Cat Ba Island has a significant biodiversity value as it is home to a number of rare and endangered species of plants and animals, with the world’s rarest primates the Golden-headed Langur. According to the study results, Cat Ba place have listed 2,380 species of animals and plants including: terrestrial plants 741 species; living animals in the forest area 282 species; mangrove plants 30 species; seaweeds 79 species; phytoplankton 287 species; plank tonic animals 98 species; sea-fish 196 species; corals 154 species. It is identified as one of the areas of highest biodiversity importance in Vietnam and is recognized as a high priority for global conservation.

Keywords: mangrove, seagrass, coral reef, phytoplankton, cat ba island

Biosphere reserves Cat Ba Island has been recognized as a UNESCO World on December 02^nd, 2004. It is the 4^th world’s biosphere reserve in Vietnam. Biosphere reserves Cat Ba archipelago including great majority of Cat Ba Island in Cat Hai district, Hai Phong city, Vietnam. Cat Ba Island is considered the richest marine biological system because of its diversity in the North of Vietnam. On the previous study,¹ they found the Langur distribution and forest cover in Cat Ba Island (Figure 1). The objective of this study was shown the distribution of mangrove, seaweed/seagrass, coral reefs and phytoplankton in Cat Ba Island.

Figure 1The white-headed langur (Trachypithecus poliocephalus), an endemic species from Cat Ba Island.1

Description of the study area

Cat Ba is the largest of 336 islands in the Cat Ba Archipelago, and also the second largest limestone island (200 km²) in the coastal zone of Vietnam.² It is located in the north-east of Vietnam in the northern section of the Tonkin Gulf and adjacent to Ha Long Bay (the world natural heritage site) (Figure 2). The total natural land area of Cat Ba Archipelago Biosphere Reserve is 26,240 ha, of which land area (island) is 17,040 ha and 9,200 ha of sea. Cat Ba Island is considered the richest marine biological system because of its diversity in the North of Vietnam.^3‒7

Figure 2Cat Ba Island.

Methods of field survey

Mangrove: All mangrove species that were identified in the survey sites were recorded. Also to obtain quantitative data, a 10m x 10m quadrate was set at a location that was representative of the site, and the following information was recorded: species composition, canopy height (highest and lowest), stem diameter (3 m from ground) and density (No. of stems/100 m²).

Seaweed/seagrass: All seaweed/seagrass species that were identified in the survey sites were recorded. Samples were collected and preserved in a 10% formalin solution for further analysis at IMER’s laboratory. Except sites AL11, survey was conducted only along the shoreline (underwater survey was not possible due to very low water visibility). Sites AL11 was surveyed by scuba diving.

Coral Reefs: Coral survey was conducted by squba diving at sites AL11. Species composition and live coral coverage were recorded at 5m interval along a 100 m line transect.

Phytoplankton: The qualitative Phytoplankton samples were collected by towing a plankton net (mesh size: 20μm, diameter: 20cm) several times in the vertical direction. Quantitative samples were collected from the surface by collecting 1L of surface water in the PVC bottle. All samples were preserved in 3ml Lugol solution for further analysis at IMER’s laboratory.

Demersal fish: Demersal fish was collected with a trawling net (mesh size: #15, width: 5m). Each trawl was conducted for 15 minutes at a speed of approximately 2 miles/hour. All collected species were measured (total length) and weighted, then preserved in 10% formalin solution for further analysis at IMER’s laboratory.

Diversity of the species composition

According to previous studies, a total of 2,380 species have been recorded in Cat Ba Island. Among of them 1,053 species are terrestrial species occupies 51.7% of the total species while the marine species occupies 48.3% with 985 species (Table 1). High biological diversity of the species included in the Cat Ba Island will be a very important basis for the exploitation of natural resources to serve different purposes of human. On the other hand it provides great significance in scientific research: evolution, ecology, environmental indicators along with a wide range of species likely to play an important role in the provision of ecosystem services such as disaster prevention and source of raw materials for the chemical industry and medicine production.

Diversity of major marine ecosystem

Coral reefs: Coral reefs are mainly distributed at the surrounding waters in the southeast of Cat Ba Island such as Cong La, Ang Tham, Ba Trai Dao, Van Boi, Cong Hip, Tung Ngon, and Coc Cheo. The coral reefs areas are the major fishing grounds for reef fisheries due to high values of the coral reef fishes and associated reef species (Figure 3). The coral reefs in Cat Ba - Ha Long area are represented for the fringing reef types and two addition types of islands connectivity and the unofficial atoll. Generally, along the limestone islands of Cat Ba have coral distribution at a depth of 3, 6, 9 and 11 meters. Morphology of the reef is determined mainly by the morphology of the ground slopes and partly by carbonate sediments originated from organisms on the reef. Due to changes of the environmental conditions under the impact of natural and human, coral reefs are now greatly reduced in terms of area and the level of diversity of the groups living together. The percentage of the live coral coverage occupies by less than 40%, meaning classification of medium and low level by the UNESCO criteria for assessment of the coral reef health.

Figure 3Locations of the field survey sites (mangrove, seaweed/seagrass and coral reef).3 Note 1 Coral was surveyed only at sites AL11; Site AL8 was not surveyed as the area was under construction activities.

Table 2 shows the hard coral species identified through the field survey. Twenty-eight species and 58 species were identified at the sites AL11 respectively. The diversity at site AL11 was lower probably due to relatively turbid conditions. Within the identified species, 4 species are listed in the Vietnam Red Book namely: Porites lobata, Acropora aspera, Acropora formosa and Acropora nobilis; which are all classified as “Vulnerable”. Porites lobata was found at the AL11 site.

Seagrass: Seagrasses are mainly distributed in Cat Ba Island in the narrow range like leopard style on tidal marshes (with the carpet of mangroves) in Gia Luan and in aquaculture ponds in Phu Long. A small area of seagrass distributes in the island of Long Chau (offshore) and Van Boi (Southeast of Cat Ba Island) where there are coral reefs with live coral cover is relatively high (Figure 4). There were only two species of seagrass have been identified in the island of Cat Ba such as Ruppia Maritima and Halophila beccarii. However, both species are not included in the Red Data Book of Vietnam, although the species of Halophila beccarii is classified at the level of “danger threatened” in the IUCN red list of threatened species, 2011.

Figure 4Distribution map of the seagrass area at Cat Ba islands.3

Table 3 shows the seaweed species identified through the field survey. Seventeen species belonging to 9 families were identified. Chaetomorpha capillaris and Enteromorpha compressa were the most common species. Some species such as Asparagopsis taxiformis, Colpomenia sinuosa, and Cladophoropsis membrannacea were identified at only specific sites. None of the identified species are included in the Vietnam Red Book. Some species in the Ulvaceae family are harvested for food stuffs in certain areas of Vietnam. Table 2 shows the seagrass species identified through the field survey. Only two seagrass species Ruppia maritima and Halophila beccarii were identified, which were found at sites AL10 and AL11 respectively. Halophila beccarii was record for the first time in the surveyed area. Although both species are not included in the Vietnam Red Book, Halophila beccarii is classified as “Vulnerable” in IUCN Red List.

Mangrove forest: The mangrove forest and the coral reefs are the invaluable natural resources of the tropical countries in general and Cat Ba in particular. They play as the sources of genetic reservation, increasing of the natural biomass and stabilization for the shoreline. Most of the mangrove forests maintain high cover in the western side of Cat Ba Island (Phu Long Natural Reserve). In near future, the fight campaign for climate change will lean much on these green corridors (Figure 5).

Figure 5Distribution map of the mangrove area at Cat Ba islands.3

Most of the mangrove areas distribute in Phu Long Commune with high relative density. The total area is 775.98 ha of mangrove forest is divided into two types: mangrove distribution outside the farming area (224.74 ha), mangrove distribution in ponds (551.24 ha) (Figure 6). Because mangrove forests in farming systems are relatively large so the long-term master plan and conservation of mangroves will be difficult due to satisfactorily resolve the relationship of private-public ownership. Table 4 shows the mangrove species identified through the field survey. Eleven species belonging to 9 families were identified. Rhizophora stylosa and Avicennia marina were the most common species in the survey area. None of the identified species are included in the Vietnam Red Book.

Figure 6Mangrove distribution in Cat Ba Island.3

Phytoplankton: Phytoplanktons are the keystone species in this habitat type, providing basic food items for the zoobenthis and fish that are the key targets of the capture fishery (Figure 7). Table 5 and Table 6 show the phytoplankton species identified through the dry and wet season surveys respectively. In the dry season, a total of 134 species were identified (Table 5). The genus Chaetoceros was found in many survey sites, which play an important role as food sources for fish and other marine species, in particular at the early stages of their life cycle. Some of the identified dinoflagellate species such as Ceratium fusus, Prorocentrum micans and Dinophysis caudate are known to cause red tide and harmful algae bloom when at high density. In the rainy season, a total of 136 species were identified (Table 6). The density of the phytoplankton has been driven by some dominante species such as Skeletonema costatum with the cell density is approximate 103 up to 4,104 cell/L; next to the Chaetoceros spp. with the density from 103 - 56,103 cell/L; Ceratium furca has the density from 2.103 to approximate of 104 cell/L (EC9 site); Oscillatoria sp.2 103 cell/L and Bacteriastrum spp. reaches 47,103 cell/L.

Figure 7Locations of the field survey sites (phytoplankton, zooplankton, zoobenthos, demersal fish and other zoobenthos).3

Table 5 to Table 6 shows the number of phytoplankton species and cell density at each survey site. For the dry season, the number of species ranged between 32 and 57 species. While there was no clear spatial trend in the cell density, relatively high levels were recorded at the EC4 and EC8 sites. The seasonal variation shows a clearly trend of reduction both interm of species composition and cell density. However, the number of species is slightly lower in rainy season while the cell density rapidly reduces up to 98.42% at the EC8 site to 22.3% at the EC11 site.

Demersal fish: Table 7 shows the demersal fish species identified through the dry and rainy season surveys respectively. In the dry season, in general, fish diversity and abundance were significantly higher in the shallow coastal survey sites (e.g. EC1, EC2, EC4, and EC7) compared to the deeper offshore survey sites. Within the identified species, two species are listed in Vietnam Red Book namely, Bostrichthys sinensis and Anodontostoma chacunda, which were found in the shallow coastal survey sites EC1 and EC2 respectively. Bostrichthys sinensis and Anodontostoma chacunda are classified as “Critical” and “Vulnerable” respectively. In the wet season, the similar trend in term of fish diversity and abundance has been found among sampling sites (shallow sites are more abundance than the off shore sites). However there are differences in the species composition of the economic species with the distribution of the family Sciaenidae to occur in 9/11 sampling sites. This family also contributes for higher biomass of the total catch at the sampling sites of EC8, EC9 in the rainy survey.

Diversity of terrestrial animals

Cat Ba National Park is tropical moist forest on limestone, which harbors a number of endemic and rare species, foremost of which is the endemic Cat Ba Langur Trachypithecus poliocephalus poliocephalus.^9‒11 These led to the discovery of new species of Goniurosaurus¹² (Figure 8) and Sphenomorphus.¹³ Among the 40 reptile species recorded from Cat Ba Island, two species are listed in the IUCN Red List (2008), seven species are listed in the Vietnam Red Data Book,¹⁴ four in the CITES appendices (2008), and five species are protected by governmental law (Decree No. 32/2006/ND-CP) (Table 8).

Figure 8(1) Cuora mouhotii¬. (2) Acanthosaura lepidogaster. (3) Pseudocalotes brevipes. (4) Goniurosaurus catbaensis. (5) Gekko gecko. (6) Gekko palmatus. (7) Hemidactylus frenatus. (8) H garnotii. (9) Eutropis multifasciatus. (10) Plestiodon quadrilineatus. (11) Scincella reevesii. (12) Sphenomorphus tonkinensis.12

Notes: IUCN: IUCN Red List, VNRB: Vietnam Red Data Book: CR: critically endangered, EN: endangered, VU: vulnerable, LR/nt: near threatened; CITES: I, II = Appendix I and II; Dec. 32: Governmental Decree No. 32/2006/ND-CP: IB = Group IB (prohibited exploitation and use for commercial purpose), IIB = Group IIB (limited exploitation and use for commercial purpose); []: Only photographic record or observation.

In this study, there are occurrences of three main key habitats: mangrove, coral reefs, and seaweeds/seagrass. These contribute for species diversity and abundance of the natural marine resources available for the local people to exploit. The distribution trends of the living resources are higher abundance in the near shore sites, lower abundance at the offshore sites. These may relate to the biological/physical driven factors such as the available of substrate/habitat, natural food resources or water current. The coastal sites provide the nursery grounds for the economic species where the mangrove forests remain as the shelters.

We would like to thank the National Hydro-meteorological Service (NHMS) for their assistance in the collection of topography map and data for this paper. We are thankful to the NHMS for this research opportunity and providing permission for using their facilities to perform the study.

The authors declare that there are no competing interests regarding the publication of this paper.

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