Blackfin Reef Shark Evacuation from Moorea Island, 2002

Blackfin reef sharks (Carcharhinus melanopterus) reside in the lagoons and on the outer slope of the barrier reefs of Moorea Island. This coral habitat is shallow enough to facilitate underwater observation of the species. Regular observation at different sites centering in the Vaihapu region (Galzin and Pointer 1985) yielded a large amount of data on their movements, ethology, and social biology between 1999 and 2007 (Porcher 2005). In 2002, all blackfin reef sharks under observation, not only by the author but also by the dive clubs holding shark dives, left the north shore of the island for a period of ten days to two weeks. This event suggests an unknown pattern or influence at work.

With the surprising new information from Johann Mourier, that these sharks easily travel between islands, I'm re-posting my account of their unexplained and unprecedented disappearance from Moorea Island in late July, 2002.

Johann began studying the same sharks that I observed, using far more high-tech methods, just as I was leaving the island, and has worked within the scientific community in French Polynesia to make sure that they stay protected, while unfolding many mysteries about these interesting sharks.

http://johannmourier.wordpress.com/

Recently he discovered that this apparently sedentary species actually swims easily to other islands! They seem content to stay in their favored region of the coral garden—most particularly the matronly lagoon females—but if an adult female goes to another island to birth, thereby returning to the place she originated, it follows that she must have initially changed islands when still a juvenile! 

I often noted surprising roaming patterns in the lagoon juveniles—they seemed to travel widely before settling down into a home range at the age of about four. And males disappearing for months at a time during the reproductive season could have been going much farther than I imagined. Then there is the remaining mystery of how such reef-loving sharks got to these islands in the first place, since they result from volcanic eruptions in the relatively recent geological past, and are so far from the continents. Johann's finding indicates that these reef sharks are quite capable of oceanic navigation and travel.

If you learn of a similar event, involving the temporary disappearance of a community of sharks from its ranges, I would be glad if you would contact me by e-mail. Thank you!

The Event :

On the evening of Saturday, July 20th, 2002, I found many visiting male blackfins in the lagoon, though it was not the reproductive season when they usually visited. The resident females were absent which was highly unusual, and so were nearly all the older juveniles. The small juveniles, including sharks less than one year old, who usually remained hidden in certain regions of thick coral, were absent that night.

One of the visiting males appeared highly agitated, twitching wildly with successive instantaneous directional changes throughout the period in which he was present, a behaviour pattern I had only noted before in sharks after being startled, and then normally in juveniles. I had never seen an adult shark behave like this for so long (20 minutes) and later associated the episode with the unique event which followed.

On July 23rd, 2002, the divers conducting daily shark dives outside the barrier reef, and the commercial tours holding shark feeding sessions in separated lagoons on the north shore of Moorea (see map), reported that no sharks had appeared that day, something that had never happened before. Only Phillipe Molle, of M.U.S.T. Dive Club, Maharepa, said that he had been holding shark dives for 17 years and on only one other occasion had no sharks attended, on the day before a hurricane struck the island. He was expecting bad weather, but none came.

No sharks were seen for the next five days, with the exception of two sightings of “small sharks” who passed in the distance and did not approach. Usually these shark dives were frequented by approximately twenty to thirty male blackfin reef sharks, who met dive boats at the site and swam without fear among the divers during the dive.

These were the only places on the island where the species was regularly observed, so the actual extent of the evacuation is unknown. 

The juvenile males began to reappear first. I identified one on July 28^th, and one resident female came back by herself to my study area on Tuesday, July 30^th. But the majority of lagoon residents, the females and many more juveniles, returned on Friday, August 2^nd. They appeared to be on the move, travelling swiftly along the lagoon from west to east, displaying no socializing behaviour nor casual circling. Many had their home ranges to the west, and within the next days most had settled back into their home ranges. Though a few more sharks returned later, the majority of the lagoon residents returned that evening, travelling swiftly together, widely spread out across the coral landscape.

By mid-August their behaviour and distribution was nearly back to normal, but a major disruption had occurred. In some parts of the lagoon, the pattern of sightings I had systematically recorded prior to the disappearance took months to re-establish itself. Some adult residents never returned, and a high fraction of the small juveniles I had identified as inhabiting the protected areas of thick coral, never returned.

The approximately 10 day evacuation involved the populations of C. melanopterus from at least two lagoons (separated by a wide, deep bay) who normally don't interact. Adult females and juveniles of all ages, as well as the entire population of males who live outside the barrier reef, left.

Discussion

Intensive investigation of the shark evacuation revealed no applicable reason for it. There was no sudden fishing effort that could have caused the sharks to flee, no pollution event, the weather was normal, the water temperature was normal, there were circle island tours moving around the island daily who reported nothing unusual, such as an incidental food source which could have attracted the sharks into the ocean. Such a food source would be visible from far away due to the vast muli-species congregation of seabirds which would be circling above it.  No other species appeared to be affected, though at that time of year I did not normally see nurse sharks, Nebrius ferrugineus., and reef white-tips, Triaenodon obesus, so regularly in the lagoon. I did not see any of these species of sharks while searching for C. melanopterus, but whether they had also evacuated was unverifiable. Female grey reef sharks, Carcharhinus amblyrhynchos appeared as usual, however, off the north-west corner of the island, which was the only dive site where they were regularly seen.

There were no military events in the area, and no earth tremors. On one of the days in which no sharks had been seen at any of the sites, humpback whales were resting in the bay with a group of spinner dolphins. Whatever had affected the sharks was not troubling the whales. Dr. Michael Poole, a marine biologist observing the whales and dolphins daily along the north shore, and frequently down the west coast and around the entire perimeter of the island, reported no change in the patterns of the movements of the marine mammals and nothing unusual in the ocean during this period.

The oceanic current does not move fast enough to explain the return of the sharks in such a short period if oceanic conditions were responsible for the evacuation (personal communication from Arthur A. Myrberg Jr.).

The occasional appearance of a juvenile shark on the limits of visibility could be a clue that the sharks were in deeper water. The reluctance to approach to eat is typical of a shark alone. It is possible that the larger sharks had gone much deeper for unknown reasons, while the juveniles preferred shallower water, so were the ones who came into visible range of divers at about 22 m in depth.

Though I contacted every elasmobranchologist I knew, posted an inquiry on the elasmo-list shark discussion board, and received many interested replies, no one reported having seen nor heard of any similar event, nor did anyone have a plausible explanation.

Professor Myrberg considered the evacuation to be in the category of things that cannot be explained at this time.

While the reason for the spontaneous evacuation of Carcharhinus melanopterus from the north shore of the island between July 23, and August 3, 2002, could not be explained, the one factor that suggested that it was a natural event was that it occurred during the period of the full moon, a time favoured by individuals for roaming.

References

Galzin R, Pointer JP (1985) Moorea Island, Society Archipelago. In: B Delesalle, R Galzin & B. Salvat (Eds). 5th International Coral Reef Congress, Tahiti, 27 May to 1 June 1985. Volume I: French Polynesian Coral Reefs: 73-102
Porcher IF (2005) On the gestation period of the blackfin reef shark, Carcharhinus melanopterus, in waters off Moorea, French Polynesia. Mar Bio 146: 1207-1211

see also : Mourier, J. & Planes, S. (2013). Direct genetic evidence for reproductive philopatry and associated fine-scale migrations in female blacktip reef sharks (Carcharhinus melanopterus) in French Polynesia. Molecular Ecology 22 (1): 201-214.