SAIL ROCK: BIOLOGICAL ENIGMA TURNS INTO DIVERS’ BEST DREAM

The exceptional clearness of the water around Sail Rock during the last week, with up to 25 m visibility, revealed the dive site’ incredible biologic diversity and density. Such circumstance offered the chance for reflecting over an apparent mystery: sometimes divers were literally caught into enormous schools (hundreds individuals) of jack fishes in rather shallow waters, while deeper waters were constantly patrolled by queen trevallies, huge schools of young barracudas, small groups of enormous barracudas (up to 1.8 m in length) and giant groupers. The overall biomass of those predators appeared so relevant to outmatch that of the preys, constituted by young individuals of the most common reef fishes, especially fusiliers and damsel fishes, but also butterfly, parrot, angel, bat, rabbit, surgeon fishes and wrasses.

Sail Rock Koh Phangan

Dive Site Sail Rock in the Gulf of Thailand

In any natural environment, the biomass of the preys is usually at least 10 times greater than the biomass of the predators (evident examples of that are the savannahs, the rain forests, the tundras etc.). This proportion is a consequence of the energy consumption  through the food web: any predator needs to ingest an amount of biomass 10 times greater than that corresponding to its growth in weight, in order to maintain its metabolic processes. So here come the enigma of Sail Rock waters: how is it possible that such environment can sustain itself through the time, when such proportion is not observed? Why so many predators are concentrated in such a small environment, when in the
waters surrounding the neighbour islands (Koh Tao and Koh Phangan) only few of them are encountered?

Trevallies at Sail Rock

Trevallies at Sail Rock

The mystery can be solved with few considerations involving oceanographic and biological factors. Sail Rock is a tiny island located in the middle of the gulf of Thailand. Due to its small size, it is continuously surrounded by currents, independently form which direction they are flowing; beyond that, there is only a negligible supply of fresh water coming from the rain falling over its surface. This means that a considerable amount of plankton is constantly supplied by the currents, 24 hours a day, all around Sail Rock. Plankton is the primary food source for the early stages of life of any fish or marine invertebrate, so its availability is determinant for their reproductive success. Most of the common reef fishes and invertebrates adopt a reproductive strategy based on high numbers, in order to  guarantee the survival of a few. When the food sources are abundant, the hatchings can easily survive the very early stages of life (larval stages), reaching millions of individuals on the juvenile and young adult stages. During these phase they become preys for big predators, like carangids (jacks and trevallies), barracudas and groupers.

That is therefore how the mystery of Sail Rock can be explained. The observable biomass of predators is apparently the same or higher than the observable biomass of adult preys. Anyway, due to exceptionally favourable conditions, preys are highly successful in reproduction, generating a number of offspring thousands times higher than their own number. Most of such offspring is constantly grazed by the predators before they can actually reach the adult stages, disappearing from the biomass account observable by the divers. Another factor that needs to be considered to explain the general abundance of fishes around Sail Rock is the lack of fishing activity over the area, which is prevented by underwater pinnacles that would destroy trawlers’ nets.

Solved this mystery, another one appears consequently: If such environment is so favourable to predators, why apex predators like sharks are not encountered over there? Well, this unfortunately has something to do with the most threatening predator ever: man. A population of bull sharks was observable around Sail Rock until few years ago, but it slowly decreased until disappearing…. At the same time, in some Thai restaurants you can still find dishes containing shark meet, and especially shark fins… Differently from other fishes, sharks have a very slow reproduction rate, and they keep on moving from one place to another, ending up in anglers’ nets who have been chasing them for decades to
supply Asiatic restaurants, determining their decimation in most of South-East Asian seas. Sharks are keystone species for the marine environment, and their disappearance is irreversibly compromising its biological balance, already threatened by common overfishing. There is only one way we can help stopping it: by acting responsibly through our own choices. If we should get into a restaurant where shark meat is offered in the menu, we should complain with the owner about it and get out, writing a negative report on Trip-Advisor. This will discourage restaurant owners to purchase more shark meat and fins, making such a dirt business end. At the same time, we should reduce as much as possible common fish consumption, in order to decrease the impact of overfishing over the oceans. Nowadays divers community is not a small one, we are millions all over the world: if we all act consciously spreading our awareness to others, we will be successful protagonists in ocean preservations, and we’ll be rewarded by the sight of thousands of fishes during our dives. So let’s take inspiration from Sail Rock, to have more and more of such amazing dive sites all over the world!

Giovanni d’Erasmo
M.Sc. in Marine Sciences; PADI MSDT; Dive Instructor at HAAD YAO DIVERS

Marine Life Thailand Whale Shark

Whale Shark in shoals of trevallies at Sail Rock

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