Sharks are crucial to marine ecosystems. They maintain a balance in populations of prey species and keep the ocean healthy by removing ill or diseased animals. They are an important resource supporting local economies through fishing and as an attraction to dive tourists. But sharks are in a global decline. Over-fishing has reduced many shark populations around the world to levels that threaten their continued existence. Shark numbers have fallen by more than 80% in many cases, and the continued existence of some species is at immediate risk in some regions. Sharks occupy every marine environment from tropical coral reefs, to temperate coastal waters, to the open ocean, to the icy ocean depths. Some species also venture into freshwater such as the bull and the river shark. Sharks have evolved into roughly 500 species and come in many sizes from the dwarf lantern shark at around 7 cm to the whale shark - the world’s largest fish - that grows up to 12 meters. Sharks eat a great variety of prey. Most sharks are predators, but some are scavengers, while whale and basking sharks are filter feeders with eating habits similar to a baleen whale. Compared to other vertebrates (animals with a backbone including mammals), sharks generally have a slow reproductive cycle. The reproductive strategy of most shark species more closely resembles those of whales, elephants and birds than other fish. Under natural conditions this slow reproductive strategy works well for sharks as they have few predators and so have no need to rapidly replenish their numbers. These traits work against sharks when they need to recover from over-fishing or other substantial losses. A slow reproductive strategy means they are unable to respond quickly to the removal of many individuals from a population . The shark’s reproductive strategy is very different to most bony ocean fish that release millions of eggs in a lifetime. As we shall see later in the course, this key difference contributes greatly to the many problems associated with shark fisheries management. Another trait that makes some shark species vulnerable to heavy fishing is their tendency to form groups based on their age, sex and/or maturity. Large, older females of many shark species produce greater numbers of stronger pups than younger females, so the sudden removal of these older females through fishing can have serious consequences for the population. Sharks are often the apex predator in their ecosystem, meaning they are at the top of many food chains. As adults they have no or few natural predators. Typical traits of apex predators is that they feed on many different species and change food sources when one prey animal becomes hard to find. In this way sharks help maintain a balance that ensures no one species over-populates and depletes the species on which it feeds. Fishers use a variety of methods to catch fish; most of them result in by-catch. By-catch refers to the part of a catch that is not the target species or is undersized. By-catch can not be landed in many regions depending on local regulations and how strictly they are enforced. When by-catch can not be landed or is not wanted it is dumped overboard, sometimes live, sometimes dead or dying. By-catch includes sharks and bony fish as well as dolphins, whales, turtles, invertebrates and seabirds. By-catch accounts for the majority of the total catch in some shrimp trawl fisheries. Tens of millions of sharks are killed as by-catch every year. Discarded by-catch is rarely accounted for in fishery records so these shark deaths are missing from official statistics . Many human activities on land have a negative effect on sharks, particularly reef and coastal species. Scientists warn that 75 percent of the world’s coral reefs are threatened from local pressures such as coastal development, pollution and over-fishing, combined with the impacts of rising sea temperatures caused by increased concentrations of CO2 and other greenhouse gases in the atmosphere. One fifth of the world’s mangroves have been removed since 1980 for land reclamation projects and aquaculture farms. Coastal development can damage important shark habitats and nurseries. Mangroves, estuaries and salt marshes provide important habitats for sharks to give birth and mature. These areas are rapidly being destroyed in mankind’s rush to accommodate a growing population. Marine debris - the rubbish we allow into the ocean kills and injures sharks through entanglement or because they eat it. Ghost nets - fishing nets that have been accidentally lost or purposefully dumped at sea - also contribute to the annual shark death toll. Courtesy of Ghost Nets Australia Other impacts include swimmer protection devices such as beach nets and drumlins with baited hooks used in Australia and South Africa. These devices kill sharks including species that are of no threat to humans, as well as many other marine animals such as dolphins, rays and turtles.