Main distribution patterns
Total relative abundance
The North Kenya banks and the Somali coast south of 2°N
Sub-area 1. Somali coast 2°-5°N
Sub-area 2. Somali coast 5°-10°N
Sub-area 3. North-east Somali coast
Sub-area 4. North Somali coast
Sub-area 5. West Yemen coast
Sub-area 6. East Yemen coast
Sub-area 7. South-east Oman coast
Sub-area 8. The Gulf of Oman
Sub-area 9. West Pakistan coast
Sub-area 10. South Pakistan coast
Mesopelagic fish
0-group fish and plankton
The group small pelagic fish consisted mainly of sardinellas, horse mackerels (including scads) and anchovies (Table 2). The geographical distribution of the group is shown in Fig. 2. Small pelagic fishes were found exclusively in nearshore - and shelf waters during both periods of coverage and no echo traces or visual surface observations were made in the oceanic part of the survey area. As a rule the largest concentrations were observed in waters shallower than 100 m - frequently shallower than 40 m.
Demersal fish consisted of a variety of species (Table 2); the dominating ones being ponyfish and threadfin bream along the Arabian coast and catfish and croaker off Pakistan. The main distribution pattern was equal to that of the small pelagic fishes. The largest concentrations were observed in nearshore and shelf waters mainly, shallower than 100 m.
The mesopelagic fish group were dominated by myctophiids (Lantern fishes). In the Gulf of Aden cardinal fish also made a significant contribution. These groups were recorded in mid-water (200-500 m) in the whole survey area where the depth exceeded 200 m (Fig. 3), with the largest concentrations in the Gulfs of Aden and Oman.
It is seen (Table 1), that by far the largest portion of the total integrated echo intensity in the area came from mesopelagic fish and 0-group fish and plankton, while only a minor portion is due to small pelagic fish and demersal fish. The indices for the different groups can not, however, be compared directly in order to arrive at weight- or volumratios between the 4 groups. Small fishes and organisms will as a rule give a much higher contribution per unit weight to the indices than larger fish. As the 0-group fish and plankton group mainly consisted of organisms less than 4 cm, and the mesopelagic fish group also consisted of small fishes, ranging from 4-10 cm, the volumes of these two groups are not that much greater than the volumes of the two other groups as it appears from Table 1.
In addition, parts of the pelagic fish volume were recorded as small schools in the near-surface layer. These schools were not "seen" by the echo sounder and consequently they were not contributing to the abundance indices, but a sonar counting technique was applied to obtain estimates of abundance. Hence the abundance indices of Table 1 will give under-estimates of the volume of small pelagic fishes.
Close to the bottom there will always be a layer where the fish is not recorded by the echo sounder. The thickness of this layer depends on the regularity of the bottom and the pulselength and beamwidth of the sounder. Therefore, the indices in Table 1 will also be under-estimates for the demersal fish group.
As the scattering properties of plantonic organisms are very poorly known, no valid conclusions concerning the volumes of plankton can be drawn from the abundance indices in Table 1. The tendency in Table 1 is, however, quite clear; the total volume of mesopelagic fish within the survey are many decades higher than the total volumes of small pelagic and demersal fish. The estimates of abundance and the main features of the biology of the small pelagic - and demersal fish categories, as well as their distribution and behaviour patterns will now be considered for each sub-area.
Only very scattered recordings were observed in this area both during spring and autumn. The main contributor to the echo recordings was the swimming crab (Charybdis edwardsi) giving catch rates of up to 180 kg/hour trawling.
Small pelagic fish
The relative high abundance of this category in spring (Table 2) were caused mainly by the contribution from the porcupine fish (Diodon maculifer). As this fish species has an extremely high target strength as compared to their length and volume (Olsen et al. 1973) the weight in Table 2 is probably largely overestimated. Catch rates of porcupine fish up to 0.4 tonne/hour trawling were obtained.
During the autumn survey, concentrations of round herring (Etrumeus teres) were found within a limited area between 3° and 4°N. Spotted herring (Herklotsichthys spp.) and sardinellas (Sardinella spp.) contributed also to the recordings in this area. The length ranges for the three species are given below:
|
Round herring: |
16-20 cm |
|
Spotted herring: |
7-10 cm |
|
Sardinella spp: |
10-13 cm |
Demersal fish
A variety of species among which snappers (Lutjanus spp.) and groupers (Epinephelus spp.) were the most dominant contributors to the demersal fish recordings. Catch rates up to 0.3 tonne/hour trawling were obtained.
In autumn bad weather prevented fishing to some extent in this area.
Small pelagic fish
During spring the porcupine fish was the most numerous contributor to the recordings also in this area. Recordings of japanese mackerel (Scomber japonicus) were observed at the bottom at 260 m depth off Ras Auad and a bottom trawl showed a catch rate of 1.4 tonnes/hour trawling. The fish was immature with a length range between 21 and 26 cm. Due to bad weather conditions fishing operations were largely prevented in this area during the autumn survey (Aug-Sep), and reliable identification of the recordings was not obtained then.
Demersal fish
The large amount of fish observed during the autumn survey (Table 2) were not identified. The recordings were made at the steepest slope at depths between 100 and 300 m 5-30 nautical miles to the south of the area where the mackerel was observed in spring. However, as the bad weather also created a lot of noise on the echo record, no conclusions can be drawn regarding their origin. In nearshore waters scavengers (Lethrinus spp.) and groupers were found and catch rates up to 0.8 tonne/hour trawling of scavengers were obtained.
Small pelagic fish
Fig. 4 A and Fig. 5 show the horisontal distribution of the main concentrations in spring and autumn. The indices and weights in Tables 1 and 2 for this area show no significant change in abundance from spring to autumn.
During the spring survey continous recordings covered an area of approximately 100 square nautical miles between the 30 m and 500 m contour lines. The fish, mainly oil sardinella (Sardinella longiceps) and round herring were observed both as a scattering layer in 10-30 m depth and as dense small schools close to the surface. Sonar counting indicated that approximately 2500 schools were present within the area, most of the schools being quite small, less than 2 tonnes. This indicate an upper abundance limit of 5000 tonnes for the total number of schools, which is not included in the estimates in Tables 1 and 2.
During the autumn survey (Fig. 5), the recordings mainly consisted of indian oil sardinella and layang scad (Decapterus macrosoma). At night the oil sardinella were recorded as scattering layers and small schools in the upper 20 m, while in daytime these gather together into larger schools upto 50 m in depth, the largest being estimated from the sonar trace to be more than 100 tonnes. The layang scad was recorded as a layer close to the bottom during the day, while at night it was apparently scattered in the whole water column. The main features of the biology of the fish observed are given in below:
|
|
SPRING |
AUTUMN |
||
|
Length, cm |
maturity |
Length, cm |
maturity |
|
|
Oil sardinella |
13 - 16 |
immature |
16 - 18 |
ripening |
|
Round herring |
13 - 17 |
immature |
18 - 20 |
spent |
|
Layang scad |
|
|
18 - 20 |
ripening |
Catch rates of layang scad amounted to 1 tonne/hour trawling in bottom trawl.
Demersal fish
The abundance of demersal fish show a significant decrease from spring to autumn. The reason for this is not known. Scavengers, sharks and rays were caught in the some few bottom trawl hauls, but in large parts of the area the irregular bottom prevented identification of bottom recordings.
Small pelagic fish
The relative large amount of this group during the spring survey (Table 2) is probably caused by erronous identification of echo traces; recordings of mesopelagic fish being taken as small pelagic fish. In the whole area recordings of pelagic fish were very scattered during both surveys. It is worthwhile noticing that during the autumn small scad, 6-11 cm in length, was caught, partly far off shore, in this area.
Demersal fish
The quantities of demersal fish observed appear to be quite small (Table 2) during both surveys.
Small pelagic fish
Tables 1 and 2 show a significant decrease in abundance from spring to autumn. The distribution of a concentration of small pelagic fish in this area is shown in Fig. 4B which presents the results from the spring survey. This concentration, which was not observed in the autumn, was recorded as a scattering layer in 0-30 m in a limited area between the 20 and the 50 m contour. The dominant species were gold stripe sardinella (Sardinella gibbosa) and two types of scad (Decapterus dayi, Selar crumenophthalmus). Oil sardinella constituted a minor portion of the catches. Measurements of head lengths showed that the oil sardinella in this area possessed significantly shorter heads than the fish off North-east Somalia and further east along the Arabian coast. Whether these data signify racial differences is, however, hot known.
No sonar traces were observed in the area and it seems likely that most of the pelagic fish abundance are included in the estimates of abundance (Tables 1 and 2). During the autumn survey crevalle (Alepes sp.) were found to be the most significant contributors to the echo recordings. The main features of the biology of the fish were:
|
|
SPRING |
AUTUMN |
||
|
Length, cm |
maturity |
Length, cm |
maturity |
|
|
Oil sardinella |
13 - 17 |
unripe |
|
|
|
Gold stripe sardinella |
10 - 14 |
immature |
|
|
|
Scads |
12 - 16 |
immature |
|
|
|
Crevalle |
|
|
9 - 12 |
immature |
The abundance of demersal fish was found to be quite low during both survey periods (Table 2).
Small pelagic fish
A significant increase in abundance appeared from spring to autumn (Table 2). During the spring survey small pelagic fishes were recorded along the entire coast between Mukalla and Quamr Bay (Figs 6 and 7). Sardines and horse mackerels were in majority but various species of ponyfishes contributed also largely to the catches.
Oil sardinella were observed in dense concentrations in an area 30 nautical mile east of Mukalla, where a purse seiner "Dhab-Dhab" operated. A sonar mapping showed approximately 200 schools within a limited area of 20 square nautical miles between the depth contours 0 m and 40 m. The majority of the schools were estimated to 2-10 tonnes, while some few probably exceeded 100 tonnes. Hence the total volume within these 20 square nautical miles were probably between 1000 and 5000 tonnes.
Further to the east oil sardinella was found more incidentally except for an area in Quamr Bay where a minor concentration of this species were recorded. The other main species within the area between Mukalla and Quamr Bay were orangefin ponyfish (Leiognathus bindus), rainbow sardines (Dussumieria acuta) and horse mackerel (Trachurus sp.).
In the autumn small pelagic fish was recorded along the entire coast (Fig. 7), although mainly in scattered concentrations. The species contributing to the recordings were: Oil sardinella, shorthead anchovy (Stolephorus heterolobus), shad (Hilsa sp.), crevalle (Alepes sp.), rainbow sardine, horsemackerel and scad. The maximum catch rates, in terms of tonnes/hour trawling were: oil sardinella - 1.0, horse mackerel - 4.3. The main features of the biology of the fish are given below:
|
|
SPRING |
AUTUMN |
||
|
Length, cm |
maturity |
Length, cm |
maturity |
|
|
Oil sardinella |
11 - 16 |
immature |
8-10 |
immature |
|
Rainbow sardine |
12 - 18 |
immature |
12 - 15 |
immature |
|
Horse mackerel (Trachurus sp.) |
11 - 16 |
immature |
16 - 22 |
unripe |
|
Scad |
16 - 19 |
unripe |
18 - 26 |
- |
The samples of oil sardinella from the autumn survey may not be representative for the stock in this area. "Dr. Fridtjof Nansen" did not observe this species in the area between Ras al Kalb and Mukalla, but "Dhab-Dhab" had good catches of oil sardinella in shallow waters ca. 20 nautical miles west of Mukalla one week after the research vessel passed that locality. Echo grams taken by Mr. Willy Ellingsen, the master-fisher man onboard "Dhab-Dhab", showed that large dense schools of oil sardinella were present in this locality during most of the autumn period.
Demersal fish
The estimates of abundance (Table 2) show a marked decrease in abundance of demersal fish from spring to autumn, but it can be questioned whether this decrease is real or not. The apparent decrease may be caused by a transfer of fish weight from demersal fish to small pelagic fish from spring to autumn. The main bulk of demersal fish in the area consisted of ponyfish, cat-fish, hairtail, shark and Bregmaceros sp., and all these fishes were partly recorded and caught pelagically. As bottom recordings of small pelagic fish as sardinella and horse mackerel were observed many places, the discrimination between the two groups was at times very difficult, and this may explain the difference in the estimates of abundance.
The dominating species during the spring survey were orangefin ponyfish between Mukalla and Ras Fartak and largehead hairtail (Trichiurus lepturus) in the Ras Fartak area. In autumn cat-fish (Arius sp.) and Bregmaceros sp. were the most significant contributors to the bottom trawl catches. The maximum catch rates in terms of tonnes/hour trawling were: Ponyfish - 1.2, hairtail - 1.8, catfish - 2.9 and Bregmaceros sp. - 0.7.
Small pelagic fish
The estimates of abundance show a considerable increase in biomass of small pelagic fish from spring to autumn (Table 2). In spring, recordings of this group were scarce, while in autumn dense recordings were made at several localities within the area (Fig. 8 A).
East of Kuria Muria the dominant species was the oil sardinella and to a lesser extent Japanese anchovy (Engraulis japonicus). The best recordings were obtained inside the 40 m depth contour and consisted of immature oil sardinella. Mature sardinella was recorded as small schools over somewhat larger bottom depths.
Very good concentrations mainly of 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 oil sardinella were recorded as large schools during the day and as layers during the night. The same difference in distribution pattern as previously described was observed; immature fishes were mainly caught in shallow waters, while the mature fishes were found between the depth contours 50 and 100 m.
The main features of the biology of the fish were:
|
|
SPRING |
AUTUMN |
||
|
Length, cm |
maturity |
Length, cm |
maturity |
|
|
Oil sardinella |
- |
- |
10 - 20 |
immature |
|
Round herring |
- |
- |
15 - 18 |
spent |
|
Japanese anchovy |
- |
- |
8 - 10 |
- |
Demersal fish
The biomass of demersal fish (Table 2) decreased also in this area from spring to autumn and the explanation for this decrease may well be similar to that for sub-area 7. During the autumn survey parts of the integrator values classified as pelagic fish in this area were due to threadfin bream (Nemipterus spp.) which was observed to be pelagically at night. At Sauquara Bay and round Masira Island very good recordings of this species were obtained (Fig. 8 B). During the day it was observed as a scattered layer along the bottom at a depth of 70-80 m. At night it was found pelagically in 30-40 m depth over the same bottom depths. The fish ranged in length from 11 to 22 cm and the mature specimens were in a spawning or near spawning stage. Catch rates for threadfin breams of 6-8 tonnes/hour trawling were obtained in bottom trawl hauls in daytime and 8 tonnes/hour trawling in a pelagic haul at night.
During the spring survey the most significant contributors to the demersal fish biomass were found to be grunter (Rhonciscus stridius) and ponyfish, and bottom trawl hauls showed maximum catch rates of respectively 1.4 and 1.9 tonnes per hour trawling for the two species.
Small pelagic fish
The estimates of abundance (Table 2) indicate that the biomass of small pelagic fish within this area was rather low during both survey periods; the recordings being very scattered in the whole area.
Demersal fish
Also the recordings obtained from demersal fish were very scattered. The most important species in the catches during spring were scavenger and grouper while threadfin bream dominated in the autumn. Only low catch rates were obtained, the maximum, containing a variety of species, being 0.8 tonne/hour trawling.
Small pelagic fish
The estimates of abundance (Table 2) indicate a very low biomass of small pelagic fish.
Demersal fish
Similarly to the small pelagic fish the biomass of demersal fish also seems to be quite low.
Small pelagic fish
The estimates of abundance (Table 2) indicate a considerable decrease in biomass from spring to autumn. During the spring survey relatively good concentrations of small pelagic fish were found in this area (Fig. 9). The fish was recorded as small schools and layers both in mid-water and close to the bottom at depths mainly shallower than 70 m. The dominating species in the catches were orangemouth thryssa (Thryssa vitrirostris), Buccaneer anchovy (Stolephorus buccaneer), Malabar cavalla (Carangoides malabaricus), rainbow sardine. In autumn, recordings of small pelagic fish were scattered in the area, the most numerous species being orangemouth thryssa, hardtail scad and Indo - Pacific Spanish mackerel (Scomberomorus quttatus). The maximum catch rates in terms of tonnes/hour trawling were as follows: Orangemouth thryssa - 7.0, malabar cavalla - 1.9, hardtail scad - 1.5. The main features of the biology of the fish were:
|
|
SPRING |
AUTUMN |
||
|
Length, cm |
maturity |
Length, cm |
maturity |
|
|
Orangemouth thryssa |
6 - 15 |
- |
12 - 16 |
- |
|
Buccanc anchovy |
3 - 8 |
- |
- |
- |
|
Malabar cavalla |
13 - 18 |
- |
- |
- |
|
Hardtail scad |
- |
- |
30 - 40 |
spent |
|
Indo-pacific Spanish mackerel |
- |
- |
32 - 44 |
spent |
Demersal fish
The estimate of abundance (Table 2) shows a marked decrease from spring to autumn similarly to the value for small pelagic fish. The main reason for this might be the same; a migration of fish out of the survey area.
During spring relative large quantities of demersal fish were recorded at depths mainly shallower than 50 m, and catfish and croaker were the most abundant species. Catch rates of respectively 1.7 and 1.2 tonnes/hour trawling of these two species were obtained in bottom trawl hauls. In the autumn catfish, croaker and threadfin breams dominated, and bottom trawl catch rates of 6.0, 4.0 and 1.1 tonnes/hour trawling were obtained, consisting of a mixture of the most dominant pelagic and demersal species.
As mentioned previously. Table 1 shows that the total biomass of mesopelagic fish is by far the largest portion of the total fish biomass within the survey area. According to the indices there is a significant decrease in abundance from spring to autumn, the autumn value being only 50 percent of the spring value. The decrease occurred in all sub-areas except for sub-area 7 where the index did not change. This variation cannot be fully explained but it may partly be caused by a change in the distribution pattern. During the day the mesopelagic fish layer was as a rule recorded between 150 and 500 m. Scatteres at greater depths than 500 m were not recorded by the integrator, with the settings used during the cruises. If a larger portion of the mesopelagic fish biomass was below 500 m in autumn than in spring this will lead to a decrease in the indices.
There was also an apparent change in behaviour between spring and autumn. During the spring survey mesopelagic fish was mainly observed as a layer of varying density in 150 - 500 m depth in daytime. Most of this layer migrated to the surface at dusk and descended again at dawn. In autumn mesopelagic fish performed dense schools during the day in 150 - 450 m depth, extending upto 80 m along the coasts of Iran and Pakistan. Whether these schools were dense enough to create acoustic shadowing effects and, thereby, give reduced integrator outputs, which in turn lead to lower abundance indices, is not known.
The dominant species in the mesopelagic fish layer were the lantern fish (MYCTOPHIDAE) and Palinurichthys spp. During spring the cardinal fish (Synagrops sp.) contributed largely to the catches in the Gulf of Aden. The largest concentrations were found along the shelfs in the Gulfs of Aden and Oman (Fig. 3). Table 4 shows the maximum catch rates in terms of tonnes/hour trawling in each sub-area. The length ranges for the most abundant mesopelagic fish groups were:
|
|
Length, cm |
|
|
Spring |
Autumn |
|
|
Lantern fish |
2 - 9 |
2 - 10 |
|
Cardinal fish |
5 - 9 |
3 - 7 |
|
Palinurichthys spp. |
- |
9 - 18 |
The main contributors to the indices of echo abundance (Table 1) were salps, yellyfish, seweed and to some extent swimming crabs. But in some few localities in nearshore waters fish larvae made significant contributions. During the spring survey the highest catch rates of fish larvae in the pelagic trawl were obtained off Mogadiscio, on the east coast of Somalia and off the Arabian coast from east of Aden eastwards to Ras al Kalb. Catches of postlarval fish were taken in Quamr Bay and south of Karachi.
Off the east coast of Somalia cardinal fish and barracuda with lengths of 10-17 mm and 25-66 mm were dominating. Along the Arabian coast anchovy larvae, 11-30 mm were the most abundant. The only catch of sardinella larvae was obtained in Quamr Bay where sardinella (mainly oil sardinella) constituted 20 percent of the total larvae catch, the remaining 80 percent being anchovy. The length range were 25-46 mm and 28-41 mm respectively for oil sardinella and anchovy (mainly Engraulis japonicus). South of Karachi the anchovy larvae caught measured 24-30 mm (Stelephorus buccanuri).
The catches from the autumn survey remain to be identified.
