3 SURVEYS IN THE ARABIAN SEA AND ADJACENT GULFS (Contd.)

Discussion of estimates of abundance

The biomass estimates of mesopelagic fish obtained in the northwest Arabian Sea with the acoustic integration technique show, according to Gjø;sæter and Kawaguchi (1980), mean densities of roughly an order of magnitude higher than found with micronekton tows and other methods elsewhere in the world's oceans. Its seems, however, likely that nekton tows and a method based on acoustic resonance estimation would give underestimates of the true densities.

The applicability of the acoustic integration technique to these very small sized fish was discussed in several of the reports, e.g., Gjø;sæter (1977), Gjø;sæter and Kawaguchi (1980); and comparison attempted with other observations of densities such as from catch rates as in Aglen et al. (1982) and Schärfe (1983). These comparisons showed reasonable correspondence with estimated densities from acoustics for fish in the D1 layer, but not in the D2 layers where catches were very low despite observed high acoustic densities. The “blurred” or “smoky” or “fluffy” appearance of the echosounder recordings from the D2 layer is noted in several reports. Therefore, an important question concerning the data is whether resonance phenomena may have affected the acoustic estimates.

A contribution to this discussion was submitted by Dalen and Ona (1984): “Resonance effects in acoustic population estimates of mesopelagic fish” (in Norwegian). This short paper considers the theoretical conditions for a possible depth relation in the backscattering sound energy from Benthosema pterotum, the main species in the mesopelagic fauna in the Gulf of Oman. The size of the swim bladder of this physoclist fish was assessed from shock-frozen specimens. The resonance frequency and its depth relationship will depend on the ability of the fish to compensate pressure-lost swim bladder volume through gas production. Gas production and absorbtion mechanisms have been demonstrated in this species, but it seems highly unlikely that the downward migration which occurs at speeds up to 4 m/min can be fully compensated. That would need a gas production capacity some 250 times greater than measured in other species with well developed secretion. Concerning the issue of energy budget, full compensation seems unlikely.

As a likely case the authors computed and demonstrated the resonance frequency/depth relationship for 30 mm fish assuming 1) a downward migration without compensation, 2) a process of compensation at an unknown rate and 3) completeness in the deep-layer and upward migration with full compensation (Figure 3.19).

Figure 3.19 Relationship between depth and resonance frequency for likely strategy of depth adaptation for Benthosema pterotum of 30 mm length. Adapted from Dalen and Ona (1984)

The paper concluded that:

• The swim bladder in the smallest fish (<20 mm) can give resonance effects in the D2 layer irrespective of strategy for adaptation.

• The high speed of descent probably results in compression of the swim bladder, and fish at certain lengths can then give resonance effects over a wide frequency spectre at depths from 100 to 400 m.

• The very low apparent trawl efficiency found in fishing experiments in the D2 layer may indicate that the acoustic estimate of this layer is too high due to resonance.

The authors consider that resonance effects in the echoes easily can result in overestimates of the biomass of 2–10 times.

The paper ends with a proposal for an instrumentation with which resonance effects could be tested, but there has been no opportunity to implement it as IMR has not been involved in any subsequent investigations of mesopelagics.

Although an instrumented field test must await a future opportunity, it is possible to use some data derived from acoustic measurements of fish density from the various depth layers in a preliminary check on whether resonance is likely to have occurred.

If resonance primarily affects the deep-layers (D2 and N2) it should be expected that the total night estimates would be lower than the day estimates since the diurnal migrations usually bring a large part of the deep-layer biomass up into the shallower N1 layer. Gjø;sæter (1977) who covers the 1975–76 surveys, found no significant difference between the day and night densities in a selection of sections, but with observations from dusk and sunrise omitted.

Consistent lower night densities were, however, evident from the Gulf of Oman where repeated surveys were made covering the main area of the Gulf of Oman three times and a smaller selected test area six times in 1981 and the main area twice in 1983. Assuming that the difference between the day and night observations in each of these sets of data is the effect of the diurnal vertical migration between the deep and the shallower layers, the decline in an unbiased estimate of density in the deep-layer from day to night should be compensated by a corresponding increase of density in the shallower layer.

If, however, the backscattering from the fish in the deep-layers was affected by resonance, the increase of the N1 over the D1 (N1 minus D1) layer would be less than the decrease of the D2 layer, from D2 to N2 (D2 minus N2) and the rate between the two would be a measure of its effect.

The results of an analysis based on Tables 5 and 6 from Aglen et al. (1982) and Table 4 from Gjø;sæter and Tilseth (1983) are presented in Table 3.22.

Table 3.22 Comparison between the increase of N1 layer and decrease of D2 layer backscattering levels due to diurnal migration of mesopelagic fish (units mm of integrator deflection)

This analysis indicated that measurements in the deep-layers increased the echo abundance, in units of integrator deflection in mm, by 1.6–3.5 times with a mean of 2.2.

Gjø;sæter and Tilseth (1983) discussed the possible reasons for the higher day than night densities observed in 1981 and 1983. A distribution close to the surface at night above the hull-mounted echosounder transducer would have such an effect, but tests with a towed upward-looking transducer did not confirm such a behaviour as a general phenomenon. Loss by too high threshold settings was also mentioned, but this would generally be expected to be greatest in the deep-layer.

The case may be summarized as follows:

• There are indications from well conducted surveys in the areas of highest concentrations of mesopelagics in the Gulf of Oman that the volume backscattering coefficient from the D2 layers is from 1.6 to 3.5 times higher than from fish in the shallow layer.

• There is a theoretical basis for relating this to resonance effects in the deep-layer.

• There is support for this in the low catch rates and apparent low catching efficiency in the deep-layers.

• The echograms from the deep-layers are reported to be different from those in the shallower layers, by being more “smoky” or “fluffy” in appearance.

It seems thus reasonable to strongly suspect that acoustic biomass estimates of mesopelagics may be influenced by resonance in the deep-layer recordings. Theoretical considerations show that resonance may also occur at shallower depths, especially for small-sized fish < 20 mm. This may perhaps explain why Gjø;sæter (1977) found no statistical difference between day and night recordings in observations from a series of sections during the 1975–76 surveys.

The observed variation of the assumed resonance effect for the data analysed may perhaps be related to differences in the size distributions of the fish, the depth of the recordings and the time of day when the main recordings are made.

The often high variance found in the biomass estimates from repeated surveys could more easily be explained in terms of variance in resonance effect than by actual short-term changes in the biomass or by survey variability.

On the other hand, the acoustic densities observed in the D1 layer seem to have been verified in a general way by the comparisons made between acoustic estimates and densities estimated from results of fishing experiments. Using swept-water-volume estimates from the area of the trawl mouth opening and towing distance Aglen et al. (1982) estimated the apparent trawl efficiency as follows: catch rate in g/m³ filtered water in relation to acoustic density in g/m³. They obtained mean ratios of 0.51 and 0.77 for D1 layers in 6 and 23 hauls in 1979 and 1981 respectively. In 1983 tows with special trawls enlarged with large-mesh frontal parts showed a mean ratio of apparent efficiency of 0.44, measured in a similar manner in 27 trials on reasonably good D1 layers.

The true catchability coefficient of the trawls for these small-sized fish is unknown. By definition it can not exceed 1, and about 0.5 is perhaps not an unreasonable level. If the true catchability is lower, the acoustic density would be an underestimate of the true density. It may perhaps be concluded that the fishing trials in a general way confirm the acoustic estimates as a minimum estimate of the fish density in the D1 layer.

Revision of biomass estimates

Although adjustment for bias caused by resonance in the D2 layer is still an uncertain process in view of the limited information available and the variation in the observed apparent effects, it still seems a worthwhile exercise to revise the biomass estimates.

Gulf of Oman (Sub-area A)

The first review was made of the biomass estimates from the best investigated area, the Gulf of Oman. Figure 3.20 adapted from Gjø;sæter and Tilseth (1983) shows the reported total estimates which range from 6 to 15 million t. The time varied gain function (TVG) of the echosounder is of special importance for fish recorded at great depths, and as reported in Chapter 2, when a new TVG card was installed in 1980, the old function was discovered to have drifted. This can have caused some of the variability in the data, but the early high short-term changes with a range of more than 50% seem to reflect real changes in biomass.

From 1981 on the TVG was controlled and the contributions from the various types of scattering layers were reported separately.

Three successive coverages in 1981 gave estimates of 8, 11 and 13 million t with a mean of 10.7 million t. On the assumption that backscattering from the deep-layers is doubled by resonance, the details of the contributions from the various layers indicate that estimates adjusted for the bias caused by resonance would be reduced by 69%, 65% and 72% respectively, which gives 5.5, 7.0, 9.4 million t and a mean of 7.3 million t.

Compared to 1981, the two acoustic coverages in 1983 showed a reduced abundance of 8.0 and 4.7 million t. Reducing the deep-layer contributions to half gives corrections to 70% and 78% respectively resulting in 5.6 and 3.7 million t a mean biomass estimate of 4.6 million t.

Furthermore, in both the 1981 and the 1983 estimates it had been assumed that the densities on the Iranian side of the Gulf of Oman were the same as those on the surveyed Omani side. If it is lower, as shown in Table 3.21, there would be a need for another adjustment reducing the mean values of 7.3 and 4.6 million t to 6.4 and 4.1 million t respectively, or an average abundance close to 5 million t for the entire Gulf of Oman.

Revisions of the assumed target strength for the mesopelagic fish might lead to further adjustments. The levels assumed in 1981 and 1983 were -35.2 dB/kg and -34.2 dB/kg (for 17 cm fish) respectively (Aglen et al., 1982; Gjø;sæter and Tilseth, 1983). These were based on an average target strength of 10 log L - 22 dB/kg fish and comparable to the value of -28.4 dB/kg for 5.2 cm fish applied in the surveys with the R/V LEMURU of mesopelagics in the area (Lamboeuf and Simmonds, 1981). This corresponds approximately to the relationship TS = 20 logL - 72 dB as commonly used for small pelagic fish. Direct observations of the target strength of the very small-sized mesopelagic fish are not available. Keeping in mind the reasonable correspondance between the acoustic densities of the D1 layers and those estimated from the fishing experiments, any further adjustment should await new information on the target strength of mesopelagic fish. Therefore, there is no reason to further adjust the assessment of 5 million t of mesopelagic fish in the Gulf of Oman in 1981– 83. From an examination of the unadjusted estimates shown in Figure 3.20 it does not seem unreasonable to accept this estimate as representing an average level for the Gulf of Oman.

Figure 3.20 Estimated abundance of mesopelagic fish in the Gulf of Oman 1975–1983, unadjusted. Adapted from Aglen et al. (1982)

Estimates of the precision of the survey estimates have not been made. The degree of coverage for the surveys in the Gulf of Oman was as follows:

The 1975–76 data include coverage of the shelf on each side (see Section 3.3). These indices indicate a reasonable survey effort in the Gulf of Oman for the type of distribution represented by mesopelagic fish.

Gulf of Aden (sub-areas E and F)

The first series of surveys indicated that the Gulf of Aden could also be expected to have a high abundance of mesopelagic fish and therefore the special surveys in 1979 and 1981 also covered this area. Figure 3.21 shows biomass estimates ranging from 4 to 27 million t. However, the area of the Gulf of Aden is almost three times that of the Gulf of Oman and the fish densities are thus lower. A few high catch rates were obtained in the 1975–76 surveys, but the more extensive fishing trials in 1979 and 1981 gave only low catch rates with means of 40–50 kg/h. This may in part be related to the fact that the D1 layer is non-existent or weak. Also a change in species composition seemed to have occurred with a reduction of the proportion of Myctophidae from more than 90% in the early surveys to only 40–60% of the total biomass of mesopelagics in 1979–81. Furthermore, the species composition seems to have varied between the layers in the later surveys.

Diurnal vertical migration also characterizes the mesopelagics in the Gulf of Aden and with the additional phenomenon of the weak D1 layer, the bias in the acoustic abundance estimate may have been greater than in the Gulf of Oman. The wide variation in abundance estimates in the more recent surveys - 4 million t in 1979 and 16 million t in 1981 - is not supported by changes in catch rates which were insignificant in both years. A comparison of catch rates from the N1 layer in the Gulf of Oman and the Gulf of Aden shows a mean of 276 kg/h of Myctophidae in 23 hauls in the Gulf of Oman, but only 26 kg/h as a mean of 8 hauls in the Gulf of Aden, a ratio of about 10. The mean acoustic densities observed in the N1 layers in the two areas show a ratio of 1.5. Thus whereas the fishing experiments in the Gulf of Oman in general provide evidence which support the existence of a considerable density of mesopelagic fish, this is not the case for the fishing trials in the Gulf of Aden apart from a few hauls of the early surveys, and it seems reasonable to conclude that the acoustic abundance calculated for this area have been affected by a bias causing a substantial, but uncertain rate of overestimate.

Figure 3.21 Estimated abundance of mesopelagic fish in the Gulf of Aden 1975–81 (adapted from Aglen et al., 1982)

Pakistan (sub-area B)

In the five 1975–76 surveys the estimates of biomass of the mesopelagics in an area of roughly 47,000 nmi² off Pakistan ranged between 5 and 8 million t with a mean of 6.6 million t. The corresponding mean density was 70 g/m² against 147 g/m² in the Gulf of Oman (Gjø;sæter, 1981).

The catch rates from the Pakistan surveys confirmed the existence of mesopelagics at high densities. The number of hauls with catches exceeding 0.5 t/h was:

The most common species was Benthosema pterotum, but various Diaphus spp. and Benthosema fibulatum were important components of some of the catches.

A rough correction based on theoretical evaluation and observations from the Gulf of Oman would be to halve the original figures giving a total biomass in Pakistan waters of about 3 million t and a mean density of 35 g/m².

Northwest Arabian Sea (sub-areas C, D, G, H and I)

There is finally a need to discuss the estimates of total biomass of mesopelagics in those parts of the northwest Arabian Sea covered by the first surveys. Figure 3.22 shows densities by subareas in the four surveys from autumn 1975 to end 1976 with data from Table 1 of Gjø;sæter and Kawaguchi (1980). Relatively high mean densities were recorded in several of the areas, some extending far seawards in as in sub-areas C and G. Outside the Gulf of Oman the mesopelagics were found only as a D2 layer between 250 and 350 m during the day and migrating to a surface N1 layer at night. Highest densities were found near the edge of the shelf where high catch rates (up to 20 t/h) confirmed a high abundance of Myctophidae in the area as shown in Figure 3.18.

Figure 3.22 Mean acoustic densities (g/m²) by sub-areas in the four main surveys autumn 1975 to end 1976
(unadjusted data from Gjø;sæter and Kawaguchi, 1980)

Based on the mean densities of the four surveys, an average biomass level of 78 million t for the total area outside the Gulf of Oman was obtained. It is reasonable to suspect that also outside the Gulf of Oman the acoustic estimates were biased by resonance. Assuming a similar bias as deduced for the Gulf of Oman would reduce the estimate for the northwest Arabian Sea by about one-third to 52 million t.

However, another approach would be to assume that the 1983–84 estimate from the special surveys of the Gulf of Oman of 5 million t is an improvement on the mean estimate of 11.8 million t from this area obtained in the four main surveys from autumn 1975 to autumn 1976. The original estimate of the mean abundance of mesopelagics in the areas outside the Gulf of Oman in these four surveys was 78 million t. An adjustment proportional to that applied above for the Gulf of Oman would reduce this to 33 million t.

Similar adjustments of the combined mean densities shown in sub-areas C, D and G in Figure 3.22 gave the following results: a reduction by one-third to compensate for resonance gives a mean density of 36 g/m² or 122 t/nmi². An adjustment based on the assumption that the 1983–84 surveys gave an improved estimate of the Gulf of Oman biomass as assessed in the 1975–76 surveys gives a mean density of 22 g/m² or 75 t/nmi².

Even if it is difficult to provide a more precise assessment of the total biomass of mesopelagic fish in the northwest Arabian Sea, there is no doubt that the area is characterized by a very high abundance of these fish. The best current estimate based on acoustics and supported by density estimates from fishing trials for the Gulf of Oman is 5 million t. For the vast highly productive oceanic area and including the Gulf of Aden where the assessment is more uncertain it seems reasonable to indicate a range of 30–50 million t.

Summary of revised biomass estimates

The revised biomass estimates and corresponding densities are presented in Table 3.23.

Table 3.23 Summary of revised biomass and corresponding densities of mesopelagic fish in the Northeast Arabian Sea (1975–84)

3.5 PAKISTAN, FURTHER SURVEYS, 1983–84

Part of the DR. FRIDTJOF NANSEN programme under the global UNDP/FAO programme GLO/82/001 “Survey and Identification of World Marine Resources” was to provide more detailed information on important areas covered by the first regional survey of the northwest Arabian Sea in 1975–76.

In 1975–76 Pakistan was included in the large-scale exploratory surveys of pelagic fish in the northwest Arabian Sea, while the programme in 1977 was a special joint effort with the Marine Fisheries Department of Pakistan and the Institute of Marine Biology, University of Karachi, with the objective of describing the fishery resources and their physical and biological environment. These first survey programmes not only covered the shelf, but also the adjacent oceanic waters (Table 3.24).

Survey objectives and effort

The 1983/84 surveys supplemented a survey by smaller vessels of the UNDP/FAO Marine Fisheries Development Project (PAK/77/033), in order to provide further information on the shelf resources and the environment.

The distribution, composition and abundance of pelagic and semi-demersal fish were described in these surveys by the acoustic method combined with trawl sampling. The January-February 1984 survey also included a programme for the study of the demersal fish resources through a system of pre-determined bottom trawl stations in co-operation with the PAK/77/033 project.

Table 3.24 DR. FRIDTJOF NANSEN surveys of the Pakistan shelf

Figure 3.23 The Pakistan continental shelf and the survey routes and stations of the DR. FRIDTJOF NANSEN survey 5–16 September 1983

In all surveys hydrographical stations were occupied in a fixed set of profiles across the shelf. In all 1975/76 and in most of the 1977 surveys these profiles were extended well beyond the shelf and combined with observations on and sampling of mesopelagic fish.

The 1975/76 and the 1977 surveys are reviewed in Section 3.3 and as regards mesopelagic fish in Section 3.4. The 1977 surveys were briefly described in IMR, 1978a and data and material collected has been used as basis for a number of theses at the University of Karachi. Brief accounts of the 1983–84 surveys were given by Nakken (1983), IMR (1984a) and Ona (1984b). A summary report reviewing the results of these surveys was prepared for a Pakistan National Workshop on Fisheries Policy and Planning (IMR, 1986f). Data from the September 1983 and the January 1984 surveys were used by Bianchi (1992b) to describe demersal assemblages of the Pakistani shelf.

Figure 3.23 shows, as an example, the survey area and the course tracks and stations of the September 1983 survey. The indices of coverage (d) for the acoustic surveys were rather high for the 1983–84 surveys.

Bottom topography

The shelf off the long Makran coast is narrow (10–20 nmi), while the area from Sonmiani Bay to the border with India is 50–70 nmi wide, except where the swatch off the Indus Delta separates the Pakistan part of the Kori Great Bank from the shelf off the Sind Coast. As described by Abildgaard et al. (1986), the bottom of the Kori Bank is mud-covered; that off the Sind Coast is also generally muddy, but with hard patches between 60 and 80 m and also at greater depth. The bottom of Sonmiani Bay is mainly soft and muddy except for the deeper parts and the grounds closer to the Makran coast, where harder bottom occurs, while that off the Makran coast is predominantly hard.

The shelf areas by depth ranges assessed from Admiralty charts are shown in Table 3.25. The areas of shallowest depth range do not include the estuaries. The Makran shelf is shallow, while that of the Sonmiani Bay-Sind coast is deeper with the widest part between 50 and 100 m.

Table 3.25 Areas of the Pakistan shelf by region and depth ranges (nmi²)

Hydrography

The Pakistan shelf waters were well covered by the hydrographic observations of the many surveys at the different seasons. The main features of interest for fisheries oceanography are related to the effects of the monsoons. The southwest monsoon June-Sept (July-Aug) results in an east-flowing current along the Makran coast and a south-flowing current along the Sind coast. The observations confirmed this situation with isopleths tilted towards the coast and a shallow mixed layer inshore. The series of hydrographic observations from the January-June 1977 surveys indicate that the tilting of the isopleths may start already in late April-early May before the onset of the local southwest monsoon. On the southwest coast of India a reversal of the north flowing current during the northeast monsoon was observed to start already in March-April with coastward tilting of isopleths, and deduced to be the effect of the onset of the anticyclonic gyre in the North Arabian Sea spun by the southwest monsoon (Longhurst and Wooster, 1990).

Figure 3.24 Hydrographic profiles from the Indus River to the southwest, September 1983 and January and June 1984

In the southwest monsoon season there is an intrusion of oxygen-deficient bottom water onto the Pakistan shelf and in September 1983 water of less than 2ml/l dissolved oxygen was found at depths between 15 and 30 m, see Figure 3.24. During the northeast monsoon the currents are reversed. The January 1984 observations showed that the mixed oxygen-rich surface layer was deep (100–150 m) and covered the bottom layers of the shelf beyond the edge. This shift between seasons of oxygen-deficient and oxygen-rich bottom water over a large part of the shelf has a marked effect on the distribution of the demersal fish which tend to avoid water with low oxygen content (Brandhorst, 1986, Bianchi, 1992b). The result is reduced availability of bottom fish in the deeper parts of the shelf in the southwest and post-southwest monsoon season and increased catch rates in bottom trawl in the inshore shallow parts.

A second important effect of the hydrographic shift during the southwest monsoon is that nutrient-rich deep water is lifted up into the photic zone resulting in an enhancement of the primary productivity. Off Somalia and Oman this process is intensive during the southwest monsoon. Brought about by wind-induced coastal upwelling it forms the basis for the high productivity of these areas. The process seems, according to available oceanographic observations, to be less intensive on the Pakistan shelf and appears mainly to be the dynamic effect of the seasonal current. During the May-June 1977 survey, however, an inshore area of low surface temperatures was observed along the east Makran coast demonstrating active upwelling at the onset of the southwest monsoon (IMR, 1978a).

On the Sind coast discharges from the Indus River cause low salinity in the surface layers resulting in eurohaline components in the fauna.

Abundance estimates

The reports already issued have served to present the main findings regarding the resources which the surveys had provided. On closer examination, only part of the data were found to be of value for a review and for further analysis. From the 1975–76 exploratory surveys the main data available are a series of acoustic estimates of small pelagic fish at different seasons and one acoustic estimate of semi-demersal fish in the pre-monsoon season (see Section 3.3).

Uncertainties exist regarding the state of the acoustic instruments during the 1977 surveys, therefore acoustic assessments from these surveys are not considered in this review. However, the catch data from the aimed fishing for identification and sampling of these surveys do represent valuable observations on the distribution and composition of the fish by depth and seasons.

In the analyses the trawl data were grouped into five surveys, numbered I to V in the various tables.

The catch data from the 1977 and the 1983/84 surveys are available at IMR as NAN-SIS files.

Acoustic estimates of small pelagic fish

In most of the 1977 and 1983–84 acoustic surveys small pelagic fish were found to be present over the major parts of the shelf inside 200 m depth both off the Makran coast and off Sonmiani Bay and the Sind coast. The densities were low over most of the area of distribution. Higher densities were only observed in limited locations inshore, and in some surveys also further out on the shelf, 40–50 nmi offshore in the Sonmiani Bay and off the Sind coast north of the swatch.

The revised estimated abundance of the small pelagic fish based on the acoustic integration technique was as follows (data from IMR, 1986f adjusted to a target strength of -34 dB/kg):

The high June estimate may be an effect of an annual production cycle in the short lived anchovies as was found on the southwest coast of India (Section 3.2). The five estimates of small pelagics in Pakistan in the 1975/76 surveys (Section 3.3), showed a mean of 750,000 t (adjusted to the level of target strength used here). A mean standing biomass of 600,000 t seems the best available acoustic estimate.

Most of the biomass of pelagic fish was found at relatively low densities and not in dense school areas. This confirms similar findings from the 1975/76 surveys and is a distributional characteristic in contrast to that found for the small pelagics off the Arabian Peninsula and Somalia, where a major part of the biomass aggregated in high-density school areas. The difference in distribution and behaviour is probably related to the predominance of Thryssa spp. and Stolephorus spp. and to characteristics of the primary and secondary production on the Pakistan shelf. Even though the overall production seems also to be high off Pakistan, it is likely to be less concentrated in time and space and less patchy than in true upwelling areas with its local upwelling cells and wind dependent processes.

Species composition of small pelagic fish

Information on the small pelagic species composition is available from the aimed fishing with pelagic trawl for identification and sampling. Table 3.26 shows that the proportion by families varies considerably between surveys. This may have been caused by the generally low sampling success of this fishing method. These data agree, however, with similar data from the 1975/76 surveys in showing dominance of clupeids, engraulids and Carangidae.

Pelagic fish also formed an important part of the catches in the bottom trawl, especially in shallow water. Table 3.27 shows the composition by families in all hauls in the 10–25 m range for the Makran coast and the Sonmiani Bay-Sind coast separately. These data indicate that here may have been a difference between the two regions with Carangidae and Engraulidae dominating the Makran shelf, and Engraulidae and Clupeidae the Sonmiani Bay-Sind shelf.

Table 3.26 Proportion by families of small pelagic fish in catches from aimed fishing with pelagic trawl. Average catch composition of all hauls per survey in %

Table 3.27 Proportion by families of small pelagic fish in catches with demersal trawl in the 10–25 m depth range. All hauls (%)

All data show low catch rates of Scomberomorus sp. and Sphyraenidae, but these non-schooling larger pelagic fish no doubt form a much higher part of the total pelagic biomass than indicated by their appearance in the bottom trawl catches. The assessment of these stocks can only be made if they are identified as a separate acoustic target, larger fish in single fish distribution. This was not part of the acoustic programme in Pakistan.

As regards composition by species, the samples obtained by aimed pelagic trawling should be expected to give the most direct evidence, but the results indicate that the number of samples was insufficient to provide a representative picture. Thus while Thryssa spp dominated the Engraulidae in the samples from the 1977 surveys, Stolephorus spp. were most common in those from the 1983/84 survey. In both survey periods Dussumieria acuta dominated the Clupeidae, but the proportions of species of the genera Sardinella, Ilisha and Tenualosa varied. Megalaspis cordyla and Decapterus russelli were the most common Carangidae in the pelagic samples from both survey periods, but the catches were in the proportions 7:1 in the first period and 1:5 in the second.

The occurrence and the catch by weight of the pelagic fish in the demersal hauls was another source of information on their composition by species. The 1977 surveys provide the most comprehensive data, see Table 3.28 which includes only the species with the highest density in each family.

Table 3.28 Species composition of main pelagic families in demersal trawl hauls in all 1977 surveys. Incidence (%) and estimated mean density (t/nmi²)

Because of the schooling behaviour of especially the Engraulidae and Clupeidae, the sample variance must be expected to be high. With the wide inner shelf off the eastern coast, Stolephorus spp. and Thryssa spp. as well as the rainbow sardine (Dussumieria acuta) are common here. Various scads, trevallies and queenfish had together with Spanish mackerels and barracudas a wide distribution, but generally low catch rates in the bottom trawl.

Biomass estimates of demersal fish

Survey IV of January 1984 with pre-determined trawl stations was used for biomass estimates applying the swept-area technique with shelf areas within depth ranges (Table 3.23) as substrata. The estimated distance between the wing tips, 18.5 m for the bottom trawl was also assumed to be its effective fishing width. Table 3.29 shows the results by main groups. The catchability coefficient was taken as 1.

There are various possible sources of bias in these estimates of which the unknown true effectiveness of the trawl is likely to be the most important. The pelagic fish is severely underestimated and the trawl estimate is of little interest, except perhaps to indicate that parts of this group, e.g., some of the Carangidae, have a demersal behaviour and therefore may not have been included in the acoustic estimate.

Table 3.29 Estimated biomass by species groups. Data from Survey IV, January 1984 (t)

The lack of coverage of the inshore waters less than about 10 m depth represents a source of underestimation. A correction has been attempted for this, assuming the density in this part to be equal to that in the 10–25 m range.

Another source of bias may result from the escape of semi-demersal fish above the headline of the trawl. The biomass of fish identified as semi-demersal was estimated in surveys III, IV and V (IMR, 1986f). With adjustment to a target strength level of -32 dB/kg (17 cm fish) as seems suitable for this kind of fish the estimates were 147,000, 40,000 and 227,000 t for the three surveys respectively. This considerable variation is most likely due to a seasonal change in behaviour. The simple mean, 138,000 t indicates that a large biomass of semi-demersal fish which is not caught by normal bottom trawls may occur in Pakistan waters.

The difference in catch composition between aimed and random fishing (Table 3.31) and the higher catch rates in aimed fishing (Table 3.29) show that some of the types of fish occur in aggregations above the bottom which are easily identified by echosounders. The mid-water behaviour of many groups of demersal fish - such as hairtails, ponyfishes, catfishes, sharks - is well known. A quantification of the bias in trawl surveys caused by mid-water occurrence is, however, difficult. The indications are that for conditions such as in Pakistan, it could be considerable.

The main data on demersal fish were obtained from the catches by bottom trawl. Table 3.30 is an overview of the mean catch rates by main groups for the Makran coast and Sonmiani Bay-Sind coast for each of the surveys.

Pelagic fish include the families Engraulidae, Clupeidae, Carangidae, Scombridae and Sphyraenidae analysed above. Demersal fish include the following 10 families which were judged to be the most common and of greatest commercial interest: Ariidae, Lactariidae, Leiognathidae, Lutjanidae, Nemipteridae, Pomadasyidae, Sparidae, Sciaenidae, Polynemidae and Trichiuridae.

The mean catch rates in Surveys I, II, III and V, where fishing was done for the purpose of identification and sampling were, as can be seen, in most cases higher than those of Survey IV where fishing was at pre-determined positions. The aimed fishing gave catch rates which may simulate those of a commercial fishery.

Table 3.30 Mean catch rates by main groups in bottom trawl hauls (kg/h)

The depth distribution of the demersal fish on the Pakistan shelf is, as already mentioned, known to be affected by the intrusion of oxygen-deficient water onto the shelf in the southwest monsoon season. In the present series of data, those from the Sonmiani Bay-Sind coast which cover a broad depth range were best suited for an analysis of this phenomenon. Table 3.31 shows the mean densities of the ten families of demersal fish by depth ranges normalized for comparison. These observations confirm the seasonal change in depth distribution, with a shift of high density towards shallow water which started already in June, but is pronounced in September.

Table 3.31 Mean densities of demersal fish by depth ranges. Sonmiani Bay-Sind coast. Normalized and rearranged by month

The composition of the catches of demersal fish varied between surveys. Table 3.32 shows the composition by families for all surveys for the Sonmiani Bay-Sind coast and for the 1983– 84 surveys on the Makran coast (where the 1977 data proved inadequate). The mean composition in the surveys in which trawl hauls were made for the purpose of identification and sampling, is shown separately for comparison with that of Survey IV in which the predetermined fishing positions and the equal density of stations at different depth ranges should give compositions reflecting the correct biomass proportions of the groups.

Table 3.32 Composition of demersal fish in bottom trawl catches (%)

The two sets of data show that, compared with random fishing, the aimed fishing overestimates the relative abundance of Sciaenidae and Trichiuridae, but underestimates that of the Nemipteridae. The aimed fishing will mostly have been directed at echosounder targets over the bottom, and is thus likely to have caught a higher proportion of semi-demersal fish. These types of fish may on the other hand be under-represented in the random fishing survey because of their occurrence off the bottom. There may also be a seasonal variation in species composition as indicated by the low proportions of Nemipteridae in Survey III, September 1983. Lutjanidae and other hard bottom groups may have been under-represented because of restricted fishing on this type of bottom.

In view of these possible biases in the data, it may be difficult to determine the true order of abundance of the demersal fish. However, the five families: Trichiuridae, Nemipteridae, Sciaenidae, Ariidae and Pomadasyidae represented more than 80% of the catches in both sets of data. The relatively abundant fauna of demersal fish on the Pakistan shelf is thus dominated by a low number of families and these are again dominated by one or a few species, a type of fauna which should be seen in relation to the highly variable environment.

The proportion of sharks and rays in the catches was at about the same level as one of the abundant teleost families. Squids were abundant in the 1977 surveys in deeper waters, 50–100 m and 100–200 m. The squids occuring in the shelf area may have a seasonal production cycle.

This composition of the catches by bottom trawl corresponds well with that described by Abildgaard et al. (1986) from the surveys carried out in Pakistan waters in 1983–85.

Species identification and assemblages of demersal fish

Problems appear to have been encountered in some of the surveys in the identification to species level. The available information can be summarized as follows:

Trichiuridae (hairtails): Both Lepturacanthus savala and Trichiurus lepturus are reported to occur in Pakistan waters (Bianchi, 1985). In the 1977 surveys the two species were reported in varying proportions, usually with a dominance of T. lepturus. In the 1983/84 surveys, however, all catches of this family were identified as T. lepturus.

Nemipteridae (threadfin breams): In nearly all of the surveys all catches of this family were reported as Japanese threadfin bream, (Nemipterus japonicus). According to Bianchi (1985), however, this species is often caught (in Pakistan waters) together with a then undescribed species of this genus. This was described in 1986 as Nemipterus randalli (Russell, 1990). The Nemipterus species were found in highest densities on the deeper parts of the shelf, 50–100 and 100–200 m.

Sciaenidae (croakers): Appeared to have been inadequately identified to the species level in some of the surveys. The data indicate an approximate order of abundance of the main species as follows: Otolithes ruber, Johnius spp., Protonibea diacanthus, Otolithes cuvieri and Argyrosomus sp. The croakers were represented over the whole shelf, at times with high catch rates even beyond 100 m depth.

Ariidae (catfishes): Identification to species of catfishes was not attempted in any of the surveys. They were found in shallow waters with hardly any catches at depths greater than 50 m. Their density may have been underestimated because of lack of survey coverage inside 10 m depth.

Pomadasyidae (grunters): The javelin grunter (Pomadasys kaakan) was by far the dominant species of this family reported from both parts of the shelf in all surveys and representing an average of 70% of the catches of grunts. The saddle grunt (Pomadasys maculatum), also reported from all surveys, but with a more restricted distribution represented 19% of the grunt catches in seven area/surveys. The grunts were mainly found inside 50 m depth, but high catch rates were occasionally obtained beyond 100 m depth.

Sparidae (breams) and false trevally: The king soldierbream (Argyrops spinifer) dominated the Sparidae with a high incidence and representing 64% of all catches of sea breams. The false trevally (Lactarius lactarius) was common in all surveys.

In a description of the demersal assemblages of the Pakistan shelf based on the September 1983 and the January 1984 surveys Bianchi (1992b) found that the shelf may be divided into two major zones: a deeper zone from 50–80 to 200 m where environmental conditions change dramatically with the season. During the southwest monsoon with low oxygen and temperature near the bottom, only a few species, such as hairtails and threadfin breams, are found here, probably because of their ability to swim to upper water layers. During the northeast monsoon, with a mixed surface layer covering the whole shelf this deeper zone is “invaded” by the rich and diverse fauna of the second, shallow zone, where relatively stable temperature and oxygen values prevail throughout the year and where most of the fish fauna from the deeper zone may find seasonal shelter. The present review, which includes the additional data from surveys I, II and V (January-June 1977 and June 1984) confirms this.

Review of findings and comparison with later research and with development of the fishery

The review deals with data from a series of surveys of the Pakistan shelf during January-June 1977 and September 1983-June 1984 which provided information on the distribution, composition and abundance of pelagic, mesopelagic and demersal fish and on their environment.

An extensive programme of oceanographic observations confirmed the well known monsoonal effect on the hydrographic regime with shoreward tilting of the isolines during the southwest monsoon and intrusion of oxygen-deficient water onto the shelf. Active upwelling was observed on the Makran coast.

Small pelagic fish were found over wide parts of the shelf in all surveys, but no area with especially high densities was identified. Catch compositions indicated that Engraulidae and Carangidae dominated on the Makran coast and Clupeidae and Engraulidae on the Sonmiani Bay-Sind coast. The relatively low catch rates of Scombridae were thought to underestimate their abundance.

Table 3.33 Summary of estimates of standing biomass and mean density for the shelf

The five families Trichiuridae, Nemipteridae, Sciaenidae, Ariidae and Pomadasyidae yielded more than 80% of the catches of teleost demersals. The mean catch rates in the aimed sampling trawling were higher than in the random trawling and were dominated by Trichiuridae and Sciaenidae against Nemipteridae in the random survey.

The estimates of standing biomass and density are summarized in Table 3.33.

Other surveys

Reporting on the results of three stratified trawl surveys with the Pakistan Governments research vessels MACHHERA and TEHKIK, (sister ships), Abildgaard et al. (1986) estimated the total demersal fish biomass on Pakistan's continental shelf between 10 and 200 m depth at:

The variation in these estimates is ascribed to an apparent annual cycle in standing biomass of the demersal fish with high availability in the early part of the year and low availability in the last part, caused by migration. The estimate from the January 1984 DR. FRIDTJOF NANSEN survey of 254,000 t agrees well with those obtained in the season of low availability. It also seems likely that the semi-demersal fish, for which there is an acoustic biomass estimate of 140,000 t, was not fully represented in the bottom trawl catches. The “best” estimate of the mean standing biomass of the demersal resources from the DR. FRIDTJOF NANSEN surveys on the Pakistan shelf in 1983–84 is perhaps 300,000–350,000 t and 600,000 t for the pelagic resources.

Brandhorst (1986) adjusted the demersal biomass estimate from 1983–85 found in Abildgaard et al. (1986) for not-covered inshore and estuarine waters, concluding with a total biomass of demersal fish of 500,000 t, which could give an annual potential yield of 180,000 t. He estimated the 1984 landings of demersals to be 110,000 t, thus leaving a 70,000 t potential for an expansion of the demersal fisheries.

The observed biomass represented partly exploited stocks. In a comparison between the biomass estimates and the reported 1981 landings, IMR (1986f) concluded that there seemed to exist potentials for an increase especially of pelagic fish, but also for demersals such as hairtails, croakers and catfishes.

Pakistan's reported landings of marine fish increased from 261,000 t in 1981 to 400,000 t in 1991 (FAO, 1986 and 1992). The landings (t) by main groups in 1983 and 1991 were according to these sources as follows:

(Reported unidentified marine fish is thought to consist mainly of small pelagics and has been included in that group, but is likely to include also demersal “trash fish” from shrimp by-catch, etc.)

The increased landings of demersal fish were mainly Ariidae, Sciaenidae, Pomadasyidae, Stromateidae and Trichiuridae. The Nemipteridae which had a high abundance in the surveys seem to be underutilized unless it is included in the unidentified group. Sharks and rays would seem to have been severely underestimated by the surveys.

3.6 OMAN, FURTHER SURVEYS, 1983–84

Survey objectives and effort

Information on the DR. FRIDTJOF NANSEN surveys in Omani waters is given in Table 3.34. For survey purposes Omani waters have been divided into three areas:

1. shelf area in the Gulf of Oman,
2. deep part in the Gulf of Oman and
3. shelf area in the Arabian Sea.

All the areas represented central parts of the first regional exploratory surveys in 1975/76. One of the main findings was a great abundance of mesopelagic fish of which the density was especially high in the Gulf of Oman. Therefore this area was chosen for a number of special surveys in 1979, 1981 and 1983 to study the distribution, biology and abundance of the mesopelagics. Finally, the shelf resources of demersal and small pelagic fish were investigated in more detail in three surveys in 1983–84.

In both the exploratory and the mesopelagic surveys the distribution, composition and abundance of the fish were determined by acoustic methods combined with sampling by trawl and experimental trawl fishing trials. The programmes of the 1983–84 shelf surveys were expanded and included investigations of demersal fish abundance using the swept-area method at pre-determined trawl stations.

Table 3.34 DR. FRIDTJOF NANSEN surveys in Omani waters

The results of the 1975/76 exploratory surveys are reviewed in Section 3.3 and the surveys for mesopelagics in Section 3.4. The results of the three shelf surveys in 1983–84 were first presented in cruise reports, (Strø;mme, 1983b, Strø;mme and Tilseth, 1984 and Strø;mme, 1984a). The results were further analysed in a final report by Strø;mme (1986). The Oman surveys are among the best described of the DR. FRIDTJOF NANSEN programmes in the Indian Ocean. In view of the importance of the area, and to facilitate comparisons with other areas, it is still considered valuable to include a description of these surveys in this review.

The continental shelf

Figure 3.25 shows a typical cruise track and the border of the shelf. Estimates of the dimensions of the shelf are given in Table 3.35. The northeast coast along the Gulf of Oman has a narrow shelf with a mean width of about 10 nmi and a steep slope. The shelf off the southeast Arabian Sea coast is wider, especially off the Bay of Masirah and the Sawqirah Bay, but also narrow west of the Kuria Muria Islands where a steep slope starts at about 100 m depth.

Figure 3.25 The Omani shelf and the cruise tracks of the March 1983 survey

Table 3.35 Dimensions of the Omani shelf

According to Johannesson (1995) much of the bottom of the Omani shelf is rough and untrawlable. Bottom of this type was found to represent 24% of the shelf of the Gulf of Oman, 27% of the shelf from Ras al Hadd to Ras Madrakah and 55% of the shelf further west, a total of 39% of the whole shelf. Thus considerable parts of the shelf could not be sampled with the bottom trawl.

Hydrography

The main oceanographical features of Omani waters are the pronounced monsoonal shifts of the hydrographical regime causing high variability of important environmental factors, and the upwelling along the Arabian Sea coast during the southwest monsoon which provides the basis for the high productivity of the region.

The environmental change which most directly affects the fish and the fisheries is probably the intrusion of cold, oxygen-deficient water onto the shelf during the southwest monsoon. This affects the distribution of demersal fish and may at times cause mass mortalities of fish indicated by abundance of fish bones over an extended area of the shelf south of Ras al Hadd observed by the CHALLENGER expedition (quoted by Brongersma-Sanders, 1957).

Figure 3.26 shows the observations of oxygen content in a profile off the Kuria Muria Islands at various times during the 1975/76 surveys. A coastward tilting of the isolines could be observed from April to October. The 1 ml/l isoline for oxygen was found at about 100 m depth in April 1975, at about 80 m in May 1976, at 10 m in September 1976 and at some 20 m in October 1975. Similar observations from the 1983/84 surveys show that the 1 ml/l isoline was observed at 50 m depth in the Sauqara Bay in late November 1983 indicating that the process may last well into the post-monsoon season.

The upwelling off the Arabian Peninsula extends to at least 400 km offshore, but is most intense in a narrow band adjacent to the coast (Currie et al., 1973), and is reported to last from April until September. The effect of the upwelling off the Arabian Sea coast of Oman was observed in the regional DR. FRIDTJOF NANSEN surveys 1975–76 with inshore surface temperatures at 24°C in April-May 1975, 23°C in May 1976 and 18°C in September 1976.

Figure 3.26 Oxygen profiles off the Kuria Muria Islands in the 1975/76 surveys

Measurements of rates of primary production made in the upwelling season off the Arabian Peninsula during the IIOE caused oceanographers to state that its productivity might be at a similar level as those of the upwelling areas off West Africa and Peru (Wooster et al., 1967).

There is a large year-to-year variability in the monsoon winds (Swallow, 1984) and this is likely to be reflected also in upwelling and primary productivity as well as in the abundance of short-lived pelagic fish.

Water with high nutrient content is advected into the Gulf of Oman from the Arabian Sea coast by surface currents and this probably forms the basis for the high densities of mesopelagic fish found here. These fish migrate vertically between surface waters and 200– 300 m and their distribution in the Gulf of Oman may be affected by the flow of heavy water from the Persian Gulf (Bakun, FAO, pers. com.), which at a depth of 200–300 m follows the Omani deep slope. This water has an oxygen content of about 2 ml/l compared to less than 0.5 ml/l elsewhere in the Gulf of Oman at this depth.

Acoustic estimates of small pelagic fish

The main data available from the 1975/76 exploratory surveys were a series of estimates of the abundance of small pelagic and semi-demersal fish with information on species and length composition and distribution (Section 3.3). The 1983/84 surveys provided data on the distribution, composition and abundance of small pelagic- and semi-demersal fish from acoustic observations with sampling, and of demersal fish from swept-area trawl programmes.

The distribution charts of small pelagic fish based on acoustic recordings from the five 1975/76 surveys (Kesteven et al., 1981, Figures 17–21) and the three in 1983/84 (Strø;mme, 1986, Figures 4–6) show that in all surveys by far the largest part of the fish was found on the Arabian Sea shelf. The recordings from the Gulf of Oman shelf showed low densities and were of limited extent. In the Arabian Sea high-density recordings were obtained in the Gulf of Masirah for all eight surveys, in the Sauqara-Kuria Muria sector in four and in the Ras al Hadd-Masirah sector in three surveys. The Gulf of Masirah was thus the most consistent area of high densities of small pelagic fish. According to Strø;mme (1986) the distribution of small pelagics fish was not limited to the shelf proper, but extended in the Sauqara area also to the surface waters immediately off the edge of the shelf.

Figure 3.27 shows the distribution of small pelagic fish on the southeast coast in the March 1983 survey demonstrating high abundance in the Gulf of Masirah.

Figure 3.27 Distribution of small pelagic fish in the Arabian Sea off Oman in March 1983

Estimates of the biomass of small pelagic fish in the Arabian Sea, obtained by echo integration are presented in Table 3.36 for all surveys. All these estimates have been adjusted to a target strength of -34 dB/kg.

Table 3.36 Acoustic biomass estimates of small pelagic fish, all adjusted to a TS of -34 dB/kg, in the Arabian Sea off Oman, 1975–84

A part of the large variation must have been caused by survey sampling variation which is likely to have been higher in 1975/76 than in 1983/84 as the degrees of coverage were only about 7 to 10 in the first compared with about 20 in the recent, more detailed surveys. It seems appropriate to question the validity of the very low estimate from the first survey, which might have been the effect of malfunction of the instruments. However, the estimate of semi-demersal fish in this survey, based on simultaneous recordings was close to those obtained in three of the four later surveys, which supports the validity of the observation of the low biomass of small pelagics in early 1975. The mean of the biomass estimates of the four subsequent surveys (Nos 2 to 5) was 1.9 million t, which is somewhat higher than the levels in 1983/84 (1.0–1.4 million t).

A likely bias in surveys 6 and 7 in 1983 is an underestimate caused by saturation both in the EKS echosounder and in the QM analog integrator at high fish densities. This problem was significantly reduced with the EK 400 and QD system used in the last survey. All but some 10,000 t of the fish was located in the Arabian Sea.

In May 1984 the highest concentration of small pelagic fish was found in the sector Masirah Island to Ras al Madraka.

The possibility of large-scale fluctuations in the stocks of small pelagic fish in this region, perhaps related to interannual variability in the southwest monsoon, is discussed below.

The 1983/84 surveys provided data for describing the density distribution of the small pelagic fish on the southeast coast. Strø;mme (1986) shows that the main part of the biomass was found in aggregations of high density. The pattern varied little between surveys and the average proportions of density levels in % of total biomass were:

Fish concentrations at densities higher than 300 t/nmi² are considered to be suitable for industrial fishing.

Species composition of small pelagic fish

Aimed pelagic trawling for species identification gave high overall catch rates and thus seems to have been successful as a sampling method. Table 3.37 shows the mean catch rates for the hauls in the Arabian Sea and their composition by main families and species of pelagic fish. For all the surveys five families accounted for 91% of the total catches, of which Carangidae represented 80% and Clupeidae 8%.

Table 3.37 Composition by families and species of catches of aimed pelagic trawling in the Arabian Sea off Oman (% of total catch by weight).

A small number of species dominated the catches of the mid-water trawl. The five species shown in Table 3.37 represented an average of 93% of the catch of the five pelagic families and 85% of the total pelagic catch. The Arabian scad (Trachurus indicus), which alone comprised 77% of the weight of the catches, was by far the most abundant species. A pelagic assemblage dominated by a few abundant species is characteristic for coastal upwelling systems. However, it seems likely that the often nearshore distribution of the Clupeidae and Engraulidae caused them to be under-represented in these data compared with the other pelagic families because the nearshore areas could not be covered.

The Carangidae, as the only pelagic family, also made a significant contribution to the bottom trawl catches, 40% of a mean catch of 1042 kg/h (27 hauls) in March/83 and 48% of a mean catch of 931 kg/h (38 hauls) in November/83. In these catches the species Trachurus indicus and Decapterus russelli appeared with roughly the same abundance in both surveys (Table 3.38). It seems, however, likely that the sampling with the pelagic trawl, which showed a large dominance of the Arabian scad, gave a better indication of the actual proportions of the two species.

Table 3.38 Species composition of catches of Carangidae in swept-area bottom trawl hauls (%)

The catches in the bottom trawl may also provide information on the distribution of the pelagic fish on the shelf in terms of distance from shore. Clupeids occurred nearly exclusively in bottom trawl hauls shallower than 20 m depth, while nearly all the high catch rates of Trachurus indicus and Decapterus russelli were from the depths of 50–100 m and 100–200 m. A similar pattern of distribution of small pelagic fish is found in other upwelling areas, e.g., the Benguela and Canary Currents, where clupeids are found mainly inshore and Trachurus sp. on the outer parts of the shelf and in the slope. This ecological difference must be seen in relation to the different position of these types of fish in the food chain, with Clupeidae closer to the inshore centres of upwelling and primary production and the small Carangidae offshore where there is a high secondary production of zooplankton.

Biomass estimates of demersal fish

Estimates of the biomass of the demersal fish for the southeast coast, Ras al Hadd to Ras Marbat based on the swept-area trawl survey method were as follows (Strø;mme, 1986, Table 8):

The shelf of the Gulf of Oman was only covered in May 1984 and the biomass there was estimated at 42,000 t. The low estimate from November 1983 was related to greatly reduced catch rates from deeper waters which occurred in connection with the presence of oxygen-deficient water on the deeper shelf (Strø;mme, 1986).

The best estimate of the swept-area trawl surveys would thus seem to be 335,000 t for the area covered in the Arabian Sea. An adjustment for the Salalah sector which was not covered of 15,000 t and inclusion of the Gulf of Oman estimate of 42,000 t raises this to a total for the country of 390,000 t.

In addition to the usual uncertainty with swept-area estimations of the true effective fishing width of the trawl, there is probably a bias of underestimation connected with the semi-demersal behaviour of many of the species. The biomass of the semi-demersal fish was estimated with echo integration as follows (adjusted to -32 dB/kg for demersal fish):

Acoustic estimates of the semidemersal fish from the first exploratory surveys (1975–1976) (Kesteven et al., 1981) showed:

Although showing high variability these data demonstrate that at times a considerable amount of demersal types of fish occur in mid-water on the Oman shelf. The most common of these semi-demersal fishes were probably the Japanese threadfin bream and hairtails. These species are likely to be under-represented in the swept-area estimates, but a quantification of this bias is not possible.

The swept-area estimate of biomass may also be biased if the mean density of demersal fish on the trawlable part of the shelf is different from that of the untrawlable rough-bottom part. Johannesson (1995) estimated the untrawlable bottom to represent 39% of the Oman shelf between the depth ranges 20–200 m. To correct for the possible bias from different mean densities he used observations of ’acoustic density’ of fish in a 6 m layer next to the bottom which represented the vertical opening of the trawl used by the RASTRELLIGER. These data showed a mean fish density over hard bottom about double of that over soft bottom. An assumed similar higher fish density over hard bottom in the DR. FRIDTJOF NANSEN surveys would raise the swept-area estimate of 390,000 t quoted above to about 550,000 t.

Species composition of demersal fish

Table 3.39 shows the catch rates of main species groups by surveys using the bottom trawl. The data are mainly from the Arabian Sea. The very high rates for pelagic fish in the May 1984 surveys result from hauls directed at echo recordings for identification and sampling.

Table 3.39 Mean catch rates by main groups in bottom trawl hauls (kg/h)

Table 3.40 Composition of demersal fish in bottom trawl catches in the Arabian Sea off Oman (%)

The category pelagic fish represents the families Carangidae, Clupeidae, Engraulidae, Scombridae and Sphyraenidae analysed above. The category demersal fish consists for 90% of the ten most important families of demersal and semi-demersal teleost fish (see Table 3.40). In the Gulf of Oman, Psettodidae replace the Serranidae.

The distribution of demersal fish species by depth on the shelf varied between surveys. The Nemipteridae, Lethrinidae and Sparidae had the widest depth ranges through the surveys and the mean rates of the combined catches of these three families are shown by depth and surveys in Table 3.41. Changes in depth distribution may be related to the presence of the oxygen-deficient bottom water: In November 1983 it was found at only about 50 m depth, in March 1983 it occurred well below 100 m and in May 1974 it occurred between 75 and 100 m (Strø;mme, 1986).

Table 3.41 Mean catch rates by depth range of the combined catches of Nemipteridae, Lethrinidae and Sparidae, Arabian Sea off Oman (kg/h)

Because of the change of depth distribution of some of the families the mean composition in the catches will only represent the true proportions of the families if the sampling density is the same at all depth ranges and remains unchanged between the surveys. These conditions are roughly met for these data, but the number of hauls at some depths has been low thus increasing the likely survey variation. For this reason a single haul with the highly unusual catch of 10 t/h of Sciaenidae was omitted from the analysis.

In spite of these limitations the composition of the demersal group of 10 families (Table 3.40), was fairly consistent between surveys with five dominating families: catfishes, threadfin breams, grunts, emperors and seabreams. They represented 80%, 90% and 88% of the catches of the demersal group of 10 families in the three surveys respectively. However, it is unlikely that the catches showed the true composition of the demersal fish. The hard bottom fauna, especially snappers and groupers, and the typically semi-demersal fish such as hairtails and threadfin breams were probably significantly underestimated compared with the other demersal families.

The five families Ariidae, Nemipteridae, Pomadasyidae, Lethrinidae and Sparidae, were also reported to be among the taxa with highest biomass density in the extensive surveys carried out with the R/V RASTRELLIGER 1989–90 (Johannesson, 1995). There were, however, differences: Nemipteridae had a relatively much higher abundance, and the total catch composition included a high proportion of rays and porcupine fishes which had very low abundance in the DR. FRIDTJOF NANSEN surveys. At least part of these differences could be attributed to differences in the trawl gear used by the two vessels and in the sampling systems applied.

Species identification and assemblages of demersal fish

Most of the samples were identified to the species level. The dominating species for the most abundant families were as follows (see Strø;mme, 1986 Annex VI for more details):

Ariidae (catfishes): giant catfish (Arius thalassinus)

Nemipteridae (threadfin breams): Japanese threadfin bream (Nemipterus japonicus). It should perhaps be questioned if the catches reported as Japanese threadfin bream consisted of Nemipterus japonicus only, or as in Pakistan included an unknown proportion of the similar species described in 1986 as Nemipterus randalli. Other species of the same genus and of the genera Scolopsis and Parascolopsis occurred with low catch rates.

Pomadasyidae (grunts): striped piggy (Pomadasys stridens) and painted sweetlip (Diagramma pictum).

Lethrinidae (emperors): spangled emperor (Lethrinus nebulosus) and L. lentjan.

Sparidae (seabreams); king soldierbream (Argyrops spinifer), Santer seabream (Cheimerius nufar) and Arabian pandora (Pagellus affinis).

In analysing the demersal assemblages of the Oman shelf, Bianchi (1992b) found two groups in shallow water and a third in deeper waters, mostly in the Arabian Sea, the latter characterized by species such as Trachurus indicus and Nemipterus japonicus which are able to migrate to avoid oxygen-depleted waters. That the Japanese threadfin bream can also perform vertical migrations was confirmed by its occurrence in pelagic trawl hauls over the mid- and outer shelf in the November 1983 survey. Bianchi (1992b) related the high abundance of the third deeper water group to the high productivity of the area caused by seasonal upwelling.

Review of findings and of later research

The three 1983/84 surveys did not cover the monsoon seasons well; a planned fourth survey in September 1984, in the late southwest monsoon, had to be cancelled due to a breakdown of the main engine. However, the November 1983 survey showed the characteristic post-monsoon intrusion of oxygen-deficient water on the shelf in the Arabian Sea and supplementary hydrographical data from the 1975/76 surveys demonstrated the general scale of the wellknown upwelling in this region during the southwest monsoon.

Small pelagic fish were found in high density concentrations during all three surveys, most consistently in the sector off the Gulf of Masirah. The distribution of these fish was not limited to the shelf proper, but at times extended to the surface waters off the shelf edge.

The catches from aimed fishing with a pelagic trawl were dominated by Arabian scad (Trachurus indicus), while the pelagic fishes in the bottom trawl catches were a mixture of this species and the Indian scad (Decapterus russelli). Clupeidae were found inshore with Indian oil sardine (Sardinella longiceps) and goldstripe sardinella (S. gibbosa) dominant.

Table 3.42 Summary of estimates of biomass and mean density in Oman obtained from the 1983/84 survey data

Table 3.42 summarizes of biomass estimates and densities. Similar high levels of density of pelagic fish are found in other coastal upwellings such as the Canary and Benguela systems. It should be remembered that the shelf of the Gulf of Oman was surveyed only once and that therefore estimates for this part are less reliable. The much lower fish densities observed for this shelf compared with those found in the Arabian Sea upwelling region were, however, not unexpected and are most likely related to differences in ecosystems.

A set of comparable data on Oman's fish resources is available from the comprehensive programme of surveys executed from November 1989 to November 1990 with the RASTRELLIGER (Johannesson, 1995). The reported estimates of total biomass for pelagic and demersal fish were:

The acoustic estimate was only based on one survey in September 1990 when pelagic fish was found in some abundance in the Arabian Sea. In the other surveys much less pelagic fish was recorded. Thus this estimate appears particularly unreliable, and it would perhaps be better to use the sum of the mean observations made in each sub-area (Johannesson, 1995, Table 3) i.e. 163,000 t. For comparison with the DR. FRIDTJOF NANSEN data an adjustment is needed from an assumed TS of -29.4 dB/kg to a TS of -34 dB/kg (which represents an increase of the biomass with a factor of 2.88).

The comparable estimate then becomes

163,000 * 2.88 = 469,440 or 470,000 t.

For RASTRELLIGER's swept-area trawl estimate the geometric mean of the catch rates was used. Using the arithmetic mean as for the DR. FRIDTJOF NANSEN leads to a 30% increase. On the other hand the estimate should be reduced by 10%, because a “catchability coefficient” of 0.9 has been assumed for the RASTRELLIGER trawl gear, against 1.0 for the DR. FRIDTJOF NANSEN.

The comparable estimate then becomes:

414,000 * 1.2 = 496,800 or 500,000 t

Thus there was a much lower acoustic estimate of pelagic fish in the 1990 survey than in 1983/84. The lower abundance is also confirmed by the difference in the proportion of scads in the swept-area trawl catches: in March and November 1983 these species represented 40– 50% of the catches against only 6–7% in 1990.

Catch statistics of a small trawler fleet operating in deeper waters in the Arabian Sea off Oman, targeting hairtails and cephalopods (Anon., 1994) provide further evidence of a reduced abundance of scads in 1989–90; the by-catches of small pelagics were as follows: 1986 5.7%, 1987 3.3%, 1988 2.4%, 1989 0.4%, 1990 0.4%, 1991 0.3%, 1992 1.2% and 1993 4.8%.

The RASTRELLIGER estimate of demersal fish abundance is based on more than 400 trawl stations. The differences in the trawl gear used by the two vessels complicates the comparison of the biomass estimates. For example, there is a large difference in catch rates of rays, which is not a group expected to change much in abundance. In the March and November 1983 DR. FRIDTJOF NANSEN surveys the mean catch rates of rays in the Arabian Sea were 3 kg/h and 36 kg/h respectively compared with 125 kg/h in RASTRELLIGER's 1990 surveys. This difference is most probably caused by the rock-hopper ground gear used with the trawl of the RASTRELLIGER. In the absence of intercalibration between the trawls, there seems little point in detailed comparisons of the results of the two surveys. The correction proposed by Johannesson (1994) to adjust for the higher apparent fish density over the hard untrawlable bottom would increase the RASTRELLIGER's estimate of demersal fish by 28% to 640,000 t, and that of the DR. FRIDTJOF NANSEN to 570,000 t.

The main difference between the findings of the two sets of surveys is the low abundance of the pelagic fish in 1990 compared with 1983/84. As noted above, a similar observation of low abundance of pelagics was made in the first 1975 survey, while pelagic fish were found in high abundance in the four subsequent surveys from late 1975 to late 1976. Such fluctuations indicate a dependence of population size on a variable environment as has been found in the anchoveta - El Niño relationship in the Humboldt Current off Peru.

In a review of information on inter-annual variability in the Arabian Sea, Luther (1991) found that evidence exists of both bi-annual and decadal variability in the southwest monsoon winds and in the consequent open-ocean upwelling. This variability could be related to the amount of monsoon rainfall on the Indian continent and seems in a wider context to be part of a global fluctuation in climate. Thus a link has been shown between El Niño/Southern Oscillation (ENSO) events in the Pacific and monsoon variability, with a weak monsoon preceding a warm ENSO phase. Studies quoted by Luther (1991) showed a period of generally low rainfall and weak monsoons from the mid 1960s to the mid 1970s.

Variations in adult stock biomass of small pelagic fish caused by changes in the intensity and duration of upwelling is likely to lag 2–3 years behind the variations in the strength of the monsoon. Parthasarathy et al. (1992) reports Indian summer monsoon rainfall indices (ISMR) from 1871 up to 1990. The year 1972 had an anomaly of -49.5% in this index and ranked as the fifth driest year since 1871: this could explain the low abundance found in the first 1975 survey. The years 1985 through 1987 had negative anomalies with both 1986 and 1987 defined as dry years. The ISMR index for 1987 showed this to be the fourth dryest year since 1871. The weak monsoon of 1987 could thus have caused the low biomass of pelagic fish in 1989–1990 found in the RASTRELLIGER surveys.

It is uncertain whether the variability of the main monsoon and the open ocean upwelling is parallelled in the coastal upwelling off the Arabian Peninsula, however, until specific data for this region becomes available, it seems reasonable to consider the hypothesis that fluctuations also occur there and these are the most likely cause of the observed low stability of the stocks of small pelagics in Oman.

Fisheries development

The total landings of Oman's fisheries fluctuated around 100,000 t in the period 1980–86. There was an increase to over 160,000 t in 1988 brought about by a sharp increase in the landings of large pelagics (Spanish mackerels and tunas), but by 1989 the total landings were down to 118,000 t and remained at that level till 1993. About 80% of the 1993 landings were by the traditional small-scale fishery and the rest by small trawlers and longliners. The high abundance shown in the surveys of such species as Japanese threadfin bream and Arabian and Indian scads is not reflected in the landings, probably due to the undesirable small size of these fish. Sardines with their predominantly inshore distribution dominated the 42,000 t landings of small pelagics. The demersal landings of about 27,000 t were dominated by seabreams, emperors, croakers, groupers and hairtails which were all identifed as important families in the surveys. Large pelagic fish (tunas and Spanish mackerels) were not covered by the surveys. Since they are top predators, the landings of about 37,000 t of this group represent an indirect utilization of the abundant mesopelagic and small pelagic fishes. A comparison of the total landings with the findings of the surveys, as presented above, indicates that there is considerable room for expansion of Oman's fisheries.

3.7 YEMEN AND NORTHEAST SOMALIA, FURTHER SURVEYS, 1984

Survey objectives and effort

Yemen's coast in the Gulf of Aden and northeast Somalia had been shown to have a high abundance of small pelagic fish, and therefore these areas were also covered by two surveys under the new programme, in February-March and in August-September 1984 respectively, the first of which also included Socotra. The results were briefly described in preliminary survey reports (Blindheim, 1984 and Strø;mme, 1984b and c) and for northeast Somalia summarized and discussed by Strø;mme (1984d).

Table 3.43 shows the operational data for the surveys. Yemen's survey 1 included Socotra, while survey 2 also covered an offshore part. Yemen's shelf is narrow with a mean width of about 10 nmi, while the east coast of Somalia, north of Ras Mabber has a mean shelf width of 20 nmi. The degrees of coverage for the acoustic surveys were good. The timing of the surveys covered the two monsoon seasons, survey 1 the northeast and survey 2 the south-west monsoon.

Figures 3.28 and 3.29 show the coasts of Somalia and Yemen respectively and the survey routes and stations in the August-September 1984 coverage.

Figure 3.28 Somalia: Survey routes and stations, August-September 84

Figure 3.29 Yemen: Survey routes and stations in the August-September survey

Table 3.43 Yemen and northeast Somalia, operational details of the 1984 surveys

Hydrography

Sets of hydrographical stations were occupied in profiles across the Gulf of Aden and off Somalia in both surveys (IMR, 1984b; Strø;mme, 1984b and c). While the February data showed a situation of stable surface layers, the observations from August during the southwest monsoon showed clear evidence of coastal upwelling with low surface temperatures inshore along the Somali coast and the coast of Yemen east of about 47°E. On the coast of Yemen low oxygen water, less than 1 ml/1, was found shallower than 20 m depth compared with 150 m depth in the February survey. A similar seasonal rise of the oxycline was observed off Somalia. In addition to the effects of the upwelling process on the productivity of the region, the regular seasonal changes in environment influence the composition of the fish assemblages and cause major changes in their distribution (Bianchi, 1992b). These phenomena must be taken into account when interpreting the results of the surveys. The effects of the upwelling off Somalia includes at times mass mortality of fish as observed along a wide stretch of the northeast coast by the DISCOVERY expedition in August-September 1964 (Foxton, 1965).

Acoustic estimates of small pelagic fish

Charts of integrator outputs derived from small pelagics indicated a difference in distribution off the Yemen coast in the two surveys; in February the fish appeared to be restricted to the narrow shelf area, while in August aggregations were also found 30–60 nmi off the coast in the Mukalla-Ras Fartak region. The offshore distribution is likely to have been a response to unfavourable environmental conditions inshore during the southwest monsoon. Such findings were not reported from the five previous surveys of the DR. FRIDTJOF NANSEN in the Aden Gulf in 1975/76 nor from the special survey for mesopelagics in 1979.

The distribution of small pelagic fish as charted by the acoustic observations in the two surveys off northeast Somalia, believed to be typical for that well-known upwelling area is shown in Figure 3.30. Nearly all the fish was found between Alula and Ras Hafun. There was no time for an offshore coverage, but previous DR. FRIDTJOF NANSEN surveys had shown the small pelagics to be mainly confined to the shelf waters. Recordings made of the deeper slope and further offshore were shown to consist mainly of mesopelagic fish (Section 3.4).

Figure 3.30 Distribution of small pelagic fish off northeast Somalia. A: Feb-Mar 1984; B: Aug 1984

There was a considerable variation in the biomass estimates of pelagic fish from the two acoustic surveys and it seems unlikely that this reflects a true change in the biomass of the stocks. The estimates from the surveys which were assumed to have given the best coverage could therefore be chosen (survey 2 for Yemen and survey 1 for Somalia):

Species composition of small pelagic fish

Identification through sampling with pelagic trawl was not very successful. Survey 1 off Yemen had catches of Sardinella gibbosa, Decapterus russelli, and Rastrelliger kanagurta in the pelagic catches and Sardinella longiceps and Megalaspis cordyla in the demersal trawl. From the offshore aggregations in survey 2 small samples were obtained of S. longiceps, Scomber japonicus and various Carangidae.

Off Somalia the pelagic trawl samples included the Clupeidae Dussumieria acuta, Etrumeus teres and Sardinella longiceps, while Carangidae included Decapterus macrosoma and D. russelli. Small amounts of Scomber japonicus appeared in nearly all catches. These species also dominated the pelagic fish which appeared in the bottom trawl samples.

Biomass estimates of demersal fish

It was no doubt difficult to sample the demersals representatively on the narrow Yemen shelf with its changing environment between the surveys. Table 3.44 shows the mean catch rates on this shelf by main groups in semi-random hauls in the two surveys. There is a clear decline in the rates in the second survey with ponyfish presumably distributed in shallow water inside the survey coverage as a result of intrusion of oxygen-deficient water onto the shelf.

Table 3.44 Yemen: No. of hauls and mean catch rates by main groups on the shelf 10–200 m (kg/h)

A swept area estimate of the fish biomass on the 6,700 nmi² shelf area using the mean densities of the February survey gave a total of 150,000 t of which the demersal group represented 32,000 t.

Also in east Somalia, Table 3.45, the mean total catch rate of the bottom trawl during the southwest monsoon season in August was less than half of that in February. The group of demersal fish, potentially commercial species, represented a higher proportion of the catches than in Yemen.

Table 3.45 Somalia: No. of hauls and mean catch rates by main groups on the shelf 10–200 m (kg/h)

A swept area estimate of the biomass on the 3,600 nmi² shelf north of Ras Mabber using the mean densities of the February survey gave 150,000 t, of which 83,000 t represented the demersal group of potential commercial species.

Species composition of demersal fish

In Yemen the dominating species among the demersals were:

This assemblage reflects mainly the composition of the demersal fish group in the first survey. In the second survey a few high catch rates of Nemipterus japonicus dominated and most of the other species did not appear in the catches.

In Somalia the dominating species in the families of demersals were:

As pointed out by Bianchi (1992a) the seabream Boops boops common in the Mediterranean and off West Africa had not previously been reported from the Indian Ocean.

Review of findings and of fishery development

The surveys showed the well-known effect in this part of the Indian Ocean of redistribution of demersal fish caused by upwelling of oxygen depleted water during the southwest monsoon. Off Yemen also the coastal small pelagic fish seemed to have been dislocated by changes in the environment during the monsoon and surface aggregations of these fish were found outside the shelf area at some 30–60 nmi off the coast, a distribution not previously observed.

Using those surveys that are assumed to have given the best coverage of the target group, the following estimates of standing biomass were obtained:

These densities may be compared with the levels found in the Arabian Sea off Oman of 150 t/nmi² and thus confirm the relatively high productivity of the waters off Yemen and northeast Somalia.

In a discussion of the potentials of Yemen's resources Sanders and Morgan (1989) indicate that the official statistics of 70,000–90,000 t total landings in the period 1980–86 may represent an under-reporting particularly of the small-scale inshore fishery for oil sardine. There are uncertainties about the level of the industrial fishery for oil sardine during the 1970s. The FAO Yearbook of Fishery Statistics quote oil sardine landings of 80,000–90,000 t up to Vol 48 (1979), but only about 10% of that for the same years, from Vol 50 (1980) onwards.

The industrial fishery with large purse-seiners in any case ceased about 1980. The 1975–76 DR. FRIDTJOF NANSEN surveys gave estimates of about 0.5 million t of small pelagic fish for the Yemen coast, (Section 3.3). Results of exploratory fishing with purse-seines at the time (Ellingsen, 1975 quoted by Sanders and Morgan, 1989) tended to confirm a high abundance of small pelagics. The February 1984 acoustic survey showed a very low stock, while the 265,000 t estimate from August 1984 included an offshore distribution. Only part of this was, however, identified as oil sardine. These various data indicate a considerable variability in the stocks of small pelagic fish on the Yemen coast, perhaps affected by interannual variability in the monsoon through changes in the upwelling (as also seems to be the case in Oman).

An assessment of the oil sardine based on commercial fisheries data (Sanders and Bouhlel, 1984) showed a more limited potential, about 20,000 t annual yield, than perhaps indicated by the acoustic surveys. This assessment related particularly to the stocks available on a 90 nmi stretch of the coast near Mukalla. There may not be any discrepancy between this assessment and the survey data since these include a wider area as well as additional species.

In later years the stocks of small pelagics seem to have been only lightly exploited. Restraints other than resource availability limit an expansion of the fishery (Sanders and Morgan, 1989).

The considerable history of attempts and efforts to develop marine fisheries in Somalia in the 1970s and 1980s were reviewed by Anon., 1987. Although basic assessments of fishery resources had been provided by resource surveys at an early stage, sufficient information concerning commercially sustainable yields had been lacking. Various attempts had been made to obtain such information. As regards the resources of small pelagic fish two large Romanian factory trawlers were used in a simulation fishing programme off northeast Somalia in 1983–84 and had regular commercial size catches with a total production exceeding 6,000 t. The area fished was identical to that in which small pelagic fish was located in the 1984 DR. FRIDTJOF NANSEN surveys. An annual potential yield of 40,000 t was estimated by the Romanians for this area. Considerable environmental, technological and biological problems were, however, experienced and no follow-up resulted in terms of development of an industrial fishery.

Under a Fisheries Exploration/Pilot Project 1984–87 (Anon., 1991b) two purse-seiners were used for exploratory fishing of small pelagics in 1986–87. They were according to the report deployed in North Somali waters where the results led to the conclusion that “There was not enough off-shore fish to justify a major purse-seining method of harvesting small pelagic species.” The programme included trials off Ras Hafun on the northeast coast where, however, purse-seine fishing is difficult in the period April-September due to wind and current conditions.

During a period of one year 1984–85 two large Japanese trawlers operated in a commercial bottom trawling feasibility study and caught well over 3,000 t of mixed demersal fish, for which, however, difficulties were experienced in finding international market acceptance (Anon., 1987). A Somalia/Egypt joint trawling venture with a smaller vessel and marketing in Egypt was economically more successful.

The problems of utilizing the marine fish production in Somalia are no doubt related to such factors as the remoteness of the northeast coast, the limited tradition in fisheries, the narrow shelf, the rough sea and wind conditions during the monsoon, the low fish consumption in the country. In these many constraints Somalia may represent a special case. The various unsuccessful efforts to develop the fisheries can on the other hand be seen as an apt illustration of the often complex process from base line resource studies to the stage of their commercial utilization.