Between 20 and 30 marine species have gone extinct in the last 500 years, according to assessments compiled through 2022. That number is lower than terrestrial extinctions, partly because the ocean is vast and harder to survey, meaning some species may have vanished before scientists ever documented them. The confirmed losses span mammals, birds, fish, and invertebrates, with human hunting, habitat destruction, and bycatch driving most of them.
Steller’s Sea Cow
Steller’s sea cow is one of the most dramatic extinction stories in marine history. This massive sirenian, a relative of modern manatees and dugongs, was discovered by European explorers in 1741 near the Commander Islands in the North Pacific. By 1768, just 27 years later, the last one had been killed. Adults could reach lengths of around 8 to 9 meters, making it one of the few true megafauna to disappear during recorded history.
The traditional explanation is straightforward overhunting. Sailors and fur traders harpooned the slow-moving animals from shore or small boats for food during expeditions. A more recent hypothesis suggests that the extinction may have been compounded by ecological collapse: as hunters wiped out local sea otter populations, sea urchin numbers exploded and destroyed the shallow-water kelp beds that sea cows depended on for food. Either way, the species highlights how even small numbers of hunters with basic tools can eliminate a large, slow-reproducing animal in a startlingly short time.
Great Auk
The great auk was a large, flightless seabird native to the North Atlantic, standing about 75 centimeters tall with small wings suited only for swimming. It nested on rocky islands from Canada to Scandinavia and was hunted for centuries for its meat, feathers, and oil. On July 3, 1844, fishermen killed the last confirmed pair at Eldey Island off the coast of Iceland. By that point, collectors knew the species was vanishing, which only increased demand for specimens and eggs, accelerating the final decline.
Caribbean Monk Seal
The Caribbean monk seal was the only seal species native to the Caribbean Sea and Gulf of Mexico. It was heavily hunted starting in the colonial era for its blubber, which was rendered into oil. Caribbean monk seals were last seen in the 1950s, and after decades of failed search efforts, the species was officially declared extinct. It remains the only seal species driven to extinction in modern times, though its two relatives, the Hawaiian and Mediterranean monk seals, are both critically endangered today.
Japanese Sea Lion
The Japanese sea lion once inhabited coastal waters and rocky islands across the Sea of Japan, from the Japanese archipelago to the Korean Peninsula. Commercial hunting for skin, oil, and organs drove a steep decline through the early 20th century, compounded by habitat loss and fisheries conflict. The last credible sighting was of 50 to 60 individuals on Takeshima (also called Dokdo) in 1951. The IUCN has listed the species as extinct since its first assessment in 1996.
Smooth Handfish
In a grim milestone, the smooth handfish became the first modern marine fish to be officially declared extinct, with the IUCN making the designation in 2020. Handfish are a family of 14 unusual bottom-dwelling species found in the waters around Tasmania, Australia. They’re related to deep-sea anglerfish and are named for their habit of “walking” along the seafloor on modified pectoral fins rather than swimming.
What made the smooth handfish especially vulnerable is its life cycle. Unlike most marine fish, it has no larval phase. Instead of producing tiny larvae that drift on currents and colonize new areas, handfish stay put. That means populations are extremely localized. If a single habitat is degraded by pollution, dredging, or invasive species, the fish living there have no way to relocate. The smooth handfish was known from a small area of coastal Tasmania, and once that habitat deteriorated, the species had nowhere to go.
Megalodon
Going further back in time, the megalodon is probably the most famous extinct sea animal of all. This giant shark disappeared roughly 3.6 million years ago during the Pliocene epoch, long before humans existed. Research published in the Proceedings of the National Academy of Sciences found that megalodon was at least partially warm-blooded, maintaining body temperatures higher than the surrounding water. That adaptation helped it become an apex predator, but it came with high energy costs.
When sea levels shifted during the Pliocene and productive coastal habitats shrank, the prey landscape changed with them. Baleen whale populations shifted in diversity and distribution. For a massive predator that needed enormous amounts of food to fuel its warm body, this was catastrophic. Smaller, cold-blooded sharks with lower energy demands survived the same period. The megalodon’s size and metabolism, once advantages, became liabilities when food grew scarce.
Why Marine Extinctions Are Hard to Count
The total number of confirmed marine extinctions sits somewhere between 20 and 30 species over the past five centuries, depending on how strictly you apply the evidence. That range exists because the ocean is extraordinarily difficult to survey. To officially declare a species extinct, the IUCN requires that exhaustive surveys across the species’ known and expected habitat, conducted at appropriate times and over a timeframe matching its life cycle, have failed to find a single individual. For a deep-sea invertebrate or a small reef fish, meeting that standard can be nearly impossible.
Some species initially declared extinct have later been rediscovered, which is why the count has actually been revised downward over the years. One review found that 36 marine species were at one point considered extinct over the past 500 years, but reexamination of the data lowered that figure to 20 to 24 confirmed cases. The true number of losses is almost certainly higher than the official count, since many marine species went undocumented before they disappeared.
Species on the Edge Right Now
The vaquita, a small porpoise found only in the upper Gulf of California in Mexico, is the marine species closest to extinction today. A 2024 survey estimated that just 6 to 8 individuals remain, down from an estimated 8 to 13 in 2023. Researchers noted that based on the population decline observed in 2018, the species was projected to already be gone by now. The fact that it has survived into 2024 is considered cautiously encouraging, but the situation remains dire.
The vaquita’s primary threat is gillnet fishing. The porpoises become entangled in nets set for shrimp and a fish called totoaba, whose swim bladder is illegally traded. During the 2024 survey, researchers observed healthy vaquitas feeding undisturbed in areas where gillnets had been removed, suggesting the species could stabilize if alternative fishing methods replace gillnets throughout its range. No calves were confirmed during the survey, however, which raises concern about whether the remaining population can reproduce fast enough to recover.