IT HAS BEEN SAID that we know more about the moon than we know about our own oceans, but just how connected are the two? We all know that the moon and sun control our tides, but we are only just starting to understand how the moon controls fish aggregations and spawning events.

It’s no new fact that the fish follow the moon phases; fishermen have been timing this for centuries. Having to fish for three or four consecutive days at the exact time to catch the most fish is far more economical than going out every day to catch a few fish, and so diaries were kept of locations and timings around the moon phases to make life easier, passing down from generation to generation around the planet. But why are huge aggregations of fish timing their reproductive stages to the moon cycles?

Over the last few years, the moon phenomenon and spawning aggregations have become an addiction of ours in Palau. For years, we would see huge schools appearing at regular times of the year at regular sites and months like clockwork.

A diver records this strange phenomenon that fishermen use to their advantage

The first real attraction to us was the Moorish idols (Zanclus cornutus) and Orangespine Unicorn fish (Naso lituratus). During the months of January and February, gathering at half moon and schooling until full moon, these fish, which are usually solitary or hang around in small schools, gather at certain sites, usually corners, in their thousands to reproduce.

Forming such large schools has its obvious advan-tages: females can find the fittest, strongest males from the group giving their offspring the best advantage to survive and continue the cycle. But gathering in such large numbers also attracts predators. Giant trevally (Caranx ignobilis), grey reef sharks (Carcharhinus amblyrhynchos), white tip sharks (Triaenodon obesus ) and Napoleon wrasse (Cheilinus undulatus) are among the list of predators ready and waiting for a quick meal, chasing the school up and down the reef attacking at every opportunity. Most will never see another year’s spawning cycle.

The most recent phenomenal spawning event to be discovered based around the moon is the bumpheads parrotfish (Bolbometopon muricatum). As an avid diver, you would have heard of large schools of bumphead seen grazing in Sipadan, but no one knew that more than 2,000+ can been seen continuing their existence here in Palau.

A lone turtle swims slowly amongst the company of thousands of snappers

Timing is the key – the moon phase or the light phase is taken into account for these fish know the exact time when their unborn young will have the best chance of survival. Early in the morning, as the moon approaches its new moon phase thousands of bumpheads gather together in their herds waiting for that exact moment.Once ready, the huge school will spill out into the blue, where a dance like ritual begins. Sexual dimorphism takes place, heads change white, bars are displayed, males are showing off their dominance and then like a firework display, the action begins. Females shoot to the surface with eager males in hot pursuit wanting to be the fastest, wanting to be the one to continue their existence. The slowest ones are out run by the strongest ones and some females can be engulfed by 10 or 15 males at once in a tight ball of fusion and excitement releasing their cloud of sperm and eggs into the blue water. Once the event has taken place, the school breaks up returning back to their resident homes and the show is over, leaving nothing more than a few bumpheads grazing on the reef like a regular day in Palau.

The full moon brings the twinspot snapper (Lutjanus bohar) together, here on one of the outer reef corners schools of up to 20,000 snappers can be seen aggre-gating in Peleliu, one of Palau’s most southern islands. Famous for its WWII history, Peleliu also has some hidden underwater secrets.

For the rest of this article (Asian Diver Issue 3/2013 Vol.126) and other stories, check out our past issues here or download digital copy here.

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