Ocean Science Series: Sharks - The State of the Science
The biological characteristics of sharks make them particularly vulnerable to overfishing. They grow slowly, become sexually mature relatively late and produce few offspring. This vulnerability is reflected in the large number of shark species that are considered to be threatened or endangered.
A review of the current scientific literature on the number of sharks killed per year, the causes of this mortality, the status of shark species worldwide and the impact on ecosystems after large predators are removed provides the following key points:
- Millions of sharks are killed every year to supply the fin trade. In 2000, for example, 26 million to 73 million sharks were killed for fins, corresponding to 1.21 million to 2.29 million tons of shark.
- Commercial fisheries targeting sharks occur throughout the world. Sharks are sought primarily for their fins and meat but also for their cartilage, liver and skin.
- The highest numbers of reported shark landings are from: Indonesia; India; Taiwan, Province of China; Spain; and Mexico.
- Shark bycatch is frequently reported in pelagic longline fisheries targeting tuna and swordfish and can represent as much as 25 percent of the total catch. This bycatch is considered to be a major source of mortality for many shark species worldwide.
- Blue sharks make up an especially large fraction of shark bycatch in pelagic fisheries (47–92 percent).
- The value of shark fins has increased with economic growth in Asia (specifically China), and this increased value is a major factor in the commercial exploitation of sharks worldwide.
- Declines in population sizes of sharks, as much as 70–80 percent, have been reported globally. Some populations, such as the porbeagle sharks in the northwestern Atlantic and spiny dogfish in the northeastern Atlantic, have been reduced by up to 90 percent.
- The removal of large sharks can negatively impact whole ecosystems by, for example, allowing an increase in the abundance of their prey (fewer sharks eat less prey), or influencing prey species through non-lethal means, by causing behavioral changes to prey habitat use, activity level and diet.
- Live sharks have a significant value for marine ecotourism (for example, recreational diving, shark feeding and shark watching) that is typically more sustainable and often more valuable than their individual value to fisheries. Whale shark tourism, for example, is estimated to be worth $47.5 million worldwide.