S E A F D E C /S C S .73: S -14
S om e C o n sid eratio n o n th e R e la tio n sh ip b etw e en E n v iro n m en tal F a c to rs an d th e D istrib u tio n o f Fisheries R esources in th e
S o u th C hina Sea and th e A n d am a n Sea by
A k ih ik o S h iro ta
M arine Fisheries R esearch D e p a rtm e n t S o u th e a st A sian Fisheries D evelopm ent C en ter
C hangi P o in t, S ingapore
A b stra c t
T his p a p e r discusses th e rela tio n sh ip b e tw e en en viron
m e n ta l fac to rs an d th e d istrib u tio n o f fisheries resources in th e S o u th C hina Sea and th e A n d am a n Sea, based on th e ocean o g rap h ic and fisheries resources d a ta co llected by R /V CH A N GI fro m N ovem ber, 1969 to M arch, 1973.
T h e fish c a tc h is rela te d m o re to th e b o tto m to p o g ra p h y th a n th e b o tto m fe a tu re o f th e sea. G o o d m ean catch ( > 200 k g /h r) was o b ta in e d in u p w elling regions a t th e steep c o n tin e n ta l slope in areas o ff th e n o r th and th e east co ast o f th e A n d am a n Sea, and in areas w ith b o u n d a ry zo n e a t th e fla t area along th e ea st co a st o f Malay Peninsula.
T h e d is trib u tio n o f fisheries resources was also related to th e h y d ro lo g ic al facto rs. In general, b e tte r ca tc h ( >
116 k g /h r ) was o b ta in e d fro m areas having d e p th s o f 4 5 —65 m , salinity range o f 3 2 .0 — 33,5 ° /o o and b o tto m te m p e ra tu re s above 2 3 ° C. H ow ever, in th e coastal w aters o ff S araw ak an d N o rth A n d am an Sea (o ff B urm a), good fishing g ro u n d s w ere ch aracterized b y lo w salinity, low te m p e ra tu re and high n u tr ie n t co n te n ts , suggesting th e in flu en ce o f fre sh w ate r discharge in th e area. A b u n d a n t o cc u rre n ce o f coastal w a te r fishes w ere fo u n d in th ese areas.
F ro m th e p re se n t analysis, i t appears th a t th e co m b in a
tio n and in te ra c tio n o f b o th h y d ro lo g ic al and to p o g rap h ic c o n d itio n s are im p o rta n t fa c to rs c o n trib u tin g to w ard s th e fo rm a tio n o f g o o d fishing g ro u n d in th e surveyed areas.
1. IN T R O D U C T IO N
I t is k n o w n th a t th e e n v iro n m en ta l fa c to rs p la y an im p o rta n t ro le in th e d is trib u tio n o f fisheries resources alth o u g h th e rela tio n sh ip is r a th e r com plex. In o rd e r to s tu d y th is rela tio n sh ip th e d a ta fro m th e o cean ographic and fisheries reso u rce survey carried o u t b y th e 3 8 7 -to n research vessel CH A N G I in th e s o u th e rn p a rt o f th e S o u th C hina Sea and th e A n d am a n Sea fro m N o v em b er 1969 to M arch 1973 w ere analysed. Special a tte n tio n was p aid to th e various fa c to rs c o n trib u tin g to w ard s th e fo rm a tio n o f good fishing grounds.
2. M A T E R IA L A N D M ETHODS
C atch d a ta fo r th is s tu d y w ere o b ta in e d b y ex p e ri
m e n ta l tra w l fishing by CH A N G I, w ith th e use o f a
4-seam ed n e t having a head ro p e le n g th o f 36 m and to w ed a t a speed o f 2.5 to 4.5 k n o ts. T he original traw l catches o b ta in ed fro m each half-degree b lo ck o f 30 square m iles w ere co n v erted in to m ean ca tc h p e r h o u r fo r th e respective m o n so o n and in te rm o n so o n seasons.
C atch d a ta o b ta in e d fro m th e prelim in ary ex p lo ra to ry v ertical h ead lin e fishing in th e A n d am an Sea in F e b ru a ry
1973 w ere n o t converted.
A nalyses o f w ate r sam ples co llected by N ansen (N an
sen) reversing b o ttle s an d m u d sam ples collected by SK -type sam p ler w ere as described in a previous p a p e r (S h iro ta e t al., 1972).
3. RESU LTS A ND D ISCU SSIO N
F ro m th e analyses o f th e c a tc h d ata, a com parative stu d y was m ad e b etw e en th e n o rth e a s t m o n so o n , th e so u th w e st m o n so o n and th e in te rm o n so o n perio d s in th e S o u th C hina Sea. A s n o c a tc h d a ta w ere available during th e so u th w e st m o n so o n in th e A n d am an Sea, co m p ariso n o f c a tc h b etw e en th e tw o m o n so o n seasons in th e region was n o t possible.
Fig. 1 V aria tio n in traw l c a tc h b y b lo c k in th e n o r th east an d so u th w e st m o n so o n s (1 9 6 9 —73).
111
Fig. 1 shows the mean catch per hour per block during the northeast monsoon (October-March) and the south
west monsoon (May—August) season. On the whole the mean catch for the respective seasons fluctuated consider
able in the South China Sea.
During the northeast monsoon season, high means catch per hour (more than 200 kg) was observed in the 7 blocks along the east coast of the Malay Peninsula and 2 blocks in the west coast of Sarawak, while in the southwest monsoon season, such high catch was observed in 3 blocks along the east coast of the Malay Peninsula and 4 blocks in the west coast of Sarawak. However, during the intermonsoon season, mean catch of similar magnitude was not observed in any block.
Taking into consideration of the above mentioned results the analysis of the environmental factors contribut
ing towards the formation of good fishing grounds was carried out.
3.1 Bottom Feature
Since the distribution of demersal fisheries resources largely depends on the different types of sea beds, which in turn are related to different current movements, there may be a relationship between catch and bottom feature in the South China Sea. The catch data superimposed onto a map illustrating the bottom feature of the South China Sea is shown in Fig. 2. As far as the trawl catch in the South China Sea is concerned no clear relation between the catch and bottom obtained from both sandy and muddy areas.
Fig. 2 Bottom feature and mean catch per hour.
3.2 Bottom topography
Fig. 3 shows the bottom topography of the South China Sea, the Straits of Malacca and the Andaman Sea with superimposed catch data. In the area off the Northeastern coast of Sarawak the depth increases sharply from 100 to 1000 m. Upwelling may easily occur when the offshore water moves towards coastal region across the steep continental slope. A similar pattern of bottom topography is also seen in the Andaman Sea from its north western to central part. It can be seen from the figure that good mean catch of more than 200 kg/hr was obtained from these areas. However similar catch was also obtained from the flat area in the southwestern part of the South China Sea where strong upwelling cannot be expected. This may be due to the presence of a boundary
zone formed along the east coast of the Malay Peninsula as explained in the next section.
3.3 Hydrological factors
Fig. 4 shows the relationship between fish catch and hydrological factors in 1971-1972 in the South China Sea. A better fish catch was obtained from areas having depths of 4 5 -6 5 m, salinity range of 32.0-33.5% and bottom temperatures above 23 °C.
Fig. 5 illustrates the surface isohalines drawn from data collected in 1970—1973 by CHANGI during both north
east and southwest monsoons. From the figure it can be seen that the sharp gradient in salinity is located in some areas where the offshore water supposedly intrudes into the coastal water especially along the east coast of West Malaysia and the northwest coast of Borneo where good 112
Fig. 3 Bottom topography and mean catch per hour.
mean catch per hour was obtained.
It may be of interest to note that the surface isohaline of 33.5% is situated close to the east coast of the Malay Peninsula. This pattern was also observed in both seasons in 1950—1955 (Wyrtki, 1961). Thus, a long and narrow strip of boundary zone is formed along the east coast of Malay Peninsula in both monsoon periods, and this may account for the relatively good catch in this area in the northeast and southwest monsoon seasons.
The good fishing grounds in the northwest region (15°00'N, 94-95°30'E) of the Andaman Sea displayed a different hydrological pattern (Fig. 6). This pattern is characterized by low temperature and salinity and high nutrient contents suggesting the influence of considerable freshwater discharge from the Irrawaddi and Sittang rivers.
The Phosphate-Salinity (P-S) diagrams show different patterns between coastal and offshore waters (Fig. 7).
There is a tendency that water with high salinity has low PO4-P contents whereas water with low salinity is accom
panied by high PO4-P contents in coastal waters. How
ever, in offshore waters such tendency has not been observed.
Fig. 4 Relationship between fish catch and environ
mental factors in the South China Sea (1971—
72)
Fig. 5 Distribution of surface salinity in the north
east and southwest monsoons (1970—73)
Fig. 6 Temperature-salinity diagrams for the Straits of Malacca and the Andaman Sea in the north
east monsoon.
In the coastal waters of Sarawak and north Andaman Sea good fishing grounds were characterized by low salinity, low temperature and high nutrient contents.
In both areas the species composition in catch greatly differed from other areas of the South China Sea especial
ly in the abundant occurrence of croaker, shad, gru nter and pomfret. However, in the offshore waters although high PO4-P contents accompanied by high salinity were found the relation between catch and hydrological condi
tion in these areas were not known.
Recently a preliminary survey on vertical handline fishing was conducted in the eastern part of the Andaman Sea. The fishing grounds were located along the slope where the depth increased suddenly from 100 to 500 m.
Fig. 7 Phosphate-salinity diagrams for offshore and coastal waters (1971—72)
The fish schools detected by the echo-sounder were very often observed along the edge of the slope. The results are summarized in Table I. Since the fish schools were located in a restricted area the topographical and hydrological factors seem to be im portant for the formation of good fishing ground.
Table I. Catch by bottom vertical handline fishing in the eastern part of the Andaman Sea in February, 1973.
Date Loca
tion
Time Start End
Operation Time (min)
Total catch (kg)
24 Feb. 997a 12:05-12:33 28 72.2
25 Feb. 1097a 13:15-13:33 18 96.8
26 Feb. 1097a 10:33—13:47 125 622.4
27 Feb. 1097c 15:27-17:28 67 149.8
28 Feb. 1097c 12:10—17:21 100 384.1
1 Mar. 1097a, c 07:0 1 -0 7 :4 8 47 79.8
4. CONCLUSION
From the above results and discussion it appears that the combination and interaction of environmental factors may determine to a certain extent the distribution of demersal fisheries resources and may contribute towards the formation of good fishing grounds. In this connection environmental factors should not be analysed separately but as a whole. However, in fishing ground along the edge of continental shelf and banks of submarine ridges the distribution of fish may depend to topographic condition
primarily and hydrological condition secondarily.
In view of our limited knowledge further studies need to be carried out on the relationship of environmental factors and the distribution of demersal fisheries resources before such resources can be fully exploited.
ACKNOWLEDGEMENTS
The author is indebted to Mr. Chen Foo Yan, Chief of the Marine Fisheries Research Department, SEAFDEC, for reading the manuscript and giving valuable criticism.
The author also extends his thanks to Dr. Satoshi Mito, Deputy chief of the Department, for kindly reading the manuscript and good suggestions. Finally, the author expresses his heartiest thanks to Mr. Mathew Chow for his valuable technical assistance.
References
Shirota, A., L. C. Lim and M. Chow, 1972. Some oceanographic characteristics of the southern part of the South China Sea. Indo-Pacific Fisheries Council, IPFC/72/SYM 41.
Wyrtki, K., 1961. Physical oceanography of Southeast Asian Waters. Naga Report, Volume 2, The University of California, Scripps Institution of Oceanography.
SEAFDEC/SCS.73: S-21
Brief Note on the Relationship of Scattering Layer and some Hydro-Biological Factors*
by Akihiko Shirota
Marine Fisheries Research Department Southeast Asian Fisheries Development Center
Changi Point, Singapore
Abstract
The analysis of echo-sounding records obtained from the South China Sea showed that scattering layer is caused by a concentrated layer of zooplankton and is related to thermocline. The occurrence of scattering layer may be used to indicate either the depth where thermocline occurs or the vertical movement of zooplankton.
Fish schools were recorded at the vicinity of the scattering layer and this phenomenon is probably associa
ted with the feeding habits of the fish.
1. INTRODUCTION
A scattering layer as recorded on an echo-sounder paper (PLate 1) is caused by the presence of zooplankton.
During the survey cruises of the research vessel CHANGI the depth of the scattering layer has been observed to be
related to the depth of the thermocline as recorded by the bathythermograph. Since zooplankton is an important source of food for juvenile and plankton-feeding fishes, studies on the relationship of scattering layer and some hydro-biological factors are of some importance to fish
eries development in the Southeast Asian region.
2. MATERIALS AND METHODS
CHANGI conducted an oceanographic survey cruise and occupied 21 stations in the South China Sea in April 1971 (Fig. 1). Three additional surveys were also carried out in April—May, June and September of 1972. The echo-sounder used was a SR-11 type (Kaijo-Electric Co.
Ltd.) with 200 KC frequency. Continuous graphical records of water temperature in the sea were obtained by the bathythermograph (BT). Zooplankton samples were collected by vertical hauling with a closing net (mouth diameter = 25 cm, mesh size = 24/cm) from various strata of the sea and the wet weight of zooplankton biomass (mg/m1 * 3) was recorded.
*
We regret that it is not possible to publish the original “plates”
as their condition is no longer suitable for printing.
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