There is an enormous diversity of marine organisms in the ocean. This is hardly breaking news, but what is surprising is that around 25% of all marine organisms spend some of their life on coral reefs.
From the smallest microalgae to the largest fish in the sea, coral reefs are a kaleidoscope of colour and diversity. One of the most critical animal groups inhabiting coral reefs are the diverse and dominating family of cartilaginous fishes, sharks.
What is a shark or a ray?
Shark’s have been around for over 400 million years. When you stop and think about that length of time it is unfathomable. They were here well before the dinosaurs and must have been doing something right to still be a dominant force in the marine world occupying most marine ecosystems on Earth.
What makes sharks different from regular, bony fish? Sharks are special due to their cartilaginous skeletons as opposed to the bony skeletons of regular fish. The sub-class of Elasmobranchs consists of 1000 species which includes around 400 species of sharks but also 600 species of stingrays.
Fearsome or friendly?
Sharks have been given a bad rap from movies and television over the years portraying them as fierce predators to be feared. This may be true if you are a small fish living on the reef, but not so for people.
Sharks are inquisitive and if they come across a foreign object in the water, will most likely go and inspect the unknown object. In the case of large sharks and humans, this often results in a bite, which unfortunately can lead to severe blood loss, but the sharks are not preying on humans. It is usually a case of mistaken identity. Plus, sharks have far more to worry about when it comes to people than the other way around.
The importance of sharks
Sharks are critical species that play an important ecosystem role, maintaining balance, creating harmony. Sharks manage the abundance of the organisms in the food chain below them and provide a barometer for ocean health.
They prey on weak, sick and injured prey in addition to keeping balance amongst other predators. This ensures species diversity and a healthy ecosystem.
Being major predators, their spatial distribution and feeding strategies affect the habits and activities of other species. The change in shark abundance has directly negatively affected the health of coral reefs and associated ecosystems.
The removal of sharks leads to an increase in larger predators, like groupers, who may have an influence on the abundance of herbivores.
This could have negative feedback loops as a reduction in herbivores can lead to an increase in macroalgal dominated reefs and a change in ecosystem balance. By preventing one species from dominating a limited resource, they enhance the diversity of the ecosystem.
Sharks are a threatened species
Shark fishing presents a potentially catastrophic threat to shark populations the world over. It is estimated that up to 73 million sharks are captured each year, mostly for their prized fins. Consequently, the IUCN (International Union for the Conservation of Nature) has estimated that almost one-third of all open ocean shark species are threatened with extinction.
There is no doubt that shark populations are in decline the world over. One of the key life history traits that compromises the ability of sharks to meet this challenge is due to their relatively slow growth and maturation rates, and their low reproduction rates. These characteristics mean that sharks are particularly vulnerable to over-exploitation.
Some fisheries don’t directly target sharks, but many are killed as unnecessary by-catch as a result of inefficient and unsustainable fishing methods. Trawl fisheries are the primary culprits, with many of the dead or dying sharks are tossed overboard.
To improve the trajectory of shark populations and the outlook for the species, it is important that more sustainable fishing methods are employed, demand for shark fin soup and related shark products is reduced and the practice of shark finning needs to be outlawed altogether.
My shark experiences on Koh Tao
Sharks have an amazing sensory system that enables them to travel vast distances, sense nearby threats and detect prey. It is likely true that if you see one shark a hundred have probably seen you.
It comes as no surprise then that sharks are not the easiest to see. On Koh Tao I have had the pleasure of seeing blacktip reef sharks, bull sharks, leopard sharks and whale sharks at Shark Island, Mae Haad, Ao Leuk, Twins, White Rock, Southwest Pinnacle, Leam Thian, Chumphon Pinnacle and Sail Rock, just to name a few.
Some experiences were momentary glimpses while others were lingering moments to share lasting entire dives.
Sharks of Shark Island
Having spent over ten years living on Koh Tao I had the pleasure of multiple encounters with many species of sharks and rays. The shallow, broad expanses of the appropriately named Shark Bay were popular for sightings of black tip reef sharks (Carcharhinus Limbatus), but they were not my favourite experience of the common reef dwelling shark.
Whilst leading a group of open water divers at the aptly named shark island I swam over a rocky outcrop in about the 7m of water and surprised a blacktip reef shark exploring an alcove on the island’s eastern flank.
My heart skipped a beat with excitement. The problem was that it disappeared so quickly that no one in my group of divers had the pleasure of witnessing this majestic creature. Worse still, few people either in my group or those back on the boat believed that I had actually seen a shark!
Shark island also harbours fond memories for me as a home to a resident leopard shark (Stegostoma fasciatum). Leopard sharks can often be found resting on the sandy seafloor, pumping water over their gills to stay oxygenated before heading off to explore the reef.
We would often find a leopard shark at around 18m at the base of the fringing reef but my most memorable experience with a leopard shark was at the most unlikely locations. The year was around 2005 and i was completing my divemaster training at Crystal Dive under the tutelage of now PADI Regional Manager for Indonesia, Paul ‘Tosh’ Tanner.
We were in the middle of a divemaster skills circuit at around 9m deep just outside the main harbour in Mae Haad. As Paul was demonstrating the next skill, a sleek leopard shark swam between our group, unfazed and unfussed.
The feeling of surprise, shock and awe was instant. Needless to say there was a momentary lapse in focus with the task at hand as we all tailed the shark for a few minutes before it disappeared out of sight.
Baby sharks of Ao Leuk
Often times if you go to the shallows of Aol Leuk bay you’ll find a number of juvenile black tip reef sharks. The site became something of a nursery where you could stand in waist deep water barely one metre deep, pop your face in the water and see these beautiful creatures swimming around.
It was ideal for budding photographers too as close up shots were much easier to capture with multiple, inquisitive sharks cruising around your legs.
Bull Sharks of Chumphon
The depths of Chumphon Pinnacle located to the north-west of Koh Tao are probably the most popular location for shark sightings. For many years the sight was home to a resident population of bull sharks (Carcharhinus leucas) that were inquisitive and reliable, making any dive trip to the famed site a real treat.
I recall on many occasions descending to the bottom at around 28m and waiting patiently for them to appear. Sure enough, they would swim out of the shadows and cruise about a foot above the seafloor. One, two, three. They would appear and disappear like shadows in the night.
As they swam off, one of my dive buddies went to follow them and I motioned for him to wait, be patient and enjoy the experience as they circled our location just checking out the scene with seemingly no real objective.
Unfortunately bottom time at those depths were all too short and I would have to leave the sanctity of the deep ocean floor and the sharks that lived there.
On one occasion I was leading a group of Advanced Open Water students along the southern wall of Chumphon Pinnacle. As I rose over a rock, a 2.5m long bull shark was coming straight towards me in the opposite direction.
I held my breath as the shark looked me directly in the eye before peeling off to my right and swimming by with nary a care or thought in the world. This time, my entire group was there to witness the occasion and when we returned to the surface the wails of sheer joy were something to behold.
The Majestic Whale Shark
I have had the pleasure of the company of the largest fish in the sea, the whale shark (Rhincodon typus). It is one of those bucket list type of experiences and luckily, a great place to potentially see one. Whale sharks are not guaranteed, but recently they have been seen regularly around this small island in the Gulf of Thailand.
My first experience with a whale shark was at Chumphon Pinnacle. We had heard that they were around and descended in hope. In the distance we could see a dark shadow that loomed ever closer and finally into focal view.
It was the large spotty fish, as we colloquially call them, cruising through the depths with a troupe of travelling remoras and sharksuckers for company.
We watched in awe as this amazing creature sailed by and disappeared effortlessly into the distance. We tried to follow, but our tiny fins were no match for this giant.
Thankfully, when whale sharks are around they tend to circle locations like Chumphon, so it wasn’t long before he returned.
This continued for the entire 30 minute dive and while I have seen whale sharks since then, the first time still remains one of my most enduring memories of the largest fish in the sea, a member of one of the most important species for our reefs and oceans.
Nathan Cook M.Sc
PADI MI 479720