According to marine ecologists, overfishing is the greatest threat to ocean ecosystems today (1). Overfishing occurs because fish are captured at a faster rate than they can reproduce (2). Advanced fishing technology and an increased demand for fish have led to overfishing, causing several marine species to become extinct or endangered as a result (3, 4). In the long-term, overfishing can have a devastating impact on ocean communities as it destabilizes the food chain and destroys the natural habitats of many aquatic species (2).

In the past, fishing was more sustainable because fishermen could not access every location and because they had a limited capacity for fish aboard their vessels. Today, however, small trawlers and fishing boats have been replaced by giant factory ships that can capture and process extremely large amounts of prey at a given time (2). These ships use sonar instruments and global positioning systems (GPS) to rapidly locate large schools of fish (1). Fishing lines are deployed with thousands of large hooks that can reach areas up to 120 kilometers deep. The trawling vessels and machines can even reach depths of 170 kilometers and can store an extraordinarily large volume of fish. Each year, these huge trawling ships comb an area twice the size of the United States. They use massive nets 50 meters wide with the capacity to pull the weight of a medium-sized plane (2). They also have several plants for processing and packing fish, large freezing systems, fishmeal processing plants, and powerful engines that can carry this enormous fishing gear around the ocean. Because these ships have all the equipment necessary to freeze and tin fish, they only need to return to their base once they are full. Even when the ships are filled, however, the fish are often transferred to refrigerated vessels in the middle of the ocean and are processed for consumption later (4). As such, industrial fishing has expanded considerably and fishermen can now explore new shores and deeper waters to keep up with the increased demand for seafood. In fact, it has been reported by the United Nations Food and Agricultural Organization (FAO) that over 70 percent of the world’s fisheries are either ‘fully exploited’, ‘over exploited’ or ‘significantly depleted’ (5). The annual total global catch of fish is 124 million metric tons, which is equivalent in weight to 378 Empire State Buildings (2).

Fishing gear is often non-selective in the fish it targets. For example, any fish that are too big to get through the mesh of a net are captured. Therefore, overfishing does not only threaten the species of fish that is targeted for food, but also many non-target species. As a result, these other species, including marine mammals and seabirds, are accidentally caught in the fishing gear and killed (6). For example, for every ton of prawn caught, three tons of other fish are killed and thrown away. Those in the trade refer to this practice of inadvertent catching of other species as bycatch (4). The FAO has pointed out that about 25 percent of the world’s captured fish end up thrown overboard because they are caught unintentionally, are illegal market species, or are of inferior quality and size. Many of the fish caught this way include endangered and over exploited species, 95 percent of which are eventually thrown away (2). Bycatch is not just limited to just unwanted fish, but rather affects all types of marine life, including whales, dolphins, porpoises, fur seals, albatrosses, and turtles. For example, tuna fisheries are indirectly responsible for the deaths of an estimated one million sharks annually due to bycatch. Small cetaceans, such as dolphins and porpoises, are also targets of bycatch as they are often caught in fishing nets. In fact, hundreds of dolphin corpses are washed up on the beaches of Europe every year, bringing attention to the growing scale of this problem (6).

Many modern fishing methods are also irreversibly destructive. For example, bottom trawling, a technique that uses extremely wide nets armed with heavy metal rollers, can crush everything in the path of the gear, destroying fragile corals, smashing rock formations, and killing several tons of fish and animals as bycatch (7). As such, these practices can wreak havoc on delicate marine ecosystems.

Not surprisingly, it has been reported that industrial fishing takes between only 10 and 15 years to wipe out a tenth of whichever species it targets (2). In fact, several marine species have already been fished to commercial extinction, and this number is rapidly increasing (1). One of the reasons for this is that the regulation of fishing vessels and the fishing industry is universally inadequate. Roughly two-thirds of the ocean is free of laws and fishing vessels only follow the laws ratified by their country of origin. However, most fishing countries have not ratified any international convention to protect the sea or marine life (2). Moreover, fishing factory ships and companies are given access to fisheries before the long–term impact of their fishing practices is understood (1).

Today, the number of fish caught worldwide is actually shrinking as the fishing industry is in decline from many years of overfishing (2). The year 1988 was the first time in human history that global wild fish catches dropped and they have continued to fall ever since. In European waters, four out of every five known fish stocks are already beyond safe biological limits (7). Illegal and unreported fishing have also contributed a great deal to the depletion of the oceans and continues to be a serious problem.

A new study conducted by the International Union for Conservation of Nature (IUCN) found that 5 out of the 8 tuna species are at risk of extinction (8). All three species of bluefin tuna, for example, are threatened with extinction and are at a population that makes their recovery practically irreversible (2). The IUCN has also reported that freshwater fish are among the most endangered species, with more than a third facing extinction. Not surprisingly, among those at the greatest risk are species like the Mekong giant catfish, the freshwater stingray, and the European eel, which are used to make some of the most expensive caviars. The Mekong giant catfish is the closest to extinction, with as few as 250 left. Overfishing has reduced the numbers of Mekong freshwater stingray by over 50 percent in Southeast Asia and has reduced the giant Mekong salmon carp population by over 90 percent (9).

As previously mentioned, shark populations have also been greatly affected by overfishing. There are already more than 135 species of shark on the IUCN’s list of endangered animals and more are being added each year. For example, the number of scalloped hammerhead shark has decreased by 99% over the past 30 years. Other species recently added to the endangered list include the smooth hammerhead, shortfin mako, common thresher, big-eye thresher, silky, tiger, bull, and dusky (10). Besides being caught as bycatch, sharks are now also being targeted by commercial fishermen for their fins which can fetch a substantial price on the Asian food market. Sharks are particularly vulnerable to exploitation because they have long life spans, are exceptionally slow to mature (taking as long as 16 years in some cases), and are relatively unprolific breeders (11). Recent reports suggest that over fishing has caused a 90% decline in shark populations across the world’s oceans and up to 99% along the US east coast, which are some of the best managed waters in the world. Because sharks are at the top of the food chain, a decline in their numbers has devastating consequences on marine ecosystems (10).

Overfishing impacts not just the particular species that is exploited, but also damages other species of fish and disrupts local ecosystems. The stability of ecological communities depends largely on the interactions between predators and prey (12). Thereby, the balance of the food chain is disturbed when certain species are removed. As a result, many ocean species are disappearing and losing their habitats. The evolutionary process of marine species is also being altered, causing cycles of premature reproduction and relative decreases in the size of fish across generations. As predators diminish, the populations of smaller fish escalate because they were previously the food source of the bigger fish. In addition, the disappearance of these species affects many other species, like seabirds and sea mammals, which are vulnerable to the lack of food (2).

A recent study found that overfishing is also decreasing the genetic diversity of fish worldwide. Diversity is projected to be reduced further if overfishing continues at the same rate (13). This has serious effects on nutrient recycling in marine ecosystems because fish species vary widely in their rates of nitrogen and phosphorus excretion. As such, altering fish communities creates divergent nutrient recycling patterns and disrupts the functioning of the ecosystem. Recently conducted studies in lakes affected by overfishing show that loss of species contributes to a decline in nutrient recycling and destabilizes the ecosystem (14).

While it is often overlooked for other environmental issues, overfishing has historically caused more ecological extinction than any other human influence on coastal ecosystems, including water pollution (5). Unfortunately, due to a lack of data, the extent of this damage has only recently been recognized (15).

Given that fishing is a food source for millions of people, attempting to solve the problem of overfishing not easy, especially for developing countries. Nevertheless, scientists and the UN Committee for Sustainable Development have called for a restoration of depleted fisheries and continue to stress the importance of stricter fishing regulations in oceans and inland waters (5). Sustainable fishing will be a necessary goal in counterbalancing depletion in fisheries and re–stabilizing coastal ecosystems.

References:

1. Overfishing – Emptying our Seas (2008). Available at http://www.greenpeace.org.uk/oceans/problems/overfishing-emptying-our-seas (25 November 2011).

2. Overfishing: Oceans are Dying (2010). Available at http://www.oceansentry.org/lang-en/overfishing/campaign.html (25 November 2011).

3. M. Floyd. Long-lived deep-sea fishes imperiled by technology, overfishing. (2007). Available at http://www.eurekalert.org/pub_releases/2007-02/osu-ldf021307.php# (25 November 2011).

4. Overfishing Fact Sheet (2011). Available at http://www.ypte.org.uk/environmental/over-fishing/29 (25 November 2011).

5. Overfishing: A Threat to Marine Biodiversity (2006). Available at http://www.un.org/events/tenstories/06/story.asp?storyID=800# (25 November 2011).

6. Bycatch – Wasteful and Destructive Fishing (2008). Available at http://www.greenpeace.org.uk/oceans/problems/bycatch-wasteful-and-destructive-fishing (27 November 2011).

7. Bottom Trawling (2008). Available at http://www.greenpeace.org.uk/oceans/problems/bottom-trawling (27 November 2011).

8. M.G. Richard. More than Half of Tuna Species Facing Extinction, But Over-Fishing Them is Too Profitable to Stop (2011). Available at http://www.treehugger.com/clean-technology/more-than-half-of-tuna-species-facing-extinction-but-over-fishing-them-is-too-profitable-to-stop.html (28 November 2011).

9. R. Gray, Overfishing and Dams Driving Freshwater Fish Towards Extinction (2011). Available at http://www.smh.com.au/environment/conservation/overfishing-and-dams-driving-freshwater-fish-towards-extinction-20110801-1i875.html. (28 November 2011).

10. A. Jha. Shark Species Face Extinction amid Overfishing and Appetite for Fins (2008). Available at http://www.guardian.co.uk/environment/2008/feb/18/conservation.aaas. (28 November 2011).

11. P. Eccleston. Atlantic Sharks Face Extinction Due to Overfishing and Shark-Finning (2008). Available at http://www.telegraph.co.uk/earth/earthnews/3418281/Atlantic-sharks-face-extinction-due-to-overfishing-and-shark-finning.html. (28 November 2011).

12. J. Bascompte, C. J. Melian, E. Sala, P. Natl. Acad. Sci. USA. 102, 5443-5447 (2005).

13. B. Holmes. Overfishing Eats Away at Genetic Diversity of Fish (2011). Available at http://www.newscientist.com/article/dn20699-overfishing-eats-away-at-genetic-diversity-of-fish.html (03 December 2011).

14. P. B. McIntyre, L. E. Jones, A. S. Flecker, M. J. Vanni, P. Natl. Acad. Sci. USA. 104, 4461-4466 (2007).

15. J. B. C. Jackson et al., Science 293, 629-637 (2001).