Orca Predation on Whales at SE Farallon Island, History of White Shark Interaction
September 2, 2024
When we think of top predators in the Greater Farallones National Marine Sanctuary, we naturally turn to the great white shark. Here in the Red Triangle, the area bounded by Monterey, Point Arena and west to the Farallon islands, 40% of all white shark attacks on humans in the USA occur. We hear a lot about these interactions between sharks and humans, but in fact shark attacks are quite rare, around one every other year in California. Adult white sharks prey primarily on seals and sealions, and occasionally dead whales, and they are impressive predators. A sudden strike by a 3000-pound white shark on a 300-pound elephant seal is truly dramatic, and a little gruesome.
However, the real apex predator is the killer whale, or Orca.
On August 22, researchers on SE Farallon Islands observed around 15 orcas aggressively hunting a group of humpback whales. Over a period of nine hours the researchers observed the orcas harassing the humpbacks, splitting the whale group apart, and focusing on one individual adult.
These whales, orcas more commonly prey on far smaller marine mammals, such as seals, sea lions, dolphins and gray whale calves. Occasionally they go after young humpbacks in their calving grounds, but they are not known to pursue healthy adult whales.
Dr. Nancy Black, founder of the California Killer Whale Project (CKWP) postulated that the recent Farallon whale predation event may have become interested in a calf in the group of humpback whales. She identified the orcas as part of the Biggs, or Transient group of orcas. These whales travel up and down between Southern California and British Columbia to feed on seals, sea lions and other whales. The Transient orcas have been observed preying on gray whale calves in Monterey Bay, and south in Baja California where the gray whales birth.
Killer whales, also known as orcas (Orcinus orca) have been categorized scientifically into separate groups, called ecotypes. Three ecotypes of killer whales overlap in Monterey and off the Farallones, including the Transients (Bigg’s), the Residents, and the Offshores. These whales have separate prey types, and distinct social groupings. In 2023, it has been determined that the Transient or Bigg’s orcas are a distinct species from the Residents which prey primarily on salmon and fish, and the other ecotypes.
THE WHALE THAT ATE JAWS
The Bigg’s Orcas have also been observed preying on white sharks at SE Farallon Island.
On 4 October 1997, whale watchers off SE Farallon Island were amazed by an Orca attacking and eating the liver of a white shark. Filmed by a wildlife tour group with Ocean Safaris , the incident caused a huge sensation among the tourists on board, but also by the other sharks in the vicinity.
From the island, Point Blue Farallon Island biologists documented a pair of killer whale females that attacked and consumed a white shark, the first time this had ever been documented. What was remarkable is the biologists, who observe and record seabirds, whales, pinnipeds on or around the island, noted a huge decline of shark predations following the orca-shark predation event.
On average, the scientists watching shark predations from SE Farallon recorded 40 pinnipeds killed by sharks per year. The season of the first Orca-white shark attack, the researchers recorded only two shark predations on seals following the orca predation. This was a decline of 95% in observed shark -seal predation compared to previous years. Tagging data later confirmed that the white sharks vacate the Farallon Islands within a day of their encounters with killer whales, moving to other locations along the California Coast. When the orcas are active around the island, the sharks leave the area, one of their favorite feeding grounds due to the plentitude of pinnipeds.
Similar episodes of a correlation of Orcas appearance with the decline in white sharks- seal predations from SEFI were recorded again in 2009, 2011, and 2013, The average number of shark pinniped kills declined by 62% compared to previous years.
Besides the three ecotypes of killer whales in Monterey and off the Farallones; the transients (Bigg’s), the residents, and the offshores, another group has been observed. CKWP has also recorded a less common group of killer whales visiting from the Eastern Tropical Pacific. Referred to as ETPs, these whales may specialize in preying on sperm whales. Though the ETP orcas are often elusive and frequently travel offshore, drone footage collected by CKWP includes the first known documentation of them foraging on a six-gill shark.
Highly intelligent, Orcas are also adaptable. It is believed that the Resident orcas may be moving into southern waters to seek alternate prey sources due to a decline in salmon. With an abundance of anchovies in the Greater Farallones and Monterey Bay National Marine Sanctuaries, hundreds of humpback whales are feeding nearshore. Overlapping with the whale season is Sharktober, when the adult white sharks return to feed on pinnipeds off the Farallon Islands. Shark Stewards leads public education trips out to the Farallones each fall helping collecting observations on whales and sharks. Although we love orcas, speaking from a shark conservationists’ point of view, we hope they pass on through and white sharks aren’t on the menu this year!
References
Revised Taxonomy of Eastern North Pacific Killer Whales (Orcinus orca): Bigg’s and Resident Ecotypes Deserve Species Status
Phillip A. Morin et al 2024 Published:27 March https://doi.org/10.1098/rsos.231368
Killer Whales Redistribute White Shark Foraging Pressure on Seals
Jorgensen, S.J., S. Anderson, F. Ferretti, J.R. Tietz, T. Chapple, P. Kanive, R.W. Bradley, J. Moxley, and B.A. Block. 2019. . Scientific Reports.