Plankton and 0-group fish
In Figs. 1-4 the cruise track, fishing stations and hydrographical stations are described. The survey grid was kept denser in localities with good recordings than in general. Bathy-thermograph stations were evenly distributed, but were particularly worked at or near fishing stations.
The total integrated echo recordings are shown in Fig. 5, and Table 1 gives the details of the fishing stations. While judging the echo recordings, the integrator values were split into four categories: pelagic fish, mesopelagic fish, demersal fish and plankton and 0-group fish.
The integrated echo recordings of pelagic fish are shown in Fig. 6. The resource was mainly confined to the shelf area, with only minor recordings at or beyond the slope.
The best recordings of pelagic fish were made in the Gulf of Oman area off the coasts of Pakistan and Iran. The integrator values were higher than obtained in the same area during the previous cruise, (Cruise No 5, 6 March to 25 June 1976). Good recordings were also made in the Gulf of Masira area and east of Ras Fartak on the coast of Oman, in the area off Aden and south of Ras Asir on the Somali coast.
Fig. 7 shows the distribution of pelagic fish in the Gulf of Oman area. The recordings consisted of various pelagic species of which carangids made up a greater part. These fish were mostly scattered in the shallow zone and though no catch of commercial size was taken, they were frequently caught with bottom trawl. Hence no single species could be picked out as the dominating one among the carangids.
Djeddaba crevalle (Alepes djeddaba) was observed in the catches off the Pakistani and Iranian coasts together with threadfin trevally (Alectis indicus) and various species of cavalla (Carangoides spp.) Hardtail scad (Megalaspis cordyla) was found in the Ras al Kuh area, and also a few Indian mackerel (Rastrelliger kanagurta) and Indo-Pacific Spanish mackerel (Scomberomorous guttatus).
On the north coast of Oman the pelagic recordings consisted mainly of bigeye scad (Selar crumenophthalmus), Malabar cavalla (Carangoides malabaricus) and frigate mackerel (Auxis thazard). These species appeared also as surface schools and were recorded within two small separated areas; north of Ash Shinas and off Ras al Ghaf. The schools which were rather small, were distributed from near shore to approximately 20 n.mi off. From an assessment based on school counting the two areas were appraised to contain 1500 and 12000 tonnes of fish respectively.
Indian pellona (Pellona ditchela) were caught in relatively good numbers in the bottom trawl west of Ras al Kuh and as stray individuals in Gwatar Bay. The major part of the catches of this species had started maturing. Mustached thryssa (Thryssa mystax) was found in smaller quantities in the same localities.
Ponyfish (Leiognathus spp.) made up a great part of the pelagic recordings, especially in the inner part of the gulf. Off Ras al Kuh a catch rate of 3.6 tonnes/hour was obtained with the bottom trawl. Of this 1.5 tonnes were orange ponyfish (L. bindus). Slender ponyfish (L. elongatus) contributed to a minor extent in the same area, while common ponyfish (L. equulus) was found on the other side of the gulf.
Another pelagic species that contributed noteworthy to the recordings were spotted sicklefish (Drepane punctata). Off Khor Rabij on the Iranian coast, it made up half of a bottom trawl catch of a total of 780 kg/hour. Silver pomfret (Pampus argenteus) and black pomfret (Formio niger) contributed also partly to the recordings.
The distribution of pelagic fish recorded off the east coast of Oman is shown in Fig. 8. South of Masira Island various clupeid species made up the majority of the recordings. Round herring (Etrumeus teres) and rainbow sardine (Dussumieria acuta) dominated and were joined by goldstripe sardinella (Sardinella gibbosa) and white sardinella (S. albella). These fishes were in immature stages. A pelagic trawl haul gave 780 kg/hour of round herring out of a total catch of 1100 kg/hour.
The clupeids in this area appeared mostly as surface schools and though the integrator values were relatively high, only a part of the stock was recorded by the echo-sounder. The schools were observed by eye and recorded by the sonar, restricted within an area north of 19° 40 N and west of 58° 40 E. However, due to shallow water, only the outer part of the Gulf of Masira was surveyed, and hence the inner limit of the school area was not clear defined. Together with the clupeids small quantities of longnose cavalla (Carangoides chrysophrys), djeddaba crevalle and round scad (Decapterus maruadsi) were also observed.
In the area south of Ras al Madraka Bregmaceros macclelandi contributed most to the pelagic recordings. A pelagic trawl haul gave 580 kg/hour, consisting almost purely of this specie.
In the Gulf of Aden the pelagic recordings were mostly located on the northern side. Surface schools were observed off Ras Fartak, from Mukkalla to Gubbat alAin and in the inner part of the gulf, west of Aden. Indian oil sardinella (Sardinella longiceps), mostly immature, dominated among the schools in the Ras Fartak area, but was only found in minor quantities in the Mukkalla area and west of Aden. In the latter locality goldstripe sardinella and spotted sardinella (S. sirm) also appeared, but only traces of them were taken in the catches. Yellowstripe trevally, the most dominant specie in this area, was joined by other carangids like round scad (Decapterus maruadsi) and cavallas (Carangoides malabaricus and C. oblongus). A few numbers of Indian mackerel were also observed.
The school area off Ras Fartak is shown in Fig. 9. After approximating the abundance it was concluded that the schools represented 50 000 tonnes of fish. Fig. 10 shows the distribution of schools in the Mukkalla-Gubbat alAin area. This area was surveyed twice, but due to wind and strong current the purse seine could not be used at these times. Though only stray individs of pelagic species were caught with bottom and pelagic trawl, it was evident that horse mackerel (Trachurus sp.) and round scad made up the major part of the schools. By counting the schools the abundance within the area was assessed to be 20 000 tonnes of fish.
The pelagic recordings in the inner part of the gulf are shown in Fig. 11. This includes also the schools recorded off Aden.
Of pelagic species, other than carangids and clupeids recorded in the Gulf of Aden, anchovy (Engraulis sp.) and Bregmaceros macclelandi were well represented in the outer part. In the inner part ponyfish (Leiognathus spp. and Gazza minuta) and whipfin mojarra (Gerres filamentosus) were recorded.
The poor recordings at the north coast of Somalia were mostly made up of slender ponyfish. A haul with bottom trawl on the shelf east of Elayu gave a catch rate of 340 kg/hour of which this specie represented about half.
On the east coast of Somalia the best pelagic recordings were found from Ras Asir to Ras Hafun. Relatively high numbers of surface schools were observed (Fig. 12) consisting mainly of Indian oil sardinella and round herring. By counting the schools the area was assessed to represent 60 000 tonnes of fish. Conditions for purse seining were fair and two trails were made. However, both were unsuccessful and only a small sample of Indian oil sardinella was caught. In the pelagic trawl hauls made in the same area, round herring dominated in the catches, with anchovy (Engraulis sp.), mackerel (Scomber sp.) and round scad participating to a smaller extent. None of these catches were of commercial size.
Except for the Indian oil sardinella, of which some were mature, the other pelagic species were all in immature stages.
Farther south on the Somalia coast anchovy (Engraulis sp. and Stolephorus heterolobus) and round herring made up most of the pelagic recordings. Smaller amounts of mackerel (Scomber sp.), scad (Decapterus sp.) and sardinellas were also observed. Off Pta Maraglie a pelagic trawl haul gave 360 kg/hour of which 200 kg were anchovy (Engraulis sp.).
On the slope north and south of Mogadiscio several schools were recorded near the bottom at a depth of about 150 m. Because of unsuitable conditions these were not identified by trawl. They were however, classified as mackerel. Single specimens of mackerel were caught on the shelf within the same area.
Along the Somali coast south of Mogadiscio and on the North Kenya Bank the recordings of pelagic fish appeared to be rather poor. Strong current in these areas made fishing operations difficult, and an attempt with the pelagic trawl at Latitude 0°45 S resulted in the loss of a complete trawl.
Length distribution and maturity stages for the dominant and most important pelagic species are shown in Figs. 13 - 24.
Mesopelagic fish gave by far the highest contributions to integrator values (Fig. 25). The recordings were somewhat higher than during the previous cruise (Report on cruise No. 5). Deep scattering layers were recorded from the open sea to the edge of the shelf. A typical diurnal vertical migration was observed. During day time one layer was observed at a depth around 200 m, ascending towards surface just before sunset. In most areas another layer alternated from around 500 m to 100 m depth in a same diurnal cyclus.
The abundance in the Gulf of Oman area was distinctly higher than elsewhere in the surveyed area.
Lantern fish (MYCTOPHIDAE) dominated in the mesopelagic group within the whole area. Other noteworthy contributors were Cubiceps natalensis and Bombay duck (Harpadon nehereus). These were mainly, distributed in the northern part. In the Gulf of Aden Palinurichthys sp. was quite abundant. It seemed that this fish substituted Cubiceps natalensis in the gulf area. Cardinal fish (Synagrops japonicus) was distributed from the Gulf of Aden and southward along the Somali coast.
Though the recordings of mesopelagic fish were relatively high, the catch rates seldom exceeded 1000 kg/hour. In several cases it was evident that the fish was too small and passed through the meshes of the pelagic trawl. The best catch of lantern fish in the Gulf of Oman area was obtained at the border between Pakistan and Iran. A pelagic trawl haul in 300 m depth gave 1000 kg/hour.
However, the best catch rate in the survey area was obtained with bottom trawl at 120 m depth south of Socotra. This gave 20 tonnes/hour of lantern fish.
Samples of the lantern fish were preserved for later analysis. Length measurements and maturity stages for the other mesopelagic species mentioned above are given in Figs. 26 - 29.
Due to the limitation of the echo sounder to record fish close to the bottom (mentioned in the Report on Cruise No. 5) the integrator deflections only provide an indication of the distribution of the bottom fish (Fig. 30).
Though the average values are relatively low, they are clearly higher than obtained during the previous coverage. The best recordings were made on the shelf in the Gulf of Oman (Fig. 31) in an area with good recordings also of pelagic fish. A great variety of different fish species appeared in the trawl catches.
Catch rates of up to 3, 6 tonnes/hour with bottom trawl were obtained at the Iranian and Pakistan coasts. In this area croakers (Otolithes ruber and Johnieops sp.) and Japanese threadfin bream (Nemipterus japonicus) were dominating species, but other fish like dog shark (Scoliodon palasorrah), lizardfish (Saurida spp.) and groupers (Epinephelus spp.) were also well represented.
Off the south coast Oman a bottom trawl haul at 275 m depth gave 18 tonnes/hour, but this, however, consisted mainly of slime head (Hoplostethus mediterraneus). In the same catches also Laemona globiceps (Gadidae), shrimp (Heterocarpus sp.) and lobster (Puerulus sp.) contributed.
In the Gulf of Aden catch rates of upto 1.9 tonnes/hour were made with bottom trawl. Lizardfish and breams (Pagellus natalensis and Nemipterus japonicus) were the dominant elements, with contributions from longface emperor (Lethrinus miniatus), snappers (Lutjanus spp.) and catfish (Arius spp.).
The recordings at the north coast of Socotra consisted mainly of emperors (Lethrinus spp.), surgeon fish (Acenthurus strigorus), snappers (Lutjanus spp.) and grouper (Epinephelus sp.). At 35 m bottom depth a trawl haul gave 2.9 tonnes/hour. Off the southern coast of the island blacksaddle goatfish (Parapeneus fraterculus) and grouper were the most dominant fishes. However, sponges were quite abundant, and upto 1.5 tonnes of it were taken in one bottom haul.
The highest catch rate at the Somali coast was 2.4 tonnes/hour obtained south of Pta Maraglie. At some places on this coast strong currents made trawling difficult, and areas with sponges lowered the fishing efficiency.
Mostly barracuda (Sphyraena sp.), bream (Pagellus natalensis) and goatfish (Mulloidichthys sp. and Parapeneus sp.) were taken in the catches, but also some snappers and grouper (Epinephelus sp.) appeared.
Also on the North Kenya Bank sponges and strong current disturbed the fishing operations. Humpnose large-eye bream (Monotaxis grandoculis) was among the most dominant species in the catches together with emperor, goatfish, snappers and groupers.
Length distribution and maturity stages for some of the dominant species are shown in Figs. 32 - 35.
On an average the plankton recordings were approximately of the same magnitude as obtained during the previous cruise (Fig. 36).
The main contributors to this group were mantis prawn (Oratosquilla investigatoris), swimming crab (Charybdis sp.), krill (Euphaciacea), and jellyfish. In the north jellyfish contributed considerably to the recordings in offshore areas. Swimming crab dominated nearer the coast. The abundance of mantis prawn increased southwards, and along the east coast of Somalia it dominated completely. South of Pta Marraglie the pelagic trawl was towed only 5 minutes in plankton recordings and gave a catch of 5 tonnes. Seventy percent were mantis prawn, and the rest swimming crab. The theoretical catch rate of this is 60 tonnes/hour.
In the area off also krill was quite abundant. Only traces of 0-group fish were spotted in the trawl catches. However, some 0-group scad (Caranx spp.) and round herring were found in the Gulf of Masira area and various carangids south of Ras Hafun.
Fig. 37 indicate the areas with surface observations. Due to rough weather in the beginning of the cruise, the observations off the coasts of Pakistan and Iran were rather poor.
Fish schools were most frequently observed at the Oman and Yemen coasts and also at the north eastern Somali coast. Dolphins were mostly seen from the Gulf of Aden and southwards. Generally upto 20 specimens appeared at a time, but off Ras al Junaiz schools of more than hundred dolphins were observed.
Snakes were rarely observed at the surface. They were caught with trawl only in the northern area, and in Gulf of Oman 1-3 snakes frequently appeared in the catches.
The horizontal distribution of temperature, salinity and oxygen in 10 m depth are shown in Figs. 38-40. The vertical distribution of the same parameters are given in five sections in Figs. 41-45.
In August and the beginning of September the SW-monsoon still persisted in the northern area. The decreasing values of temperature and salinity towards shore indicate that upwelling still occurred along the Oman coast (Fig. 38).
Along the east coast of Somalia a south-going current was observed in the surface layer during October. However, while using the trawl it was evident that at greater depths the north-going current still persisted. The variation of current directions at different depth was obviously linked with the transitional period of the monsoons.
Table 1. R/V Dr. Fridtjof Nansen. Record of fishing operations. PTR: Pelagic trawl, BTR: bottom trawl, BLL: bottom long line, KTR: Krill trawl, PSE: Purse seine, Fish names: FAO Species Identification Sheets for Fishery Purposes.
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
(Table 1. Continued)
Fig. 1. Survey grid and stations, Karachi to Gulf of Aden.
Fig. 2. Survey grid and stations, Gulf of Aden to Mogadiscio. Symbols as Fig. 1.
Fig. 3. Survey grid and stations, Somali coast south of Mogadiscio. Symbols as Fig. 1.
Fig. 4. Survey grid and stations. North Kenya Banks. Symbols as Fig. 1.
Fig. 5. Total echo abundance, average integrator deflection in mm per nautical mile.
Fig. 6. Pelagic fish, average integrator deflection in mm per nautical mile.
Fig. 7. Distribution of pelagic fish in the Gulf of Oman area. Single hatching: Scattered recordings, double hatching: dense recordings.
Fig. 8. Distribution of pelagic fish off the east coast of Oman, Symbols as Fig. 7.
Fig. 9. Distribution of fish schools off the south-east coast of Oman.
Fig. 10. Distribution of fish schools, Mukkalla to Gubbat AlAin on the coast of Peoples Demacratic Republic of Yemen.
Fig. 11. Distribution of pelagic fish in the area off Aden. Symbols as Fig. 7.
Fig. 12. Distribution of fish schools, Ras Asir to Ras Hafun on the Somali east coast.
Fig. 13. Indian oil sardinella, Sardinella longiceps. Length frequency and maturity distribution. A and B) Ras Fartak, South-Yemen, C) Northeast Somalia coast.
Fig. 14. Indian pellona, Pellona ditchela. Length frequency and maturity distribution. Ras al Kuh, Gulf of Oman.
Fig. 15. White sardinella, Sardinella albella. Length frequency and maturity distribution. A) East coast of Oman, B) Gwatar Bay, Gulf of Oman.
Fig. 16. Goldstripe sardinella, Sardinella gibbosa. Length frequency and maturity distribution. A) East coast of Oman, B) East of Aden, South-Yemen, C) Pta Marraglie, Somalia.
Fig. 17. Round herring, Etrumeus teres. Length frequency and maturity-distribution. A and B) South of Masira Island, Oman, C and D) Ras Asir, E) Pta Marraglie, F) El Arar, Somalia.
Fig. 18. Malabar cavalla, Carangoides malabaricus. Length frequency and maturity distribution. A) Ras al Kuh, B) Ras al Ghaf, C) Ash Shinas, Gulf of Oman. D) Aden, South Yemen, E) Armaleh, Somalia.
Fig. 19. Round scad, Decapterus maruadsi. Length frequency and maturity distribution. A) South of Masira Island, Oman, B) Ras Fartak, C) Aden, South Yemen, D) Armaleh, E) Ras Hafun, Somalia.
Fig. 20. Indo-Pacific Spanish mackerel, Scomberomorus guttatus, Length frequency and maturity distribution off Ras al Kuh, Gulf of Oman.
Fig. 21. Shorthead anchovy Stolephorus heterolobus Length frequency and maturity distribution off Pta Marraglie, Somalia.
Fig. 22. Anchovy, Engraulis sp. Length frequency and maturity distribution. A) Ras Asir, B) Pta Marraglie, Somalia.
Fig. 23. Moustacked thryssa, Tryssa mystax. Length frequency and maturity distribution. A) Ras al Kuh, B) Gwatar Bay, C) Khur Rabij, Gulf of Oman.
Fig. 24. Silver pomfret, Pampus argenteus. Length frequency and maturity distribution. A) Gwatar Bay, B) Ras al Kuh, Gulf of Oman.
Fig. 25. Mesopelagic fish, average integrator deflection in mm per nautical mile.
Fig. 26. Bombay-duck, Harpadon nehereus. Length frequency and maturity distribution. A) Ras Ormara, Pakistan, B) Gwatar Bay, C) Khur Rabij, Gulf of Oman, D) Ras al Madraka, Oman, E) Ras Fartak, South-Yemen.
Fig. 27. Cardinal fish, Synagrops japonicus. Length frequency and maturity distribution. A) Karachi, Pakistan, B) Somali north coast, C) Ras al Ara, South-Yemen, D) Pta Marraglie, Somalia.
Fig. 28. Palinurichthys sp. Length frequency and maturity distribution. A and B) Ras Fartak, C) Ghubbat al Ain, South-Yemen, D, E and F) north Somali coast, G) Ras al Ara, South-Yemen.
Fig. 29. Cubiceps natalensis. Length frequency and maturity distribution. A) Ghubbat al Ain, South-Yemen, B) north of Socotra, C and D) Ras al Hadd, Oman.
Fig. 30. Demersal fish, average integrator deflection in mm. per nautical mile.
Fig. 31. Distribution of demersal fish in the Gulf of Oman area. Single hatching: Scattered recordings, double hatching: dense recordings.
Fig. 32. Bream, Pagellus natalensis. Length frequency and maturity distribution. A) Ghubbat al Ain, South-Yemen, B and C) Pta Marraglie, D and E) El Arar, Somalia.
Fig. 33. Japanese threadfin bream, Nemipterus japonicus. Length frequency and maturity distribution. A) Khur Rabij, B) Ras al Kuh, Gulf of Oman, C) Ghubbat al Ain, D) Aden, South-Yemen, E) Armaleh, Somalia.
Fig. 34. Longface emperor, Lethrineus miniatus. Length frequency and maturity distribution. A) Elayu, Somalia, B) Ras al Ara, South-Yemen, C) Socotra.
Fig. 35. Tiger-toothed croaker, Otolithes ruber. Length frequency and maturity distribution A) Gwatar Bay, B) Ash Shinas, C) Khur Rabij, Gulf of Oman.
Fig. 36. Plankton and 0-group fish, average integrator deflection in mm per nautical mile.
Fig. 37. Surface observations. F: Fish school, D: Dolphin, W: Whale, T: Turtle.
Fig. 38. Temperature, t °C, at 10 m depth.
Fig. 39. Salinity, S , at 10 m depth.
Fig. 40. Oxygen, ml/l, at 10 m depth.
Fig. 41. Temperature, salinity and oxygen in the section South west-Kori Great Bank, Pakistan.
Fig. 42. Temperature, salinity and oxygen in the section Ras al Ghaf-Khur Rabij, Gulf of Oman.
Fig. 43. Temperature, salinity and oxygen in the section Ghubbat al Ain-Elayu, Gulf of Aden.
Fig. 44. Temperature, salinity and oxygen in the section Pta Marraglie-South east, Somalia.
Fig. 45. Temperature, salinity and
oxygen in the section Mogadiscio-South east, Somalia.