Fish are aquatic vertebrates that are primarily distinguished from other animals by their use of gills for extracting oxygen directly from water, the presence of fins for stability and propulsion and the presence of skin typically covered in scales (although a few species can breathe air and some lack scales or even jaws). Unlike mammals and birds, most fish, along with reptiles and amphibians, are ectothermic, meaning their internal body temperature adjusts to match their surrounding environment.
Although they share a backbone with other vertebrates, their skeletal structures vary widely, ranging from the flexible cartilage found in sharks and rays to the bony frameworks of salmon and goldfish. They also vary immensely in size, from tiny reef dwellers to enormous species such as the whale shark. These diverse adaptations allow fish to inhabit nearly every aquatic ecosystem, from the deepest ocean trenches to high-altitude mountain streams.
Fish make up by far the largest share of vertebrate species, with 36,000 to 38,000 described species, accounting for more than half of all vertebrates as a whole. This is because they have had much longer to evolve (more than 500 million years), along with an exceptionally wide array of habitats and traits. After fish, reptiles are the next largest group of vertebrates by species count at about 10,700, followed by birds at around 10,400.
In addition to their obvious role in water, fish also play an important role in many land habitats by transferring nutrients and providing an indispensable energy source for terrestrial wildlife. Migratory fish, such as Pacific salmon, frequently travel hundreds of miles inland through major river networks, depositing substantial quantities of marine-derived elements like nitrogen and phosphorus deep into interior watersheds.
Numerous terrestrial predators and scavengers, including bears and raptors, rely heavily on seasonal fish runs to satisfy their high energy requirements for survival and reproductive success. When they catch these fish, they often carry them further into the woods where uneaten scraps and waste enrich the soil and thus facilitate the growth of trees and understory plants, truly creating "salmon-powered forests."
Although the share of farmed fish in the human diet has increased rapidly in recent decades, wild fish remain foundational to cultural diets, coastal economies and global food security. Globally, fish and other seafood provide roughly 15 to 17 percent of all animal protein consumed by humans, with wild-caught and farmed sources sharing that vital role roughly equally. While fish constitutes a small percentage of protein intake by weight, its caloric and dietary value is greatly magnified by its density of high-quality nutrients and essential fatty acids.
As with so many other types of organisms, fish are in the midst of a major ecological crisis. Overfishing, habitat fragmentation from dams, pollution and climate change are severely threatening aquatic populations worldwide, disrupting both marine ecosystems and the terrestrial ecosystems that depend on them.
At first glance, overfishing may not initially appear to be a problem because the total volume of wild-caught fish has remained surprisingly stable over recent decades, at between 86 and 94 million metric tons per year since the late 1980s. However, the composition of what is being caught has shifted dramatically. Large, long-lived predator species such as large tunas, cod, sharks and groupers have experienced steep population drops. To maintain the total weight, fisheries have increasingly targeted smaller, short-lived species lower down the food chain, such as sardines, anchovies and krill.
According to Food and Agriculture Organization (FAO) assessments, the percentage of global marine fish stocks fished within biologically sustainable levels has steadily declined, now down to about 62.3 percent. The most endangered species are typically those with slow life cycles that also face high commercial demand. Cartilaginous fish, such as sharks, rays and sawfish, grow slowly, mature late, and produce few offspring, making them uniquely unequipped to survive heavy targeting for the fin trade and accidental bycatch.
Similarly, luxury seafood markets have driven large open-ocean predators like bluefin tuna, eels, and large reef groupers to severe depletion through advanced industrial fishing. Meanwhile, freshwater sturgeons, prized for caviar, are critically endangered because their decades-long path to maturity collides with relentless overfishing and river-blocking hydroelectric dams.
Although far from sufficient, some progress is being made on protecting these species. One way is through the implementation of strict catch quotas and regional management. A prominent success story is the Atlantic bluefin tuna, which has steadily recovered in recent years from near-collapse. Another vital tool is the increasing establishment of "no-take zones" in critical marine habitats, coral reefs and spawning grounds, giving local fish populations safe havens to rebound.
Additionally, the mandatory use of turtle excluder devices and modified fishing hooks/nets in industrial trawling has drastically reduced accidental deaths for endangered sharks, rays and sea turtles in regulated fisheries. Finally, a growing movement to tear down obsolete river dams, particularly in parts of North America and Europe, has reconnected hundreds of miles of historical spawning habitat for migratory species like salmon and sturgeon.