New research from Monterey Bay Aquarium and badociated institutions published today in Nature Scientific reports It challenges the idea that the great white sharks are the most formidable predators of the ocean. The study "Killer whales redistribute the feeding pressure of white sharks on seals" shows how the great white hunter becomes the hunted, and the sea elephant, the common prey of sharks and killer whales, emerges as the winner .
"When faced with killer whales, white sharks will immediately abandon their preferred hunting ground and will not return for up to a year, although killer whales are only pbading by," said Dr. Salvador Jorgensen, principal research scientist at the Bay of Aquarium. Monterey Author of the study.
The research team, which included aquarium scientist Scot Anderson of Jorgensen and Monterey Bay, and the research partners of Stanford University, Point Blue Conservation Science and Montana State University, documented four encounters among the top predators in the South Farallon Island in the Farallones National Marine Sanctuary. outside of San Francisco, California. The scientists badyzed the interactions using data from 165 white sharks tagged between 2006 and 2013, and compiled 27 years of studies of seals, killer whales and sharks in the Farallones.
"The research in this paper combines two really robust data sources," said Jim Tietz, co-author of the study and a Farallon biologist at Point Blue Conservation Science. "By complementing the new shark tagging data from the Aquarium with the long-term monitoring of Point Blue fauna at the Farallon Islands National Wildlife Refuge, we were able to conclusively show how white sharks exit the area when they appear the orcas. "
In all the cases examined by the researchers, the white sharks fled the island when the orcas arrived and did not return until the following season.
The elephant seal colonies in the Farallones also benefited indirectly from the interactions. The data revealed four to seven times fewer predatory events in elephant seals in the years that the white sharks remained.
"On average, we documented about 40 elephant seal predation events by white sharks in the southeast of Farallon Island each season," Anderson said. "After the orcas appear, we do not see a single shark and there are no more deaths."
Each fall, between September and December, white sharks gather in Farallones to hunt young elephant seals, usually spend more than a month circling in the southeast of the island of Farallón. Transient killer whales also feed on elephant seals, but only appear occasionally on the island.
To determine when orcas and sharks occurred in the area, the researchers compared data from shark tags with field observations of killer whale sightings. This made it possible to demonstrate the result in the rare cases in which the predators found each other.
The electronic tags showed that all white sharks began to clear the area in a matter of minutes after the orcas' brief visits. Sometimes killer whales were only present for less than an hour. Then, the tags found that white sharks piled up in other elephant seal colonies along the coast or headed for the coast.
"These are huge white sharks, some are over 18 feet long (5.5 meters), and they usually rule here," Anderson said. "We have been observing some of these sharks for the past 15 to 20 years, and some of them even more than that."
The study findings highlight the importance of interactions between major predators, which are not well documented in the ocean.
"Normally we do not think about how fear and risk aversion could play a role in the configuration of large predator hunting and how that influences ocean ecosystems," Jorgensen said. "It turns out that these risk effects are very strong even for large predators like white sharks, strong enough to redirect their hunting activity to less preferred but safer areas."
The researchers did not draw conclusions about whether orcas attack white sharks as prey or harbad competition for high-calorie sea elephants.
"I think this shows that food chains are not always linear," Jorgensen said. "The so-called lateral interactions between major predators are well known on Earth, but they are much harder to document in the ocean, and because this happens infrequently, it may take us longer to fully understand the dynamics."
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Salvador J. Jorgensen et al., Killer whales redistribute the feeding pressure of white sharks on seals. Scientific reports (2019). DOI: 10.1038 / s41598-019-39356-2
A study finds that white sharks flee the feeding areas when orcas are present (2019, April 16)
retrieved on April 16, 2019
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