A summer day off Southern California’s coast can feel deceptively calm and somewhat predictable. Surfers paddle through rolling waves, families crowding the shoreline, and tourists taking photos of the view. It’s picturesque and what one normally sees when this part of the world is on a movie screen.

But the movies usually leave out that beneath the calm-looking ocean surface, juvenile white sharks cruise through shallow nursery habitats that they return to again and again (unless the movie is about a shark encounter, then that’s sometimes all they focus on besides the orange spray tans and general California opulence). Above, these coastal areas offer a reprieve from the hustle-and-bustle of Hollywood for many Californians. For those below, these coastal areas offer young sharks warm water, abundant food and relative safety during one of the most vulnerable stages of their lives. “Nearshore beach environments throughout the Southern California Bight serve as critically important nursery habitats for juvenile white sharks. Along these beaches, sharks form loose aggregations in the summer and fall months, when ocean temperatures are warm and seas are calm. These conditions allow for rapid growth and enhanced juvenile survivorship, while high prey densities in nearshore ecosystems maximize foraging opportunities,” explains Jack T. Elstner , a PhD student at Scripps Institution of Oceanography at University of California San Diego .

Then Tropical Storm Hilary arrived.

In August 2023, Hilary became the first tropical storm in the 21st century to significantly impact Southern California; it brought strong winds, historic rainfall, flooding and dramatic changes to coastal ocean conditions. For humans, the disruption was obvious. For marine life, the consequences were less visible. “Tropical storms pose significant threats to coastal ecosystems. However, we still know surprisingly little about how marine species actually respond to storm events, especially highly mobile marine predators like white sharks,” said Elstner. So what happens when a top predator that depends on a carefully chosen nursery habitat suddenly faces a storm unlike anything it has experienced before? In a study led by Elstner, a team examined the response of juvenile white sharks to Hilary, providing some answers.

Understanding how animals respond to environmental change is one of ecology’s central questions. Scientists have spent decades studying predictable shifts such as seasons, tides and long-term climate patterns. Extreme events are a different challenge, as storms arrive quickly, alter habitats dramatically and often disappear before researchers can collect enough data to understand what happened. That gap in knowledge is becoming increasingly important as climate change influences weather patterns around the world and extreme events are expected to become more frequent and, in some cases, more intense. Understanding how wildlife responds can help scientists predict which species are likely to adapt and which may face greater risks.

To investigate the sharks’ response, researchers turned to acoustic telemetry, a technology that allows scientists to monitor animals remotely. The team had already tagged juvenile white sharks inhabiting a well-known nursery area near Torrey Pines and Del Mar, California . Each shark carried an acoustic transmitter that emitted a unique signal and an array of underwater receivers spread across a couple of miles recorded those signals whenever tagged sharks passed nearby. By the time Hilary arrived, the researchers were tracking 22 juvenile white sharks. Before the storm, the animals showed remarkable loyalty to the nursery; at any given hour, scientists could typically detect between 16 and 19 individuals within the monitored area. Then conditions changed almost overnight. As the storm approached, barometric pressure dropped. Winds strengthened. Rainfall surged. Most importantly for the sharks, sea surface temperatures plummeted by over 42 degrees Fahrenheit (six degrees Celsius) in just 12 hours (i.e., water that had been approximately 68°F/20°C cooled to below 57.2°F/14°C).

The sharks responded, although that part itself wasn’t as surprising. "Other marine species have been shown to evacuate from nearshore habitats during storms. We wanted to see if juvenile white sharks respond in the same way," explains Elstner. “After the storm passed, we downloaded detection data from our receivers and noticed a stark drop in shark detections during the storm. This motivated us to look into what exactly caused sharks to leave nursery habitats.” During peak storm conditions, the number of sharks detected within the nursery dropped dramatically with more than half of the tagged individuals temporarily disappearing from the monitored area. Statistical models suggested that the probability of a shark leaving the nursery increased more than 28-fold compared to normal conditions.

Most sharks returned within days. Nearly all had reappeared within three weeks. Only one individual left and never returned during the study period.

Temperature emerged as the strongest predictor of shark departures. While white sharks possess specialized adaptations that allow them to maintain body temperatures above surrounding water, juveniles are not as efficient at retaining heat as adults. Previous studies have shown that young white sharks prefer warmer waters and often leave nursery habitats when temperatures fall below certain thresholds. Hilary pushed temperatures right past those thresholds. Other environmental factors likely contributed as well. Falling barometric pressure, declining salinity caused by heavy freshwater runoff and increased wave activity all appeared to influence the sharks’ decisions. Together, these cues may have signaled that conditions were becoming unsuitable. Thus, "this behavioral response was somewhat expected. During the storm, rapid drops in sea surface temperature, increased turbulence, and abrupt changes to nearshore biogeochemistry likely made conditions within nursery habitats quite unpleasant. Therefore, we were not surprised when sharks left their aggregation site in search of more favorable habitats."

But, where exactly did the sharks go?

The team cannot say with certainty. However, one intriguing clue emerged from a nearby monitoring site in La Jolla Cove. “On the day of the storm, several sharks were also detected by a separate receiver array in La Jolla Cove, possibly indicating that these more sheltered habitats served as a nearshore refuge area for some individuals. We expect that many individuals evacuated to deeper waters further offshore,” said Elstner. In fact, one tagged shark was recorded at a depth of 433 feet (132 meters) during the storm, raising the possibility that some sharks sought refuge in deeper, calmer canyon habitats rather than undertaking long-distance migrations offshore.

Still, bigger questions remain. What happens if storms become more frequent? What if cooling events last longer? Could repeated disruptions alter nursery use, growth rates or survival? And are younger sharks, which appeared more likely to leave during Hilary, especially vulnerable in a changing climate? “Disruptions to nursery habitats have the potential to produce long-term, population-level impacts. Therefore, understanding how juvenile white sharks respond to, and recover from these disturbances is critical in a changing climate,” agreed Elstner. The answers are still emerging, but this study offers hope that rather than passively enduring the storm, young white sharks showed flexibility, mobility and resilience. In an era of increasing environmental uncertainty, those traits may prove just as important as sharp teeth for an animal on the rebound from extinction in the northeastern Pacific .

This is all, of course, fascinating stuff. But there was one key thing the researchers remembered when interpreting the results: Storms do not just affect animals. They can also interfere with research equipment. Strong winds, turbulence and underwater noise can make acoustic receivers less effective, which made Elstner and this team question if suddenly detecting fewer animals during a storm meant the animals were actually gone or if the equipment had simply become less reliable due to the ongoing storm. To answer that question, the team developed a new modeling approach that accounted for changes in receiver performance. By using synchronization transmitters already built into the monitoring network, they were able to estimate when environmental conditions reduced detection efficiency and correct for those effects statistically. It may sound like a technical detail, but it is a significant scientific advance as improving the reliability of ecological data during storms could benefit research on everything from fish and sharks to marine mammals.

The sharks — and equipment, sort of — demonstrated something conservationists hope many species possess in an era of increasing environmental instability: resilience. And in a rapidly changing ocean, understanding that resilience and flexibility in both of these “players” may prove just as important as understanding the storm itself.