Overview of recent marine research activities on Koh Phangan. Please click the Read More link in the individual projects to access full proposal

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Key fish species controlling macro algae growth

Coral reefs are biodiversity hotspots which provide resources, coastal protection, income from tourism and building materials to the local communities depending on them. However, many coral reefs have been known to undergo phase shifts from coral dominated systems to macroalgae dominance. This is commonly associated with a reduction of herbivorous fishes, for example through overfishing. While grazing fishes, which make up most of the herbivorous fish abundance regulate small turf algae, they are unable to feed on established macroalgae. Browsers, on the other hand, are able to consume these chemically and morphologically defended algae. However, browsers are generally of large body size and thus are especially targeted by fisheries. Therefore it is important to learn more about which species are able to help the reversal of phase shifts from macroalgae dominance to coral dominance.


The aim of the project is to identify important browsing species in Mae Haad and in the Marine Park Ang Thong and compare these two different coral reefs in terms of what species feeds on macroalgae and how much biomass can be removed. Additionally, local fisheries will be assessed with regard to whether these species are removed from the local bay Mae Haad. In the long run these findings may aid in implementing management efforts to increase the resilience of Koh Phangan’s coral reefs.


To observe which fish species remove macroalgae, thalli of abundant macroalgae (Turbinaria sp., Padina sp.) will be collected from the back reef, weight and transferred to the reef crest, where they will be filmed. Feeding behaviour on the assays will be analysed for which species is feeding, how many bites this species is taking and how big these species are. Using this information, the relative importance of species for macroalgae removal can be evaluated. The same set-up will be deployed in Mae Haad and in Ang Thong to test for differences. In order to gain some knowledge of the local fisheries, the landing pier will be visited to identify the fish groups being fished and to get some rough ideas on the quantities. Also, personal communication with some of the long-tail boat captains can give insight into the local fishing practices.



Project Progress

Macroalgae assays deployed in Mae Haad

Macroalgae assays deployed in MPA

Hours of Video watched


Project Progress

Coral Algae interactions observed

Line point intercepts

Marked Coral Algae interactions

Effect of coral growth forms on coral-algae interactions

Coral reefs are one of the most diverse ecosystems worldwide, with half a billion people depending on their ecosystem services and functions. With ongoing pressure of anthropogenic impacts such as coastal development, destructive fishing methods, pollution and river runoff as well as ocean acidification and rising sea surface temperatures related to climate change, coral reefs have experienced an onward decline in coral cover during the last decades. In most cases the coral cover decline is accompanied by an increase in macroalgae abundance, leading towards a phase shift from coral dominated towards an algae dominated reef. For possible conservation and restoration strategies it is fundamental to understand how corals and algae interact and how this interaction is influenced. A recent study found evidence for coral growth forms being an important factor in the competitive process of coral algal interactions, suggesting a trade-off between effective defense mechanisms (massive corals) and high growth rates (branching corals).


The aim of the study is to provide information about the abundance and character of coral-algal interactions in different locations around Koh Phangan, revealing which algae seem to be more harmful in contact with corals. Furthermore the influence of different coral growth forms on the progression of coral-algal interactions will be investigated.


Line-point intercepts (3x25 m) are used in order to investigate the benthic cover and the abundance of coral-algal contacts. Furthermore the character of coral-algal interactions will be classified to determine which algae are harmful and which ones seem to have no impact on corals. In Mae Haad, interactions of the most abundant alga and corals from different genera and with different growth forms will be marked and monitored weekly over the whole study period to gain information about the short-term competitive dynamics and the influence of coral growth forms on the interaction.


Effect of structural coral reef complexity on fish communities

Coral reefs are complex habitats regarding both their topography and inhabitants. These inhabitant organisms in one way or another have to interact directly with their habitat. An example of habitat-organism interactions are functional fish groups such as herbivores and corallivores. Known to provide certain ecological roles, functional fish groups shape the composition of coral reef habitats, similarly as it shapes theirs by providing shelter and/or food sources. The interaction between structural reef complexity and inhabiting fish communities is an important topic to reef degradation and management.


The aim of the study is to determine the role of structural reef complexity to important herbivorous and corallivorous fishes of Mae Haad. Simultaneously, this study will identify the species contributing most to both functional fish groups and the structures responsible to their presence on the reef. The final study outcome is supposed to make the management of Mae Haad’s reef more effective by increasing the conservation emphasis on certain fish species as well as structural elements.


Rugosity measurements: chain & tape method (as per Risk 1972).

Fish abundance measurements: Underwater visual census of corallivores & herbivores with belt transects (as per Green & Bellwood 2009).

Benthic cover measurements: Line-point intercept survey of structural reef elements (e.g. corals, macroalgae, sponges, rubble, sand; as per Knudby & LeDrew 2007).



Project Progress

Belt transects surveyed

rugosity chain counts




Macroalgae can be defined as large, multicellular, photosynthetic organisms. The term macroalgae can also encompass three groups; red algae (rhodophyta), green algae (chlorophyta), and brown algae (phaeophyta). Macroalgaes structure consists of three main parts:

  • lamina/blade = leaflike structure where photosynthesis takes place.
  • stipe = stemlike structure that provides support, although may be absent in some.
  • holdfast = specialised structure that attaches the algae to substrate.

The most important abiotic factor that allow macroalgae to survive include sunlight availability. This limits how deep they can inhabit in seawater as they need to be shallow enough to obtain sufficient light for photosynthesis.

Macroalgae are environmentally important as they provide nutrients for the marine ecosystem being one of the main primary producers. Also, macroalgae can create a unique habitat for other organisms such as invertebrates and juvenile fish.





The world’s coral reefs are in decline,with many exhibiting a phase shift from coral to macroalgal dominance [1–6].Thischangeisoftenassociatedwithhabitatloss and overharvesting of herbivorous fishes, particularly parrotfishes and surgeonfishes [6–9]. The challenge is to reverse this decline and enhance the resilience of coral-reefecosystems[10,11].Wedemonstrate,byusing a large-scale experimentally induced phase shift, that the rapid reversal from a macroalgal-dominated to a coral- and epilithic algal-dominated state was not a result of herbivory by parrotfishes or surgeonfishes. Surprisingly, phase-shift reversal was primarily driven by a single batfish species (Platax pinnatus), a fish previously regarded as an invertebrate feeder. The 43 herbivorous fishes in the local fauna played only a minor role, suggesting that biodiversity may not offer the protection we hoped for in complex ecosystems. Our findings highlight the dangers faced by coral reefs and other threatened complex ecosystems: Species or functional groups that prevent phase shifts may not be able to reverse phase shifts once they occur. Nevertheless, reversal is possible. The critical issue is to identify and protect those groups that underpin the resilience and regeneration of complex ecosystems.



McCook et al. (2001) reviewing the coral-algal interactions, proposed 6 mechanisms by which algae can compete with corals: 1/overgrowing, 2/shading, 3/abrasion, 4/allelopathy, 5/recruitment barrier and 6/epithallial sloughing. All those mechanisms have been more or less documented and the recent work of Rasher and Hay (2010) demonstrate the significance of chemical ecology in the coral-algal-herbivore interactions and the impact on coral reef. Numerous native as well as non-indigeneous algae (invasive species) have been involved in algal overgrowing and reef degradation and there are a limited number of reports of the reversal of the phase shifts (Stimson & Conklin, 2008). Several groups of macroalgae including Halimeda, Caulerpa, for the greens, Dictyotales (Dictyota, Lobophora, Padina) and Fucales (Turbinaria, Sargassum) for the brown and Acanthophora, Asparagopsis, Euchema, Gracilaria and Portieria for the reds are currently associated with algal blooms and coral reef degradation. In New Caledonia, various coastal shallow reefs and outer reef slopes are facing overgrowing of certain species of Halimeda, Dictyotales (Dictyota and Padina), Sargassum and Asparagopsis deeply changing the seascape in the affected areas.

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