6. Summary of findings and estimates of sustained yields

During each of the survey periods in the post-monsoon autumn season September-November 1979 and the pre-monsoon spring season March-April 1980 the Burmese waters between the border with Bangladesh in the north and that with Thailand in the south was covered twice. The observations of the distribution and abundance of the fish resources were pooled for each season so that two sets of estimates are available, one from September-November 1979 and one from March-April 1980. Observations of the hydro-graphical environment demonstrate clear seasonal changes in the distributional characteristics of temperature, salinity and oxygen which are likely to affect the fish resources in various ways.

The acoustic recording of fish were classified into two main groups: "small pelagic fish" and "other fish", the latter consisting mainly of demersal and semi-demersal forms. By the help of nearly 400 fishing stations the species- and size composition of the fish were sampled. The findings from this work are presented in a number of charts and tables which demonstrate the most common types of fish by season and area.

The acoustic observations together with information on the size of the fish form the basis for assessments of the fish biomass at the time of the surveys. For navigational reasons inshore areas of depth less than about 15 m could not be covered in these surveys. These uncovered parts of the shelf comprised about 30% in the autumn and 20% in the spring. The biomass have been raised by simple area ratios to compensate for the uncovered shelf parts.

The assessments of total biomass were (thousand tons).

	Autumn	Spring
Small pelagic fish	620	1330
"Other fish"	540	960
Total	1160	2290

The biomass during the spring is about twice that of the autumn. For small pelaqic fish the figure is more than doubled while there is an increase of about 80% in "other fish". The most likely explanation of this difference is that it is caused by a seasonal fluctuation in biological production of these generally short-lived fishes. Similar seasonal changes in biomass have been observed in nearby and comparable areas such as the coast off Pakistan and the SW coast of India. The mean densities of biomass measured over the shelf (down to 200 m depth) were 17 tons /n.m.² in the autumn and 34 tons/ n.m² in the spring.

The biomass of small pelagic fish was found to be highest in the Delta and on the Tenasserim coast:

Small pelagic fish (1000 tons )
	Autumn	Spring
Arakan	180	170
Delta	370	640
Tenasserim	70	520
	620	1330

"Other fish" was found in highest abundance in the Delta area: Other fish (1000 tons)

Small pelagic fish (1000 tons )
	Autumn	Spring
Arakan	130	120
Delta	290	710
Tenasserim	120	130
	540	960

The total biomass and mean densities in tons/n.m.² were distributed on the areas as follows:

	Autumn		Spring
	Biomass	Mean	Biomass	Mean
	Density		Density
Arakan	310	28 t/n.m2	290	26 t/n.m2
Delta	660	19 t/n.m2	1350	2 39 t/n.m2
Tenasserim	190	9 t/n.m2	650	31 t/n.m2
	1160	17 t/n.m2	2290	34 t/n.m2

From this it is seen that the relatively limited shelf area along the Arakan coast holds quite high densities of fish biomass both in the autumn and in the spring.

The densities measured as biomass per unit shelf area is thought to be meaningful when used as here to compare seasons or nearby areas. As a general measure of production it is of somewhat doubtful value since biological productivity in some areas extends outside the shelf and in others may be limited to only a part of the shelf.

A few words must be said about the likely systematic errors in these assessments. An attempt has been made to adjust for the incomplete areal coverage of the shelf on the assumption that mean fish densities in the inshore shallow areas are the same as on the outer shelf. Inshore areas are often especially rich in fish and the adjustment may thus be an underestimate. There are two further sources of bias which also lead to underestimation. The acoustic system does not cover the depth layer from the surface down to well below the depth of the transducer at about 15 m, nor will it obtain echoes from fish very close (within about 1/2-l m) to the sea bottom. These effects on the total biomass estimates cannot be quantified, but they are not likely to represent more than a minor fraction.

Only a part of the biomass can be harvested on a sustained basis. For long-lived larger fish the harvestable part is smaller than for smaller shortlived fish. The transformation from standing biomass to sustained yield is also complicated by the observed fluctuations of the biomass and the uncertainties regarding the effects on the stocks of the present fishery which is reported to be about 300 000 tons/year. A yield estimate on the conservative side is obtained by using the simple mean of the two biomass values. This would give an average standing stock of about 1 million ton of small pelagic fish and 750 000 tons of demersal and semi-demersal fish. Assuming the yield fractions for these to be 0.5 and 0.25 (based on assumed values of natural mortality M of 1.0 and 0.5 respectively) the annual potential yields would be 0.5 million tons of small pelagic fish and nearly 200 000 tons of demersal- and semi-demersal fish.

Higher estimates of the sustained yields are obtained if it is assumed that the curves describing the seasonal fluctuations in biomass approximates a sinus form. The mean available biomasses are then well over 1.1 million tonnes of pelagic fish and 800 thousand tonnes of "other fish". A possible "upper" limit for the yield estimates can be obtained by assuming the natural mortality for small pelagic fish to be 1.2 and that of "other fish" to be 0.7. The latter may include a fishing mortality component. The yield fractions will then be 0.6 and 0.35, and the sustained yields will be 670 thousand tonnes and 290 thousand tonnes.

The estimates can be summarized as follows (1 000 tonnes)

	Lower estimates		Higher estimates
	Mean biomass	Yield	Mean biomass	Yield
Pelagic fish	1 000	500	1 100	670
Demersal and semi-demersal fish	750	200	830	290
Total	1 750	700	1 930	960

Both of these estimates indicate a considerable potential for increase of the present catch provided the types of resources are economically and technically acceptable. And with the development of the various components of a fishery information will be obtained which can help improve the assessments obtained here.

A total yield of the order of 700-900 thousand tons agrees reasonable well with several previous estimates. In "Notes on Fisheries in Burma" dated 6 January 1978 the following assessments are quoted:

		Tonnes
S. Jones & Banerjee	1968	1 575.000
Prasad R.et al	1970	726.000
Gulland	1972	625.000
Menon	1977	1 512.000

The first and last of these figures probably refer to the total standing biomass.

The fishing experiments although carried out for the main purposes of identification and sampling, also provided valuable information on catch rates. On the average catch rates in survey-fishing will be lower than in commercial fisheries. The review of the fishing results below show that the obtained catch rates in a general way confirm the relatively high abundance of fish resources in Burmese waters.

	BOTTOM TRAWL		PELAGIC TRAWL
	Oct-Nov /79	Mar-Apr /80	Oct-Nov /79	Mar-Apr /80
ARAKAN
Number of stations	45	28	11	22
Mean total catch per hour	609	1285	132	238
Max. Catch rates*	6390	5254	378	680
DELTA
Number of stations	44	57	26	39
Mean total catch per hour	506	513	232	559
Max. Catch rates*	2260	1380	1142	5340
TENNASSERIM
Number of stations	16	26	7	30
Mean total catch per hour	894	1041	83	297
Max. Catch rates*	3930	5067	504	1062

* Means of 3 highest catches.