Orcas visit the island

On November 2, 2009, orcas from two distinct groups arrived at Southeast Farallon, the main island of the Farallon Islands, off the coast of San Francisco. They remained there from 12:48 to 15:30, a total presence of 2 hours and 42 minutes.

Sharks disappear after the visit

On that day, 17 marked great white sharks were present at the island, and 16 of them were not seen again before the following season. Sixteen out of 17 were no longer detected by the receivers.

Study reveals the pattern

This finding comes from a study led by Salvador Jorgensen, a researcher at the Monterey Bay Aquarium, published in 2019 in Scientific Reports. The team analyzed data from 165 marked great white sharks between 2006 and 2013 and added 27 years of records on seals, orcas, and sharks at the Farallon Islands.

Orcas and sharks avoid each other

The study documents four encounters between the two predators within the Greater Farallones National Marine Sanctuary. The 2009 incident is the clearest example. Two groups of orcas remained 2 hours and 42 minutes near Southeast Farallon in November 2009. Sixteen of the 17 marked great white sharks were not seen again before the following season.

Tracking sharks with technology

To track the sharks, researchers attracted them to their boat using a seal-shaped lure made of carpet, then placed electronic tags on them. Receivers then recorded their presence around the island. On November 2, 2009, these receivers counted 17 marked great white sharks at Southeast Farallon.

Sharks vanish after orcas leave

7 hours and 50 minutes later, after the orcas passed, none were within range. Sixteen of the 17 did not return before the following season, which was July 2010 or later. Less than three hours of visitation, at least eight months of absence.

Orcas’ presence disrupts shark behavior

In years when orcas were present, the predation of sharks on seals decreased by a factor of 3.5 to 7. 2 hours and 42 minutes of visitation, then an island without sharks. The orcas did not stay in one place; they crossed the waters around the island like one would cross a neighborhood.

Sharks flee regularly

However, this pattern does not result from an isolated accident. In each of the cases examined by the researchers, the sharks fled the island upon the arrival of the orcas and did not return until the following season. A calendar coincidence would not explain such regularity.

Sharks avoid orcas out of fear

A fear that lasts almost a year. Each autumn, between September and December, great white sharks gather around the island to hunt young sea lions. Orcas, on the other hand, rarely pass through the Farallon during this period. When they do appear, the sharks leave the area.

How sharks detect orcas

According to Salvador Jorgensen, the sharks immediately abandon their preferred hunting ground and do not return for at least a year, while the orcas are merely passing through. Regarding what alerts them, the researcher offers an intuition: he thinks that sharks detect orcas by smell, but notes that this requires future research.

Uncertainty about orca behavior

One question remains unresolved by the study. The authors do not conclude whether orcas target sharks as prey or if they are simply displacing a competitor to secure the calorie-rich sea lions. Only one confirmed direct predation of orcas on great white sharks has been recorded at Southeast Farallon. It is the incident of 1997, filmed off the Farallon Islands.

Sharks flee without being hunted

For the 2009 incident, the study reports no kills: it describes a flight. The seals, big winners of the orcas' passage. In years when orcas approach within 3 kilometers of the island, the predation of sharks on seals decreases by a factor of 3.5 to 7.

Sharks shift hunting areas

The authors speak of a negative correlation between the number of seals killed by sharks each year and close encounters with orcas. This statistical link is based on 27 years of field data. The seals benefit from the orcas' presence.

Sharks avoid danger, seals gain

The mechanism is not a direct hunt on sharks. Fear is enough to drive sharks to less preferred but safer hunting areas. Their departure also frees up seal resources that orcas could potentially benefit from. The predation pressure thus shifts in space and time, rather than simply disappearing.

Study details and findings

The study was published on April 16, 2019, open access, under the title "Killer whales redistribute white shark foraging pressure on seals." It is based on four documented encounters, 165 marked sharks over eight years, and 27 years of field data. The details of November 2, 2009, with precise minute-by-minute timings, come from this series of data.