STUDIES ON PLANKTON, PRIMARY PRODUCTION AND FISH IN THE INNER BRUNEI BAY by

The Brunei Bay is a relatively shallow enclosed water area (Figure 1), approx. 45 x 45 km in size. Depth figures normally range between 30 and 40 meters. The bay is influenced by a number of rivers which discharge into the area, the largest being Padas in the north and Limbang in the south. The annual mean flow of Padas is estimated at 200 m per sec and the maximum over 1550. The rivers contribute with large quantities of sediment into the bay, and this sediment is covering the seabed and cause turbidity of the water. The total amounts of suspended solids brought to Brunei Bay from the river Padas has been measured at 2000—3000 tos per day or about 1 million ton per year. The vegetation in the area north, east and south of Brunei Bay consists'of primary and secondary rain forest, rubber plantations and other crop, and mangrove swamps. In 1982-1983 large areas of tropical forest on Borneo was affected by fire. It has been estimated that, in East Kalimantan only, some 3,5 million hectares of forest were destroyed. The fires also affected close to 1 million hectares of tropical vegetation in western Sabah (MALINGREAU et al 1985) and it seems probable that there have been indirect effects also in the marine environment due to thess fires. An area in the eastern central portion, the Gunong Lumako Forest Reserve, is since the last 5 years subject to extensive forestry operations involving systematic clear-cutting and plantation. This area is also being industrialized, involving for example the construction of a wood—, pulpand papermill complex, the construction of an ocean harbor and various other infrastructures. The study reported here was carried out in 1984 and 1985 to provide some basic information on the marine ecosystems of the inner Brunei Bay. The present results cover studies of the diversity and abundance of the plankton, primary production, and fish fauna of the inner Brunei Bay. A separate report covers the state of the coral reefs in the area (LINDEN et al, 1988), and another report various physico-chemical parameters of the water and sediment (in preparation). The only plankton study available from Brunei Bay is the result of a preliminary study of the species composition of zooplankton carried out by MOHAMED et al (1984). Also with regard to the South China Sea in general, few studies have been reported. However, some baseline work has been carried out in the Straits of Malacca and along the east coast of Peninsular Malaysia (CHUA & CHONG 1973, CHU A1984).


INTRODUCTION
The Brunei Bay is a relatively shallow enclosed water area (Figure 1), approx.45 x 45 km in size.Depth figures normally range between 30 and 40 meters.The bay is influenced by a number of rivers which discharge into the area, the largest being Padas in the north and Limbang in the south.The annual mean flow of Padas is estimated at 200 m 3 per sec and the maximum over 1550.The rivers contribute with large quantities of sediment into the bay, and this sediment is covering the seabed and cause turbidity of the water.The total amounts of suspended solids brought to Brunei Bay from the river Padas has been measured at 2000-3000 tos per day or about 1 million ton per year.The vegetation in the area north, east and south of Brunei Bay consists'of primary and secondary rain forest, rubber plantations and other crop, and mangrove swamps.In 1982In -1983 large areas of tropical forest on Borneo was affected by fire.It has been estimated that, in East Kalimantan only, some 3,5 million hectares of forest were destroyed.The fires also affected close to 1 million hectares of tropical vegetation in western Sabah (MALINGREAU et al 1985) and it seems probable that there have been indirect effects also in the marine environment due to thess fires.An area in the eastern central portion, the Gunong Lumako Forest Reserve, is since the last 5 years subject to extensive forestry operations involving systematic clear-cutting and plantation.This area is also being industrialized, involving for example the construction of a wood-, pulp-and papermill complex, the construction of an ocean harbor and various other infrastructures.The study reported here was carried out in 1984 and 1985 to provide some basic information on the marine ecosystems of the inner Brunei Bay.The present results cover studies of the diversity and abundance of the plankton, primary production, and fish fauna of the inner Brunei Bay.A separate report covers the state of the coral reefs in the area (LINDEN et al, 1988), and another report various physico-chemical parameters of the water and sediment (in preparation).The only plankton study available from Brunei Bay is the result of a preliminary study of the species composition of zooplankton carried out by MOHAMED et al (1984).Also with regard to the South China Sea in general, few studies have been reported.However, some baseline work has been carried out in the Straits of Malacca and along the east coast of Peninsular Malaysia (CHUA & CHONG 1973 In the present study qualitative and semi-quantitative samples of phyto-and zooplankton were collected in Brunei Bay during November-December 1984 and April 1985.In addition, the amount of chlorophyll a was determined and the primary productivity was studied using the 14 C technique. No previous results of scientific investigations of the fish fauna of Brunei Bay are available.However, a checklist of marine fishes of West Sabah, based on collection made in March 1974, have been published by CHUA& CHAW (1978).In order to study the fish fauna of the area in Brunei Bay a number of fishing efforts using survey nets and a beach seine net were conducted in November-December 1984.

METHODS
Phytoplankton were sampled using a standard phytoplankton net (mesh size : 50 um, diameter : 15 cm).One vertical haul from the sea bed to the surface was taken at each station.The samples were preserved in neutralized 4% formaldehyde solution.Qualitative and semi-quantitative analysis were carried out using an inverted microscope.
For the determination of chlorophyll a samples of 500 ml of sea water collected at the surface and in the deep water were filtered through 0.45 u fibreglass filters in the field.The filters were stored in a freezer until they were analyzed for chlorophyll a according to standard methods.
Primary production was measured using 14 C-technique according to standard methods.Water samples were incubated in glass flasks for 3 to 5 hours at 5 different depths (usually 0.5, 5, 8, 12 m and 0.5 m above the bottom).Dark bottles were incubated at 0.5 m and at the largest depth.Four uCi of carrier free NaH 14 CO 3 were added to all bottles.After incubation the samples were acidified and the samples were aerated for 20 minutes.Ten ml of the samples were transferred to scintillation vi&ls and counted in instagel (Packard Instruments) in an Intertechnique SL 40 liquid scintillation counter.The uptake of carbon was calculated according toGARGAS (1975).
Samples of zooplantkon were obtained by vertical net hauls from bottom to surface, using a Unesco WP-2 net with 90 um mesh size.The samples were preserved in 4% formaldehyde solution.Counting and determination of groups were performed using an inverted microscope.
For fish sampling standard survey nets of 36 m length, 1.5 m height, divided into 12 sections, each with a length of 3 m with mesh size varying from % to 3 inches were used.Ten nets were utilized each time.Fishing was carried out in the day during 2½ to 5 hours, or through the night during 13 hours.The stations are shown in Figure 2.
For nearshore fishing a beach seine net measuring 28.8 m in length and 1.5 m in height with a mesh size of 6 mm was used.At each station 4 to 7 shootings of the net were conducted, each covering an area of approx.2.500 m 2 .At each occasion the shootings were carried out around noon and the stations are shown in Figure 2. Mar. Res. Indonesia Vol.28, 1992: 55-80

Phytoplankton
At least 78 species of phytoplankton were recorded during the two sampling periods.The results are shown in Table 1.In November-December some 71 species were obtained, while in April some 48 species were recorded.Also with regard to abundance, higher figures were noted in November-December compared to April.The dominating genera in November-Desember were diatoms such as Bacteriastrum, Chaetoceros and Rhizosolenia These are large organisms which occurred in high abundance.In April these genera were also noted but, with few exceptions, much less abundant than five months earlier.Instead blue-green algae (Oscillatoria) dominated all samples collected in April.This group was only found occasionally during November-December.
Hence, larger species diversity and abundance were recorded in November-December compared to April.The situation in April could indicate that lower concentrations of nutrients were available, resulting in lower phytoplankton activity.These observations were confirmed in the studies of chlorophyll and primary production (see below).
The results of the chlorophyll a analysis, shown in Table 2, indicate two to three times higher figures during November-December compared to April.These results are in agreement with the results of the studies of the species diversity and abundance reported above.
The primary production measured as C 14 -uptake was 3 times higher in November-December compared to April (Table 3).If a 12 hr daylight period is assumed, the autumn value is on average slightly over 550 mg C m day , and the spring value slightly over 150 mg C. Assuming that the year is divided into autumn and spring production, the integrated production on the whole year may be calculated as 100-150 g C m~2.Such figures are in general in accordance with what would be expected from coastal tropical areas.RYTHER (1969) reports values in that range.However, the difference in primary production between autumn and spring, may seem remarkable for a tropical area.This difference is possibly at least partly determined by the loss of nutrients and differences in turbidity due to land run-off.The annual precipitation in the area approaches 4000 millimeter/year, and the most pronounced rainy period is October to January.

Zooplankton
The results of the studies of the zooplankton population are shown in Tables 4 and 5. Generally the most common group is crustaceans with copepods frequently being abundant.Shrimp larvae were recorded in most samples.Tunicates and chaetognates appeared to be slightly more common in April compared to November-December.Otherwise no obvious differences could be observed between the two sampling periods.

Fish
The results of the fishing using survey nets are shown in Tables 6 to 10.In total some 54 species were caught (Table 6).The largest number of species and total weight were obtained when fishing was carried out through the night (although it should be remembered that at this occasion fishing was carried out two to three times longer than during the day).The most abundant groups (total number) were sardins (Clupeidae), ponyfish (Leiognathidae) and catfish (Ariidae).Some species were obtained during the night only; such as catfish, snook (Centropomidae) and croakers (Scianidae).Triggerfish (Balistidae) were caught during daylight only.By weight the most important groups were catfish, sardins and herrings, therapons (Teraponidae), spadefish (Ephippidae) and rays (Rajidae).
The results of the sampling using theseine net are shown in Tables 11 to 15.Some 42 species were caught using the beach sein (Table 11).The most common groups were juveniles of mullets (Mugilidae), snooks, and silversides (Atherinidae).By weight the most important groups were tongue fish (Cynoglossidae) and flounder (Bothidae).
Despite the relatively limited effort, the study indicate that the Sipitang area is inhabited by a diverse fish fauna.Many of the approximately 240 species reported by CHUA and CHAW (1978) for West Sabah occured in the area.Several of the species obtained in the present study are of high commercial importance (jacks) and cavalla had a retail price of 6.50 to 7.30 Ringgit* kg -1 in Sipitang in 1983; the corresponding figure for mackerel was 5.60 to 7.20 Ringgit kg -1 ).In addition it is also clear from the shallow water samplings .thatthese areas function as nursery and feeding areas for junveniles of commercially important species.Studies similar to the present one were carried out on the east coast of Peninsular Malaysia (TAN, 1984).Although the methods used were not exactly the same as those used in the present study, the results indicate that much larger number of specie^ and larger quantities of young fish were obtained in the present study compared to what was found in the study at Peninsular Malaysia.
In conclusion, the present study has provided some basic information on the marine ecosystem of Brunei Bay.The investigation has shown a large variation in plankton diversity, biomass and productivity during the year.In November-December more species were present, the chlorphyll content was higher and the primary productivity higher than during April.This difference is likely to be at least partly related to the increased flush-out of nutrients from land during the rainy season.Furthermore the study has shown a rich fauna of fish in a relatively limited area.Plankton, Primary Production And Fish In The Inner Brunei Bay (Olof Linden, et.al.)Table 7. Fish species, number of individuals (A) and total weight in grams (B) for each species, caught by survey nets at 2 m depth 400 m NW of Tg Merintaman.Fishing was carried out during 13 hrs in the night, November 1984.

Table 5 .
Zooplankton caught in Brunei Bay in April 1985.x = single specimen; xx = occasional; xxx = abundant; xxxx = very abundant