The White Shark, among other things the protagonist of the film Steven Spielberg’s shark, is the largest predatory fish on Earth. And yet he too may not be invincible: when he meets a killer whale on his path, another one of the most powerful predators (in this case the film reference for film buffs is Michael Anderson’s Murder Orca ) he pulls back and leaves the field – the ocean – free. Even the colossal shark, therefore, from predator could become predated.
And between the two litigants is the third to enjoy: to benefit from his departure is the sea elephant , a mammal of the seal family, the preferred prey of the white shark. To describe these dynamics is a study coordinated by the Monterey Bay Aquarium in Monterey, California, which has analyzed for years, through structured surveys, the habits of these great predators. The results have been published in Scientific Reports.
The authors analyzed the displacements of 165 white sharks with a chip that followed them from 2006 to 2013. In addition, they carried out surveys for 27 years on the habits of white sharks, killer whales and sea elephants at the Farallon islands , off the coasts of San Francisco. “The research in this paper combines two very solid data sources , ” says Jim Tietz , biologist and coauthor of the study. “We were able to definitively show how the white sharks spin away when the killer whales appear.“
To study whether and when orcas and sharks were present in the same stretch of ocean, the authors compared the data from observations that lasted 27 years with those collected through the electronic tag affixed to sharks. In all the cases examined, the white shark has moved away from the island as soon as the killer whales arrived, taking off within a few minutes, and does not return until the following season. Normally its destination are other colonies of sea elephants along the coast, or it heads offshore. And this happens even if the orcas are often just passing through, and are found near the coasts of the Farallon islands for less than an hour. The animals studied, it is right to point out, are “huge white sharks” , commented Scot Anderson, co-author of the study,“Some are 5.5 meters long and usually lay down the law here”.
Probably the white sharks flee for fear or risk aversion and competition, elements that could also play a role in the hunt of these great predators and influence the ocean ecosystem. But the authors have so far not drawn conclusions on the reasons behind the removal: it could be due to the fact that the orcas consider the white sharks prey, or that they simply want to compete with them to capture the sea elephants.
Sea elephants , on the other hand, seem to be benefiting from all this: the events in which these animals are predated fell sharply in the years when sharks were far away. During the 27 years of observations the authors have detected on average 40 predatory attacks for each season, with a peak in the months of October and November, to the detriment of the sea elephants. In the years in which the killer whales were present, with distant sharks, these events fell until they were no longer registered.
The research, the authors explain, highlights the importance of interactions among the major predators, which are not yet well documented. “I believe this shows that the food chain is not always linear”, concludes Salvador Jorgensen , first author of the paper. “The so-called lateral interactions between the major predators on land are quite well known while it is more difficult to document those occurring in the oceans. And since this happens so rarely, it may take a little longer to understand the dynamics.“