OBSERVATIONS ON THE ARTISANAL PRAWN FISHERY IN THE, SHALLOW COASTAL WATERS OFF CHILAW DURING 1994-1995

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INTRODUCTION
Prawns are considered as a luxury food commodity in international trade. They supply an increasmg population with proteins and provide significant earnings of foreign exchange. The prawn industry has thus assumed major importance in recent years in Sri Lanka.' The most productive fishing grounds and the majority of species of high commercial interest are found in the tropic^.^ Most of the artisanal production of prawns in Sri Lanka comes from lagoons." The penaeid prawns of Sri La~llra from estuaries and sea are important commercial fishery resource^.^ Thirty one species of prawns have been recorded off Sri Lanlra." Fishery for prawns in estuaries, lagoons and the shallow coastal areas of' the seas off Sri Lanka has a long history. Prior t o the introduction of the mecharlization to the fishing industry in Srj Lanlra prawn resources have been ' Corlusponding author extensively exploited using the traditional fishing methods in the different regions of the country. Prawns contribute around 3%) by weight of the total marine landings of the island. Out of this 20% comes from Negombo and Chilaw which are the major prawn landing centres of the country."n addition cultured prawns contribute significantly to the foreign exchange earnings from aquatic products and the percentage contribution has varied from 48% to 70% in the recent years.7 Artisanal prawn fishery has been in existence in the seas off Chilaw over the years. A ban on prawn trawl fishery was imposed in Chilaw in 1992. Since then the artisanal prawn fishery has been mainly responsjble for prawn catches from the Chilaw area. The present study was conducted to investigate the impact of the ban imposed on prawn trawl fishery on the traditional prawn fishery in the shallow coastal waters off Chilaw. The present investigations of the traditional prawn fishery included studies on catch and effort, species, length composition of catches and specifications of the crafts and gear.

METHODS AND MATERIALS
The artisanal prawn fishery in the shallow coastal areas off Chilaw was studied for a two year period from January 1994 to December 1995. Initially a frame survey was carried out along the study area to identify fish landing centres which represent different craftlgear combinations in the fishery. From around 08 fish landing centres scattered along the study a r e a 9 w o major fish landing centres, Chilaw and Weljhena were selected for sampling.
Every second week, the selected fish landing centres were visited to collect information on fish catch, fishing effort, gear details, species composition of the catches and the length measurements of the commercially important species. Of the total number of crafts operated, 20 -40 percent were sampled a t each sampling site.
The total number of crafts operatedlday was taken as the unit of measure of the effort of this fishery. These values were derived for each month by averaging total number of crafts operated on sampling days. This was used to calculate the percentage of crafts operated in each month a t the sampling site and this was then extrapolated to obtain an estimate for the study area.
The average catch/craft/day was considered as the catch per unit effort for each month. The total catch for a day was estimated by multiplying the total riunlber of crafts operated on a sampling day by the average catch per craft sampled. To obtain the monthly total catch, the estimated daily total catch was multiplied by the number of fishing days in each month (generally around 23 -26).

Fishing crafts and gear
The prawn resources in the shallow coastal areas off Chilaw are harvested by around 200 traditional log rafts (LR) scattered throughout the study area. In addition there were around 80 fibre reinforced plastic (FRP) boats operated around the sea mouth of the Chilaw Estuary especially during the period from January to April of each year.
The fishing gear used are trammel nets (TN) which are made up of nylon multifilament twine. Each net consists of number of equal size net pieces (each piece is 1500 mesh long and 65 mesh wide). The size of the net is determined by the number of these net pieces. The number of such units used for a single operation varied from 4 -20 (Mean = 11.7 SD = 3.55) for log rafts while for FRP boats they varied from 8 -32 (Mean =15.4 SD = 6.41) ( Table 1).

Fishing effort
The traditional prawn fishery in the shallow coastal areas off Chilaw is highly seasonal due to the inability of carrying out fishing operations during the south west monsoonal period with prevailing unfavourable weather conditions. The fishing operations were generally therefore restricted to the periods January -April and October -December during both years studied.
The total annual fishing effort (number of fishing operations) in the study area was estimated a t 20,808 for the year 1994. Of this fishing effort, 83% was carried out by traditional log rafts and the remaining 17 % by FRP boats. The following year, the fishing effort in the study area dropped from close to 20,000 fishing operations to slightly over 18,800 fishing operations which is a drop of about 9 %. The composition of the annual fishing effort does not show considerable change during the two years studied. There is a tendency for an increase in effort during the period November to January (Fig. la). The variation pattern of the effort was the same for the two years covered during the present investigation.

Catch rates
The catch rates of the two different craftlgear combinations does not show considerable variations. Highest catch rates of 9.96 kgloperatjon and 10.67 Irg/operation for the years 1994 and 1995 were recorded for the craft/gear combinations log raftltrammel nets and FRPItrammel nets respectively. During the present investigation the average catch rates recorded for the craftlgear combinations log raftltrammel nets and FRPItrammel nets were 3.74 kg/boat/day and 7.25 kg/boat/day respectively. In general the avetage catch per effort dropped for both craftlgear combinations between 1994 and 1995. The variation patterns of the catch rates of the two different craft/gear combinations were found to be si~nilar in both years studied (Fig. lb). There is a trend for the catcldeffort to increase jn October -November and decline to a minimum in March.

Total catch
The annual fish catch from the traditional prawn fishery in the study area was estimated a t 92 MT in 1994 and 68 MT in 1995. This was approximately a 26 94, reduction in the fish catch. Contribution of the trammel nets operated by the log' rafts to the total production of the fishery was around 69 %. The variation pattern of the total production was the same for both craft types and for the two years studied (Fig. lc). Generally the period November to February could be considered as the peak period for the fishery though high catches were observed in the month ofApril of both the years studied.

Species and size composition of the catches
A total of 3 1 species of finfish and shellfish belonging to 14 families with some commercial value were identified among the catches (Appendix 1). Almost all the finfish and shellfish species caught are of marine origin. Although a number of species are fished and have some commercial value, only a few dominate the catches in terms of weight and/or value. Of the important crustacean species P indicus, P. merguiensis, P semisulcatus and Portunus pelagicus (sea crab) dominated the catches and Leiogn,athus spp. and sciaenids were among the finfish species caught. Two major craft/gear combinations used in the study area (FRP/TN and LR/TN) have not shown a difference in catch composition ( Fig. 2a and Fig. 2b) since both targeted the more valuable prawns.
Due to the narrow range of the mesh sizes used in the traditional prawn fishery (32 -50 mm, 38 being the most common) there is no considerable size range

Penaeus semisrilcatus
In In of'the prawn species caught in this fishery. Length frequency data (Fig. 3) showed that the total lengths of the major prawn spe~ies caught in this fishery (P indicus, I? m,erguicl~sis and I? .scnzisulcatus) vary from 9.9 cm to 19.8 cm, 13.8 cm to 21.8 cm and 11.6 cm 19.5 cm respectively. Small prawns (Metapenaeus spp.) were recorded only among the catches of seasonal fishing operations and small meshed gillnets particularly in JanuaryIFebruary period. The total lengths of the Metapenaeus dobson,i caught by seasonal fishing operations and small meshed gillnets varied from 7.1 cm to 13.6 cm and 7.3 cm to 13.7 cm respectively.

In
Seasonal fishing operations for small p r a w n s Observations were also made on the seasonal fishing operations for small prawns in the study area carried out on an irregular basis. During the present study, these operations took place only in the months of January and February of 1995. The gear used was an encircling gillnet, rectangular in shape (length = 12.3 m, height = 2 m and mesh size = 7 mm stretched mesh). The fishing unit consisted of a FRP boat and a traditional log raft with the crew number varying from 4 to 6. There were around 20 such fjshing units in the study area. The estimated average catch rate for the seasonal fishing operations was 62 kgloperation. The total productivity from these operations was estimated at 17.36 MT.

DISCUSSION
The variation pattern of the catch rates of the different crafugear combinations were found to be similar during both years studied. There is a trend for the catchieffort values to increase in October -November and decline to reach a minimum in March. High catch per unit effort values were observed a t t h e beginning of the north east monsoon. Similarly a period with low catch rates coincides with the tail end of the north east monsoon. It is understood that the total. catch rate of the prawn fishery is determined by the catch rate of the finfish to a great extent. In addition, the period of high catchleffort for prawns coincides with the estimated spawning season of the penaeid prawns in the west coast of Sri Lanka (August to F e b r~a r y ) .~, " In the study area, the total annual production from the artisanal prawn fishery obtained for the 1994 period was 92 MT which was comparatively higher than the production obtained for the 1995 period which was 68 MT. In addition the estimated total production from the seasonal fishing operations for sma1.l prawns in the seas off Chilaw in 1995 period was around 17 MT. However, the comparatively high production obtained in 1994 period could have been probably due to the high average catch rate and the fishing effort of both craftdgear combinations in 1994 which led to a high productio~l in the 1994 period. The production from the artisanal prawn fishery in the study area is quite low when compared to the production from the prawn trawl fishery in Chilaw area during the periods of 1979-1980 and 1980-1981. Total productions from the prawn trawl fishery in Chilaw area for the periods 1979-1980 and 1980-1981 have been estimated to be 364 MT and 359 MT respect,ivel y." Contribution of prawns to the total productivity of the artisanal prawn fjshery in Chilaw was around 24% whlch is comparatively low when compared to that of the prawn trawl fishery m Chilaw area which was around 37 %.However, the prawn catches from the artisanal prawn fishery were dominated by the large and more economically important prawn species such as P. indicus, P rnerguiensis and P. sernisulcatus and the contribution of small prawns (especially t h e Metapen.aeus spp.) to the prawn catches was found to be insignificant. This could have been probably due to the structure of the trammel nets employed in the artisanal prawn fishery which were especially designed to catch large prawn varieties. However, according to ~a~a k o d y , ' the contribution of small prawns to the prawn catches from the trawl fishery in the Chilaw area was significant and it was around 26 %).
Prior to the termination of the prawn trawl fishery in the coastal waters off Chilaw in 1992 these two fisheries (artisanal prawn fishery and the prawn trawl fishery) were mainly responsible for prawn catches from Chilaw area. However, after the imposition of the ban on prawn trawl fishery in 1992, the prawn resources in the shallow coastal waters off Chilaw are exclusively exploited by the traditional prawn fishermen which provide livelihood for more than two thousand people scattered throughout the study area. The present investigation shows that the small prawn varieties which made significant contribution to the prawn catches from the trawl fislzery are presently exploited by the seasonal fishing operations which last only for very short period during Januarymebruary with the appearance of massive schools of small prawns in the coastal waters off Chilaw.
According to the common understanding of the fishermen these fishing operations are carried out more or less on an irregular basis and during the present study seasonal fishing operations were viable only in 1995 for a short period of a couple of weeks. Therefore, it is possible that this resource is under exploited a t the moment. However, prior to making any conclusion on this aspect a detailed study should be conducted to understand the distribution, migration and the stoclr abundance of the small prawn resources in the study area. Findings of the present investigation would be useful as baseline information for such a detailed study.