Over 365 species of coral reef fish belonging to 61 families have been recorded in the Gulf of Thailand and around Koh Tao (Scaps, 2006). This includes many small colorful fishes found amongst the corals, as well as the world’s biggest fish, the Whale Shark.This number is climbing though, as we work to document and publish more species found around our area. We conduct monthly monitoring activities to assess the biodiversity and abundance of fish around Koh Tao. Below you can find out a bit about our local fishes.
The first fishes are believed to have evolved about 500 million years ago, the most primitive fishes that we find today are called jawless fishes, which include Hagfish and lampreys. These fish lack the bone or cartilage that we associate with most vertebrates, and illustrate the link between our invertebrate and vertebrate groups. Slightly more advanced are the cartilaginous fishes, which is a very ancient group having a flexible skeleton made of cartilage instead of bone. This group includes families of fish such as the sharks and rays, which are some of the reef’s top predators
Sharks evolved as much as 420 million years ago, and have physically changed very little since then. Which is to say that in evolutionary terms they have not needed to change, for most of their history they have been optimally adapted to their environment. Today there are about 350 living species of sharks.
Sharks have a very streamlined, fast body (called fusiform), which is true of most predatory fish. They hunt using 6 senses; smell, sound, sight, touch (they sense vibrations in the water with their lateral line), electroreception, and lastly taste. Sharks, like humans are late to reach sexual maturity and have very few, but well developed, offspring. Male Sharks can be identified by claspers located in the pelvic region (like small ‘arms’ used for holding onto a female during mating).
Sharks are rightfully called the ‘kings of the sea’, because in most marine food chains they are the top predator, and thus are a top-down control on the entire ecosystem. Sharks maintain the ecosystem balance and they play a big role in nutrient cycling and export from reefs.
The three most common species of sharks on Koh Tao are Whale Sharks, Black Tip Reef Sharks, and Bull Sharks.
It is estimated that over 100 million sharks are killed by humans each year, and populations of most species of sharks are in sharp decline.
Contrary to popular belief, sharks are not dangerous to humans. According to local reports, only 3 shark attacks have occurred in the Gulf of Thailand in the last 100 years, and all were on fisherman, not divers of snorkelers. The chances of seeing a shark during the EMP are not high, but at many of the island’s deeper dive sites they can be found quite regularly (albeit less than a few years ago).
If you do see sharks, please report it on our reporting data base at www.marineconservationkohtao.com. Even if you do not see sharks, you can still report your dive information to SharkTrust.Org, because saying that you dived and didn’t see sharks is just as important as if you did. Also be sure to check out groups like Project Aware to find out how you can help to protect sharks.
Rays (Family Dasyatidae)
Rays are closely related to sharks; they have cartilagenic skeletons and very advanced electrosensory skills. Although there are occasionally Eagle Rays or Manta Rays around Koh Tao, the primary type of ray found during the EMP is the Jenkins Ray and Blue Spotted Ribbon Ray.
Sting rays and Ribbon Rays feed primarily on benthic invertebrates such as crabs and shrimps. They are primarily nocturnal hunters, and will usually be found under coral heads or buried partially in the sand during the day. The Jenkins ray is dull colored, has a diamond shaped body, and can grow over 1 meter in width. The Blue Spotted Ribbon Ray is smaller, has a rounded body, and is covered in bright blue spots.
Rays are considered to be top predators, and so thus important in top-down controls of the trophic structure. They are sensitive to changes in the environment, and also allow us to gain information on the abundance of crustaceans on the reef.
Moray Eels (Family Muraenidae)
Moray eels are actually a type of bony fish, they are not related to snakes as some may infer. There are over 200 species of moray eels worldwide, but the most common species on Koh Tao is the White Eyed Moray Eel.
Moray eels have a serpentine shape, and live in burrows or crevices and holes in corals and rocks. They feed on crustaceans, mollusks, and small fish. They are nearly top-predators, but sometimes become prey for barracuda or sea snakes.
As predators, eels are important in regulating the balance of the reef. They are also sensitive to habitat destruction and declines in water quality.
Butterfly fishes (Family Chaetodontidae)
Our first group of familiar coral reef fishes are the butterfly fishes. Most of the butterfly fish are very colorful and live in close association with corals. All of the butterfly fish have a similar body shape, which is plate-like (thin, tall, and round). They are the opposite shape of a shark’s mouth, which illustrates how important a role predation has played in their evolution. Butterfly fish are the easiest fish to spot while conducting the EMP, and usually occur in pairs but sometimes can be found in schools.
Butterfly fishes are thin, tall, and plate-like to avoid predation. To further avoid predation, the tail of most butterfly fish looks just like the head, and often they have a line over their eye for disguise. Together these confuse predators, who don’t know which direction to sneak up on the fish, or which way the fish is going to swim to get away.
Butterfly fish feed on coral polyps, micro-invertebrates, and algae, so live only in healthy reef areas. In fact, graphs of coral abundance and butterfly fish abundance tend to be very closely correlated. In many areas they are a prized fish for the aquarium trade due to their bright colors, but on Koh Tao are mostly threatened by fishing activities or habitat destruction.
Groupers (Family Serranidae)
Groupers are ambush predators; comparing them to the butterfly fish, they appear stronger, more streamlined, and faster. Groupers are generally demersal (live along the bottom) and can be found on top or under rocks and corals. They lie and wait for small fish or invertebrates to come close and then use their draw-bridge like mouth to suck the prey in. Their lower jaw is spring loaded and open and closes in a fraction of a second, but in doing so increases in volume several fold, creating a current that few prey species can escape.
Groupers are a predator, but are also a prey species for larger fish; thus they are a vital link in the food chain. They are also very sensitive to fishing and habitat destruction.
Parrot fish (Family Scaridae)
Parrot fish have a very fitting name for many reasons; they tend to be brightly colored, have a parrot like beak, and swim with their pectoral fins in a bird-like fashion.
On Koh Tao, parrot fish are one of the most abundant and effective fish grazers of micro-algae. They use their beak to scrape algae from rocks and dead coral which prevents algae from overtaking the corals, opens up clean areas for settlement of coral larvae, and regulates nutrient levels in the reef. In some areas of the world, parrot fish can grow quite large and will actually use their beak to bite chunks off of the coral. By doing so, they cause a minor disturbance to the reef, but also help to create white sandy beaches.
Juvenile parrot fish tend be a more dull green color, and are sometimes confused for the moon wrasse
Rabbit fish are a common reef fish, and resemble the simple fish profile that everyone can recognize. They have a nose which resembles a rabbit, and also play a very similar ecological role to rabbits. The 3 most common species on Koh Tao are: the Double Barred Rabbit fish, the Java Rabbit Fish, and the Gold Saddle Rabbit fish.
Rabbit fish are important reef herbivores feeding on benthic algae. They are also an important fish as prey for sharks and food for humans.
Rabbit fish have poisons spines and should be handled carefully when removing from nets found while diving.
At night rabbit fish can be found sleeping with their dorsal and anal fin spines extended, making them a more difficult meal for nocturnal predators.
Snappers (Family Lutjanidae)
Snappers are a predatory fish, and have a perciform shape. They appear strong and streamlined when compared to the herbivores fishes. There are several species of snapper on Koh Tao, the most common of which are the Spanish Flag Snapper, The Russel’s Snapper, the Checkered Snapper, and the Blackspot Snapper.
Snappers are secondary predators which feed on crustaceans and other fish. They are also a favorite of the fishing industry, and can be used to assess fishing related threats.
Snappers can be solitary or in schools, and are usually quick to flee from divers.
Surgeon Fish (Family acanthuridae)
Surgeon fish are mostly herbivores fishes, and derive their name by the spines protruding from the tail, which look like a surgeon’s scalpel. On Koh Tao they are quite rare, but easily spotted by their unique body shape and colorful spine (usually orange or yellow).
Due to their relative rarity on Koh Tao they are used as an indicator of reef biodiversity. They are sensitive to changes in habitat and by fishing pressures.
Sweetlips are part of the emperor fish family, which can grow quite large. Their body shape appears like a mix between the groupers and snappers. Adults are generally white or dull colored with black spots, while juveniles are brightly colored and very ornate.
Sweetlips are important predator fishes that feed on crustaceans and other benthic invertebrates. They are a favored fish by the fishing industry, and are indicators of fishing pressure on the reefs.
Juveniles and adults have very different appearances, and live in very different areas of the reef. The juveniles can usually be found in the shallow reef areas, and will stay in the same place for about 1 month while developing. Adults are more likely found on the deep reef or in more sandy areas where they can blend in easily with the surroundings.
Triggerfish (family Balistidae)
Triggerfish are large reef fishes with an oval body and a strong mouth. They swim using their dorsal and anal fins in an undulating motion. They are notoriously territorial, and can be quite aggressive. They have two ‘Triggers’ which are used to warm intruders and lock themselves into cracks in the reef while sleeping. The two most common species on Koh Tao are the Titan Trigger fish and the Yellow Margin Triggerfish, occasionally the Clown Triggerfish and the Pink Tail Triggerfish .
Trigger fish are important top predators on the reef. They help to control the populations of coralivores gastropods and echinoderms (like the Crown of Thorns). They are very intelligent fishes and often use tools such as rocks to break open clams or other shelled organisms.
Triggerfish on Koh Tao often attack when they feel threatened, to avoid problems do not swim directly towards or over a triggerfish, but instead remain calm, lay low, and wait for the fish to move away from you.
Red Breasted Wrasse (Family Labridae)
The wrasse family of fish is very large and diverse. They are one of the most abundant families of fish on the reefs of Koh Tao, but we will only record one species, the Red Breasted Wrasse. This species is one of the larger Wrasse on Koh Tao, and are easily identified by their black and white striped body and red head/breast.
Red Breasted Wrasse are large fish which require a high abundance of marine invertebrates to survive. They are an indicator of crustacean abundance and also fishing threats.
Check out some more photos of the amazing fish life around Koh Tao in our gallery
Others (coming soon)