Fishing down the food web - AbsoluteAstronomy.com

The idea that fishing down the food web was happening on a global level was first proposed by the fisheries scientist Daniel Pauly

Daniel Pauly

Daniel Pauly is a French-born marine biologist, well-known for his work in studying human impacts on global fisheries. He is a professor and the project leader of the Sea Around Us Project at the Fisheries Centre at the University of British Columbia. He also served as Director of the Fisheries...

and others in an article published in the journal Science

Science (journal)

Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

in 1998. Large predator fish with higher trophic level

Trophic level

The trophic level of an organism is the position it occupies in a food chain. The word trophic derives from the Greek τροφή referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism...

s have been depleted in wild fisheries. As a result, the fishing industry

Fishing industry

The fishing industry includes any industry or activity concerned with taking, culturing, processing, preserving, storing, transporting, marketing or selling fish or fish products....

has been "fishing down the food web", targeting fish species with trophic levels that are progressively decreasing. The authors were criticized for the mismatch between the simplicity of the modelling approach and the categorical conclusions by Caddy et al. in a later Science issue the same year. A more elaborated review contradicting Pauly et al. appeared in 2010, indicating that the Mean Trophical Level (MTL) in the worlds oceans are stable or increasing
Over the last 50 years, the abundance of large predator fish, such as cod

Cod

Cod is the common name for genus Gadus, belonging to the family Gadidae, and is also used in the common name for various other fishes. Cod is a popular food with a mild flavor, low fat content and a dense, flaky white flesh. Cod livers are processed to make cod liver oil, an important source of...

, swordfish

Swordfish

Swordfish , also known as broadbill in some countries, are large, highly migratory, predatory fish characterized by a long, flat bill. They are a popular sport fish of the billfish category, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood...

and tuna

Tuna

Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

, has dropped 90 percent. Fishing vessel

Fishing vessel

A fishing vessel is a boat or ship used to catch fish in the sea, or on a lake or river. Many different kinds of vessels are used in commercial, artisanal and recreational fishing....

s now increasingly pursue the smaller forage fish

Forage fish

Forage fish, also called prey fish or bait fish, are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding...

, such as herring

Herring

Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...

s, sardine

Sardine

Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....

s, menhaden

Menhaden

Menhaden, also known as mossbunker, bunker and pogy, are forage fish of the genera Brevoortia and Ethmidium, two genera of marine fish in the family Clupeidae.-Description:...

and anchovies, that are lower on the food chain

Food chain

A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

. “We are eating bait and moving on to jellyfish and plankton” says Pauly. Beyond this, the overall global volume of fish captured has been declining since the late 1980s.

Mean trophic level

The mean trophic level is calculated by assigning each fish or invertebrate species a number based on its trophic level

Trophic level

. The trophic level is a measure of the position of an organism in a food web

Food web

, starting at level 1 with primary producers

Primary producers

Primary producers are those organisms in an ecosystem that produce biomass from inorganic compounds . In almost all cases these are photosynthetically active organisms...

, such as phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

and seaweed

Seaweed

Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

, then moving through the primary consumers at level 2 that eat the primary producers to the secondary consumers at level 3 that eat the primary consumers, and so on. In marine environments, trophic levels range from two to five for the apex predators. The mean trophic level can then be calculated for fishery catches by averaging trophic levels for the overall catch using the datasets for commercial fish landings.

Ecopath

Pauly's team used the catch data from the FAO

Fão

Fão is a town in Esposende Municipality in Portugal....

which it fed into an Ecopath

Ecopath

Ecopath with Ecosim is a free ecosystem modelling software suite. It was initially a NOAA initiative led by Jeffrey Polovina, but since primarily developed at the Fisheries Centre of the University of British Columbia. In 2007, it was named as one of the ten biggest scientific breakthroughs in...

model. Ecopath is a computerised ecosystem model

Ecosystem model

An ecosystem model is an abstract, usually mathematical, representation of an ecological system , which is studied to gain a deeper understanding of the real system.Ecosystem models are formed by combining known ecological relations An ecosystem model is an abstract, usually mathematical,...

ling system. The functioning of an ecosystem

Ecosystem

An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

can be described using path analysis to track the direction and influence of the many factors controlling the ecosystem. The original Ecopath model was applied to a coral reef

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

food web

Food web

. Scientists tracked tiger shark

Tiger shark

The tiger sharks, Galeocerdo cuvier, is a species of requiem shark and the only member of the genus Galeocerdo. Commonly known as sea tigers, tiger sharks are relatively large macropredators, capable of attaining a length of over . It is found in many tropical and temperate waters, and is...

s at the top of the food web and collected data on their feeding behaviour, what they ate and how much. Likewise, they collected feeding data on the other organisms in the food chains down to the primary producers, such as algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

. This data was fed into an Ecopath model, which then described the energy flow, in terms of food, as it moved from the primary producers up the food web to the apex predator. Such models allow scientists to compute the complex effects that occur, both direct and indirect, from the interactions of the many ecosystem components.

The model showed that over the last 50 years the mean trophic level of fish catches has declined by somewhere between 0.5 and 1.0 trophic levels. This decline applied both globally, on a world wide scale, and more locally on a scale specific to oceans, that is, for the separate FAO subareas: the Atlantic, Indian and Pacific Oceans, and the Mediterranean-Black Seas.

FAO critical rejoinder

Pauly's team argued in their 1998 paper that the larger, more valuable predatory fish

Predatory fish

Predatory fish are fish that predate upon other fish or animals. Some predatory fish include perch, muskie , pike, walleye, salmon.Levels of large predatory fish in the global oceans are estimated to be about 10% of their pre-industrial levels...

, such as tuna

Tuna

, cod

Cod

and grouper

Grouper

Groupers are fish of any of a number of genera in the subfamily Epinephelinae of the family Serranidae, in the order Perciformes.Not all serranids are called groupers; the family also includes the sea basses. The common name grouper is usually given to fish in one of two large genera: Epinephelus...

, had been systematically overfished, with the result that fishing effort was shifting to less desirable species further down the food chain. This "fishing down the food web," said Pauly, would in time reduce people to a diet of "jellyfish and plankton soup". The colourful language and innovative statistical modelling of Pauly's team triggered critical reactions. Later in the same year, Caddy and his team from the FAO

Fão

Fão is a town in Esposende Municipality in Portugal....

argued a counter position in a paper also published in Science. They argued that Pauly's team had oversimplified the situation and may have "misinterpreted the FAO statistics". The response of Pauly's team was published in the same paper, claiming that the corrections suggested by the FAO, such as accounting for aquaculture

Aquaculture

Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...

, actually made the trend worse.

The concerns raised by the FAO were further exhaustively countered by Pauly and others in 2005. Other researchers have established that "fishing down" also applies to smaller, regional areas, such as the Mediterranean, North Sea, Celtic Sea, and in Canadian, Cuban and Icelandic waters.

Marine Trophic Index

In 2000, the Convention on Biological Diversity

Convention on Biological Diversity

The Convention on Biological Diversity , known informally as the Biodiversity Convention, is an international legally binding treaty...

, an international treaty aimed at sustaining biodiversity

Biodiversity

Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

which has been adopted by 193 member countries, selected the mean trophic level of fisheries catch as one of eight indicators for immediate testing. They renamed it the "Marine Trophic Index" (MTI), and have mandated that member countries report over time on changes in ocean trophic levels as a primary indicator

Ecological indicator

Ecological indicators are used to communicate information about ecosystems and the impact human activity has on ecosystems to groups such as the public or government policy makers. Ecosystems are complex and ecological indicators can help describe them in simpler terms that can be understood and...

of marine biodiversity

Biodiversity

and health.

The "Marine Trophic Index" is a measure of the overall health and stability

Ecological stability

Ecological stability can refer to types of stability in a continuum ranging from resilience to constancy to persistence. The precise definition depends on the ecosystem in question, the variable or variables of interest, and the overall context...

of a marine ecosystem or area. The index is also a proxy

Proxy (statistics)

In statistics, a proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained...

measure for overfishing

Overfishing

Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....

and an indication of how abundant and rich the large, high trophic level fish are.

Changes in the Marine Trophic Index over time can function as an indicator

Ecological indicator

of the sustainability

Sustainability

Sustainability is the capacity to endure. For humans, sustainability is the long-term maintenance of well being, which has environmental, economic, and social dimensions, and encompasses the concept of union, an interdependent relationship and mutual responsible position with all living and non...

of a country’s fish resources. It can indicate the extent that the fishing effort within a country's fishing grounds is modifying its fish stock

Fish stock

Fish stocks are subpopulations of a particular species of fish, for which intrinsic parameters are the only significant factors in determining population dynamics, while extrinsic factors are considered to be insignificant.-The stock concept:All species have geographic limits to their...

s. A negative change generally indicates that larger predator fish are becoming depleted, and an increasing number of smaller forage fish

Forage fish

are being caught. A zero or positive change in the Marine Trophic Index indicates the fishery is stable or improving.

The ecology of fishing down

Ecologically, the decline in the mean trophic level is explained by the relationship between the size of the fish captured and their trophic level

Trophic level

. The trophic level of fishes usually increases with their size, and fishing tends to selectively capture the larger fishes. This applies both between species as well as within species. When the fishing is intense, the relative abundance of the larger fish positioned high in the food chain

Food chain

is reduced. Consequently, over time, small fishes start to dominate the fisheries catches, and the mean trophic level of the catches declines. Recently the market value of small forage fishes and invertebrates, which have low trophic levels, has sharply increased to the point where they can be considered to be subsidizing fishing down.

Daniel Pauly

Daniel Pauly

has suggested a framework for the ecological impacts fishing down can have on marine ecosystem

Marine ecosystem

Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine...

s. The framework distinguishes three phases:

Pristine – the first phase. Pristine environments are the states ocean ecosystem were in before fishing made strong impacts. Some outlier areas of the South Pacific may still be pristine. For most of the world what these pristine states may have been can only be inferred from archeological data, historical accounts and anecdotes. In pristine ocean environments, the biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...

of the large predator fish are 10 to 100 times greater than their present biomass. This implies a large supporting biomass of small prey fishes and invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s. On the seafloor, the benthos
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

is dominated by deposit feeders which prevent resuspension of sediments, and filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

s which keep the phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

down. Thus the water column tends to be oligotrophic, free of both suspended particle
Particle (ecology)
In marine and freshwater ecology, a particle is a small object. Particles can remain in suspension in the ocean or freshwater, however they eventually settle and accumulate as sediment. Some can enter the atmosphere through wave action where they can act as cloud condensation nuclei...

s and of the nutrients that leach from them.

Exploited – the second phase. Exploited is the phase we are currently in. It is characterized by declines in the biomass of large predator fish, declines in the diversity, size and trophic level of captured fish, and declines in the benthos. Bottom trawlers
Bottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...

progressively destroy the biogenic structures built over many years on the seafloor by the filter and detritus feeders. As these structures and animals that were filtering the phytoplankton and consuming the detritus (marine snow
Marine snow
In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he...

) disappear, they are replaced with the polyp stages of jellyfish and other small errant benthic animals. Storms resuspend the marine snow, and the water column gradually eutrophies
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...

. In the early part of this phase, cascade effect
Cascade effect
A Cascade Effect is an unforeseen chain of events due to an act affecting a system. If there is a possibility that the cascade effect will have a negative impact on the system, it is possible to analyze the effects with a consequence/impact analysis...

s compensate these declines with the emergence of new fisheries for opportunistic feeders, such as squid, shrimp and other invertebrates. But eventually these decline also.

Fully degraded – the third phase. The dead zone
Dead zone (ecology)
Dead zones are hypoxic areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated...

is the biological endpoint of a fully degraded marine ecosystem. The dead zone is a zone with excessive nutrients in the water column, resulting in the depletion of oxygen and the elimination of multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

s. The abundant detritus and marine snow is processed by bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

rather than by the benthic animals. These dead zones are currently growing throughout the world in places such as the Bohai Sea
Bohai Sea
Bohai Sea , also known as Bohai Gulf, Bohai, or Bo Hai, is the innermost gulf of the Yellow Sea on the coast of Northeastern and North China. It is approximately 78,000 km2 Bohai Sea , also known as Bohai Gulf, Bohai, or Bo Hai, is the innermost gulf of the Yellow Sea on the coast of...

in China, the northern Adriatic Sea
Adriatic Sea
The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula, and the system of the Apennine Mountains from that of the Dinaric Alps and adjacent ranges...

, and the northern Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

. Some estuaries, such as the Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

estuary, also display features associated with a fully degraded marine ecosystem. In Chesapeake Bay, overfishing eliminated the benthic filter feeders such as oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

s, and most predators larger than a striped bass
Striped bass
The striped bass is the state fish of Maryland, Rhode Island, South Carolina, and the state saltwater fish of New York, Virginia, and New Hampshire...

, the current apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...

. One hundred and fifty years ago, the oysters formed giant reefs and filtered the waters of Chesapeake Bay every three days. Because the oysters are gone, pollution entering the estuary from rivers now produces harmful algal blooms.

Fishing through the web

Essington† TE, Beaudreau AH, Wiedenmann J (2006) "Fishing through marine food webs" PNAS, 103 (9): 3171-3175.
Shopping list gets longer -- not less choosy -- in some of world's largest fisheries University of Washington news, February 14, 2006.
National Research Council (Authors) (2006) Dynamic Changes in Marine Ecosystems: Fishing, Food Webs, and Future Options National Academies Press
National Academies Press
National Academies Press was created by the United States National Academies, to publish the reports issued by the National Academy of Sciences, the National Academy of Engineering, the Institute of Medicine, and the National Research Council. It publishes nearly 200 books a year on a wide range...

. ISBN 978-0-309-10050-2

Farming up the web

While the mean tropic level in wild fisheries has been decreasing, the mean tropic level amongst farmed fish has been increasing.

Herbivores	Intermediate predators	Top-level predators
Production trends across different trophic levels in the Mediterranean
The production of bivalves, such as mussel Mussel The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The... s and oysters, increased from about 2,000 tonne Tonne The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI... s in 1970 to 100,000 tonnes in 2004	The mariculture Mariculture Mariculture is a specialized branch of aquaculture involving the cultivation of marine organisms for food and other products in the open ocean, an enclosed section of the ocean, or in tanks, ponds or raceways which are filled with seawater. An example of the latter is the farming of marine fish,... of species such as sea bream increased from 20 tonnes in 1983 to 140,000 tonnes in 2004	The production of the bluefin tuna has gone from almost zero in 1986 to 30,000 tonnes by 2005

As an example, the table above shows trends in the trophic levels of fish farmed in the Mediterranean. However, the farming of bluefin tuna is restricted to a fattening process. Juvenile tuna are captured from the wild and put in pens for fattening. Wild stocks of bluefin are now threatened, and the fisheries scientist Konstantios Stergiou and colleagues argue that the “fact that the capacity of tuna farms greatly exceeds the total allowable catch indicates lack of conservation planning in development of the tuna-fattening industry, which, ideally, should have been linked to fisheries management policies, and may lead to illegal fishing."

Also, fish farming in the Mediterranean is a net fish consumer. Large amounts of animal feed is needed to feed a high trophic fish like the bluefin tuna. This feed consists of fishmeal processed from forage fish

Forage fish

es like sardine

Sardine

s and anchovies

Anchovy

Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...

that humans would otherwise consume directly. In addition to ecological issues, this raises ethical issues. Much of the fish suitable for direct human consumption is being used to grow higher trophic level fish to indulge a relatively small group of affluent consumers.

External links

Fishing down food chain' fails global test Scientific American, 10 November 2010.

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Mean trophic level

Ecopath

FAO critical rejoinder

Marine Trophic Index

The ecology of fishing down

Fishing through the web

Farming up the web

See also

External links