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Overfishing is a situation where one or more fish stocks are reduced below predefined levels of acceptance by fishing activities. More precise definitions are provided in biology and bioeconomics. Biological overfishing occurs when fishing mortality has reached a level where the stock biomass has negative marginal growth (slowing down biomass growth). Economic or bioeconomic overfishing in addition to the biological dynamics takes into consideration the cost of fishing and defines overfishing as a situation of negative marginal growth of resource rent. A more dynamic definition may also include a relevant discount rate and present value of flow of resource rent over all future catches.

Overfishing therefore may be sustainable, but in a non preferable way. Ultimately overfishing may however lead to depletion in cases of subsidised fisheries, low biological growth rates, critical biomass levels, etc.

Glaring examples exist of the outcomes from overfishing in areas like the North Sea and the East Coast of Canada and the United States. The result has been not only disastrous to fish stocks but also to the fishing communities relying on the harvest.

The ability for nature to restore the fisheries is also dependent on whether the ecosystems are still in a state to allow fish numbers to build again. Dramatic changes in species composition may establish other equilibrium energy flows which involve other species compositions than before.
Overfishing can do more than endanger our food supply. Some scientists say it causes widespread changes in ocean ecology -- degradation of reefs, destruction of bottom grasses, eutrophication of estuaries. Although fertilizer, runoff, sedimentation and exotic species are destroying ecosystems, they argue that overfishing came first -- and if it's controlled, ecosystems often recover. 

fishery: the organized harvest of a certain species of fish or shellfish. We speak of "the Monterey Bay squid fishery" to mean all the squid caught in Monterey Bay.

Overfishing
There's a Limit to Fish in the Sea

Ocean fish are wildlife—the last wild creatures that people hunt on a large scale. Once it seemed the ocean would supply an endless bounty of seafood. Today, we're discovering its limits. Between 1950 and 1994, ocean fishermen increased their catch 400% by doubling the number of boats and using more effective fishing gear. In 1989, the world's catch leveled off at just over 82 million metric tons of fish per year. That's all the ocean can produce. Sending more boats won't help us catch more fish.

Fisheries boom, then bust
Overfishing means catching fish faster than they can reproduce. Overfishing pushes the fish population lower and lower, until fish are so few that fishermen can't make a living any more. Many fisheries have already collapsed, throwing thousands of people out of work. All over the world, fishery after fishery booms as we send in more boats, then busts as the fish population crashes.

Chilean seabass live at least 40 years, orange roughy at least 100. A Pacific rockfish caught in 2001 was 205 years old—born when Washington was still president! Such slow-growing fishes are very vulnerable to overfishing; choose seafood from our Green List instead.
Off New England, cod were once so plentiful that boats had trouble pushing through them. Now the cod are nearly gone, and a centuries-old fishing tradition is ending. Other overfished species include Atlantic swordfish, Atlantic bluefin tuna and many kinds of West Coast rockfish. When one kind of fish is no longer plentiful, fishermen must move on to new species. Monkfish and sharks were once discarded as "trash fish," but now they're valuable—and are themselves overfished! Overfishing has also forced fishermen to look deeper for new species like orange roughy and Chilean seabass.


Around the world, people are eating more seafood than ever before. Demand is increasing due to growing populations, and because health-conscious consumers are choosing seafood more often. To help supply the global demand for seafood, people are raising fish, shrimp and oysters like farmers raise cattle and chickens. Today, almost 20% of our seafood comes from farms. The ecological impact of fish farming depends on which species are raised, how they are raised and where the farm is located.

Fish farms depend on wild fish
While farmed fish and shellfish can supplement our seafood supply, they can't replace the variety and abundance of seafood from the wild. Most seafood farms depend on healthy wild populations to supply eggs or young that the farmers raise for market. Many fish farms also depend on wild fish, like anchovies, as food for the farmed fish.

Farmed shellfish are a Best Choice
Most oysters on the U.S. market, and many of the clams and mussels, are farm-raised. These shellfish filter tiny plankton out of the water for their food, so they need no supplemental feeding. Shellfish can even improve water quality as they clear the water of excess plankton. And because shellfish for human consumption must come from clean water, shellfish farming often spurs efforts to keep coastal waters clean.

Clams are raised in special beds on sandy shores, where their harvest does little to disturb the ecosystem. Oysters and mussels are often raised in bags or cages suspended off the seafloor, doing little damage as they're harvested.

Net-pen farming can be a messy business
Many farmed fish, including most farmed salmon, are raised in net pens, like cattle in a feed lot. Thousands of fish concentrated in one area produce tons of feces, polluting the water. Diseases can spread from fish in the crowded pens to wild fish. Antibiotics and other drugs used to control those diseases leak out into the environment, creating drug-resistant disease organisms. And if farmed fish escape their pens, they can take over habitat from wild fish in the area. While the U.S. has laws to protect the environment around coastal fish farms, many nations that supply farmed fish to U.S. markets do not.

Shrimp farming can harm the coast
In Thailand, Ecuador and many other tropical nations, coastal forests of mangroves once sheltered wild fish and shrimp, which local people caught to feed their families. Mangroves also filter water and protect the coast against storm waves. Many mangrove forests have been cut down and replaced with shrimp farms that supply shrimp to Europe, Japan and America. After a few years, waste products build up in the farm ponds and the farmers have to move on. The local people are left with no shrimp farms—and no mangrove forest.

Far from the sea may be best
The best way to raise fish may be inland, far from coastal waters where wild fish feed and breed. Tilapia, a plant-eating fish, is easy to raise, and produces protein for people without using wild fish as feed. Catfish and trout are raised inland in the United States. All of these fish can be delicious alternatives to ocean-farmed shrimp and salmon. Even shrimp and salmon farming can be moved inland, where wastes are easier to handle. U.S. shrimp farmers are experimenting with enclosed, recirculating systems that filter wastewater and can be located far from the coast.
Monterey Bay Aquarium


THE THREATS TO OUR OCEAN FISHERIES:
Overfishing, Bycatch and Marine Habitat Destruction

Most everyone is familiar with the plight of the great whales, efforts to save endangered sea turtles, and the tragedy of dolphins dying in nets set for tuna. Less well known is that tuna and many other species of marine fish are in deep trouble, too. In fact, an alarming decline in fish populations poses a more disturbing and potentially more dangerous threat to life in the ocean. As fish decline, so does the sea, into a biologically unproductive and unstable environment. Strong conservation measures, and broad-based public support for implementing them, are badly needed.

Once man thought the seas held an endless supply of fish and couldn’t be destroyed. We may no longer believe that, but we continue to behave as if it were true. For decades now we’ve been taking fish from the sea much faster than it can replace them, with dire consequences. According to the National Marine Fisheries Service, 90 fish species found off the shores of the U.S. have been depleted. Many are in danger of being wiped out. Fish and shellfish at risk include bluefin tuna, cod, flounder, swordfish, blue marlin, Atlantic lobster, red snapper, salmon and a number of sharks, to name just a few from a long list that grows longer every year.

The numbers of most species of marine fish are at an all-time low, and the chief culprit is overfishing to meet an unprecedented demand for seafood. Modern, technologically-advanced fishing fleets have the capacity to push most fish populations to the brink. In addition to what is harvested, over 20 million tons of fish and other marine animals - about one-fourth of the global catch - are killed and discarded yearly by fishermen using huge nets, multi-mile longlines and other indiscriminate gear. Some fleets throw away more fish than they keep. This wasted "bykill" is a problem in almost every fishery.

When fish populations decline or collapse, everyone is the poorer. It decreases the supply of protein available from the sea, causes substantial losses to the economy, brings hardship to fishermen, disrupts traditional ways of life and limits recreational opportunities.

The immediate threat posed by overfishing is aggravated by the long-term threat of large-scale changes to marine ecosystems. Most salt water fish spend all or at least part of their lives in coastal waters, where their environment is continually assaulted by pollution and development. The massive destruction of wetlands and other vital habitats directly reduces the number of fish the ocean can support. Without healthy, properly functioning coastal ecosystems, fish cannot grow and reproduce – in a word, they can’t survive.

Just as ominous are unforeseen and possibly permanent changes in the ocean food chains caused by overfishing. Overkill at the top – widespread depletion of the ocean’s apex predators, the sharks, tunas and billfishes – can upset predator-prey relationships that took millions of years to evolve. Depleting fish populations can alter and diminish the genetic and species diversity of the ocean world.
National Coalition for Marine Conservation 

Once vibrant wildernesses teeming with diverse marine life, many of today's ocean ecosystems face the devastating effects of overfishing. A number of species have gone ecologically extinct-causing dramatic changes in ocean ecosystems. Rebuilding overfished species will actually increase the amount of fish that can be caught sustainably.

Overfishing occurs when fish are caught faster than they can reproduce. . Many marine scientists now believe that overfishing is the biggest human impact on the world's oceans. A recent study in the prestigious journal Science showed that overfishing makes ocean ecosystems more vulnerable to harm from other human impacts like pollution.

Evidence of overfishing abounds throughout U.S. waters, including the near-disappearance of fish that were once abundant, and the shrinking sizes of average-sized fish. Today, many fish are caught before they are old enough to reproduce.

Overfishing probably contributes to declines of marine birds and mammals, by reducing their food supplies. Depletion of fish populations is actually an accepted goal for most fishery managers. Fishermen are encouraged to achieve maximum exploitation by "fishing down" populations to about half their original size. Such goals ignore the role of fish as an integral part of marine food webs. 
Ocean Conservancy


The Overfishing Dilemma

Once regarded as an unlimited resource, the oceans of the world have fallen prey to destructive fishing practices. Once a colorful underwater jungle full of diversity, many of today's oceans are feeling the effects of overfishing, which occurs when fish are caught faster than they can reproduce. Many of the most abundant fishing waters are now void, victims of regional extinction.

The results of overfishing and irresponsible fishing practices:



Reduction of food supplies for marine mammals and other sea life.


Fish can no longer be found in once abundant habitats and fishing zones


Shrinking of the average fish size (many fish are now being harvested before they reach breeding age and/or size)

Disruption of the food chain (The ocean ecosystem is a delicate balance in which all species play an important role in its success. Eliminating crucial pieces of this ecosystem affects the overall health and safety of our oceans. In fact, the impact may not be realized until it is too late to correct the situation.)

The importance of responsible fishing practices, and the rebuilding of overfished species provides the following:



Improves the health of the oceans


Increases sustainable fish stocks


Increases the number of fish that can be harvested with out significantly damaging populations


Protects the livelihoods of fishermen and those dependent on the fishing industry


Protects ocean ecosystems from other damaging human impacts (i.e. pollution)


Ensures the survival of marine birds and mammals

How You Can Help!

Sea Food Surveillance: Buying safe seafood is a simple way to tell the fishing industry you will not support unsafe fishing practices and support the protection of marine mammals and sea life. Visit our Sea Food Surveillance and learn which seafood choices are ecologically responsible.
Endangered Creatures, Habitats & Organisms Fund, Inc.

Centuries of overfishing by man have emptied the world's oceans of giant fish, whales and other large sea creatures, destroying coastal environments.


It hasn't been generally understood how much has been lost and what might be regained

Jeremy Jackson, report co-author
So says a report by 14 universities and international scientific bodies, which takes a historical perspective on the declining health of the sea.

It paints a picture of coasts once teeming with herds of walrus-like mammals, tens of millions of sea turtles and shoals of giant cod.

Today, whales, manatees, sea cows, monk seals and many other large animals have disappeared altogether in many waters.

Changing oceans

The key cause, the authors say, is human exploitation of marine bounty from prehistoric times until the present day.

"Virtually all coastal ocean communities were dominated by very large populations of very large animals," said Jeremy Jackson, lead author of the report. "What you would have seen around the coast would have been dramatically different from what you see today."

Chesapeake Bay BBC
Chesapeake Bay: The whales and large sharks have gone
It is the virtual disappearance of key marine creatures such as cod, oysters and sea turtles that has led to the recent collapse of coastal ecosystems, he told BBC News Online.

Professor Jackson, of the Scripps Institution of Oceanography, University of California, San Diego, US, brought together a team of historians, palaeontologists, archaeologists and biologists to create a historical view of the oceans.

They investigated fossil records, historical archives, archaeological data and ecological studies from the past century to track ecological changes.

Giant turtles

The study suggests that there were once 40 million giant green turtles in the Caribbean alone, while estuaries such as Chesapeake Bay in North America would have been teeming with whales and large sharks.

Shellfish were in such abundance that they interfered with navigation and there were enough oyster reefs in Chesapeake Bay to filter all the water in three days.

Whales BBC
People are not aware of how much has been lost, say researchers
"In Maine, we have evidence from Indian middens dating back 5,000 years that our coastal zone was dominated by large predatory fish such as cod," said co-author Robert Steneck from the University of Maine Darling Marine Center, Walpole, US.

"The average size of cod for thousands of years was about a metre long," he added, "which is impressive considering the fishers used crude hooks made of deer bone with line made of deer intestines."

The researchers say that overfishing precedes pollution, destruction of habitats, disease, and human-induced climate change. This historical perspective could suggest new goals for coastal management and restoration, they add.

"It hasn't been generally understood how much has been lost and what might be regained," said Professor Jackson.

The research is published in the journal Science. 

Overfishing Long Ago Tied to Modern Ecosystem Collapse

Hillary Mayell
for National Geographic News
August 7, 2001

Overfishing that took place hundreds if not thousands of years ago is a key culprit in the collapse of coastal marine ecosystems today, an international group of researchers reports.

Up until now, scientists have tied the current collapse of the world's coastal ecosystems almost entirely to recent human impacts—pollution, increased nutrient runoff, and climate change.

Fishing Boat

Fishing Boat in Clarence
Strait, Alaska

A fishing boat trawls for salmon in Ketchikan, Alaska.

Photograph by Kevin Fleming/CORBIS
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By looking at historical evidence, the researchers were able to draw a picture of ancient oceans teeming with life in an abundance heretofore unimagined.

The picture today is dramatically different: dying coral reefs, dwindling populations of marine mammals, fish, and shellfish, shrinking seagrass beds, increased invasions of alien species, noxious algal blooms, and more virulent and frequent outbreaks of disease.

This current state of affairs can be attributed at least in part to the actions of aboriginal coastal populations, say the authors of the two-year study, which was published August 3 in Science.

"Up until now weve been attributing the collapse of coral reef ecosystems to pollution and global warming," said co-author Karen Bjorndal, a marine ecologist at the University of Florida's Archie Carr Center for Sea Turtle Research. "While that's certainly a factor and something society must address, ecosystem collapse was set in motion long before modern activities contributed."

The authors hope their work will help reorient current conservation and restoration practices away from quota systems and no-fishing zones to a more broad-based ecosystem approach.

"We need to change the way we think about our coastal seas—not pristine, but damaged, and equally not hopeless, but salvageable," said co-author Roger Bradbury of the Australian National University in Canberra.

Long-Term Domino Effect

In their report, the authors note that large marine vertebrates—whales, manatees, dugongs, sea cows, monk seals, crocodiles, codfish, jewfish, swordfish, sea turtles, sharks, and rays—are now functionally or entirely extinct in most coastal marine ecosystems.

They found that the depletion of these species through overfishing and overharvesting sets off a domino effect that can have impacts even centuries later.

To draw a picture of what marine ecosystems looked like eons ago, the 19 researchers who contributed to the study examined marine sediment evidence from about 125,000 years ago, archaeological information from early human coastal settlements some 10,000 years ago, and European trade records from the 15th century to the present.

In every case they looked at, overfishing by humans preceded ecosystem collapse.

Removing important marine species has a profound effect on the food chain, which ultimately leads to ecosystem breakdown, the authors say. The impacts of historic depopulation of sea turtles is one example.

"The accepted wisdom among sea turtle researchers has always been that sea turtle stocks were in pretty good shape when Columbus arrived, and that it wasn't until the Europeans started to arrive that the populations began to crash," Bjorndal explained.

When the scientists examined archaeological evidence of coastal Amerindian settlements, they found that sea turtles were an important food source for the people who lived there. Over the 100 to 200 years that followed, the amount of turtle remains found in ancient trash dumps diminished until there were no more traces of sea turtles as a food source.

The findings challenge a common assumption held by marine biologists that the consumption or use of a species by indigenous groups generally has a negligible or strictly localized impact.

"We had always thought that the impact of subsistence-level fishing would be limited to a local area," said Bjorndal. "But sea turtles travel long distances to forage for food and then return to their nesting site. By overharvesting the species at a local level, the Amerindians had a region-wide impact on the ecosystem."

Now, several hundred years later, the depletion of sea turtle populations is having a profound effect on the health of coral reefs in the Caribbean, Bjorndal said.

Sea turtles were one of many species that controlled the growth of algae. Other algae-eating species were also slowly eliminated over time, until only the sea urchin remained. In the 1980s, sea urchin populations plummeted following a well-documented outbreak of disease.

With no plant-grazing species left, the reefs were swamped by an overgrowth of algae, which killed many corals and prevented new ones from growing.

Common Pattern

The researchers found the same pattern in all the other cases they studied.

Co-author Jim Estes, a research ecologist with the Western Ecological Research Center in Santa Cruz, California, described the chain of events that occurred in the North Pacific after aboriginal Aleuts greatly reduced sea otter populations starting about 2,500 years ago.

Sea otters are the major predators of sea urchins. As their major predators were removed from the ecosystem, sea urchin populations soared.

Overgrazing by the sea urchins eventually killed off the kelp beds, resulting in changes in wave action, water quality, and siltation rates. These changes, in turn, had a major impact on other near-shore flora and fauna.

In another instance, the Chesapeake Bay now has vast areas in which algae is so abundant that the level of oxygen in the water is inadequate to support other organisms. The authors tie this process, known as eutrophication, in part to the collapse of oyster populations caused by overfishing in the 19th century.

Management Implications

"Our study shows that marine ecosystem collapse is not entirely due to recent factors, and that to really understand what's happening we need to view the problem in its proper historical perspective," said Bjorndal.

Estes said the emphasis on recent human activities as the cause of ecosystem collapse may have arisen in part because ecological data on coastal marine systems has only been collected and studied since the 1950s. As a result, researchers' insight is limited mainly to the recent structure and function of ecosystems.

The new report points to the need to manage marine ecosystems for long-term effects and not just immediate problems.

"For ecosystem restoration and management to be effective, we need to go back far enough in time to truly identify the problems and set our goals appropriately," said Bjorndal.

The study also highlights the importance of maintaining the biodiversity of an ecosystem, she added.

"It's not enough to bring back one species," said Bjorndal.

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"In a healthy marine ecosystem, there are a suite of animals that fill the same ecological niche," she said. "The loss of redundancy—where numerous species fill the same environmental niche—leaves an ecosystem extremely vulnerable." 
National Geographic

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