BENTHOS AND FISH SURVEY OF THE RIG REEF AT TWO-FATHOM ROCK (APRIL 1999)

Date: 10, 18, 24 April 1999
Author: Novi E. Yus

(With Ingrid van Houwelingen, Steve Holyoak, Hj. Ramli B Hj. Shahdan, Hans Dols, Simon Cartmer, Suzy Cartmer, Patrick Allman-Ward). most photo’s by Steve Holyoak


MANAGEMENT SUMMARY......

INTRODUCTION......

Some History:......

METHODS......

Benthos......

Fish......

RESULTS......

Benthos......

Fish......

DISCUSSION......

Benthos......

Fish......

MACROBENTHOS on the artificial reef......

Algae......

Porifera......

Cnidaria......

Hard coral

Soft coral

Tunicates (ascidian)......

Echinodermates......

Bryozoans......

Molluscs......

Crustaceans......

FISH DISTRIBUTION on the artificial reef......

LIST OF SPECIES RECORDED......

1. Pelagic fish......

2. Serranids and related fish......

3. Lutjanids and related fish......

4. Butterfly fish and Angel fish......

5. Pomacentrids......

6. Wrasses......

7. Other reef fishes......

CARNIVORES......

HERBIVORES......

HERDEVORES......

OMNIVORES......

HARD CORAL FEEDER......

CHARTS

Chart - 1Diversity and abundance of carnivorous fish.

Chart – 2Relative abundance and absolute abundance of fish species perday/station.

TABLES / FIGURES

Table - 1Species & Families of macrobenthos. On the ‘New Rig Reef’ well jacket 1 & 2, jacket 3, Jacket 5, Jacket 6, Jacket 7 and on the ‘Old Rig Reef ‘ well jacket 9.

Table - 2 Families, Species, and Feeding guilds of fishes at Rig Reef.

Table - 3 Summary of total fish species and numbers in each feeding guild.

Table - 4 The most abundant fish species based on biomass on days: 1, 2&3.

Table - 5Total number of macro invertebrate species recorded during the 1999 survey.

Table - 6 Categories used to indicate relative cover of organisms found at jacket 1, 5 and 8.

Table - 7 Relative abundance of benthic organisms surveyed.

Table - 8 Comparison of coral recruits recorded on jacket 6 and 7 at Two-Fathom Rock in May 1996 versus April 1999.

Figure - 1Location map Two fathom rock area

Figure – 2Location map Rigreefs at Two Fathom Rock

Appendix - 1 Numbers of Species & Families of fish at the rigreef (rigfish.xls)

Appendix - 2 Rigreef poster page 1 in A3 format

Appendix - 3 Rigreef poster page 2 in A3 format

REFERENCES

BENTHOS AND FISH SURVEY OF THE RIG REEF AT

TWO FATHOM ROCK (April 1999)

MANAGEMENT SUMMARY

In April 1999 the “rig reefs” (deposited by BSP in 1994) at Two-Fathom Rock in the near shore coastal waters of Brunei Darussalam were surveyed for an assessment of the floral and faunal communities found in this artificial habitat. The survey follows up on a 1996 survey by the University of the Phillipines. Organisms attached to the jackets, as well as fish species in the vicinity, were qualitatively and quantitatively described.

Data from the benthic survey reveal the dominance of algae, soft coral and other opportunistic organisms such as sponges, feather star and bivalves. This indicates that the community in the rig reef is most likely in its early stages of successional development. In contrast to earlier surveys, hard coral recruits were observed to occur more frequently at the top portions of the rig jackets where the rig reef environment is most favourable (good light penetration, steady current flow and less turbid water)

Of the 86 fish species recorded on the three days survey, 46 different species were carnivorous. Amongst the carnivores the pelagic (visitor) fish only contribute 5 species but with a significant number of individuals. Most numerous were the 18 species of planktivorous fish (plankton-feeding) dominated by big aggregations of small territorial and juvenile fish.

During the three days, seven of the structures were surveyed. The variability of the pelagic fish is a function of time, however numbers of local dwellers are thought to be relatively constant.

Because of the steady food supply in the form of smaller fish, some of pelagic fish, such as sphyraenids and carangids, tend to aggregate in the vicinity of rig reef. The survey found an aggregation of important commercial food fish at the artificial reef. Groupers (Serranidae), snappers (Lutjanidae) and rabbitfish (Siganidae) are some of the commercially important fish that have been attracted to the reef.

On rig jacket 9 adult fish dominated over juveniles and small fish, suggesting that this part of the reef primarily functions as a feeding area and secondarily as shelter.

Well jackets 1, 2, 3, 5, 6 and 7 are abundant with young fish, these structures serve primarily as breeding and nursery grounds and secondarily as a feeding area.

The abundance of young over adult fish suggests a certain imbalance. While only a small amount of the adult fish were groupers (Seranidae), the pelagic fish in comparison, were visiting in big numbers. Groupers are very territorial and easily trapped. They come to structures like rig reefs to get protection from predators. Their relative absence on the reef indicates possible overfishing in the rig reef area.

  • The 1999 survey encountered an attractive looking and partially mature coral growth and well established stock of both pelagic and carnivorous fish together with some breeding juvenile planktivorous fish.
  • There was a marked difference between the result from the 1996 quick look survey that had found the majority of growth concentrated on the lower part of the structures. Currently the growth is far more prolific in the upper part of the structures (in the light and away from the silty bottom).

INTRODUCTION

The 1999 survey was conducted in three days by Divers from the Panaga Sub Aqua Club and the Brunei Muzeum on request of BSP’s environmental department. The aim was to check any developments on the rigreefs since the previous survey by the University of the Philippines in 1996.

The Two Fathom Rock area forms a good location for the artificial reef experiment. Since, except for coral reef areas, much of the substrate in Brunei’s coastal areas is either barren sand or mud which is unfavourable to fish aggregation or settlement. As a result, fish and other marine organisms aggregate in the vicinity of, natural or artificial, underwater structures especially if these are close to an existing coral reef area.

Some History:

  • In 1985, the Department of Fisheries, Ministry of Industry and Primary Resources of Brunei Darussalam launched the Artificial Reef Programme in an effort to enhance local fish productivity. As part of the program, artificial reefs were established at Two-Fathom Rock (about 7 km offshore Berakas). Investigations carried out during phase 1 of the ASEAN/US coastal resources Management Project in Brunei indicated that underwater structures associated with oil platforms could harbor a fairly diverse fish and invertebrate community (Chou and White 1987).
  • 1988 BSP places two redundant rigs (SWA-58 and SWA-45) in Two Fathom rock reef complex
  • 1989 studies by Da Silva indicate the artificial reef functions as breeding ground for fish and supply to surrounding coastal waters while marine organisms start to colonize the structures
  • In June 1994, the SWA-7 and SWA-9 jacket/platforms were placed at Two fathom rock. Followed by a further 3 structures (SWA–8, SWA–35 and AMVJ–2) In July 1994. The structures were placed in locations already marked in admiralty charts as warning commercial shipping to keep clear of the area, consequently no additional ‘Notification on mariners’ advising of navigational hazards were needed.
  • In June 1995, clearance was obtained from the Fisheries Department for disposal of helideck from GADP–1 at Scout rock.
  • In 1996 a survey by university of the Philippines indicate start of recolonisation of the structures deposited in 1994
  • 1999 discussion about decommissioning of platforms leads to current follow up study. A team from the Panaga Sub Aqua club, with help of Brunei Muzeum staff, repeats the 1996 survey. The results of which are presented below.

METHODS

The rig reef at Two-Fathom Rock in the near shore coastal waters of Brunei Darussalam was surveyed in April 1999. During three days (5 dives) the different structures were scanned for encrusting organisms and fish abundance

The main objective of this investigation was to study the diversity and distribution of the invertebrates inhabiting the artificial reef. Surveys were conducted on seven structures, one on the “Old rig reef” (SWA 9 placed in June 1994) and six on the “New rig reef”(SWA-7, SWA–8, SWA-35, SWAMVJ-2) (placed in July 1994). See fig(1&2)

For comparison, invertebrates occurring on sea floor around the artificial reef were also investigated.

Benthos

All encrusting and other attached organisms found on the submerged well jackets were recorded and their relative percentage cover was estimated in April 1999.

Divers on SCUBA swam around each structure, assessing all horizontal, vertical and cross bars within each segment for benthic cover, the methods used were mainly qualitative, since it was unrealistic to hope to carry out quantitative surveys on all invertebrates in the time available.

Two of the well jackets (Jacket 6 and 7. fig, 1) were selected for detailed examination. Each well jacket was arbitrarily divided into segments, vertical planes of about 5 m in length and depth, encompassing the entire exposed surface area of the structure. Organisms were recorded using benthic lifeform categories developed by the ASEAN countries and Australia

In this survey, the category “OT” was expanded to take into account the presence and abundance of various invertebrate groups. All records of hard coral recruits, on the other hand, were pooled into a single category and the genera of coral found on the well jackets were noted. Estimates of the percentage cover of each lifeform were then recorded according to the system used for manta tow surveys (English et al. 1993) with the addition of the category 0 (zero) to indicate the absence of a particular lifeform (Table 6). Notes were made of other conspicuous taxa that were found in the rig reef. Photographs of the fish and benthic organisms were also taken during the survey.

Fish

Divers on SCUBA swam around “New Rig Reef” rig jackets 1, 2, 3, 5, 6, 7, and “Old Rig Reef” jacket 9 to conduct a representative census of the fish communities in the vicinity of the artificial reef. The survey was conducted in three days. The first day observed the rig jacket 3, 5, 6, 7 and the second day observed the rig jacket 9 and the third day observed on rig jacket 1, 2.,Fish were recorded as species along with estimate of their size and numbers.

RESULTS

Benthos

On the 6 structure encountered at Two-Fathom Rock the Visibility at the first and second survey was very good, but during the third survey visibility in the water column was low due to adverse weather (rain and overcast sky).

Table 2 lists a summary of data from jackets 3, 6 and 7. Averages for the upper and lower segment of each jacket are included, as well as a pooled average (indicated as grand average) of the two structures. At the three structures the abundance of organisms shows a similarity. In general the upper part of the structure is covered by invertebrates, algae and soft coral occupying nearly all available space. Mainly algal assemblages and macroalgae cover the lower tubulars. The variety of organisms occurring in each jacket was remarkably similar.

Pooled estimates of benthic lifeform cover at the 3 well jackets reveal the overall dominance of, algal assemblages, soft coral, and sponges. Other sessile (attached) organisms encountered in the artificial reef were; red coralline and other macro algae, tunicates, octocoral (sea fan), oysters, barnacles, and hard coral. Other conspicuous invertebrates found on the structures were holothurians (sea cucumbers), nudibranchs (sea slugs), gastropods and bivalves.

Fish

Although the rig reef at Two-Fathom rock is located on barren sand it is surrounded by coral reef. This enables interaction between fishes from the coral reef and the artificial reef. The rigreef is a complex environment for two reasons:

  • The high species diversity of predators and prey organisms makes it difficult to recognize a general trend.
  • The open system nature of reef ecosystem prevents quantification of interaction through unknown immigration and emigration of predator and prey.

While the current study only deals with fish on the artificial reef, a comparison with the surrounding coral reef area could be included in possible follow up work. Based on the three different surveys (two on the new rig reef jackets and one on the old rig reef jacket) we can only draw limited conclusions. There is a distinct variability in the amount of fish encountered on two days on one location (influence of weather and turbidity). However on the second location (Old Rig Reef, jacket 9) the total amount of fish is several factors larger than observed during any of the dives at the jackets closer to the buoy.

A total of 82 species of fish belonging to 31 families were recorded at the rig jackets. Carnivorous fish 46 species, Planktivorous fish 18 species, and herbivorous fish 11 species. Most abundant are the Planktivorous fish with 2558 individuals, (including juveniles).

The total number of fish found in the first second and third surveys is 2106, 771 and 1226 respectively (see table 9).

DISCUSSION

Benthos

The normal succession in coral reef development follows several distinct stages of sucessional development. First the Blue and Green algae appear (the algea prepare the substratum by removing possible poisonous substances). Later planula corals and coralline green and red algea will colonise and build on the cleaner substratum. The larger soft and hard corals can later grow on the prepared substratum.

From both the surveys in 1996 and 1999 the dominance of (pioneering phase) organisms in the well jacket, such as algae, and sponges indicates that the rig reef is still in its early stages of successional development. The life strategies of these pioneer organisms are characterised by their ability to produce a great number of offspring at a time and their fast settlement, growth and maturation rates. These traits give them the advantage of rapid colonisation in any given area, edging out and smothering propagules from slower growing organisms such as hard corals.

The next step in the successional development, such as colonisation of coral recruits on rigs, was found on some of the well jackets, such as well jacket 9. A variety of sizeable hard corals, in boulder-shaped, laminate and encrusting forms are attached the structure. Substantial soft corals have already been established on a large part of the available surfaces. Together with Green and red coralline algae the soft corals cementing effect helps to maintain and build up the reef structure and contribute Calcium salts to strengthen the hardcoral skeletons (Chou Loke Ming, 1988). The start of a transition to hard corals indicates the reef development is moving in the right direction.

The benthic sessile (attached) organisms have two different communities on the upper and lower parts of the well jackets. Close to the seafloor, less organisms and hard coral recruit are found and alga occupy the largest percentage of the space due to low light conditions brought about by shading and light attenuation with depth. Apart from the algae, the few other organisms grow to a large size (lack of competition?).

On the Upper part of well jacket, at 9 m to 12-m depth from the surface (msl), the rig reef structure provides a firm substrate for attached or encrusting organisms. The 3 Dimensional structure of the rigreef imitates a natural setting. Shallow there is more light, clear water and a current that brings plankton and other food. Corals and invertebrates are better able to settle and grow at the upper part of well jackets,

One distressing fact about the rig reef is: the low amount of hard coral recruits on the lower part jacket 1, 6 and 7. While in 1996 the university team reported: “The hard coral despite of algal dominance, were still able to recruit onto the artificial structures. A great number of coral spat (as well as coral genera) were observed in the lower segments of the rigs than in the upper portions” (Helen.T.Yap, report survey 1996).

Now the situation is virtually reversed with more hard corals on the upper segments compared to the lower part. Whereas the previous survey recorded 15 genera of hard coral recruits at the lower part (table 8). The 1999 survey found only 3 genera of young corals in a similar position.

In general fine silty bottoms provide unstable substrates for the settling of most benthic organisms. Just a few organisms can live in that area were they can adapt to the condition found on the sea floor and lower part of well jackets. Gorgonian and sea whip are the most tolerant species to sediment turbidity. However common brown sponges (Suberites), blue sponges Jaspis, sea fans Melithaea, soft corals Sarcophyton, sea whip Junceela, black corals Antipathes, and barnacles Balanus were sometime partially buried in the sand.