22 juvenile great white sharks fled San Diego nursery during Storm Hilary, later returned

When Tropical Storm Hilary hit Southern California in August 2023, people on land were bracing for flooding, strong winds and record rainfall. Underwater, a group of young great white sharks off San Diego were reacting to the same storm in a very different way: they left their nursery habitat and moved into more sheltered waters before returning once conditions improved.A study from UC San Diego’s Scripps Institution of Oceanography and the Shark Lab at California State University, Long Beach provides the first direct evidence of juvenile white sharks responding to an extreme weather event like this. Using acoustic tags to follow 22 sharks before, during and after the storm, researchers found that many temporarily abandoned the area as Hilary hit, then came back within days or weeks.A storm that changed the coastTropical Storm Hilary was a rare event for Southern California. It was the first tropical storm to directly affect the region this century, and although it weakened from a Category 4 hurricane before landfall, it still caused serious damage across the southwestern United States and parts of Mexico.On the ocean side, the storm brought a sudden drop in sea-surface temperature, rougher conditions and coastal flooding. For juvenile great whites, that created a fast-changing environment in a nursery zone they normally rely on for warmth, food and safety. Why this nursery mattersYoung white sharks in Southern California spend their early years in shallow coastal habitats where waters are calmer and warmer, and prey like stingrays and small fish are easier to find. One of the best-known nursery areas sits off Torrey Pines and Del Mar, where juveniles gather in summer and fall before eventually moving into adult habitats.That kind of nursery is important because it gives young sharks a better chance to grow without as much pressure from larger predators. But it also makes them vulnerable when the environment changes suddenly.What the researchers trackedTo understand how Hilary affected the sharks, the team followed 22 juvenile great whites, 13 females and 9 males, that had already been fitted with acoustic transmitters. Two underwater receiver arrays recorded more than 524,000 detections, creating a detailed picture of shark movement through the storm.Before Hilary arrived, the nursery regularly held 16 to 19 tagged sharks per hour. During the storm, that number fell by about half, dropping to as few as eight. Most of the missing sharks returned shortly after the weather improved, and nearly all were back within three weeks. Only one shark left as the storm arrived and was never detected again in San Diego that year.Where did they go?The researchers could not trace every individual exactly, but they did notice a clear rise in detections around La Jolla Cove. That area is next to a submarine canyon, which gives sharks fast access to deeper water and may offer a more sheltered place during rough coastal conditions.One shark with a pressure sensor made the deepest dive recorded in the study, reaching 132 meters, or 433 feet, in La Jolla Canyon on the day the storm hit. That dive may have helped the shark get away from the turbulence near the surface.The scientists think some sharks may have used La Jolla Cove as a temporary refuge while others went elsewhere. In other words, they did not all respond the same way: just as animals, and people, often react differently when conditions become dangerous. The role of cold waterOne of the biggest clues came from the temperature shift. During Hilary, sea-surface temperatures in the nursery dropped from 20°C to 13.9°C (68°F to 57°F ) in less than 24 hours. That sharp cooling lined up closely with the sharks’ departure.Juvenile white sharks are regional endotherms, meaning they can regulate body temperature better than many fish. Adults tolerate cold water relatively well, but younger sharks are more sensitive to sudden temperature changes. The researchers believe the juveniles likely moved toward warmer water until conditions stabilised.Why this matters for climate changeThis study is about one storm, but its implications are bigger than that. As climate change increases the frequency and intensity of extreme weather events, scientists need to know how marine species respond when ocean conditions shift quickly.That matters especially in California, where people and white sharks are increasingly using the same coastal waters. Better understanding shark behaviour during storms can help scientists and coastal managers make smarter decisions about conservation, recreation and safety.A rare look at shark behaviourWhat makes this study stand out is that it captured a natural response in real time. The sharks were not reacting to a lab experiment or a routine seasonal change. They were responding to a major storm that altered their habitat almost overnight.Lead author Jack Elstner said the study offers the first direct evidence of storm-response behavior in white sharks. Co-author Chris Lowe added that the finding shows how some sharks sought shelter from the storm, while others did something different, much like humans do during extreme weather.The bigger pictureHurricane Hilary caused more than $900 million in damage in the United States and led the National Hurricane Center to issue its first-ever tropical storm warning for Southern California. For the sharks, it also served as a warning of another kind: even a protected nursery can become unstable when weather turns violent.The good news is that the sharks mostly came back. That suggests juvenile white sharks may be resilient enough to ride out short-term disruption, at least in this case. But the study also shows how quickly a nursery can be disturbed when ocean conditions change fast.Catch the latest World News and Live updates. Download the TOI app.