Sharks & Other Fish
A whale shark in the Red Sea. (Photo by Simon Thorrold, Woods Hole Oceanographic Institution)
What are they?
Fish are aquatic animals that were among the first vertebrates (animals with a backbone) to evolve on Earth. They are divided into two broad groups: teleosts, which have a bony skeleton and symmetrical tail and include most familiar fish species, and elasmobranchs, which have a skeleton made of cartilage and include sharks, rays and skates. There are about 30,000 species in the two groups combined (equal to the number of all other vertebrate species combined) in existence today.
Bony Fish (Teleosts)
Bony fish are found around the world in both salt and fresh water, from the tropics to the Arctic and Antarctic, and from the coasts to the deepest parts of the ocean. They range in size from the pygmy goby, at one-third of an inch (9 millimeters), to the marlin, which can grow to nearly 12 feet (3.5 meters).
The behaviors and life cycle strategies of fish is almost as varied as the number of species. Some fish can survive being frozen for long periods, while others have adapted to life near hydrothermal vents. Some fish have even evolved mechanisms to deal with high levels of pollution.
Cartilaginous Fish (Elasmobranchs)
Sharks, skates, and rays are also found around the world and, unlike bony fishes, have a skeleton made of cartilage. They also lack the swim bladder found in other fish species, often relying instead on a large, oily liver to control their buoyancy.
There are about 500 species of sharks, several of which are among the most recognizable fish species on Earth, including the white shark, one of the ocean's largest predators, and the whale shark, which is the largest fish in the ocean. Because they are wide-ranging animals, many basic questions about sharks remain unanswered, including where many species migrate, when and where they mate and give birth, and what they eat. This lack of information has made it difficult to estimate shark populations and challenging to design conservation measures for even the most threatened species.
Sharks are generally slow to mature—white sharks likely take at least 20 years to reach reproductive age—and only produce two to ten pups per litter. This means shark populations are especially vulnerable to human activities such as fishing. Each year, between 50 and 100 million sharks are killed, mainly for their fins, which are used to make a traditional Chinese soup. This level of harvesting is unsustainable and has dramatically reduced populations of many shark species around the world.
Why are they important?
Fish serve important ecological and economic functions. Ecologically, they are both predator and prey, providing food for other animals (including birds and land mammals), and serve to keep the numbers of prey species in check, many of which could destroy important ecosystems such as coral reefs and mangroves if their numbers are allowed to grow. At the same time, fish move nutrients through aquatic food webs and provide a link between ocean and terrestrial ecosystems.
The incredible diversity of fish species provides an important insurance policy for the ocean against changing conditions. This diversity means that fish as a group have evolved a wide range of behaviors and characteristics, from migratory behaviors to reproductive strategies. Many fish species have adapted the potential to survive a changing climate, varying levels of food availability, or even sudden pollution spills. Resilient ocean ecosystems in turn are able to provide critical services to humans, including storm and flood protection, nutrient and sediment cycling, carbon storage, and oxygen production.
Economically, fish are an important source of food for people around the world. An average of 20 percent of global protein demand is met by fish and shellfish, a number that has been rising faster than population growth in recent years. As a result, many parts of the ocean are over-fished, leading to localized collapses of marine ecosystems and a rising reliance on aquaculture and farm-raised fish. Fishing is also in important recreational and cultural activity that provides income to inland and coastal communities around the world.