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6. Summary of findings, discussions, conclusions


6.1 Summary of survey findings.
6.2 Results of fishing operations.
6.3 Discussion of the findings.
6.4 Brief conclusions.

6.1 Summary of survey findings.

The overall objective of the survey was to describe and assess the demersal, semi-demersal and pelagic resources available on the Sri Lankan coast principally over the shelf banks. Because of the extreme shallow waters of the Palk Bay and -Strait this area could not be included.

Observations on the type of bottom was processed and presented in a bottom-type chart. This confirms previous findings that areas of good trawling ground are limited, mostly to the shallow inshore parts of the shelf, and the shallow northern area.

Hydrographic observations enabled a description of the watermasses along and on the continental shelf, the depth of the thermocline and the oxycline. The content of dissolved oxygen below the oxycline was low, in most cases around 1 ml/l. In this environment significant quantities of commercial types of fish will not be found. On the north-west, north-east and east coast the depth of the oxycline was only about 50 m, while it was found at about 100 m or more off the west, south-west and south coast. The observed variations in fish distribution may be related to this difference.

A detailed investigation of the deep-water trawling ground in the Gulf of Mannar was undertaken as a special first task of the work. The trawlable area was located and mapped, but found to be of rather limited extent, only about 2 by 6 nautical miles. Catches comprised deep sea lobster, deep sea prawn and deep-water fish in the approximate ratios 1:3:8. Catch rates for lobster ranged up to 140 kg per hour's tow, for prawn up to about 600 kg and for fish up to about 1.3 tonnes. The average catch rates of seven hauls: 55, 170 and 520 kg per hour respectively for the three types of resources are higher than those reported in earlier surveys, but a substantial sustained yield cannot be expected from this small fishing ground.

The main survey effort provided a chart of the distribution of the observed echo intensity of fish in the coastal areas covered. Through a classification of the echo records and from results of fishing experiments different types of resources were identified. The two most important were demersal and semi-demersal larger fish such as snappers, groupers, breams and trevallys on the one hand and smaller pelagic schooling fish such as scads, sardines, silver bellies, etc. on the other. The relative distribution of these resources are shown in Figs. 16 and 17.

On the basis of certain assumptions concerning the conversion of observations of echo abundance to measures of standing biomass, preliminary estimates of quantities of biomass were made for various areas of the coast. These findings can briefly be shown as follows:

Area

Biomass 1000 tonnes

Bank area inside 100 fathoms nm2

Types of resource

Negombo-Galle

220

1350

Mostly demersal and semi-demersal

Hambantota

100

940

Demersal with some small pelagic

East Coast

120

1300

Pelagic and demersal

Trincomalee to Mullaittivu

50

500

Mostly small pelagic

Pedro Bank

30

1020

Demersal and pelagic


The average density of fish biomass in the whole area covered was about 100 tons per (nm)2. The sustained yield forms only a minor part of the standing biomass. For long-lived larger fish the harvestable fraction is smaller than for smaller short-lived species. Assuming these proportions to be 0.2 and 0.5 respectively, the annual potential yields from the five survey areas would amount to approximately 70 000 tonnes of demersal fish and 100 000 tonnes of pelagic species. The main part of the demersal and semi-demersal fish are available on the south-west and southern part of the coast. A rough assessment indicates that only about a third of the biomass on the Pedro Bank would be available for bottom trawling, indicating a potential annual yield on this bank of about 2000 tonnes for this type of fishery.

6.2 Results of fishing operations.

The fishing operations primarily served to identify the echo targets and provide samples of fish, but some limited conclusions may also be drawn concerning the possible commercial usefulness of the operated gears. Bottom trawling was fairly successful in a few localities, but in general the distribution of demersal or semi-demersal fish did not coincide with the rather limited areas of good trawl bottom. Catches with the pelagic trawl were poor, but the rather few hauls made do not provide a basis for evaluating the possible usefulness of this type of gear in Sri Lankan waters. The same reservations must be taken with regard to the results of the trap fishing. Fishing with bottom longlines did, however, produce envouraging catch rates in several localities, notably on the Hambantota Banks. It seems reasonable to expect that with a better knowledge of the local fishing grounds and the most suitable bait, hauling time etc., this gear could be put to good commercial use. The problem of losses of hooks and of captured fish from high abundance of sharks was evident, but this could be reduced by reducing the fishing time of the lines. Exploitation of the shark resources with special shark lines would after some time also help to alleviate this problem.

6.3 Discussion of the findings.

The main finding of the survey is represented by the assessed biomass of approximately 0.5 million tonnes and its distribution along the stretch of coast covered by the survey from Negombo to the Pedro Bank, an area of around 5 thousand nm2 of shelf. The sustained yield from this biomass is assessed at about 170 000 tonnes of which 70 000 tonnes are estimated to be valuable demersal and semi-demersal food fish. The mean yield density corresponds to 100 kg/ha for all the resources and 40 kg/ha for demersal resources respectively, both of which represent high values compared with many other areas. It seems likely, however, that a shelf area such as that of Sri Lanka exposed to the strong and changing ocean currents related to the monsoon systems should have a relatively high biological productivity. The resources also extend offshore beyond the shelf, and the reference to productivity per unit shelf area is thus of limited relevance.

Various limitations and reservations should be kept in mind when evaluating these findings. They represent only a part of the fish resources of Sri Lanka's shelf because of the incomplete coverage of the north-west coast and the exclusion of the northern Palk Bay and Palk Strait areas. This latter represents about 2800 nm2 of shelf area. Also the extreme inshore shallow waters were incompletely covered, this being particularly the case in the north-west 2 area where the estimated biomass was only 40 000 tonnes on a 1500 nm2 shelf coast. Results of fishing experiments have indicated relatively high abundance of certain types of resources at least in some parts of these northern shallow waters (see Annex I).

In order to enable a rough assessment of the potential also in these northern uncovered areas one may make the simple assumption that the density of biomass found in that part of the north-west area which was actually covered by the 2 survey, viz. 50 tonnes per nm2, extends over the whole northern grounds. This represents half the mean density of biomass found in the surveyed area from Negombo to Pedro Bank. These waters would then hold some 170 000 tonnes of fish biomass. The major part of the well over 200 000 tonnes biomass from the north-west and north coasts will be small short-lived species eg. silver bellies, and the potential yields may be assessed at 70 000 tonnes of small sized fish and 10 000 tonnes of larger food fish. With these rough assessments added, the total potential yield for the Sri Lanka shelf is indicated to be 250 000 tonnes of which about 80 000 tonnes is valuable food fish. This assessment includes the major part of the present catch of about 100 000 tonnes.

This total potential of a quarter million tonnes per year may well turn out to be a conservative estimate. It does not specifically identify the important resources of large pelagic fish, tunas and tuna-like fish which are probably only partly included in the biomass assessment. The current annual catch of tunas, bill fishes and sharks at and beyond the shelf edge amounts to about 20 000 tonnes. A decline of catch rates indicate that these stocks may already be affected by exploitation.

The trawl fishery potential of the Pedro Bank of only 2000 tonnes is somewhat lower than previous estimates, which range from 3000 tonnes upwards. (Anon (1975)). From evaluations of previous fishing experiments and commercial results on the bank it seems probable that the available stock on this ground may fluctuate seasonably.

In general it should be stressed that the results reported represent the state of the resources of the time of the survey. In nearby and comparable areas such as the Pakistani coast and the south-west coast of India large-scale seasonal fluctuations of the biomass of short-lived species such as sardines and mackerel have been demonstrated (Anon (1978) and Anon (1976b)). It is likely that such seasonal changes also occur in Sri Lankan waters, and in addition there are possible changes in available biomass caused by migrations which in particular may affect larger fast-moving surface pelagic fish such as tunas and tuna-like species.

6.4 Brief conclusions.

The survey describes the distribution of the biomass of commercial fish along the coast from Negombo to Pedro Bank, the total standing weight of which is assessed at approximately 0,5 million tonnes. On the basis of some simple assumptions concerning the mean density of biomass in the northern shallow waters (Palk Bay and -Strait) not covered by the survey, the total biomass of Sri Lanka's coastal shelf and imediately adjacent waters is assessed at close to 3/4 million tonnes. The sustained annual potential yield from these resources is indicated to be about 250 000 tonnes, of which about 80.000 tonnes represent large demersal or semi-demersal fish. This estimate may well prove to be on the conservative side, but it should be used as preliminary advice that the present level of catches of about 700 000 tonnes can at least be doubled.

An important finding is the demonstration of good resources of demersal and semi-demersal fish on the south-west coast from Negombo to Galle and on the Hambantota banks.

Demersal resources were rather sparse on the Pedro Bank at the time of survey.

In general the diversity of types of resources, their distribution and the nature of the shelf grounds suggest conditions for a diversified fishery based on small fishing units.

Since it is likely that considerable seasonal changes occur in the abundance of the fish resources and in their distribution, a repetition of this type of survey at other seasons is highly recommended. Investigations of the resource potentials of the shallow northern areas are also essential for providing the basis of resource information needed for development planning.


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