in coastal regions and in small islands
An ecological assessment of Ulugan Bay, Palawan, Philippines, CSI info 12
Coastal resources assessment
In June 1998, a series of coastal resources assessment exercises were carried out in Ulugan Bay and Puerto Princesa City. The assessments were conducted by the National Committee on Marine Sciences of the UNESCO National Commission of the Philippines, with support from the UNESCO Jakarta Office. Participants came from a broad range of institutions and organizations active in Ulugan Bay and Puerto Princesa, and included staff from the Puerto Princesa City Government, the Puerto Princesa Subterranean River National Park, the Philippine Navy Western Command, and the Palawan Council for Sustainable Development, as well as local, national and international nongovernmental organizations (NGOs). The assessment had two primary objectives:
To provide input for the development of a fisheries and biodiversity database on the coral reef, seagrass and mangrove resources of Ulugan Bay and the related fisheries and seaweed resources. The database was envisioned as a tool for use in the elaboration of a model for sustainable coastal resource management, a key objective of the UNESCO-UNDP project currently under way in the bay.
To train participants in the identification, quantification and documentation of the ecological resources of Ulugan Bay and in the application of this information to resource management. Through the application of ecological data to socio-economic and management issues, participants were able to develop their understanding of integrated coastal planning for sustainable tourism and the enhancement of living standards.
The following analysis and discussion demonstrates the value of a scientific and ecological assessment of coastal resources, not only with a view to obtaining ecological data in itself, but also for the transfer of technology and skills when such an exercise is conducted in a participatory manner. The exercise provides a ‘rapid ecological assessment’ of the bay and, while not representing a complete and rigorous analysis, can nevertheless be used in initial planning and management of the bay’s resources.
|Ulugan Bay: View of Rita Island|
Integrated coastal management principles are only useful if they are founded on solid scientific and socio-cultural grounds. This foundation, in turn, is attainable only through scientific acquisition of relevant data, their effective dissemination, and eventual validation and use in the management decision-making process. In the Philippines, the failure of a number of management systems has been traced in large part to the inability of implementers to realize and apply these basic principles of research and development. Hence, assessment and documentation of the coastal and marine resources of Ulugan Bay are essential for future development activities.
Approach and methodology
The coastal assessment activities were preceded by consultations with officials from the five barangays surrounding the bay. This process was necessary to ensure that the local government and communities were informed of the nature and objectives of the exercise. Furthermore, it helped to formalize the role of the local community in the process. Participants at these introductory sessions were invited to air any reservations and concerns they felt about the activity, and agreements were reached on a number of critical issues.
Following the local community consultations, a pre-assessment briefing was held to ensure that all materials and theoretical requirements for the exercise were prepared and in place.
The Advanced Training Course for Coastal Zones Assessment and Zonation was held in December 1997, with the objective of training participants in the fundamentals of ecological assessment. The training course included both theoretical and practical elements.
During 1998 the assessment was conducted. A post-assessment briefing was held, during which a preliminary review and evaluation of the entire activity was carried out and prepared for inclusion in the activity reports. This organization of activities ensured the full co-operation and commitment of all parties.
The assessment study started with the selection of the study sites from satellite images provided by a recent United Nations Economic and Social Commission for Asia and the Pacific (ESCAP)/United Nations Environment Programme (UNEP) project. Sites were selected based on their representation of the prevailing coastal and underwater conditions of the bay and its vicinity, and the likelihood of reliable results.
In addition to the focus on habitat types, the selection of specific sites considered a number of criteria conventionally applied for the determination of protected areas, as follows:
ecological importance: the strategic location of the site, its function as a source of larvae or propagules for downstream areas, its function as a nursery or spawning ground, the variety of its habitats, and the presence of rare or endangered species;
state of naturalness: considering the degree of protection of the site from the influence of man’s activities;
economic importance: the site’s existing and potential contribution to the economic wellbeing of the local communities;
social importance: the site’s existing and potential significance for the local, national and international community due to its heritage, historical, cultural, traditional, aesthetic, educational, or recreational qualities;
scientific importance: the value of the site for research, education and monitoring;
practicality and feasibility: the site’s social and political acceptability as a research location, the extent of community support, accessibility for tourism and recreation, compatibility with existing uses and management practices, ease of management.
The study sites and associated communities are shown in Figure 2 and Table 1.
|Figure 2. Location of the study sites in Ulugan Bay|
Based on the criteria above, a series of ecological field assessments were performed with reference to the following focus areas: coral reefs, coral fish, seagrass beds, seaweeds, mangrove forests, seaweeds and water transparency. Habitats were assessed for their species composition, abundance, and intensity of environmental and human impacts. The status of coral reefs was measured based on the cover of six life form categories at seven sites, or stations. In the case of seagrasses, the abundance of the eight species present in the bay was determined at five stations in terms of density and frequency. The mangrove communities were assessed at ten sampling stations through measurement of frequency, diameter at breast height (dbh), number of cut and dead trees, and number of seedlings and saplings. A water transparency profile of Ulugan Bay was made using a Secchi disc and a Geographical Positioning System (GPS). The results of these assessments are discussed in the following sections.
|Assessment of coral reefs using the line
intercept transect method
In the assessment of coral reefs, the line intercept transect (LIT) method was used (English et al., 1994) to assess the sessile benthic community of the coral reefs. Along 150 m transect lines laid at the 5 m isobath, the community was characterized using six life-form categories (Acropora, hard coral, soft coral, dead coral, macroalgae, other) to provide a morphological description of the reef. Divers swam along lines placed roughly parallel to the reef crest at the desired depths along three 33.33 m transect lengths (TL). In a 150 m line, the three TLs were the first-, middle- and end-segment of the line. For future monitoring, the location of each site was recorded and marked on the reef with a permanent metal mark. A team of at least three personnel was required – two divers and a person in the boat. All observers were familiar with the definitions of each life-form and they spent 30–45 minutes in the water at the beginning of each field trip, comparing and standardizing their interpretations of the various life-forms. Table 2 shows the percentage cover of the six lifeforms and two other categories (silt and sand) at the seven sites.
|Table 2. Percentage benthic cover at the study sites|
|site 2: Rita-Manaburi, site 4: Oyster Bay,
site 6: Umalagan I,
site 8: Bulalakaw, site 9: Buenavista I, site 14: Sabang, site15: St. Paul Bay
Live Acropora and non-Acropora represent on average 42%, dead corals cover 21% of the reef, while soft corals were the least dominant group. Sand, silt and rocks cover on average 19%.
Comparing the various sites, healthy coral communities were relatively more abundant at the stations of Buenavista, Bulalakaw and St. Paul. At St. Paul, non-Acropora was particularly prevalent. In contrast, dead coral dominated at Oyster Bay. A general trend was noticeable in that the branching or digitate forms of coral were relatively more abundant than the massive or submassive forms at the cleaner or more exposed sites such as Buenavista, Bulalakaw, Sabang, St. Paul and Rita-Manaburi. This contrasted sharply with the more protected stations, such as Oyster Bay and Umalagan, where the more massive/submassive forms dominated.
This observation is consistent with the general statement that the larger surface area in branching corals is an adaptive mechanism designed to help the coral cope with the lower level of nutrients and the stress imposed by crowding and shading from neighbouring coral species at the more exposed sites. Correspondingly, massive corals are better adapted to more protected, silted environments. In extending these arguments, it is often stated that the relative proportion of these two main coral forms – branching and massive – at a given site can be seen as an indicator of the site’s degree of disturbance or siltation.
On the whole, the corals of Ulugan Bay are at a relatively late stage in the succession process, rather than a transitional phase. This was indicated by observations such as the preponderance of hard corals in contrast to the relatively low abundance of soft forms, and was particularly marked at the more protected stations of Oyster Bay and Umalagan, where soft coral forms were all but absent.
Coral reef fish populations were assessed by visual census along 150 m transects during daylight hours using SCUBA in conjunction with the Line Intercept Transect (LIT) for corals. Fish counts were made every 5 m along the line with a width of 10 m. All fish present within the belt transect were identified, counted and their standard lengths estimated (in cm). Juvenile fish were also monitored using the same method but with a narrower width of 2 m to facilitate observation.
Where applicable, the diversity index (Shannon-Wiener Index – Hs) was determined and the community similarity was estimated (Sorensen’s Similarity Index – SSI) (Zar, 1984).
Figure 3. Direct relationship between live coral cover
A total of 3,000 individual fish were recorded at six transect sites. They represented 63 species, 35 of which are of commercial value (see Table A, Appendix IV, for further details on fish data). Of the 25 families recorded, Labridae, Pomacentridae and Chaetodontidae were represented by the highest number of species with 10, 9 and 8 respectively. This result is consistent with similar studies undertaken elsewhere in the Philippines (Alino, 1994; Nanola et al., 1994).
In Sabang and Rita-Manaburi, the fish community shows the highest similarity (see Table B, Appendix IV) and presents the highest number of species (29) and low diversity (Hs = 1.6–1.7). Evenness values were significantly reduced by the dominance of a few species (Sabang: Abudefduf sp., Rita-Manaburi: Pterocaesio diagramma and Pomacentrus sp.).
Offered the highest degree of protection from waves and wind of all sampling sites, and located adjacent to thick mangrove vegetation, Umalagan had the lowest number of fish individuals and the weakest association with the other sites. However, some similarities were noted with neighbouring Bulalakaw – similarity index of 56% (see Table B, Appendix IV). Rita-Manaburi had the overall highest species diversity with 34 recorded species, and also the highest number of individuals: 1,028.
Of interest is the direct relationship between fish community evenness and live coral cover (see Figure 3). These results emphasize the strong dependence of the fish community on the benthic sessile community.
|Seagrass survey in shallow coastal waters|
Data on seagrasses were collected inside 50 x 50 cm quadrats placed at regular intervals along selected transects, perpendicular to the shore. The transects represented prevailing gradients in water depth, substrate, and exposure to wind and waves. The structure of the plant communities was assessed in terms of their composition, abundance, and status in relation to prevailing habitat conditions. In addition to % frequency and cover, density – the number of shoots per unit area, a more reliable index of abundance in discrete communities – was also used. Because of the inherent heterogeneity in the morphology of the species, frequency and/or density rating were used in the case of species with discrete habits (e.g. can be counted individually).
A series of photographs were taken periodically of representative quadrats using a Nikonos V underwater camera. The photographs not only allow speedy collection of data in the field, but also provide a permanent record of the quadrat, which is useful for long term monitoring of growth, mortality and recruitment (Gittings et al., 1990). Monitoring of seagrass over time using observation and photography can also be a useful technique for detecting sediment smothering (Rogers, 1990).
The results of the surveys at the five seagrass stations demonstrated that seagrass species dominance in the bay is highly site-specific. The highest overall density was recorded for Halodule pinifolia at Buenavista, with 876 individuals per m2. Details of species preponderance and density are provided in Figure 4 below and in Table C, Appendix V.
Among the five most dominant species in Ulugan Bay, the following sequence of decreasing ecological abundance (with values expressed in relative density) was established:
Enhalus acoroides (up to 82.8 %)
Halodule uninervis (up to 53.2 %)
Cymodocea serrulata (up to 46.5 %)
Halodule pinifolia (up to 43.3 %)
Cymodocea rotundata (up to 37.2 %)
It appears that the bay, as represented by the seagrasses at the five stations, is characterized by varying stages of ecological development. In turn, these stages represent the varying degrees of perturbation the stations are exposed to, whether natural or man-made. Seagrass vegetation proceeds from apparently more stable, highly diverse, mixed seagrass communities at Buenavista and Manaburi to the stable but less diverse, climax vascular vegetation found at Umalagan (Enhalus acoroides and Thalassia hemprichii) and Oyster Bay (Cymodocea serrulata, Halodule uninervis and Halophila ovalis), to less stable, similarly low diversity pioneering vegetation at Tarunayan (Halodule uninervis).
As referred to in the section on coral reefs, Umalagan and Oyster Bay represent deep indentations in the bay and as such are protected from wave action and are characteristically muddy. In contrast, Tarunayan, Buenavista and Manaburi are more frequently exposed to wave action, hence have coarser sediments. The pioneering and ecologically sensitive Halophila ovalis was observed in high density but only at one location, Oyster Bay.
In terms of diversity, the highest number of species observed at any one location was seen at Buenavista, where seven species were recorded (of the bay’s eight species, only Halophila ovalis was absent). For details of individual species at the sampling stations see Figure 4. It is interesting to note that the distribution of seagrasses among the sites follows a pattern such that more protected sites have fewer species. However, as can be seen in Table C, Appendix V, the relative frequencies of these are correspondingly higher. Similarly, more exposed sites featured more species with overall lower frequencies. This observation demonstrates the classic inverse relationship between diversity and dominance, the latter expressed in relative density.
The total area within the bay covered by coral reefs and seagrasses is 12.4 km2, based on an analysis of the satellite imagery employed in the assessment exercise. This figure is 24% higher than has been previously reported (Walters, 1996), and represents a coverage of 18% of the total bay area. Whether this figure represents an improvement in the cover after two years, or is simply a function of different data collection methods, is an important subject for further study. It should be noted, however, that the figure used in the present study was obtained from the latest satellite imagery, while the 1996 figure was the result of an approximation.
Seaweed communities at the study sites were closely associated with seagrasses and coral reefs, i.e. were recorded along transects where these habitats were surveyed. Thirteen different species of seaweed were found at the seagrass sites and eight species were recorded at the coral reef sites (see Table D, Appendix VI). It should be noted that the seaweed data may be incomplete, as seaweeds have different requirements when compared to seagrasses, coral reefs and mangroves. Of the seaweeds recorded in the bay, a number have significant economic potential and are used as food and for medicinal purposes (see Table E, Appendix VI).
The mangroves were assessed at twelve sampling sites located in the most representative mangrove areas of the bay. Five 500 m2 quadrats were established randomly at each site. Within each quadrat, data on the following parameters were collected: number of trees per species, diameter at breast height (dbh), number of cut and dead trees and number of seedlings and saplings.
||Measuring the diameter at breast height of a mangrove|
||Line transect through mangroves|
The ten dominant tree species are listed below, ranked in order of decreasing density (average number of trees per 500 m2):
Rhizophora apiculata (47.2)
Rhizophora mucronata (17.5)
Bruguiera gymnorrhiza (11.9)
Aegiceras floridum (3.2)
Xylocarpus granatum (2.1)
Sonneratia alba (1.8)
Bruguiera cylindrical (0.2)
Ceriops tagal (0.2)
Xylocarpus moluccencis (0.1)
A surprisingly high percentage of the trees were young, with dbh values between 5 and 20 cm. Only one tree, a Xylocarpus granatum recorded at Sabang, had a dbh value of over 70 cm. In fact, it was only at Sabang that the mangrove trees were relatively evenly distributed in terms of dbh values. At the other locations, the distribution of trees was skewed towards the narrower, younger categories (see Table F, Appendix VII).
These findings show a predominantly young mangrove vegetation, which could indicate that the community is not at a stable stage and/or is recovering from a perturbation (i.e. harvesting). Over-harvesting results in the trees not attaining long lives and substantial growth. The distribution of trees belonging to various dbh classes varied between the sampling stations, and even between localities within each station. At stations in close proximity to human habitation, such as Bulalakaw, Buenavista, Tagnipa and Tagabenit, the mangrove tree populations were notably younger than average. In combination with physical evidence of cut trees (see below), this indicates that trees are cut for local household use.
In contrast, at stations further removed from human settlements (such as Kayulo and Umalagan) or at stations under protection regimes (such as Oyster Bay and Sabang), trees had in general attained bigger trunk diameters and greater heights. At all sites, trees were generally younger at the frontal mangrove zones closer to the water body.
That the Ulugan Bay mangrove populations are dominated by recruitment over-harvesting was supported by the assessment of cut and dead trees. Overall, cut and dead trees occur more frequently and abundantly among the younger trees, although cut trees were observed among all dbh categories (see Table G, Appendix VII).
The abundance values of the seedlings reflect the degree of potential for recruitment of the trees and hence, their rate of colonization or recolonization of a given area. In Ulugan Bay, seedling populations were assessed at all locations, revealing Bruguiera gymnorrhiza to have the most abundant seedlings with an average density of 78.2 individuals/500 m2. The other species have the following density: Rhizophora apiculata (53.6), Rhizophora mucronata (45.1), Aegiceras floridum (12.6) and Sonneratia alba (10.0).
Saplings are somewhat older than seedlings, indicating their ability to survive the prevailing conditions for a period of 1–3 years after the seedling stage. Again, Bruguiera gymnorrhiza exhibited the highest total abundance at 60.8 individuals/500 m2. A ranking similar to that recorded for seedlings was established for the remaining species. The results show that the ratio of seedlings to saplings is generally slightly lower than 1. This shows that during the period of study, recruitment is greater than mortality in the overall development of the mangrove system.
Water transparency analysis was carried out at the sampling sites using a Secchi disk. In general, Ulugan Bay waters are clear throughout the year, with mean readings of 11.4 m (±6.58 SD). From the isolines (sampling points with similar depths), waters ranged from transparent, clear to very clear (disk readings greater than 10 m). These were noted predominantly at the more open, northern half and middle portions of the deeper part of the bay. On the other hand, relatively turbid waters (depth readings less than 10 m) were noted at the eastern, exposed sides of Rita Island and the southern and eastern bay margins. Consistently, the more turbid areas with depth readings less than 5 m were found at the mouths of the rivers inside Tarunayan and Oyster Bay, in small coves along the southwestern half of the bay and in shallow areas along the southeastern sections.