Wild fisheries

Marine and inland production

According to the Food and Agriculture Organization (FAO), the world harvest by commercial fisheries in 2010 consisted of 88.6 million tonnes of aquatic animals captured in wild fisheries, plus another 0.9 million tons of aquatic plants (seaweed etc.). This can be contrasted with 59.9 million tonnes produced in fish farms, plus another 19.0 million tons of aquatic plants harvested in aquaculture.

Lakes

Worldwide, freshwater lakes have an area of 1.5 million square kilometres. Saline inland seas add another 1.0 million square kilometres. There are 28 freshwater lakes with an area greater than 5,000 square kilometres, totalling 1.18 million square kilometres or 79 percent of the total.

Pollution

Pollution is the introduction of contaminants into an environment. Wild fisheries flourish in oceans, lakes, and rivers, and the introduction of contaminants is an issue of concern, especially as regards plastics, pesticides, heavy metals, and other industrial and agricultural pollutants which do not disintegrate rapidly in the environment. Land run-off and industrial, agricultural, and domestic waste enter rivers and are discharged into the sea. Pollution from ships is also a problem.

Plastic waste

Marine debris is human-created waste that ends up floating in the sea. Oceanic debris tends to accumulate at the centre of gyres and coastlines, frequently washing aground where it is known as beach litter. Eighty percent of all known marine debris is plastic - a component that has been rapidly accumulating since the end of World War II. Plastics accumulate because they don't biodegrade as many other substances do; while they will photodegrade on exposure to the sun, they do so only under dry conditions, as water inhibits this process.

Discarded plastic bags, six pack rings and other forms of plastic waste which finish up in the ocean present dangers to wildlife and fisheries. Aquatic life can be threatened through entanglement, suffocation, and ingestion.

Nurdles, also known as mermaids' tears, are plastic pellets typically under five millimetres in diameter, and are a major contributor to marine debris. They are used as a raw material in plastics manufacturing, and are thought to enter the natural environment after accidental spillages. Nurdles are also created through the physical weathering of larger plastic debris. They strongly resemble fish eggs, only instead of finding a nutritious meal, any marine wildlife that ingests them will likely starve, be poisoned and die.

Many animals that live on or in the sea consume flotsam by mistake, as it often looks similar to their natural prey. Plastic debris, when bulky or tangled, is difficult to pass, and may become permanently lodged in the digestive tracts of these animals, blocking the passage of food and causing death through starvation or infection. Tiny floating particles also resemble zooplankton, which can lead filter feeders to consume them and cause them to enter the ocean food chain. In samples taken from the North Pacific Gyre in 1999 by the Algalita Marine Research Foundation, the mass of plastic exceeded that of zooplankton by a factor of six. More recently, reports have surfaced that there may now be 30 times more plastic than plankton, the most abundant form of life in the ocean.

Toxic additives used in the manufacture of plastic materials can leech out into their surroundings when exposed to water. Waterborne hydrophobic pollutants collect and magnify on the surface of plastic debris, thus making plastic far more deadly in the ocean than it would be on land. Hydrophobic contaminants are also known to bioaccumulate in fatty tissues, biomagnifying up the food chain and putting great pressure on apex predators. Some plastic additives are known to disrupt the endocrine system when consumed, others can suppress the immune system or decrease reproductive rates.

Toxins

Apart from plastics, there are particular problems with other toxins which do not disintegrate rapidly in the marine environment. Heavy metals are metallic chemical elements that have a relatively high density and are toxic or poisonous at low concentrations. Examples are mercury, lead, nickel, arsenic and cadmium. Other persistent toxins are PCBs, DDT, pesticides, furans, dioxins and phenols.

Such toxins can accumulate in the tissues of many species of aquatic life in a process called bioaccumulation. They are also known to accumulate in benthic environments, such as estuaries and bay muds: a geological record of human activities of the last century.

Some specific examples are

Eutrophication

Eutrophication is an increase in chemical nutrients, typically compounds containing nitrogen or phosphorus, in an ecosystem. It can result in an increase in the ecosystem's primary productivity (excessive plant growth and decay), and further effects including lack of oxygen and severe reductions in water quality, fish, and other animal populations.

The biggest culprit are rivers that empty into the ocean, and with it the many chemicals used as fertilizers in agriculture as well as waste from livestock and humans. An excess of oxygen depleting chemicals in the water can lead to hypoxia and the creation of a dead zone.

Surveys have shown that 54% of lakes in Asia are eutrophic; in Europe, 53%; in North America, 48%; in South America, 41%; and in Africa, 28%. Estuaries also tend to be naturally eutrophic because land-derived nutrients are concentrated where run-off enters the marine environment in a confined channel. The World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, and East Asia, particularly in Japan. In the ocean, there are frequent red tide algae blooms that kill fish and marine mammals and cause respiratory problems in humans and some domestic animals when the blooms reach close to shore.

In addition to land runoff, atmospheric anthropogenic fixed nitrogen can enter the open ocean. A study in 2008 found that this could account for around one third of the ocean’s external (non-recycled) nitrogen supply and up to three per cent of the annual new marine biological production. It has been suggested that accumulating reactive nitrogen in the environment may have consequences as serious as putting carbon dioxide in the atmosphere.

Acidification

The oceans are normally a natural carbon sink, absorbing carbon dioxide from the atmosphere. Because the levels of atmospheric carbon dioxide are increasing, the oceans are becoming more acidic. The potential consequences of ocean acidification are not fully understood, but there are concerns that structures made of calcium carbonate may become vulnerable to dissolution, affecting corals and the ability of shellfish to form shells.

A report from NOAA scientists published in the journal Science in May 2008 found that large amounts of relatively acidified water are upwelling to within four miles of the Pacific continental shelf area of North America. This area is a critical zone where most local marine life lives or is born. While the paper dealt only with areas from Vancouver to northern California, other continental shelf areas may be experiencing similar effects.

Habitat destruction

Fishing nets that have been left or lost in the ocean by fishermen are called ghost nets, and can entangle fish, dolphins, sea turtles, sharks, dugongs, crocodiles, seabirds, crabs, and other creatures. Acting as designed, these nets restrict movement, causing starvation, laceration and infection, and—in those that need to return to the surface to breathe—suffocation.

Overfishing

Some specific examples of overfishing.

Loss of biodiversity

Each species in an ecosystem is affected by the other species in that ecosystem. There are very few single prey-single predator relationships. Most prey are consumed by more than one predator, and most predators have more than one prey. Their relationships are also influenced by other environmental factors. In most cases, if one species is removed from an ecosystem, other species will most likely be affected, up to the point of extinction.

Species biodiversity is a major contributor to the stability of ecosystems. When an organism exploits a wide range of resources, a decrease in biodiversity is less likely to have an impact. However, for an organism which exploit only limited resources, a decrease in biodiversity is more likely to have a strong effect.

Reduction of habitat, hunting and fishing of some species to extinction or near extinction, and pollution tend to tip the balance of biodiversity. For a systematic treatment of biodiversity within a trophic level, see unified neutral theory of biodiversity.

Threatened species

The global standard for recording threatened marine species is the IUCN Red List of Threatened Species. This list is the foundation for marine conservation priorities worldwide. A species is listed in the threatened category if it is considered to be critically endangered, endangered, or vulnerable. Other categories are near threatened and data deficient.

Marine

Many marine species are under increasing risk of extinction and marine biodiversity is undergoing potentially irreversible loss due to threats such as overfishing, bycatch, climate change, invasive species and coastal development.

By 2008, the IUCN had assessed about 3,000 marine species. This includes assessments of known species of shark, ray, chimaera, reef-building coral, grouper, marine turtle, seabird, and marine mammal. Almost one-quarter (22%) of these groups have been listed as threatened.

An ambitious project, called the Global Marine Species Assessment, is under way to make IUCN Red List assessments for another 17,000 marine species by 2012. Groups targeted include the approximately 15,000 known marine fishes, and important habitat-forming primary producers such mangroves, seagrasses, certain seaweeds and the remaining corals; and important invertebrate groups including molluscs and echinoderms.

Freshwater

Freshwater fisheries have a disproportionately high diversity of species compared to other ecosystems. Although freshwater habitats cover less than 1% of the world’s surface, they provide a home for over 25% of known vertebrates, more than 126,000 known animal species, about 24,800 species of freshwater fish, molluscs, crabs and dragonflies, and about 2,600 macrophytes. Continuing industrial and agricultural developments place huge strain on these freshwater systems. Waters are polluted or extracted at high levels, wetlands are drained, rivers channelled, forests deforestated leading to sedimentation, invasive species are introduced, and over-harvesting occurs.

In the 2008 IUCN Red List, about 6,000 or 22% of the known freshwater species have been assessed at a global scale, leaving about 21,000 species still to be assessed. This makes clear that, worldwide, freshwater species are highly threatened, possibly more so than species in marine fisheries. However, a significant proportion of freshwater species are listed as data deficient, and more field surveys are needed.

Fisheries management

A recent paper published by the National Academy of Sciences of the USA warns that: "Synergistic effects of habitat destruction, overfishing, introduced species, warming, acidification, toxins, and massive runoff of nutrients are transforming once complex ecosystems like coral reefs and kelp forests into monotonous level bottoms, transforming clear and productive coastal seas into anoxic dead zones, and transforming complex food webs topped by big animals into simplified, microbially dominated ecosystems with boom and bust cycles of toxic dinoflagellate blooms, jellyfish, and disease".