3.1 Description of work
3.2 Hydrography
3.3 Surface observations and sonar contacts
3.4 Bottom conditions
3.5 Distribution and abundance of pelagic fish
3.6 Distribution and abundance of demersal fish
3.7 Crustaceans
3.8 Biomass of fish
The area of operation was covered twice and the coverages during the two cruises are shown in Figs. 1 and 2 (a and b).
During the first cruise the operations started at the Burma - Bangladesh border in the north, and the area was covered with transects approximately 30 nautical miles apart south to the border between Burma and Thailand. The transects started approximately at the 15 m contour line and had a maximum seaward extension of 110 nautical miles. Seven hydrographic sections were worked, three off Arakan, two in the Delta area and two off the Tenasserim coast. A total of 70 trawl stations were carried out for sampling and identification purposes.
During the second cruise the effort was concentrated within areas which on cruise no. 1 were found to be of particular interest concerning fish distribution and abundance. Thus, along the Arakan coast most of the work was done in the areas off Sandoway and further south; in the Delta area the areas with bottom depths between 15 and 80-100 m were extensively surveyed, as were also the nearshore areas off Tavoy; and finally, exploratory fishing with bottom trawl was conducted at the deep water trawling ground off the Tenasserim coast. No hydrographic observations were made during the second cruise, but a total of 97 trawl stations were carried out. The results will to a large extent be considered area-wise; the Burma waters being divided into three sub areas:
1. The Arakan coast, North of 16° N
2. The Delta area, Between 14° N and 16° N
3. The Tenasserim coast, South of 14° N.
(Figs. 3, 4 and 5)
Fig. 3 shows the salinity distribution at 5 m depth. The salinities in the upper layer were particularly low in the Delta region where the large Irrawaddy and Salween rivers have their outlets. Northward along the Arakan coast the surface layer salinity increased gradually. The salinity distributions in Figs. 3 and 4 thus give the impression of a low-salinity coastal current flowing to the west in the Delta area and bending northward along the Arakan coast; the current being maintained by the freshwater influx to the Delta area.
The upper 75 m of the entire area was characterized by temperatures between 27 and 30°C, by salinities lower than 34‰ and oxygen contents between 4.0 and 5.0 ml/l. Below 75 m the temperature and the oxygen content decreased rapidly with depth. Off Arakan the oxygen content was observed to have a minimum between 150 and 300 m and values below 0.2 ml/l were observed here. Further south, in the Andaman sea, the lowest oxygen contents (just below 0.7 ml/l) were observed at depths of 200 m or more and no distinct intermediate minimum were found.
In most of the sections, and particularly in the Delta areas (section IV and V), the oxygen isolines showed a tendency to run parallel to the bottom at depths between 50 and 100 m. Even small changes in the direction of the isolines, for instance related to seasonal changes in the dynamic stability of the water masses, may thus lead to large variations in oxygen content over vast bottom areas.
The visual observations made at the surface are summarized in Fig. 6 together with the sonar contacts.
Off Arakan observations of whales (probably sperm whale) were made during both cruises. Dolphins were observed far offshore at the southern Tenasserim coast during cruise no. 1, and during the second cruise a very large school was sighted at approximately 16°N. The observations of tuna-like fish included both tunas, larger carangids and Spanish mackerel, and it is believed that perhaps Spanish mackerel were most frequently observed.
During cruise no. 1 some rather weak sonar contacts were obtained in the open ocean off the Arakan coast. These echoes were probably caused by schools of flying fish. None of the schools could be observed on the echo sounder and small schools of flying fish "took off" when the vessel approached the targets.
The visual observations of small pelagic fish included sardinella, mackerel (Rastrellinger sp.) and scads (Decapterus sp.). Although anchovies probably made up the larger portion of the pelagic fish biomass, and most of the sonar echoes originated from anchovies both in the areas off Arakan and off Tavoy, schools of anchovies were probably seldom sighted at the surface as the individuals in the surface schools mostly appeared to be considerably larger than most of the anchovies within the area.
Fig. 7 shows the bottom conditions as observed along the cruise tracks by the 38 kHz echo sounder. Bottom type 1 is suitable for all kind of bottom trawls, while on bottom type 2 only trawls with bobbins can be operated successfully. The bottom types 3 and 4 do not permit bottom trawling at all.
Off Arakan, the slope was too steep and the slope bottom too rough to permit bottom trawling at depths greater than 120 - 150 m. This was also experienced in the north and western parts of the Andaman Sea, while south of 13°N the bottom trawl was operated successfully down to 450 - 500 m depth in an area with even and steeping bottom at depths between 200 and 500 m (Fig. 7b). This area was limited by a steep slope to the west, starting at depths between 450 and 500m, and a ribbon of very rough bottom to the east at depths between 200 and 300 m. The observations (Fig. 7b) thus indicated a large deep water trawling ground from 13°N and southward, probably extending to the south beyond Burmese waters.
In many places, particularly off Arakan, isolated small rocks or mud volcanoes were observed in areas with even flat bottom. These irregularities were detected by the sonar, so during bottom trawling the sonar was frequently used to map the bottom conditions ahead of the vessel in order to avoid gear damage.
3.5.1 The Arakan coast
3.5.2 The Delta and Tenasserim areas
Figs. 8 and 9 show the distributions of echo integrator values originating from pelagic fish. In Fig. 8 the values in brackets are either taken from the other cruise or interpolated, in order to be able to work out abundance indices for the entire coast for both coverages.
During both cruises recordings of pelagic fish were obtained along the coast south of Cheduba Island (18°40' N). Further north recordings were scarce. The fish - mainly anchovies (Stolephorus sp.), juvenile sardinella and mackerel - were observed in small schools and scattering layers in 0 - 30 m depths over bottom depths between 20 and 60 m. The schools were quite small and gave no significant sonar echoes. Fig. 9a presents the distribution of pelagic fish observed during the second cruise. The highest densities were found off the southern part of the Arakan coast and the seaward extension was limited to 12 - 15 nautical miles. At the outer parts of the continental shelf and/or further seawards no recordings of pelagic fish were made during any of the two cruises.
The catch rates of pelagic fish were low. With pelagic trawl the best catch rate off Arakan was 450 kg/hour, the catch consisting of a mixture of Indian anchovy and ilisha. One bottom trawl haul showed a catch of 800 kg/hour of moustached thryssa mixed with demersal species, but on average the catch rates of pelagic fish were less than 150 kg/hour with pelagic trawl and less than 75 kg/hour in the bottom trawl hauls (Table 1).
Length distributions of the pelagic species are given in Table 2.
During both cruises a dense scattering layer was observed at 50 - 90 m depths over bottom depths of 80 - 110 m in the Delta area. The layer, which ascended to the surface at dusk, was mainly composed of small squid, juvenile lantern fish and small pelagic shrimps, "common" pelagic fish species contributed only insignificantly to the recordings. Isolated and small concentrations of anchovies and sardines were observed at several localities in the Delta area between the 20 and 50 m contour lines. Further offshore the pelagic fish recordings were scarce and originated from small schools of scad (Decapterus sp.) and mackerel.
During the first cruise very dense concentrations of pelagic fish were observed within a limited nearshore area off Tavoy at bottom depths between 20 and 50 m. The fish, mainly anchovies, sardinella and mackerel, were recorded as dense schools from the surface down to 20 - 30 m depths. Some of the schools were excellent sonar targets and were recorded at ranges of 1500 - 2000 m. The largest were estimated to amount to several hundred tonnes of fish. During the second cruise the recordings in this area were considerably more scattered. This might partly have been caused by a northward migration between the two coverages, as indicated by the high density patch at 14°N (Fig. 9b), and partly by the pelagic fish being distributed more close to the bottom during the last coverage when a bottom trawl haul in the area gave a catch rate of 10 tonnes per hour, of anchovies (Thryssa sp.).
Further southward along the coast of Tenasserim, recordings of pelagic fish were scarce and scattered. During cruise no. 1 some small schools of scad (Decapterus sp.) were observed in offshore areas. In the near shore areas/surface schools of tuna-like fish were quite frequent south of 12°N (Fig. 6a).
On average the catch rates of pelagic fish were low, both in the pelagic and in the bottom trawl hauls, less than 40 kg per hour in the Delta area and less than 70 kg per hour along the Tenasserim coast (Tables 4 and 7).
Length distributions of pelagic fish in the Delta and Tenasserim areas are given in Tables 5 and 6, 8 and 9.
3.6.1 The Arakan coast
3.6.2 The Delta area
3.6.3 The Tenasserim coast
Figs. 10 and 11 show the distributions of echo integrator values originating from demersal fish. The values in brackets were arrived at in the same way as for pelagic fish.
Tables 1, 4 and 7 present average catch per hour trawling of each fish family (or group) according to depth in the three areas.
(Fig. 11a, Table 1).Echo recordings of demersal fish were limited to waters with bottom depths shallower than 50 m. In deeper waters echo traces indicating demersal fish were seldom observed and the bottom trawl catches were quite low (Table 1). The increased catch in the depth statum 76 - 150 m in Table 1 was mainly caused by one haul on the northern border of the area which amounted to 470 kg/hour. The mean value over four hauls - excluding the highest - amounted to 55 kg/hour, a figure which fits better in with the values obtained both in the deeper and shallower depth slices. Thus, both the echo-recordings and the catches indicated that the highest concentrations of demersal fish were found in the nearshore shallow water areas and that the densities became quickly reduced with increasing depth or distance from the shore.
The composition of the catches is given in Table 1. Grunts (Pomadasyidae), pomfrets (Stromateidae) and croakers (Sciaenidae) made up the bulk of the food fish in the catches, but also ponyfish (Leiognathidae) and sharks and rays contributed largely. The length distributions (Table 3) show that small-sized fish were predominant both for grunts and croakers.
(Fig. 11b, Table 4).Comparing the two distribution maps in Fig. 11 it may look as if the recordings of demersal fish in the Delta area were more scattered than off the Arakan coast. The main difference between the integrator outputs in the two areas was that the Arakan coast was characterized by relatively high concentrations within a few limited areas, while in the Delta region the demersal fish recordings represented by "medium" integrator outputs 5 - 15 mm/n.mil covered considerably larger areas, thus resulting in higher indices of total abundance of fish. This was also reflected in the trawl catches with bottom trawl (Table 4), which showed considerably higher values than those experienced off the Arakan coast, particularly within the depth zones deeper than 26 m. The predominant species in the catches were croakers, sharks and rays and polenemus, while ponyfish and grunts contributed much less than off Arakan. Length distributions of the fish are given in Tables 5 and 6.
(Fig. 11b and Table 7).The observations indicated that the densities of demersal fish along the Tenasserim coast were considerably less than in the Delta area. The bulk of the catches in bottom trawl hauls was ponyfish (Table 7), while croakers, grunts, snappers and other quality fish were caught in much less quantities than in the two other sub-areas. However, the coverage within the large land-locked areas of the Tenasserim coast were poor during both surveys, and the results arrived at with "Dr. Fridtjof Nansen" may not reflect the true picture in the many fjords and straits in the area. Crocodile fish (Peristedion weluri) were observed in many catches between 150 and 300 m, but the average catch rate of 140 kg/hour in Table 7 was mainly contributed to by one large haul.
Length distributions of the fish are shown in Tables 8 and 9.
3.7.1 Shrimps in shallow waters
3.7.2 Deep sea shrimps and lobster (Puerulus sewellii)
Off the Arakan coast the catches of shrimps were strictly limited to nearshore shallow waters. The best catch rate recorded was 70 kg/hour at 15 m depth just north of Sandoway. The shrimp catches decreased rapidly with depth and distance from the coast and at 25 - 30 m bottom depth the catch rates were between 0 and 10 kg/hour. An intensively trawled area near Sandoway gave an average catch rate of 13 kg/hour. For the total Arakan coast the average was 17 kg/hour within the depth zone 10 - 25 m (Table 1).
In the Delta area the average catch rates were higher than off Arakan within all depth zones (Table 4). But here the shrimp catcher in the shallow water areas - less than 75 m bottom depth - were dominated by small-sized white shrimps which have little or no commercial value, and catch rates up to 150 kg/hour of these animals were experienced. In the one deep water haul (Table 4) made within this area the catch composition was similar to the observations made in corresponding depths off Tenasserim.
The catch rates of shrimps in shallow waters off Tenasserim showed a similar trend as off Arakan; a marked decrease with depth and distance from the coast (Table 7).
As mentioned previously, a number of trawl hauls were carried out on a deep water trawling ground off the Tenasserim coast. The average catch rates obtained are presented in the last three columns in Table 7. Both deep sea lobster and shrimps were present in most of the hauls, and the best catches occurred at bottom depths between 260 and 330 m; the maximum catch rates being 28 kg/hour of lobster and 150 kg/hour of shrimps. Both the size and quality of the shrimps appeared to be commercially acceptable. The deep sea lobster varied considerably in length (Table 11) and the length distribution had two distinct modes. A number of the largest lobsters (> 13 cm) were ovigerous females.
3.8.1 Acoustic estimates
3.8.2 Estimates based on catch rates
Preliminary estimates of fish of fish biomass were out on the basis of the acoustic observations and the catch rates with the bottom trawl.
Indices of echo abundance were calculated for each sub-areas with the following results:
Indices of echo abundance (mm. n.m.2)
|
AREA |
PEL. FISH |
DEM. FISH |
||
|
CR. 1 |
CR. 2 |
CR. 1 |
CR. 2 |
|
|
Arakan |
29 300 |
78 800 |
28 200 |
36 500 |
|
Delta |
77 400 |
83 300 |
34 600 |
49 300 |
|
Tenasserim |
62 600 |
36 500 |
33 700 |
39 100 |
|
Total |
169 300 |
198 600 |
96 500 |
124 900 |
Now, since the scattering cross section or fish increases proportionally to length squared, and weight of fish increased proportional to the cube of the length, the scattering cross-section per unit weight decreases linearly with length. Adopting the above mentioned value for C, this gives:
where L is measured in cm.
The length frequency distributions (Tables 2, 3, 5, 6 and 8, 9) show large variations in length ranges, but it appears that most of both the pelagic and demersal specimens were small sized. Choosing an overall average length of pelagic fish to be 8 cm and the corresponding value for demersal fish to be 16 cm this lead to:
C (pel) = 2 tonnes/mm nm2 and C (dem) = 4 tonnes/mm nm2When these factors ware multiplied with the total abundance indices from cruise no, 2 we arrived at the following estimates of biomass:
Pelagic fish biomass = 400 000 tonnesThese estimates should be considered as a first approximation and used with utmost care. They are, as we have seen, extremely sensitive to length variations and more data on species composition and length distribution are needed in order to increase confidence in the results.
Demersal fish biomass = 500 000 tonnes
Most probably both estimates are too low. Due to the near bottom dead zone of the echo sounder - 0.5 to 1.0 m within most of the area - an unknown quantity of demersal fish was not recorded by the echo sounder. The larger pelagic species were recorded only to a minor extent, and the surface schools of small pelagics were not included in the estimate. It should also be kept in mind that, particularly in the Delta area and at the coast of Tenasserim, large shallow water and land-locked areas were not covered by the survey.
The catch rates in bottom trawl hauls showed large variations and although the number of hauls was quite high, their distribution cannot be considered even enough to allow estimation of biomass from catch rates for the entire coast. However, both within a limited area off Sandoway at the Arakan coast, and in the Delta area, the conditions for a reasonable estimate based on observed catch rates were fulfilled.
The Sandoway area extends 40 nautical miles along the coast from 18° N to 18°40' N between bottom depths of 15 and 35 m, the average seaward extension thus being approximately 18 nautical miles.
The Delta area extends 40 nautical miles in the north-south direction, extending 95° to 97° E, thus being 120 nautical mile long. The bottom depths vary from 20 to 90 m.
The details of the calculation are summarized below.
|
Area |
nm2 |
Number of hauls |
Average catch rates |
Swept area |
Density |
Biomass tonnes |
|
Sandoway |
700 |
12 |
420 |
0.03 |
13.2 |
9000 |
|
Delta |
5000 |
27 |
530 |
0.03 |
17.1 |
85000 |
The estimates of biomass arrived at represent the total catch composition in the bottom trawl hauls, which also include some pelagic species. But Tables 1 and 4 show that these contributions were low.
The two areas investigated represent only a small fraction of the total shelf waters of Burma, and it is difficult to judge whether the results support those arrived at from the acoustic data or not. Bearing in mind that the acoustic estimate amounts to 500 000 tonnes of demersal fish within the total investigated area, the amount 85 000 tonnes within the Delta area appears to be a bit low.
It should be noted that, these numbers include all species and size groups, and that more observations on species and length compositions within the different areas are needed in order to arrive at more reliable estimates of biomass, as well as estimates of possible yield. A more detailed analysis and discussion of stock sizes and potential yields will be undertaken after the spring survey in March - April.
