- Both the great white shark and the killer whale or orca are fearsome top predators. But of the two massive animals, the killer whale may be the more formidable one, a new study has found.
- Researchers monitoring white sharks, lion seals and orcas around California’s Southeast Farallon Island have found that every time orcas pass through the area, the great white sharks vanish and don’t return to their hunting grounds until the next season.
- The researchers aren’t sure why the sharks move away as soon as orcas arrive. It could be because orcas may be targeting white sharks as prey, or the killer whales could be bullying their competition out of the way to gain access to the island’s elephant seals.
In the ocean, both the great white shark (Carcharodon carcharias) and the killer whale or orca (Orcinus orca) are fearsome top predators. But of the two massive animals, the killer whale may be the more formidable one, a new study has found.
Salvador Jorgensen, a marine ecologist at the Monterey Bay Aquarium in California, and his colleagues first noticed this while studying white sharks around California’s Southeast Farallon Island (SEFI). They had lured the sharks to their research boat using a seal decoy made of carpet fabric, then inserted electronic tags into the sharks to keep track of their movements along the California coast.
In 2009, they noticed something odd: 17 of their tagged white sharks had been regularly feeding on elephant seals around SEFI for months. But on Nov. 2 that year, when orcas from two different pods arrived at SEFI, not staying for too long, all 17 sharks disappeared within the next eight hours. Most of them didn’t return until the next season several months later. Jorgensen and his colleagues observed this same fleeing behavior among white sharks on three more occasions in the following years.
In fact, when the researchers combined information on 165 great white sharks they’d tagged between 2006 and 2013 with data collected on lion seals and orcas as part of a long-term wildlife monitoring program at Farallon Islands National Wildlife Refuge, they found that the sharks’ fleeing behavior had a consistent pattern.
Great white sharks gather around SEFI each fall between September and December to hunt for young elephant seals. Orcas also hunt elephant seals, but they only come by occasionally. And when they do, the sharks withdraw from the area.
“When confronted by orcas, white sharks will immediately vacate their preferred hunting ground and will not return for up to a year, even though the orcas are only passing through,” Jorgensen, lead author of the study published in Scientific Reports, said in a statement.
The elephant seals seem to benefit from the arrival of the orcas. Whenever killer whales were sighted in the region, fewer seals were hunted that season.
“On average we document around 40 elephant seal predation events by white sharks at Southeast Farallon Island each season,” said Scot Anderson, a scientist at Monterey Bay Aquarium and co-author of the study. “After orcas show up, we don’t see a single shark and there are no more kills.”
The researchers aren’t sure why the sharks move away as soon as orcas arrive. It could be because orcas may be targeting white sharks as prey: in 1997, a pair of orcas killed a white shark that had interrupted their meal, consuming only its liver. In 2017, five white sharks that had washed ashore on a South African beach had their livers missing from their otherwise intact bodies. Orcas had been spotted nearby, and the bite marks on the sharks’ bodies suggested that the whales had punctured their bodies to feast on their calorie-rich livers. The orcas could also be bullying the sharks, their competition on the food chain, out of the way to gain access to the elephant seals.
“I think this demonstrates how food chains are not always linear,” Jorgensen said. “So-called lateral interactions between top predators are fairly well known on land but are much harder to document in the ocean. And because this one happens so infrequently, it may take us a while longer to fully understand the dynamics.”
Citation:
Jorgensen, S. J., Anderson, S., Ferretti, F., Tietz, J. R., Chapple, T., Kanive, P., … & Block, B. A. (2019). Killer whales redistribute white shark foraging pressure on seals. Scientific Reports, 9(1), 6153. doi: 10.1038/s41598-019-39356-2
This story first appeared on Mongabay
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