Mass aggregation at Palau's Rock Islands
The green bumphead parrotfish (Bolbometopon muricatum) is one of the diver's favourite fishes. A school of more than 50 of these fishes swimming by is simply breath-taking.
The green bumphead parrotfish is one of the largest parrotfish species. These reef giants weigh nearly 50 kilogrammes and measure almost 1.5 metres across. They are found at the reefs of the Indian and Pacific Oceans, the Red Sea in the west and Samoa in the east, and even all the way to the coasts of Japan's Yaeyama Islands in the north and Australia's Great Barrier Reef in the south.
Being primarily coral-eaters, the bumphead parrotfish's distinctive feature is their beak-like teeth that partially cover their fleshy lips. At the back of their throats are pharyngeal teeth that they use to grind down hard coral.
Every year, during certain phases of the lunar cycle, the fish aggregates in large numbers to spawn. Little is known about the species' reproductive cycles – until several years ago when a aggregation of over a thousand individuals was discovered in Palau. This was the largest aggregation of bumphead parrotfish to date.
This event takes place at every new moon phase, during which the parrotfish gather at a sandy reef slope in the Rock Islands at Palau. The reason why the parrotfish has chosen this specific location to spawn is still unclear.
As the sun rises, the parrotfish emerge from the shallows, coming together like a waterfall from the lagoon where they had spent the night. At first, it's a loose, rhythmic formation that gets progressively denser. Soon, it is a large mass that meanders around the reef. Occasionally, there is a chomping sound as some individuals steal a bite as the events unfold.
After a while, the alpha males show their dominance by swimming upwards, extruding their fins out. At this time, their heads turn white. Often, you can hear them banging their heads with one another, signalling that the action is about to start.
Then, the huge school of parrotfish swims off into the blue water. All their heads – both males and females – have turned white. Their bands and bars down their flanks have emerged. The ritual dance of these sexually driven fish thus begins.
The females shoot up to the surface, closely followed by males in hot pursuit. This can last up to an hour, after which the school of parrotfish dispense back to their resident populations, and things go back to normal.
One theory for this strange behaviour is that the females are trying to find the fastest and strongest males to mate with, so their offspring would have the best genes.
As the parrotfish dispense, the water has turned a murky, cloudy mass of gametes, which drift along with the outgoing tide, ready to begin the first part of their life cycle. After five to six days, they will start to feed on algae and copepods something they will do for the next three months or so. Those that survive will return to the lagoon to take refuge in the shallows, remaining there for the next two to three years. After this time, they will join the adults in the outer reefs where they themselves will start to reproduce.
Being slow to mature, the bumphead parrotfish start their reproductive life when they are about 50 centimetres. Considering that they can live up to 40 years old and reach 1.5 metres across, this is relatively slow and may not to be able to sustain the species' population growth.
Although occasionally hunted by large sharks, the biggest threat to the bumphead parrotfish is people. The parrotfish habitually return to the same place to sleep, making them very vulnerable to spear fishermen, especially after the invention of the underwater dive light.
They are classified as “vulnerable” by the IUCN (International Union for Conservation of Nature), and are on the Red List of Threatened Species. Moreover, they are part of the “Management Unit Species” (MUS) plan for the coral reef ecosystems of the Western Pacific, which deals with fish management.
In 1994, Palau has banned the bumphead parrotfish's export; and in 2005, the country has gone on to impose a complete ban on the fishing of this species in their waters.
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