5. Assessment of biomass

The surveys provide data for quantitative estimates of the fish usually expressed as tonnes of standing biomass. The most important data derive from the acoustic system, but the trawling programme of Survey II also provide a basis for an estimate of demersal fish biomass.

5.1 The acoustic method and its limitations

The most favourable conditions for acoustic research are when the main fish biomass aggregates in dense concentrations, the various species does not form spatial mixtures, and the presence of plankton does not screen the fish registrations on the echosounder. Unfortunately the conditions in Pakistan are far from any of these ideal conditions. Most of the biomass forms scattered distributions, with only very few dense patches; at most locations the variety of species is great and at times the plankton screens hinder an accurate estimation of the rather scattered fish densities. All this are factors which lowers the precision of the acoustic estimates. By survey repetition one can, however, partly offset these limitations. In addition to the three surveys described in this report, results from a number of similar previous surveys are available for comparison.

5.2 The acoustic estimates of biomass

The total estimates by regions and surveys are given in Table 7.

Table 7. Total estimates of biomass, survey I - III (thousand tonnes).

	Makran Coast	Sonmiani Bay Sind Coast	Total
Survey I - 5-16 Sep 83	450	600	1050
Survey II - 20 Jan-2 Feb 84	150	500	650
Survey III - 2-12 June 84	300	700	1000

The total of all Pakistan is estimated to 1050, 650 and 1000 thousand tonnes during the three surveys respectively, average 900 thousand tonnes.

Split on regions the Makran coast is estimated to hold 450, 150 and 300 thousand tonnes during the three surveys and the corresponding figures for the region from Sonmiani Bay to Sind Coast are 600, 500 and 700 thousand tonnes. Average estimates are 300 thousand tonnes for the Makran Coast and 600 thousand tonnes for Sonmiani Bay - Sind.

We have no supporting information to explain the decrease in biomass during the second survey, which in fact goes contrary to the expected seasonal tendency based on previous surveys. Part of the decrease can be due to a migration of fish outside the Pakistan EEZ. It seems very likely, in fact, that some of Pakistans fish stocks, particularly from the pelagic community are shared with neighbouring states.

It might also be possible, however, that the resources are considerably underestimated during the second survey due to bias introduced through the survey setup. As the first and last survey are in closer agreement, we consider them as more representative for the actual biomass level. To conclude the general biomass picture, we suggest the following figures as representative for the level of the resources. Rounded figures, thousand tonnes:

Makran Coast	Sonmiani Bay Sind Coast	Total
350	650	1000

Due to the dominant presence of hairtails both in the pelagic and in the demersal fish communities it has been difficult to split the total biomass into the categories small pelagic fish and demersal fish as usually done in the course of the surveys. Through a post-analysis of the acoustic records and the composition in the catches we have set up approximate figures to express the relative weight of small pelagic and demersal fish in the total fish estimates. These are shown in Table 8.

Table 8. Approximate ratios in percent between pelagic and demersal biomass by regions and surveys.

	Makran Coast	Sonmiani Bay Sind Coast	Total
Survey I	65:35	65:35	65:35
Survey II	80:20	90:10	87:23
Survey III	50:50	70:30	65:35

Average rounded figures based on the first and last survey will be:

	Makran Coast	Sonmiani Bay Sind Coast	Total
Small pelagic	55%	65%	60%
Demersal	45%	35%	40%

Applying these ratios to the total biomass figures above, one comes up with the following estimates. Thousand tonnes:

	Makran Coast	Sonmiani Bay Sind Coast	Total
Small pelagic	190	420	600
Demersal	160	230	400
Total	350	650	1000

As mentioned above, due to a general picture with many intermingling species both in the demersal and in the pelagic communities, the precision of these estimates are likely to be relatively low. Based on experience from similar surveys we would therefore recommend the use of a 25% range above and below these values within which the true figure is likely to be. One could also expect that this would take account of any main factors of bias in the results. For fishery development purposes it is advisable to use the most conservative of these estimates and rather adjust these in the course of later improved estimates.

It can thus be concluded that the standing fish biomass on the Pakistan shelf in 1983/84 is estimated to have been as follows (thousand tonnes):

	Makran Coast	Sonmiani Bay Sind Coast	Total
Small pelagic	140-240	320-520	450-750
Demersal	120-200	170-290	300-500
Total	260-440	490-810	750-1250

“Dr. Fridtjof Nansen” has carried out previous surveys in Pakistan waters. In 1975-76 five coverages were made under the Indian Ocean Programme covering the whole Arabian Sea. In 1977 five surveys were carried out covering the Pakistan shelf. During the first series of surveys the total biomass varied between 330 and 1110 thousand tonnes with 620 thousand tonnes as an average. During the second series, in 1977, the total biomass was estimated from 250 to 1300 thousand tonnes, with 630 thousand as an average figure. These results have a wider range than found in the surveys described here and they include some very low estimates of biomass. These may partly reflect seasonal changes in biomass caused by fluctuations in short-lived species and by migrations outside the area. But one has also observed in this area and on the SW coast of India an on-shore as well as in off-shelf migration of fish caused by upwelling during the SW monsoon, which may bring part of the resources outside the range of the surveys. The extent of this migration is likely to vary from year to year. As a whole it is thought, therefore, that the results of previous surveys tend to support the present findings.

Estimates of average densities on the shelf can be useful in comparative studies. The shelf off Makran is 3380 nm² and that from Sonmiani Bay to Sind is 9390 nm². This gives densities of 77-129 tonnes/nm² for Makran and 53-88 tonnes/nm² for the Sonmiani - Sind region. Average values for the whole Pakistan shelf is 59-98 tonnes/nm².

For comparison similar studies from other regions of the world, from surveys carried out with the R/V “Fridtjof Nansen” with the same methodology, have given the following average densities (tonnes/nm²).

West Sahara, Senegal, Guinea:	100-110
Northeast Somalia:	90
Mauritania:	75
Ivory Coast, Ghana:	60-80
Tanzania, Mocambique:	30-40
Burma, Bangladesh:	25
Kenya:	18

The shelf off Pakistan thus belongs among the richer regions in terms of standing stock per unit area.

5.3 Trawl survey estimates

During the second survey, upon request from Department of Fisheries, Karachi, a programme of non-aimed trawling at preselected locations was carried out. 82 stations were worked and on the basis of these, an estimate of the demersal biomass have been calculated by the swept-area method. Applying a catchability coefficient equal to 1.0, i.e. assuming that all the fish in the path of the trawl are caught, one obtains a total estimate of 250 thousand tonnes of demersal fish for the shelf off Pakistan. Pelagic fish have been excluded from the calculations when present in the bottom trawl catches. A catchability coefficient equal to 1.0 tends to give quite conservative estimates. In addition, some of the demersal fish is outside the 6 meter vertical bottom interval swept by the bottom trawl. Thus the 250 thousand tonnes estimated from the trawl survey is in fair agreement with the 260-440 tonnes of demersal fish estimated from the acoustic surveys.