A study was conducted for a 12-month period, from June 2008 to May 2009 based on the catch data to determine the catch composition of Set Bag Net (SBN) that is a type of fishing net laid down to catch Acetes shrimps that move along the estuary of Merbok River in the state of Kedah, Peninsular Malaysia. Bag net is a static traditional gear with a cod end mesh size measuring 0.5 cm. The results showed that total catches of the bag net over a year period were constituted of three major catch groups viz. Acetes shrimps (89%), juvenile fishes (9%) and other non-Acetes shrimps (2%). The annual mean percentage composition of sergestid shrimps comprised of A. japonicus, A. vulgaris and A. indicus were found to be 89, 7 and 4%, respectively. The gear is very selective and besides Acetes there was also a small proportion of fish juvenile being caught. Seven species of juvenile fishes were recorded throughout the catch period however; their mean total abundance was very low (9%). Penaeid shrimp was the only group of non-Acetes shrimp been recorded. Based on the analyzed catch composition data, it could be concluded that SBN is Acetes-selective and not causing significant great damage to the juvenile fish population inhabiting the study area.
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The Set Bag Net (SBN) is locally used as a static gear to trap the sergestid shrimps (locally known as udang geragau) in the estuarine waters of Merbok River in the state of Kedah, Peninsular Malaysia. It is a fixed narrowing net, similar to a trawl net, set in the tidal stream by attaching it to hold-fasts. It has a rectangular mouth kept open by two vertical bamboo poles. The net is held in a fishing position against the current by linking the extended sides of the net (wing tips) to hold-fasts by means of long bamboo poles and steel wires. The hold-fasts are two wooden stakes fixed some distance apart in the sea bed, so that the net is parallel to the direction of the current. The mesh size decreases from 3.0 cm at anterior section to 0.5 cm at the cod end. Similar types of nets are also been used in other Acetes fishing areas in the Peninsular (Amin et al., 2008a; Oh et al., 2010). Bag net fisherman normally carried out its operation during the daytime by setting the mouth of the net against the flow of tide. The SBN traps sergestid shrimps that drift with current or do not swim fast enough along the estuarine bank. There is large population of Acetes shrimps that live along the shallow western coastal waters of the Malay Peninsula and a significant proportion of coastal fishermen are involved in the fishing of this small shrimps.
Despite its long years of operation, no published data is yet available on its catch composition. Although some studies on population biology of the genus Acetes have been earlier reported (Amin et al., 2008a-c, 2009a-c; Arshad et al., 2007, 2008), information is still particularly scarce on the fishery of SBN in the coastal waters of Kedah. The fishery statistics available is inadequate due to the fact that most Acetes shrimps caught are locally consumed and they are not been landed at fish landing jetties in the country. Over years, it is predicted that the small mesh size net used in the operation may lead to the overexploitation of aquatic species co-existing in the estuarine area. Thus, the present objective of the study was to examine the set bag net catch composition and the annual temporal variation of these SBN catches in the study area.
MATERIALS AND METHODS
Study area and sampling gear: Samples were collected monthly from the local SBN fisherman of Merbok River (N 5° 40' 48" and E 100° 22' 5''), coastal area of Kedah, Peninsular Malaysia (Fig. 1) between June 2008 and May 2009. The fisherman used SBN for fishing sergestid shrimps in the estuary of Merbok River.
Geographical local of the sampling station (•) in the estuary of Merbok River, Kedah. Peninsular Malaysia
The SBN is fixed mostly between 2 and 3 m depth along the shallow waters fringing the upper estuary. The mesh size of SBN is around 0.5 cm at cod end. Fresh samples for study were taken from fisherman immediately after hauling and preserved in 10% formalin for laboratory analysis.
Sample processing: In the laboratory, 200 g of Acetes sub sample in three replicates were taken from the total catch of every month. The three major groups comprising of the Acetes sp. other non-Acetes shrimps and juvenile fishes were sorted out during the twelve consecutive months.
Species identification and data analysis: Acetes specimen was taxonomically identified under a dissecting microscope with particular reference to the study of Omori (1975). Identification for juvenile fishes and other decapod shrimps were done based on the study of Mohsin and Ambak (1996) and Lovett (1981), respectively. Each taxa group was individually counted. Average percentage composition of different taxa groups was calculated from monthly sub sample data.
Catch category and monthly abundance: There were three main groups of fish resources observed from the catches of bag net in the study area (Fig. 2). The dominant one was Acetes shrimps (89%) and followed by the fish juveniles (9%) and other non-Acetes shrimps (2%). Monthly variation of different groups of SBN catches (No. of individuals / 200 g sample) are presented in Table 1. The average monthly number of Acetes was calculated at 1941.18 (individuals/200 g sample). The highest abundance (5116 individuals/200 g sample) of Acetes samples was observed in May and the lowest number (861 individuals/200 g sample) was found in October. While for the non-Acetes shrimps category, the annual average number was 43.81 (individuals/200 g sample). The lowest abundance (6 individuals/200 g sample) was recorded in April and the peak abundance (106 individuals/200 g sample) was seen in October. The estimated annual average number of juvenile fishes was 202.72 (individuals/200 g sample). The highest number (632 individuals/200 g sample) of juvenile fish was observed in the month of June while the lowest (60 individuals/200 g sample) was recorded in March.
Composition and temporal variation of Acetes shrimps: Three different species of sergestid shrimps (A. japonicus, A. vulgaris and A. indicus) were recorded in the study area (Table 2). The overall annual percentage composition of A. japonicus, A. vulgaris and A. indicus were found to be 89, 7 and 4%, respectively.
Annual percent composition of SBN in the estuary of Merbok River, Kedah, Peninsular Malaysia
Monthly variation of catches (No. of individuals/200 g sample) of SBN in the estuary of Merbok River, Kedah, Peninsular Malaysia
|Table 2:|| |
Monthly species composition (No. of individuals/200 g) of SBN in the estuary of Merbok River, Kedah, Peninsular Malaysia
Higher density (>80%) of A. japonicus was recorded almost every month in the study area (Table 2).
Composition and temporal variation of juvenile fishes: Seven species of juvenile fishes were collected together inside the set bag net in the study area. They were Sphyraena puthamiae, Trichiurus lepturus, Lagocephalus lunaris, Setipinna taty, Monodactylus argenteus, Pampus chinensis and Scatophagus argus. Highest number of juvenile fishes was observed in the month of June in the study area (Table 2).
Non-Acetes shrimps: Only prawn from family Penaeidae was recorded as the other non-Acetes shrimp of the SBN catches during the study period. The highest number (106 individuals/200 g sample) of non-Acetes shrimps was found in October and the lowest number was recorded in April (2 individuals/200 g sample).
Set Bag Net is effective to be used as an alternative fishing gear for the catching of Acetes resources besides trawl and push nets. In fact its application has been quite extensive in the country although the actual choice of the Acetes fishing gear is somehow limited by the sea bottom contours and tidal flow. There is no apparent published information regarding the catch composition of SBN fishery in the 60s and 70s in Malaysia. Catch composition of Estuarine Push Net (EPN) from Malacca and Estuarine Set Bag Net (SBN) from Johor state have been recently reported from Peninsular waters (Amin et al., 2008a-c; Oh et al., 2010).
In the present study, it was found that Acetes shrimps showed the highest level of total catches and showed highest percentage (89%) while juvenile fishes (9%) and other non-Acetes shrimps (2%) were present in insignificant proportion. This trend of catch composition showed similar consistency with the study reported by Tham (1954) for Singapore waters where the major catch composition was also Acetes. Statistics gathered shows wide fluctuation in abundance of Acetes with month as this shrimp is known to show great fluctuation in existence. However, the trend of catch composition group is similar with the Estuarine Push Net (EPN) in the Malacca state where push net is adopted as the main fishing gear, Acetes shrimps (90%) was also the main catch followed by the juvenile fishes (9%) and other non-Acetes shrimps (1%) (Amin et al., 2008a).
Three species of Acetes shrimps A. japonicus (89%), A. vulgaris (7%) and A. indicus (4%) were identified from the present study. Two of three species are the main species that are known to occur in the western coast of Peninsular Malaysia. Amin et al. (2008a) reported three species of Acetes shrimps (A. indicus 87%, A. japonicus 12% and A. intermedius 1%) from the push net operation in the coastal waters of Malacca, Peninsular Malaysia. It is observed that A. japonicus is the dominant Acetes species in the Kedah estuarine waters. This is in contrast to the southern region of the western coast of Peninsular where A. indicus is the dominant species (Amin et al., 2008a). This information further refines the species and geographical distribution of Acetes in Malaysian waters. The existing results again support the previous data of the Acetes operation in Johor state (bordering Singapore) by Oh et al. (2010). Both the studies (Amin et al., 2008a; Oh et al., 2010) showed that the gear is very species selective.
In Bangladesh it is a destructive gear because most of the catches are juvenile (Khan et al., 1994). About 185 species of fish are exploited by fishermen operating estuarine set bag nets in Bangladesh. These include 15 species of penaeid shrimp, three nonpenaeids, nine freshwater prawns, three crabs, three molluscs, 90 pelagic finfish and 62 demersal finfish (Islam et al., 1992). The bag net has been the important Acetes fishing gear to the traditional fishermen in Peninsular Malaysia due to its simple operation and cheaper to use. Although, seven species of juvenile fishes were recorded from this study their total abundance was very small (9%). Therefore, it could be suggested that the bag net operation is not showing significant damaging effect to the juvenile fishes inhabiting the estuarine area.
This study is part of a research project funded by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia (grant No. 05-01-04-SF0613). The authors would like to thank MOSTI for providing financial support to carry out this research work and UPM for the provision of the research facilities and technical assistance.
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