Warmer Sea Temperatures and Shifting White Shark Populations are Altering Patterns of Human-White Shark Encounters
On October 1, a Bay Area sailor identified as Felix Louis N’jai was reported missing while swimming with two friends at Wildcat Beach, a remote area in the Pt. Reyes National Seashore. Witnesses reported seeing a shark nearby, with accounts of a pool of blood where the swimmer was last seen. Suspected as the victim of a great white shark, Mr. N’jai has not been recovered and a shark attack is yet to be positively confirmed. However, it is highly likely Mr N’jai was victim to the first recorded fatality by shark in Marin County. This beach along Drakes Bay is in an area known to have high frequency of white sharks, including shark attacks on surfers at nearby Dillon Beach. This area of coastline falls within the region notoriously called the Red Triangle.
Surfers, swimmers and ocean enthusiasts along California’s central coastline are well aware of the hazards in the ocean, including powerful surf, thick fog and the strong currents of the nearshore waters. In the fall months an often unseen hazard lurks in the salty waters: the great white shark (white shark). With a large population of the few areas of the world home to aggregations of white sharks (Carcharhodon carcharias), California ocean recreationists have one of the highest risk of encountering white sharks in the world. An early assessment in 2006 determined around thirty-eight percent of recorded great white shark attacks on humans in the United States were bounded in an area of central California what became popularly known as the Red Triangle. At the time, this was eleven percent of the worldwide total.
The vast majority of attacks in California occur north of Point Conception, California (34°30’ N latitude), with only a single attack off Washington State. In the first scientific evaluation of human- white shark encounters along the west coast, California, McCosker and Lea determined that 80% of attacks have occurred from Humboldt County south to Monterey County, and 62% of total attacks have taken place along an approximately 100 mile stretch of coast between Marin County and Monterey County and out to the Farallon Islands (30 miles offshore). Attacks south of Point Conception are fewer because of the rarity of nearshore pinniped colonies, the favored food of adult white sharks. The abundance of attacks further north is related to the abundance of pinnipeds overlapping a high level of water use by humans in the region.
Curtis et al. (2012) found that only three fatal incidents involving white sharks occurred in Southern California in their later assessment of human- white shark encounters in the California segment of the Northeast Pacific population (NEP) of white sharks. However, a more recent evaluation published by Feretti et. al, in the journal Frontiers in Ecology and Environment indicates that the counties included in the “Red Triangle” (Monterey, San Mateo, San Francisco, Marin and Sonoma) accounted for only 73 out of the 201 total incidents (36%) and 46 of the 122 injuries and fatalities (38%).
Over the last 30 years, since 1993, there have been six fatalities in California attributed to white sharks. Five of these occurred north of Point Conception, with one in Southern California, in San Diego County. However, in recent years there has been an increase in non fatal human-white shark encounters in the south.
In 2012 Curtis, reported an increase in human shark encounters with white sharks from Point Conception south. Contrary to past reports of white shark incidents being concentrated in the Red Triangle, incidents with white sharks occur statewide, with some of the highest numbers in Southern California according to Ugoretz (2021). This increase in human shark encounters is correlated with the increase in the white shark population since protection of the species was initiated in June 1994 by the California Department of Fish and Game (now Fish and Wildlife).
The northeast Pacific population of sub-adult and adult individuals (white sharks) primarily consists of two discrete aggregations at Guadalupe Island, Baja CA Sur, Mexico (Domeier and Nasby-Lucas, 2008) and central California, USA. Adult white sharks from these two population segments seasonally migrate from North American waters to the north Pacific subtropical gyre called the White Shark Café’(Jorgensen et al., 2010. The first population assessment of white sharks in the California segment of the Northeast Pacific population was performed by Chapple et al in 2011. Using tag- recapture analysis and photographic IDs of dorsal fins between 2011 and 2018, the team estimated the adult shark population in the central California region at 219. The scientists estimated this number at approximately half of all mature and sub-adult ENP white sharks. In response to this study, Burgess et al, 2012 made an estimate of a range from 2,148 to 2,819 for the population of young, juveniles, subadults and adult white sharks in the NEP population.
Estimates of the Guadalupe Island population consisting of adult and subadult used tourist-based cage diving activities to monitor white sharks from 2014 to 2019 within the Guadalupe Island Biosphere Reserve. The data indicated a gradual increase in the overall abundance of white sharks with an age-structure shift, as young of the year and juvenile sharks were more prevalent during the latter part of the study period (2016–2019). The arrival of young of the year and juvenile white sharks coincided with regional changes in oceanographic conditions off California and Baja California.
Chapple, and later studies by Kanive in the central Pacific California aggregations focused on adult and subadult sharks because younger sharks are almost entirely absent in the region. White sharks are birthed in the warmer waters in the Southern California Bight and in Baja California. The bays and beaches serve as nursery sites and is where most juveniles and Young of Year and sub adults swim. Young white sharks consume fish and rays until they approach maturity. The range of the larger body- massed subadult and adult white sharks shifts northwards to points and islands where large aggregations of seals occur. The most recent estimate of the north central adult population (includes Ano Nuevo, the Farallon Islands to Point Reyes) at around 300 adult white (Kanive et al 2021). The authors also concluded the population of adult sharks in the central coast region is increasing slightly.
However, like the Guadalupe Population, an increase in young and juvenile white sharks has been observed north of Point Conception, a phenomenon previously undocumented. The westernmost promontory in California Point Conception creates a distinct sea temperature demarcation. This sea temperature break serves as a physiological barrier to young and sub-adult white sharks that have a preferential range of temperatures between 18 and 22 degrees C.
During the 2014–2016 North Pacific marine heatwave, unprecedented sightings of juvenile white sharks (Carcharodon carcharias) emerged in central California. Researchers onserved a dramatic increase in white sharks from 2014 to 2019 in Monterey Bay that was overwhelmingly comprised of juvenile sharks less than 2.5 m in total body length. These observations contradict the previosuly established life-history of white sharks, where juveniles remain in warmer waters in the southern California Current. The growing presence of juvenile sharks above 34° N suggests that climate change may be revising basic aspects of the established spatial population structure for this white shark population, and perhaps others.
Estimating population size and dynamics of sharks is challenging. Large distances are traveled by adults during migrations. When pregnant, females separate from the main population to gestate and pup in warm shallow regions like the Southern California Bight, Baiae de Vizcaino and in the Gulf of Califronia in Baja California. A separate 2021 study suggests the white shark population for the eastern north Pacific, especially those listed in the Gulf of California, might be underestimated. Researchers found that the mortality rates for these white sharks might be underestimated as well, as an illicit fishery for the species was uncovered in the Gulf of California, suggesting that fishers were killing many more white sharks than has been previously understood.
This spatial shift of range may create conflicts with commercial fisheries, the conservation of white sharks and endangered sea-otters, and may present public safety concerns. Warmer water temperature may impact the availability of fish and other prey that piniipeds, the primary diet of mature white sharks, may in turn reduce their populations, or cause those populations to move, potentially impacting the size and range of white sharks of the NEP.
Since record keeping began by the California Department of Fish and Game between, 82 attacks have occurred in Southern California out of the 215 records (38%) between 1950 and 2023. With a shift northward of white sharks, the Red Triangle may be more like the Red Trapezoid, with more white shark encounters shifting northwards with increasing sea surface temperatures.
Humans can reduce their risk of an attack or encounter by following prudent guidelines (link above). The chances of being bitten by a shark are quite low.
According to the California-focused study by Stanford scientists:
- Scuba divers are 6,897 times more likely to be hospitalized for diving-related decompression sickness than for white shark bites.
- Ocean-goers are 1,817 times more likely to drown than die from a shark attack.
- Scuba divers have a 1-in-136 million chance of being bitten.
- Surfers have a 1-in-17 million chance of being bitten.
The best way to avoid a shark encounter is to avoid locations and seasons where risk is highest, the study’s authors advise. For example, the riskiest time and place for California surfers is October and November in Mendocino County. That risk can be reduced 25-fold if you surf in March, and more than 1,600-fold if you surf in March between San Diego and Los Angeles.
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