Basking sharks are classified as Endangered on the IUCN Red List, with a global population trend that is still decreasing. Decades of commercial hunting decimated their numbers, and their biology makes recovery painfully slow. Today, even though targeted fishing has largely stopped, a mix of ongoing threats continues to suppress the population.
A Reproductive Rate That Can’t Keep Up
The single biggest reason basking sharks struggle to bounce back is how slowly they reproduce. Females are thought to reach sexual maturity around 18 years of age, and gestation lasts an estimated two to three years, one of the longest of any vertebrate. That means a single female might produce only a handful of litters in her entire lifetime. When a population drops sharply, this kind of reproductive timeline means recovery takes generations, not years.
Making things harder, almost nothing is known about their breeding behavior in the wild. Pregnant females are so rarely encountered that only one confirmed observation exists in the scientific literature, dating back to 1776. Scientists believe pregnant females separate themselves from the surface aggregations where basking sharks are typically spotted, which means we lack even basic data on how many pups are born per litter or how often females breed successfully.
Historical Overfishing Collapsed the Population
For centuries, basking sharks were hunted for their oil, meat, fins, and especially their enormous, vitamin-rich livers, which can make up roughly a quarter of the animal’s body weight. Liver oil was used in lamps, industrial lubricants, and later as a source of squalene. Targeted fisheries operated across the North Atlantic and Northeast Pacific through much of the 20th century, killing thousands of sharks per year at their peak.
Some regional populations were driven so low that fisheries collapsed on their own, simply because there weren’t enough sharks left to sustain commercial operations. In certain areas, governments even ran eradication programs, treating basking sharks as nuisance animals that damaged fishing nets. The cumulative result was severe population declines across their range. No formal global population estimate exists today, which itself reflects how depleted and poorly understood the remaining population is. Scientists note that numbers may now be beginning to stabilize following the end of targeted fishing and the introduction of legal protections, but recovery from this level of depletion will take decades given the species’ biology.
Bycatch and Fishing Gear
Even though directed basking shark fisheries are now banned in most countries, the sharks still die in commercial fishing operations. Basking sharks feed at the surface by swimming with their mouths open, filtering enormous volumes of water for tiny zooplankton. This habit puts them directly in the path of gillnets, trawl gear, and other equipment deployed in the same coastal and shelf waters they prefer. Because they’re large (adults typically reach 6 to 8 meters), entanglement is often fatal. The animals can become wrapped in netting and drown, since they need to keep moving to pass water over their gills.
Bycatch is difficult to quantify because many incidents go unreported, particularly in regions with limited fisheries monitoring. But for a species that reproduces as slowly as the basking shark, even modest levels of accidental mortality can meaningfully slow population recovery.
Illegal Fin Trade
Basking sharks were among the first shark species listed on CITES Appendix II, back in 2003, meaning international trade in their parts is supposed to be regulated and documented. In practice, enforcement has been weak. A 2025 study published in Science found widespread illegal trade in fins from CITES-listed shark species nearly a decade after protections took effect. Researchers compared trade records with surveys of Hong Kong’s shark fin market, the world’s largest, and found that fins from listed species remained common despite minimal trade being officially reported. Eighty-one percent of shark fin-exporting nations had never reported any trade of these species at all.
Basking shark fins are among the most valuable on the market because of their size. A single set of fins can sell for tens of thousands of dollars. This financial incentive means that even with legal protections on paper, sharks continue to be targeted or retained as bycatch and their fins quietly enter international trade networks.
Ship Strikes
Basking sharks spend much of their time at or near the surface, which puts them at risk of collisions with vessels. While most collision research has focused on whale sharks, the findings are relevant: a study in the Proceedings of the National Academy of Sciences found that 92% of tracked whale shark movements overlapped with persistent large vessel traffic, and collision-risk estimates correlated with reported mortality. Basking sharks share the same surface-feeding behavior and occupy busy shipping lanes in the North Atlantic and European coastal waters.
The true death toll from ship strikes is almost certainly underestimated. Large sharks sink when they die, so the vast majority of collision fatalities are never observed or recorded.
Pollution and Microplastics
Filter-feeding animals face a unique pollution risk: they can’t be selective about what enters their mouths. Basking sharks process thousands of liters of water per hour, and anything suspended in that water, including microplastics smaller than 5 millimeters, gets ingested along with their food. Research measuring chemical markers of plastic exposure found higher concentrations of a plasticizer breakdown product in basking shark muscle tissue compared to fin whale blubber, suggesting meaningful absorption of plastic-derived chemicals.
The long-term health effects on basking sharks are still poorly understood, but in other species, these chemicals interfere with hormone function and reproduction. For a species that already reproduces slowly, even subtle impacts on fertility could compound the population problem.
Climate Change and Shifting Food Sources
Basking sharks depend entirely on dense patches of zooplankton, the tiny crustaceans and larvae that bloom seasonally in productive coastal waters. Rising ocean temperatures are already shifting where and when these blooms occur. NOAA has flagged this as a direct concern for basking sharks: changes in zooplankton seasonality or abundance could force them to abandon historical foraging grounds and search for food in unfamiliar areas.
This matters because basking sharks appear to rely on predictable aggregation sites for feeding and likely for mating. If plankton blooms shift poleward or become less concentrated, sharks may expend more energy searching for food, arrive at breeding areas in poorer condition, or fail to encounter mates altogether. For a species with no formal population estimates even in well-studied regions like UK waters, the potential for climate-driven disruption to go unnoticed until it’s severe is a real concern.
Why Recovery Is So Difficult
What makes the basking shark’s situation especially precarious is the way these threats interact with each other. A species that matures at 18, gestates for up to three years, and produces small litters has almost no capacity to absorb additional mortality from any source. Historical overfishing created the crisis, but the combination of bycatch, illegal finning, ship strikes, pollution, and habitat disruption from climate change means the population faces ongoing pressure from multiple directions even after the fisheries stopped.
Legal protections exist on paper. CITES regulates international trade, and many countries have banned targeted fishing. But enforcement gaps, particularly in the fin trade, mean these protections haven’t fully translated into population recovery. Until the gap between regulation and reality closes, basking sharks will continue to recover far more slowly than their biology alone would predict.