The first whales were observed at the end of April in the north of French Polynesia. Credit: MMPoole 2015
Humpback whales are back in Polynesian waters. The Mata Tohora association, in a message published on May 21 on Facebook, reports that the first jumps of the year were observed on April 25. The association does not specify why it withheld this information for a month, but we can assume that it is for the same reason that it does not wish to reveal its exact location: to protect the animals from curious people. “It’s not in the Australs, but further north”, evades Agnès Benet, president of the association. It would be for the moment a few individuals, two or three adults per island, on four different islands, which “are well shown, near the ribs”, according to this biologist. She sees the signs of the first wave of humpback migration towards the warmer waters of the Society Islands and the northern Tuamotus.
Humpback whales make a journey of more than 6,000 km each year from their feeding grounds near Antarctica to the warmer waters that serve as their breeding and calving grounds. This migration from South to North takes place at the end of the austral summer. In general, we can start to observe them from mid-May in the Austral Islands, then in June in Tahiti. The greatest population density is found off the Society Islands between August and October. At the end of the austral winter, the whales begin their descent towards polar waters, but some have been observed much later, during the holidays.
Ever earlier migrations?
Therefore, the presence of humpback whales so early in the season raises questions: are the very early or very late sightings of these cetaceans off the Society Islands a sign of a recent change in their migratory habits? For the president of Mata Tohora, no doubt, the “whale season” starts earlier and ends later. She says that for 5 years, the association’s observers and its network of specially trained fishermen have shown that the stays of cetaceans in the waters of northern French Polynesia are increasingly long and no longer completely comply with the periodicity “normal”.
Agnès Benet is formal: “At first, we thought it was random, but this earliness has become very regular. We’ve been observing them for 20 years. Our observers, our network of fishermen, are trained and attentive and all of them tell us about changes in the migratory behavior over the last 5 years”. For the president of Mata Tohora, these visible changes are due to the pressures that global warming and human activity put on both the feeding grounds and the breeding grounds of the whales.
The observation of an early migration of a few whales is not necessarily significant on the scale of the whole population, according to biologist Michael Poole. Credit: MMPoole 2016
“Let us be sure of the fact, before worrying about the cause”, recalled Fontenelle. This is essentially the posture of Michael Poole, a researcher specializing in marine mammals in Polynesia for 35 years and owner of a company offering “eco-tours” to observe these mammals. He wishes to weigh the scope of the observations made by the association: “It is not because we observe a few individuals that we can say that the migration period has really begun”. At the level of individuals, there are always behaviors that are in the margins, but they are not necessarily significant of a change in behavior of the entire population of humpback whales that frequent Polynesian waters, “unless hundreds of individuals are observed in May”, he insists.
The researcher then speaks of a “observation bias” : There are more whales, since the ban on its hunting and, above all, more observers. Early migrations of small numbers of individuals have always existed, he says, but they were less observed. “Just because the absolute number of sightings is increasing doesn’t mean the frequency of the phenomenon is also increasing.” Scientific data, according to him, do not allow for the moment to confirm this extension of the migratory period of humpback whales. “As a scientist, for the moment, I am not worried about the subject of an extension of the migratory period of humpback whales”, he summarizes.
Ocean warming threatens our tohorā
This does not mean, however, that the warming of the oceans will have no effect on the behavior of our tohorā. If the waters of northern French Polynesia warm up by a few degrees during the winter, it is likely that humpback whales will gradually take up residence around the archipelagos further south, in the Gambiers or the Australs, whose waters are cooler, believes Michael Poole. These cetaceans prefer temperatures that are neither too cool nor too hot for resting or giving birth (see our box). According to the biologist, such a change is not yet observable, but the hypothesis seems probable to him. The other crucial point concerns the impact of overfishing and ocean warming on the availability of krill populations. These tiny crustaceans living in schools in cold waters are the favorite food of humpback whales. A scarcity of their main food resource, or even a displacement of schools of krill in search of a more favorable climate, would constitute a major threat to the survival of humpback whales.
Written by Antoine Launey on Tuesday, May 24, 2022 at 7:11 p.m. | Read 1421 times