There's Been a Great Decline in Basking Shark Sightings in California

Why's it so hard to find the second largest fish in the ocean?

Basking shark (Cetorhinus maximus), underwater view, Baltimore, Cork, Ireland
George Karbus Photography / Getty Images

In the mid-1900s, basking sharks were a relatively common sight off parts of the California coast. Now, the huge fish are rarely spotted.

New research confirms a major decrease in basking shark sightings after the 1970s and 1980s in the California Current Ecosystem (CCE) from San Diego north to San Luis Obispo.

Basking sharks are the second-largest fish in the world. They usually range from 23 to 26 feet long, although the largest basking shark recorded was more than 40 feet in length. But not a lot is known about these massive animals, such as their life span or where they mate or give birth. Researchers don’t know much about their population numbers but the information they have suggests that the species needs investigation.

Basking sharks are classified as endangered by the International Union for the Conservation of Nature (IUCN) Red List with their population numbers decreasing.

Lead author Alexandra McInturf was a Ph.D. candidate with the University of California, Davis Department of Wildlife, Fish and Conservation Biology at the time of the study and currently is a postdoctoral fellow at Oregon State University. She had been studying basking sharks for five years in Ireland, and her coauthors had been studying them even longer in California.

“Unfortunately, our collective knowledge provided a potential cause for concern, with the number of sightings reported in California few and far between over the last two decades. Comparatively, though sightings are also fewer than in previous centuries in places like Ireland and the broader North Atlantic region, they have not declined to the same degree,” McInturf tells Treehugger.

“Basking sharks are typically variable in their presence, so we do expect some years where none are observed. However, we needed a bigger dataset to see if there were any longer-term declines that we should be aware of.”

The National Oceanic and Atmospheric Administration (NOAA) had collected data on basking shark sightings since the 1960s, so the researchers used that information to analyze why the sharks might not be spotted so often anymore.

The authors examined aerial surveys for small fish conducted by NOAA Fisheries between 1962 and 1997. They also studied additional information collected between 1973 and 2018 that includes fisheries data, tagging and research efforts, and public observations.

The maximum number of shark sightings in one year was nearly 4,000 in 1965. Since the 1990s, the most that were reported were about 60.

“Again, they don’t show up every year, but that comparison suggests that the number of sightings has declined by orders of magnitude,” McInturf says.

Researchers also found that the size of the groups that sharks form and travel in decreased dramatically. There were about 500 sharks in a group in the 1960s then fewer than 10 per group since the 1990s.

“If these groups help facilitate mating, the decline in group size could indicate a reduced mating success and an inability to rebound from current numbers,” McInturf says. 

The findings were published in the journal Frontiers in Marine Science.

Explaining the Drop in Sightings

There are several possible reasons why basking shark sightings have declined so much. The simplest explanation is that this reflects a decrease in population size.

“This is likely linked to fisheries pressure in the area in the early to mid-20th century,” McInturf says, pointing out that other studies have supported this explanation. “Additionally, the basking sharks in California are thought to be part of one single migratory population that moves up through Western Canada. During this same time period (the mid-1900s), they were actually so abundant in Western Canadian waters as to be considered a pest species for other water users, like fishermen, and they were consequently eradicated, which could have also contributed to this decrease.”

There’s still the possibility that they are being captured accidentally (as bycatch) or on purpose for their fins or other parts in other parts of the Pacific, the researchers suggest.

So little is known about basking sharks because they are elusive, unpredictable animals.

“Basking sharks can be fickle creatures! They tend to arrive in coastal ‘hotspots,’ or gathering areas, and sometimes in big groups. But, they don’t do this every year, and the numbers that do show up can be variable. In general, their presence can be hard to predict, especially as sightings have declined,” McInturf says.

They’re most often spotted when they are at the surface dining on zooplankton, but they don’t need to surface to breathe.

“We do know they spend significant amounts of time offshore and at depth,” McInturf says. “To study that requires putting tags on the sharks that can record their behaviors and movements when we cannot see them. But, that’s a challenge when you aren’t sure of where or when they might show up.”

When they do show up, they are easy to spot. They are about the size of a small school bus and may resemble a whale or even a great white shark swimming at the surface of the water. But they appear to swim slowly, moving their head back and forth with their mouth open as they take in water to filter out the plankton. They can be identified by a large dorsal fin.

Basking Shark Conservation

The study offers suggestions for basking shark conservation and notes that the coastal areas from Monterey Bay to Baja California are important habitats for basking sharks. They suggest, for example, a coordinated effort in documenting shark deaths and sightings to create better population estimates.

“We advocate for water users to be aware of basking sharks, avoiding them when in boats and reporting their presence if they see them on the water. That will help us continue to monitor the population,” McInturf says.

“We also need to continue to track the presence of basking shark products in restaurants and markets, to see if ongoing mortality is happening through these avenues. We hope that by bringing awareness to the plight of basking sharks in California, we can start to bring further focus to this issue, which will ideally lead to even more research.”

Correction—March 9, 2021: This article was corrected from a previous version that referred to basking sharks as mammals.

View Article Sources
  1. McInturf, Alexandra G., Muhling, Barbara, Bizzarro, Joseph J., et al. "Spatial Distribution, Temporal Changes, and Knowledge Gaps in Basking Shark (Cetorhinus maximus) Sightings in the California Current Ecosystem." Frontiers in Marine Science, 17 Feb. 2022. doi:10.3389/fmars.2022.818670

  2. "Basking shark (Cetorhinus maximus)," Oceana.

  3. McClain, C.R., Balk, M.A., Benfield, M.C., et al. "Sizing ocean giants: patterns of intraspecific size variation in marine megafauna." PeerJ, vol. 3, no. e715, 13 Jan. 2015. doi:10.7717/peerj.715

  4. Lead author Alexandra McInturf, Ph.D. candidate at the University of California, Davis Department of Wildlife, Fish and Conservation Biology at the time of the study, and currently a postdoctoral fellow at Oregon State University

  5. "Basking Shark (Cetorhinus maximus)," IUCN Red List.