisted of immature Indian oil sardinella. The mature sardinella was recorded as small schools over deeper water.
Very good concentrations mainly of the Indian oil sardinella were found south of Masira Island together with lesser quantities of round herring. These concentrations were rapidly moving towards south - west along the Oman coast. The mature oil sardinella caught were spawning while the round herring were newly spent. The oil sardinella were recorded as large schools during the day and as layers during the night. The same difference in distribution pattern previously described was observed between the mature and immature fish. Immature oil sardinella were mainly caught in shallow water, while mature fish were found between the depth contours 50 and 100 m. Parts of the integrator values classified as pelagic fish in this area were due to threadfin bream (Nemimpterus spp.). This was observed to be pelagic during the night (see the section about Demersal fish). North of Masira Island to Ras Al Hadd only insignificant concentrations of pelagic fish were recorded. Fig. 15 A shows the distribution of echo integrator readings for pelagic fish between Kuria Muria Island and Masira Island.
In the Gulf of Oman only small echo recordings of pelagic fish were found. On an average, however, they were slightly higher than on the first coverage. No clupeoids were caught in this area and the catches of other pelagic fish were small. The dominant species were round scad, blackfin crevalle (Alepes melanoptera) and the crevalle (Alepes kalla). The crevalles had newly spawned and the round scad were maturing. A few surface schools of larger CARANGIDAE such as hardtail scad were observed in the. Gulf of Oman.
Only a very small part of the integrator values recorded off the Pakistani coast could be ascribed to pelagic fish. Compared to the first coverage the values observed were less than 50%. The pelagic fish caught in this area were mainly taken with the bottom trawl. The most numerous species were Orangemouth thryssa (Thryssa vitrirostris), hardtail scad and Indo-Pacific Spanish mackerel (Scomberomorus guttatus).
Fig. 16-33 show the length distributions and maturity stages of the dominant pelagic species.
Fig. 34 shows the echo integrator readings of mesopelagic fish. There is a decrease in echo abundance from the first coverage. It was only along the south-eastern Oman coast that a slight increase of the echo abundance was observed. The reduction was most marked in the Gulf of Oman.
The main part of the mesopelagic layer performed a diurnal vertical migration from 150 - 450 m during daytime to the surface during night. In the whole area except along the eastern and northern Somali coast parts of the mesopelagic layer formed dense schools during daytime extending up to about 80 m along the Iranian and Pakistani coast. These schools consisted of only lantern fish (MYCTOPHIDAE) and were mostly observed in the vicinity of the continental slope.
The dominant species in the mesopelagic layer were lantern fish and in some areas such as the Gulf of Oman and off the Pakistani coast the catches consisted practically of these species alone. The eastern Somali coast seems to have the greatest species diversity. Major other species present, were snake mackerel (Thyrsitoides sp.), lizard fish (Saurida spp.), cardinal fish (Synagrops japonicus) and hairtail (TRICHIURIDAE). Along the northern Somali coast also hatched fish (Argyropelecus sp.) and Cupiceps sp. were found. Along the coast of South-Yemen and Oman up to Ras Al Hadd the catches from the mesopelagic layer also included species such as Palinurichthys spp.
Lantern fish with mean length of 4.5 cm were caught at rates of up to 10 tons/trawl hour at trawl station 168 and at up to 3 tons/trawl hour at station 219. The mean length of the fish in this catch was 3.2 cm. Most fish of this size will most likely filter through the meshes of the trawl. A bottom trawl (trawl station 189) showed a catch rate of 4 tons/trawl hour of the species Palinurichthys spp. and smallhead hairtail (Lepturacanthus savala).
Length distributions of the most important mesopelagic species are shown in Fig. 35-37.
The integrator recordings classified as demersal fish appear in Fig. 38. Along the Pakistani coast the echo abundance of these species shows a marked decrease from the previous coverage. In the rest of the survey area the values were approximately the same. As observed last time the highest recordings were usually found in waters shallower than 50 m.
Along the eastern and northern Somali coast the dominant species were the scavengers (Lethrinus spp.), ponyfish (Leiognathus spp.), grunter (Pagellus sp.), groupers (Epinephelus spp.) and Red Sea herder (Emmelichthys sp.)
The dominant species along the South-Yemen coast were Bregmaceros sp. and catfish (Arius spp.). The maximum catch rate along the Somali and South-Yemen coasts was 2.5 tons/hour.
At Sauquara Bay and partly south of Masira Island very good recordings were obtained of threadfin bream (Nemipterus spp.). Fig. 15 B shows the distribution of the echo integrator readings of this specie. During the day it was observed as an apparent scattered layer along the bottom at bottom depth of 70-80 m. During the night it was found pelagic in 30-40 m at the same bottom depth. Bottom trawls during the day gave catch rates of 6-8 tons/hour. The fish was concentrated in a narrow band approximately 3 nautical miles broad and 80 nautical miles long parallel to the shore. The threadfin bream were in near spawning and spawning stages. Length distribution and maturity stages are shown in Fig. 39.
The most numerous commercially important fish in the trawl catches from the Gulf of Oman were the threadfin breams.
Off the coast of Pakistan the dominant species were croakers SCIANIDAE, grunts (POMADYSIDAE) and threadfin breams. Catch rates up to 7 tons/trawl hour were obtained, consisting of a variety of demersal and pelagic species.