4 SURVEYS IN THE NORTHEAST INDIAN OCEAN AND SOUTH CHINA SEA (Contd.)

4.3 SURVEYS OFF BANGLADESH, PENINSULAR MALAYSIA, WEST THAILAND AND NORTH SUMATRA, 1979–80

Survey objectives and effort

Brief assignments of exploratory nature were undertaken in the Eastern Indian Ocean and the South China Sea, supplementary to the more extensive work in Sri Lanka and Myanmar. These included Bangladesh (two coverages), the east and west coasts of Peninsular Malaysia, the west coast of Thailand and the north and west coasts of Sumatra. They formed part of the general programme of resource evaluations started by the IOP, while the assignments in the South China Sea and Malacca Strait were made in response to a request from the UNDP/FAO South China Sea Fisheries Development and Coordination Programme (SCSP). The findings of these surveys were reported as follows: Bangladesh by Saetre (1981), Malaysia by Aglen et al. (1981a), Thailand by Aglen et al. (1981b) and Indonesia by Aglen et al. (1981c). Because of the limited coverage of the surveys, the results as regards the magnitude of the resources and their distribution should only be regarded as indicative. They were, however, of interest in these areas which at the time were sparsely researched. The results regarding faunistic composition may be of interest in general and for indicating the state of exploitation at the time.

The particulars for each of the surveys are listed in Table 4.33. The coverage included the shelf waters from about 10 m to about 200 m depth and, where conditions permitted, also the upper slope. The uncovered shallow inshore parts were extensive, especially in Bangladesh, and also on the west coast of Thailand. Off the west coast of Malaysia the survey extended seawards to the medium line between Malaysia and Sumatra. The degree of coverage for the acoustic survey varied considerably, but demonstrated an adequate or even excessive effort. Unfavourable monsoon weather restricted work during part of the Thailand survey.

Table 4.33 Survey data for Bangladesh, Malaysia, Thailand and Indonesia

Survey methods

Acoustic

As described in Chapter 2, the conversion factor C used in the acoustic method for estimating the biomass of fish from the observed integrator readings, was for all of these surveys based on a target strength of -36 dB/kg (for 17 cm fish). This represents a very low target strength and a 2 dB higher level, -34 dB/kg now seems more appropriate. This results in a reduction of the biomass estimates presented in the survey reports by Sætre (1981) and Aglen et al. (1981 a,b and c) by about one-third.

Trawl

For the swept-area estimates of demersal fish in Bangladesh, Sætre (1981) assumed a trawl efficiency coefficient of 0.5. In this review the coefficient is assumed to have been equal to 1 as was used for the corresponding estimates in all other surveys dealt with in this section.

Species identification was mostly based on Fischer & Whitehead (1974), Smith (1972), and Carcasson (1977). The records of the catch composition in the bottom trawl from the survey of Sumatra have been used by Bianchi (1996) in a study of the demersal fish assemblages in this area. Similar analyses of the corresponding data from Malaysia and Thailand are being prepared.

Bottom topography

Analysis of the echo recordings with reference to the suitability of the type of bottom for demersal trawling showed that the Bangladesh shelf was mostly smooth and even, but with a steep slope from 160–180 m onwards, and a steep canyon in the western part. The shallow shelves off eastern and western Malaysia were also generally smooth, but interspersed with rocks and corals at certain depth ranges. Off western Thailand the middle main shelf was found to be mostly smooth, but the shelf edge was rough and there were rocks and corals inshore. Off north and west Sumatra rough bottom with rocks and corals and steep slopes prevailed.

Hydrography

The most notable features of the hydrographic observations from Bangladesh were the low surface layer salinities, especially in November-December brought about by the river runoffs, and the lifting of the transitional layer from about 70 m depth in November to 30–40 m in May. It must be expected that the distribution of the demersal fish was affected by this change which brought oxygen deficient water up over the shelf to a depth of 20–30 m. The shoreward tilting of the isopleths in May indicates upwelling which may enhance the primary productivity in the Bay of Bengal.

Off the west coast of Thailand the thermocline was found at about 100 m depth and did not enter the shelf. Tropical surface water also covered the shelf off west and east Malaysia. The shelf off north and west Sumatra was fully tropical with the thermocline located at 100 m depth and more off the shelf edge.

4.3.1 Bangladesh, 1979–80

Figure 4.16 shows the survey area and the coverages in the 1979 survey.

Figure 4.16 Bangladesh: Survey area, course tracks and stations in November-December 1979

Biomass estimates of pelagic fish

Two acoustic coverages, in November-December 1979 and May 1980 show no substantial concentrations of small pelagic fish over the part of the shelf covered. The general finding was a patchy and low-density distribution. Relatively high catch rates of clupeids and engraulids in the shallow depth range 10–24 m indicated, however, that the densities of these fish might have been higher in the extensive areas inside 10 m depth which could not be covered by the survey.

The acoustic estimates of the biomass of pelagic fish over the shelf covered were: 38,000 t in November-December 1979 and 76,000 t in May 1980. The part of the shelf in Bangladesh inside 10 m depth which could not be covered by the surveys is very extensive: about 7,000 nmi². If it is assumed that the density of pelagic fish here was the same as in the area between 10 and 100 m depth (about 9,400 nmi²) where the pelagic fish was observed, then raised totals of 66,000 t in November-December and 133,000 t in May would represent the whole Bangladesh shelf. These are likely to be underestimates because of the generally negative bias of this first generation of acoustic equipment.

Seasonal changes in the biomass of short-lived small pelagic fish are well known. The biomass would be expected to be highest during and after the season of high primary production, which in Bangladesh would be associated with the northeast monsoon, and this thus fits the observations made. The surveys indicated that the exploitable stock of small pelagics was in excess of 130,000 t at its peak.

Only a few of the 23 hauls with the pelagic trawl had catch rates which indicated that the target fish had been successfully sampled. Pelagic fish appeared, however, in many of the catches of the bottom trawl even to the depth range 75–99 m, and the data from the 111 hauls with this gear represent a better source for evaluating their composition. Table 4.34 shows a largely similar pattern in each of the surveys, the scombrids, carangids and clupeids formed more than three-quarters of the catches of the pelagics. Indian mackerel (Rastrelliger kanagurta) gave the high scombrid catches, while round scad (Decapterus maruadsi) was the most common carangid. Ilisha megaloptera and Sardinella fimbriata dominated the clupeids.

Table 4.34 Bangladesh: Composition of pelagic fish by families. Mean catch rates in bottom trawl by surveys (kg/h)

Biomass estimates of demersal fish

The acoustic recording of semi-demersal fish showed only low densities and gave little information on their distribution over the shelf. The distribution of mean catch rates in the bottom trawl by depths, see Table 4.35, showed a marked difference between the surveys. In November-December 1979 relatively high catch rates of demersal fish were obtained down to a depth range of 75–99 m, while in May 1980 the rates were low beyond 25 m depth with an increase in the 10–24 m range. The overall mean catch rate of demersal fish in the May survey was less than half of that obtained in November-December. It seems likely that this change was caused by the observed of influx of oxygen-deficient water, present over a large part of the shelf in May 1980, an environmental effect which is well known from the Southwest Coast of India and other parts of the western Indian Ocean.

Table 4.35 Bangladesh: Mean catch rates in bottom trawl by depth ranges and surveys; Survey 1: Nov-Dec 1979; Survey 2: May 1980 (kg/h)

Table 4.36 Bangladesh: Depth distribution of most common families of demersal fish. Mean catch rates in bottom trawl by depth ranges and surveys; Survey 1: Nov/Dec 1979; Survey 2: May 1980 (kg/h)

The mean catch rates by the more common families are shown for depth ranges in Table 4.36. Catfishes, croakers, threadfin breams and, in shallow water Bombay duck, represented about three-quarters of the demersal fish in both surveys. The lower catch rates beyond 25 m depth in the second survey are evident for most groups. Rates were low beyond 100 m depth also in the first survey, demonstrating the limit of the productive surface layer. Among the key species were the catfish (Arius thalassinus), the Bombay duck (Harpadon nehereus), the croaker (Chrysochir aureus) as the most abundant Sciaenidae and the threadfin bream (Nemipterus japonicus).

Table 4.37 shows the biomass of the fish identified as demersal (Table 4.36) estimated from the catch rates of the bottom trawl by the swept-area method. The reduction in the total estimates from 109,000 t to 47,000 t between the surveys was then presumably caused by the avoidance by the demersal fish of oxygen deficient bottom water. The high density in the 10– 24 m range in the May survey is additional evidence for this movement of the demersal fish.

Table 4.37 Bangladesh: Density and biomass of demersal fish by depth ranges

The abnormally high density in the 50–74 m depth range in November-December 1979 is due to a single 4 t/h catch. If this station is excluded, the biomass estimate for this survey is reduced to about 80,000 t. An adjustment should be made for the inshore part of the shelf not covered by the surveys. Assuming that the density in these shallow parts is the same as that observed in the adjacent depth range 10–24 m, the additional biomass would be 55,000 and 78,000 t in the first and second survey respectively. The total estimates then become roughly similar: 135,000 t and 125,000 t. If the average of these is accepted as the best estimate, the standing stock of demersal fish of 130,000 t for the shelf proper down to 100 m depth was observed to have a mean density of about 8 t/nmi².

Review of findings

An overview of biomass estimates by major groups is shown in Table 4.38. The total marine fishery resources of Bangladesh were thus estimated at 280,000 t with a density of 17 t/nmi² over the shelf to 100 m depth. The main uncertainty attached to these results relates to the extensive shallow inshore part of the shelf, representing 43% of the shelf area to 100 m depth, from which there were no observations and where fish densities were assumed to be equal to those of the adjacent outer shelf.

Table 4.38 Bangladesh: Overview of abundance estimates of pelagic fish from acoustic surveys and other resources from bottom trawl survey.

4.3.2 East coast of Peninsular Malaysia, 1980

Figure 4.17 shows the area of investigation with course tracks and stations. The coastal area, defined as that of 10–25 m depth, is about 6,000 nmi² and the offshore area, 26–75 m depth, is 28,000 nmi².

Figure 4.17 Malaysia, east coast, 10–25 June 1980: Survey area and course tracks and stations

Pelagic fish

In the acoustic survey small concentrations of pelagic fish were found near the shore at 10–25 m bottom depth, while the recordings in the offshore area were very scattered. In some areas much of the pelagic recordings were squid. Schools of pelagic fish were often seen close to the bottom and the trawl catches usually gave a mixture of pelagic and demersal fish.

Table 4.39 shows the catch rates of pelagic families in the bottom trawl and confirms their inshore distribution. The mean catch rates were low with carangids and the semi-demersal leiognathids dominating. The total species diversity was very high with for instance more than 20 species of carangids. Of these, yellowstripe trevally, (Selaroides leptolepis) dominated, while bigeye scad (Selar crumenophthalmus) was also common. Longfin mojarra (Pentaprion longimanus) was a common gerreid species.

Table 4.39 Malaysia, east coast: Mean catch rates of main pelagic families in bottom trawl by depth ranges (kg/h)

Table 4.40 Malaysia, east coast: Mean catch rates of main demersal families and other demersal groups in bottom trawl (kg/h)

Demersal fish

Acoustic recordings of fish identified as semi-demersal were low. The composition of the main families in the bottom trawl catches is shown by depth ranges in Table 4.40. There was a high diversity of species (about 110 identified species from 60 families). Catch rates were highest in the coastal area and reduced to about one-third offshore. Catfishes, snappers, goatfishes and threadfin breams were the most abundant. Catches of squid were common in the offshore area.

Biomass estimates

The acoustic estimates were based on mean integrator values and mean fish size for each of six sub-areas of the shelf. Table 4.41 shows the abundance of pelagic fish for the inner and outer parts of the shelf. The highest densities of pelagic fish and the main part of their biomass were found on the inner shelf. Semi-demersal fish were recorded with low densities in both parts.

Table 4.41 Malaysia, east coast: Abundance estimates and biomass of pelagic fish from acoustics and of demersal fish from swept-area trawl data

Table 4.41 also shows that the densities of demersal fish estimated by the swept-area method from the bottom trawl hauls are by far highest in the coastal area, 10–25 m depth.

The total fish biomass estimated at the time of the survey was thus about 280,000 tonnes.

4.3.3 West coast of Peninsular Malaysia, 1980

Figure 4.18 shows the area of investigation and the course tracks covering the shallow shelf which hardly reaches more than 70 m depth.

Figure 4.18 Malaysia, west coast, 5–15 July 1980: Survey area and course tracks

Pelagic fish

Pelagic fish were recorded over most of the shelf, but in low densities except for the coastal area north of Penang and smaller patches further south.

As on the east coast a considerable part of the bottom trawl catches consisted of pelagic fish. Table 4.42 gives the mean catch rates of the main pelagic families and shows that about two-thirds of the pelagic fish was caught inside 50 m depth.

Carangids dominated the true pelagics and, with 13 identified species only of this family, the diversity was also here high. The yellowtail scad (Atule mate) and the yellowstripe scad (Selaroides leptolepis) were the most common. Indian mackerel (Rastrelliger kanagurta) dominated the scombrids.

Table 4.42 Malaysia, west coast: Mean catch rates of main pelagic families in bottom trawl by depth ranges (kg/h)

Demersal fish

The acoustic recordings of fish identified as semi-demersal were weak and showed little variation throughout the area. The composition of the main families in bottom trawl catches is shown in Table 4.43. With about 60 species representing 30 families the diversity of species was not as high as on the east coast. For most groups there was a reduction of mean catch rates with depth, but with one haul at 90 m giving an unusually high rate of big eye. Grunts, croakers and hairtails were the most common groups inshore and snappers, threadfin breams and bigeyes offshore.

Table 4.43 Malaysia, west coast: Mean catch rates of main demersal families and other groups in bottom trawl by depth ranges (kg/h)

Biomass estimates

The acoustic abundance estimates were based on mean integrator values and fish size calculated for 5 sub-areas. The mean biomass densities of pelagic fish were as shown in Table 4.44 highest on the inner part of the shelf ranging out to 30–50 nmi offshore. The acoustic recordings of semi-demersal fish did not show such a trend and were in general low.

Table 4.44 Malaysia, west coast: Abundance estimates of pelagic fish from acoustics and of demersal fish from swept-area trawl data

The densities of demersal fish from swept-area calculations did not demonstrate any trend by depth. Sampling in shallow water was, however, sparce due to limitation on trawl operations caused by widespread commercial fishing.

The mean density of pelagic fish on the west coast shelf, 11 t/nmi² is somewhat higher than that found on the east coast, 6 t/nmi². Since the biomass of small pelagic fish is often found to have seasonal fluctuations, the difference in the observed densities may not represent differences in annual production. The density of demersal fish was about the same on both shelves.

4.3.4 West coast of Thailand, 1980

Figure 4.19 shows the survey area with cruise tracks and stations. Strong monsoon winds, rough bottom on parts of the shelf and commercial fishing inshore restricted some of the survey activities.

Figure 4.19 Thailand, west coast, 16 July-3 August 1980: Survey area and course tracks

Pelagic fish

The acoustic survey showed a patchy distribution of pelagic fish with moderate abundance in three areas: in the north near the Myanmar border, inshore south of Phuket and in the south near the border with Malaysia. The recordings showed that pelagic or semi-pelagic fish such as carangids were often found close to the bottom during daytime, while at night they occured as scattering layers in mid-water.

Four out of a total of 16 hauls with pelagic trawl had apparently successful catches of the target species, mostly engraulids and clupeids. Pelagic fish appeared in the bottom trawl catches in the proportions shown in Table 4.45. Some 25 carangid species were identified of which yellowstripe scad (Selaroides leptolepis) and Indian scad (Decapterus russelli) were the most common. Indian mackerel (Rastrelliger kanagurta) dominated the scombrids. It seems likely that engraulids and clupeids are under-represented in the bottom trawl catches.

Table 4.45 Thailand, west coast: Mean catch rates of main pelagic families in bottom trawl by depth ranges (kg/h)

Demersal fish

Semi-demersal fish were recorded in the same geographical areas as the pelagic fish. The composition in the bottom trawl catches of the main families of demersal fish is shown in Table 4.46. The diversity of the demersal fauna was high with more than 150 species from about 80 different families. Snappers, goatfishes, threadfin breams and hairtails were the most common families. The catch rates were highest in the 26–50 m depth range, but sampling in the shallow range 10–25 m was probably inadequate and biased. Some hauls at greater depths in the slope showed the presence, although in low abundance, of deep water shrimp and lobster as found further north off Myanmar (see Section 3.5).

Table 4.46 Thailand, west coast: Mean catch rates of main demersal families and other groups in bottom trawl by depth ranges (kg/h)

Biomass estimates

The acoustic estimates were based on mean integrator values and mean fish size for a northern, central and southern parts of the shelf divided at the latitudes 8° 20' and 7° 20'. Table 4.47 shows that the density of pelagic fish was highest over the central shelf. The acoustic estimate of fish identified as semi-demersal was 26,000 t, the same as that obtained for demersal fish from the swept-area calculations. It seems likely that inadequate and biased sampling of the shallow waters (10–25 m depth) caused an underestimate of the demersal fish in this range.

The west coast of Thailand has extensive inshore areas of less than 10 m depth which could not be covered by the survey. These represent about 20% of the shelf area, and if it is assumed that the mean fish density here was the same as that estimated for the outer shelf, the survey's estimate of total biomass was about 125,000 t.

Table 4.47 Thailand, west coast: Abundance estimates of pelagic fish from acoustics and of demersal fish from swept-area trawl data

4.3.5 North and west coast of Sumatra, 1980

Figure 4.20 shows the survey area, the course tracks and stations. The investigations were mostly limited to the generally narrow shelf (10–100 m) depth with some bottom trawling also in the deep slope (200–350 m). The shelf edge was often found to be marked with shallow grounds and coral reef peaks extending in some places almost to the surface. Particularly south of Kutanibong, reefs made navigation difficult and caused an uneven coverage. Rough bottom was in fact predominant over large parts of the survey tracks and the demersal fish fauna related to this type of bottom was no doubt undersampled.

Pelagic fish

Pelagic fish tended to stay close to the bottom during daytime and raised slightly and scattered at night. Real high densities of fish were not observed in any area, but the best recordings were made on the shelf inside the islands south of latitude 2°N.

The recordings of pelagic fish were seldom sufficiently above bottom to be identified by pelagic trawling, but catches in the bottom trawl provided clues to the identification. As shown in Table 4.48 the semi-pelagic ponyfishes dominated which may explain the proximity of the recordings to the bottom. There were also some carangids and scombrids. Species diversity was high with more than 20 carangid species among which Malabar cavalla, (Carangoides malabaricus) and bigeye scad (Selar crumenophthalmus) dominated.

Figure 4.20 Sumatra, north and west coast, 6–30 August 1980: Survey area and course tracks

Table 4.48 Sumatra, north and west coasts: Mean catch rates of main pelagic families in bottom trawl by depth ranges (kg/h)

Demersal fish

The geographical distribution of the recordings of fish identified as semi-demersal was similar to that of the pelagic fish. The composition of the main demersal groups in the bottom trawl catches is shown in Table 4.49. The main part of the catch was taken at depth shallower than 75 m. Snappers, grunts, barracudas and hairtails were among the most important groups. The diversity of the bottom fauna was high with more than 160 species from about 80 families.

Table 4.49 Sumatra, north and west coast: Mean catch rates of main demersal families and other groups in bottom trawl by depth ranges (kg/h)

Biomass estimates

The acoustic estimates were calculated from mean integrator values and mean fish size in four areas: the north shelf; the northwest shelf to latitude 3° 30'N; the shelf around the islands; and the inner shelf south of Singkilbaru. The seaward limit was set at the outer limit of commercial fish recordings. Table 4.50 shows that the density of pelagic fish was highest on the inner shelf in the southern part of the survey area.

In the trawl survey the highest density of demersal fish was found in the 26–50 m depth range. Many parts of the shelf were, however, undersampled because of rough bottom and it seems likely that the density of demersal fish and their biomass was underestimated by the survey.

Four trawl hauls on the deep slope (230–350 m) showed a mean catch rate of 143 kg/h with bigeyes (Priacanthidae), greeneyes (Chlorophthalmidae), deep water sharks and shrimp dominating.

The total biomass within the surveyed area was estimated at about 225,000 t.

Table 4.50 Sumatra, north and west coast: Abundance estimates of pelagic fish from acoustics and of demersal fish from swept-area trawl data