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Submarine acoustic techniques

2026-04-27T15:19:43Z

Dronkers J: Created page with "==Introduction== File:Singlebeam.gif|thumb|right|350px|Figure 1: Singlebeam. The transducer emits sound (blue cone), which is reflected (white rings) and partly received ag..."

==Introduction==

[[File:Singlebeam.gif|thumb|right|350px|Figure 1: Singlebeam. The transducer emits sound (blue cone), which is reflected (white rings) and partly received again.
<ref name="fig1">http://www.divediscover.whoi.edu/tools/sonar-singlebeam.html</ref>]]

In water, sound can travel much farther than light or any other form of radiation because it is only minimally absorbed by the surrounding medium. For this reason, sound is the primary means of remote sensing and imaging underwater. Instruments that use sound to identify objects in the water column and to determine depth are called SONARs (SOund NAvigation and Ranging).<ref name="een">http://books.mcgraw-hill.com/EST10/site/spotlight/underthesea/pdf/EST_Remote_sensing_%20of_%20fish_YB.pdf</ref>

There are two types of SONAR: passive and active.<ref name="twee">http://www.nauticalcharts.noaa.gov/hsd/SSS.html</ref> Passive SONAR systems listen to sounds produced by animals (e.g. whales) or objects (e.g. ships and submarines). Active SONAR systems generate specific sound waves themselves and then analyze their reflection (echo). These “echo sounders” have more diverse and complex applications and are therefore discussed further here.

The four main acoustic underwater techniques are the singlebeam and multibeam echosounders, the side-scan sonar, and OAWRS (Ocean Acoustic Waveguide Remote Sensing). All consist of a projector that generates the sound waves and a signal receiver or hydrophone that receives the echo. When the transmitter can also receive, it is called a transducer.<ref name="³">Makris, N. C., Jagannathan, S. and Ignisca, A. 2010. Oceanography. Ocean Acoustic Waveguide Remote Sensing: Visualizing Life Around Seamounts 23, p2.</ref>

Based on either the travel time or the energy of the reflected waves, depth or material properties can be determined, respectively. The characteristics of SONAR systems are partly determined by the transmitted frequency. Lower sound frequencies undergo less absorption and therefore travel farther than higher frequencies. Although lower frequencies allow a larger area to be monitored, this is usually accompanied by a loss of image quality.

The simplest technique is the singlebeam echosounder, developed at the beginning of the 20th century. The development of this and subsequent SONAR systems is largely due to military research, as they are ideal tools for detecting submarines and mines. In the scientific field, they are used, among other things, for producing bathymetric maps of seabed topography and for studying fish populations and dynamics.

Fish can be detected because of their reflective properties. Most fish have a swim bladder, which can scatter more than 90% of the incident acoustic energy. Some species, such as Atlantic mackerel, do not have a swim bladder but still reflect sound through their bones and muscle mass. Fish without a swim bladder therefore produce weaker echoes.

==Singlebeam echosounder==
Until the beginning of the 1960s, singlebeam echosounders (Figure 1) were mainly used for depth measurements.<ref name="²">http://www.ldeo.columbia.edu/res/pi/MB-System/sonarfunction/SeaBeamMultibeamTheoryOperation.pdf</ref> This system uses a single vertically directed acoustic pulse (“ping”), comparable to a searchlight. The transducer receives part of the echo, from which the depth is calculated based on the travel time of the pulse.

The echosounder is towed back and forth by a ship to survey or analyze the seabed over a larger area. The beam can be wide (“wide beams”), allowing a relatively larger surface area to be covered, but at the expense of image quality. More expensive “narrow beams” provide a clearer image but cover a smaller zone. Partly because one system provides too low an image resolution and the other covers too small an area, the singlebeam method is not efficient enough for scanning entire regions. The method has limitations in both time and space, meaning that only a partial picture can be obtained of, for example, fish dynamics and abundance.<ref name="³"/>

==Multibeam echosounder==

[[File:Multibeam.jpg|thumb|right|350px|Figure 2: Multibeam. The transducer emits sound in several adjacent individual beams.<ref name="fig2">http://annaroseandthesea.blogspot.be/</ref>]]

Around 1960, the multibeam echosounder (Figure 2) was developed. It consists of multiple single narrow beams arranged together. The transducer is positioned on the underside of the ship in such a way that it produces a fan-shaped array of sound waves. As a result, the seabed is scanned by a continuous line of measurement points perpendicular to the sailing direction of the ship.<ref name="³"/> The width of this line on the seabed is called the swath width, which can be expressed either in meters or as the angular width in degrees.

The transducer measures both the travel time and the intensity difference between transmission and reception, allowing it to determine depth and seabed characteristics, respectively. Using computer processing, the seabed topography can then be visualized. Depth is usually represented using color codes, where blue areas are deeper than red areas.
The intensity of the reflected signal provides information about the hardness, texture, and morphology of the seabed. A flat, hard surface reflects more acoustic energy than a soft, uneven substrate such as sand. In this way, the multibeam system can classify the seabed into different categories. To relate these acoustic classes to the actual seabed type, physical sediment samples must be collected for verification.<ref name="multi4">http://www.sgmeet.com/osm2012/viewabstract2.asp?AbstractID=12875</ref><ref name="multi2">http://www.infomar.ie/data/DataProcessing.php</ref>

==Side-scan sonar==

[[File:Sidescan.jpg|thumb|right|350px|Figure 3: Side-scan sonar. Two transducers emit sound waves from a towfish.
<ref name="fig3">http://gralston1.home.mindspring.com/Sidescan.html</ref>.]]

Unlike singlebeam and multibeam echosounders, a side-scan sonar (Figure 3) is used primarily to determine the composition of the seabed rather than its depth. This is possible because different materials have specific sound-absorbing and sound-reflecting properties. Some materials, such as metal and newly formed volcanic rock, are highly reflective, whereas others, such as clay and silt, reflect much less sound. Strong reflectors produce strong echoes with high energy (high amplitude), and vice versa.<ref name="²"/>

The amount of reflected energy can be measured, and with knowledge of the acoustic properties of common materials, an image of the seabed texture can be formed. The result is a sonogram: an image with a two-color gradient that represents the energy spectrum of the reflected signal. The best results are obtained in calm seas and when the survey vessel follows a straight course. <ref name="drie">http://www.abc.se/~pa/mar/sidescan.htm</ref>

The system consists of two transducers mounted on either side of the device, allowing measurements to be made both to the left and right of the ship. As with a multibeam system, a line is measured perpendicular to the ship’s direction of travel. Unlike singlebeam or multibeam systems, the side-scan sonar is usually mounted on a towfish rather than on the ship’s hull. A towfish is a torpedo-shaped carrier that can be towed close to the seabed. It is also possible to mount the sonar on submersibles or on autonomous underwater vehicles (AUVs).

Because side-scan sonar usually provides little information about depth, while multibeam systems provide less detailed information about composition, the two techniques are often used together as complementary methods.<ref name="twee"/>

==Ocean Acoustic Waveguide Remote Sensing (OAWRS)==

[[File:OAWRS.png|thumb|right|450px|Figure 4: Schematic representation of the OAWRS system used in the Gulf of Maine in 2006.<ref name="³"/>]]

Ocean Acoustic Waveguide Remote Sensing (OAWRS) is a recent development for the near-instantaneous imaging and continuous monitoring of fish populations on a very large scale.<ref name="°">Jagannathan, S., Bertsatos, I., Symonds, D., Chen, T., Nia, H. T., Jain, A. D., Andrews, M., Gong, Z., Nero, R., Ngor, L., Jech, M., Godo, O. R., Lee, S., Ratilal, P. and Makris, N. 2009. Ocean Acoustic Waveguide Remote Sensing (OAWRS) of marine ecosystems. Marine Ecology-Progress Series. 395: 137-160.</ref> The surveyed area can cover thousands of square kilometers, which is at least ten thousand times larger than what is possible with conventional techniques. OAWRS is rarely used for seabed mapping; instead, it is mainly applied to locate distant populations of fish or other organisms, such as krill. The technique provides information on the horizontal spatial distribution of populations, their behavior, and fish abundance. With more traditional survey methods, the research vessel often comes so close to the organisms that their normal behavior is disturbed, leading to a less realistic representation of natural conditions. The large spatial scale of OAWRS eliminates this unwanted effect.<ref name="°"/><ref name="³"/>

This acoustic underwater technique was developed by the Massachusetts Institute of Technology (MIT) in Cambridge, United States. In 1995, submarine mountains and ridges were successfully visualized. The first successful test for fish population research took place in 2003 along the east coast of the United States. In a single second, an area larger than ten thousand square kilometers was analyzed. A second successful study was carried out in 2006 on Georges Bank, where the schooling behavior of herring during spawning was investigated.

Acoustic signals are transmitted by several vertically suspended sources. These signals are reflected by both the sea surface and the seabed, creating so-called waveguide modes, or vertical standing waves, that span the full water depth. Because the waves spread outward in a circular pattern from their source, a large three-dimensional area can be scanned.
The frequencies used are close to the resonance frequency of the swim bladder, which causes most fish with swim bladders to produce strong echoes. These echoes are received by an array of receivers towed behind a ship.

In the future, it may become possible to use fixed systems instead of ship-based systems, allowing continuous long-term monitoring.<ref name="°"/>

OAWRS is considered a highly promising technique because it can provide a more accurate picture of fish dynamics and abundance at regional scales.<ref name="³"/> However, for high-resolution surveys, conventional fish-finding sonars (CFFS)—such as singlebeam, multibeam, and side-scan sonar systems—will remain necessary.<ref name="een"/>

Recent studies have also shown that the sound waves produced by OAWRS may affect whale behavior, indicating that potential ecological impacts must be carefully considered.<ref name="+">Risch, D., Corkeron, P. J., Ellison, W. T. and Van Parijs, S. M. 2012. Changes in Humpback Whale Song Occurrence in Response to an Acoustic Source 200 km Away. Plos One 7(1): e29741</ref>

==Related articles==
:[[General principles of optical and acoustical instruments]]
:[[Instruments for bed level detection]]
:[[Acoustic monitoring of marine mammals]]
:[[Marine mammals' health as an indicator of ecosystem health - tools for monitoring]]

==External links==
http://en.wikipedia.org/wiki/Multibeam_echosounder
http://nl.wikipedia.org/wiki/Side_scan_sonar
http://nl.wikipedia.org/wiki/Singlebeam

==References==
<references/>

{{author
|AuthorID=588
|AuthorFullName=Van Beveren, Elisabeth
|AuthorName=Elisabeth Van Beveren}}

[[Category:Coastal and marine observation and monitoring]]

Mud consolidation and desiccation

2026-04-27T09:18:22Z

Dronkers J:

A layer of freshly deposited mud will consolidate under its own weight as pore water is gradually expelled upward to the free surface (and sometimes downward into the underlying sediment, a case not considered here). As consolidation proceeds, the mud layer becomes denser and its thickness decreases. Desiccation of mud occurs when the expelled water evaporates into the air.

Consolidation of freshly deposited mud is a slow process. The consolidation time is approximately proportional to the square of the thickness of the mud layer. Thin layers thus consolidate much faster than thick layers.

==Poro-elastic consolidation model==

[[File:MudConsolidation.jpg|thumb|350px|right|Fig. 1. Theoretical mud consolidation curve (solid line). From Verruijt (2001<ref name=V1>Verruijt, A. 2001. Soil Mechanics. Lecture notes Technical University Delft</ref>). The dotted line represents the approximated Eq. (3) for <math>t_v<0.6</math>.]]

A theoretical curve for the consolidation of a freshly deposited unconsolidated mud layer is shown in Fig. 1.

The consolidation degree is expressed by the factor

<math>U \equiv \dfrac{\Delta h}{\Delta h_{\infty}} \, , \qquad (1)</math>

which represents the ratio of the layer compaction <math>\Delta h = h_0-h</math> at time <math>t</math> to the final compaction <math>\Delta h_{\infty} = h_0 -h_{\infty}</math> after a very long time. The initial layer thickness is <math>h_0</math> and the thickness at time <math>t</math> is <math>h</math>.

If it is assumed that consolidation can be described as a poro-elastic process<ref>Terzaghi, K. 1925. Erdbaumechanik auf Bodenphysikalischer Grundlage. Franz Deuticke</ref>, the rate of consolidation of a thin mud layer depends on the dimensionless time

<math>t_v=\dfrac{c_v \, t}{h^2} \, , \qquad (2)</math>

where <math>c_v=\dfrac{K}{\rho g (m_v+n \beta)}</math> is the consolidation coefficient<ref name=V1/>, <math>\rho=</math> pore water density, <math>g=</math> gravitational acceleration, <math>K=</math> mud permeability (hydraulic conductivity), <math>m_v=</math> coefficient of mud volume compressibility, <math>n=</math> mud layer porosity and <math>\beta=</math> pore water compressibility. The pore water compressibility is very small and can usually be neglected.

The consolidation curve of Fig. 1 can to a good approximation be represented by

<math>t_v<0.6 \; : \; t_v=\dfrac{\pi}{4} U^2 \quad \text{and} \quad t_v>0.6 \, : \; t_v=-0.085-0.405 \, \ln(1-U) \; . \qquad (3)</math>

Common values of the mud consolidation coefficient are in the range <math> c_v \sim 10^{-8} \text{ to } 10^{-7} \; \text{m}^2/\text{s} </math> although lower values may occur for highly organic or very soft estuarine muds. Fig. 1 then indicates that the consolidation of a mud layer of 1 m thick will generally take more than a year.

The value of the coefficient of volume compressibility <math>m_v</math> for a particular mud sample is usually determined in the laboratory by means of an [https://en.wikipedia.org/wiki/Oedometer_test oedometer test]. The coefficient of consolidation, <math>c_v</math>, can likewise be determined from the same test by observing the time-dependent settlement of the sample and interpreting the degree of consolidation using Eqs. (1) and (2).

The theoretical consolidation curve shown in Fig 1 is based on the simple poro-elastic model described in the appendix of the article [[Wave-induced seabed liquefaction]]. A more elaborate model of mud consolidation, taking account of the variation of the coefficients of permeability and compressibility as consolidation proceeds, was proposed by Gibson et al. (1981<ref> Gibson, R. E., Schiffman, R. L., & Cargill, K. W. (1981). The theory of one-dimensional consolidation of saturated clays: II. Finite non-linear consolidation of thick homogeneous layers. Can. Geotech. J., 18:280–293</ref>).

==Desiccation of mud==

[[File:MudCracks.jpg|thumb|right|300px|Fig. 2. Crackled desiccated mud surface. Photo credit Hannes Grobe, Creative Commons Licence CC-BY-SA-2.5]]

The poro-elastic consolidation model assumes that the mud deposit is fully saturated and horizontally uniform. However, when the mud surface is exposed to air, evaporation occurs not only from the expelled pore water at the surface, but also from the pore water within the upper layer of the deposit. As a result, this surface layer becomes unsaturated and starts to desiccate.

Desiccation generates matric suction in the unsaturated layer, which draws pore water upward from the underlying saturated mud by capillary forces. The increase in matric suction raises the effective stress and causes shrinkage of the mud skeleton<ref>Fredlund, D.G. and Rahardjo, H. 1993. Soil Mechanics for Unsaturated Soils. Wiley & Sons</ref>.

Because this shrinkage is generally non-uniform and is partly restrained by the underlying material, tensile stresses develop within the surface layer. When these stresses exceed the tensile strength of the mud, desiccation cracks form. These cracks allow air to enter the soil and further accelerate drying.

This process produces the characteristic polygonal cracks on the surface of dried mud deposits
<ref>Kodikara, J.K., Barbour, S.L. and Fredlund, D.G. 2000. Desiccation Cracking of Soil Layers. Procs. Asian Conf. in Unsaturated Soils, UNSAT ASIA 2000. Balkema, pp. 693-698</ref> (see Fig. 2).

==Related articles==
:[[Dynamics of mud transport]]

==References==
<references/>

{{author
|AuthorID=120
|AuthorFullName=Job Dronkers
|AuthorName=Dronkers J}}

[[Category:Physical coastal and marine processes]]
[[Category:Sediment]]

The Tragedy of the Commons

2026-04-25T20:35:58Z

Dronkers J: Created page with " ==Meaning of the Tragedy of the Commons == The "Tragedy of the Commons" refers to a paper which was written by biologist Garret Hardin in 1968 <ref name=H68>Hardin, G. 1968...."

==Meaning of the Tragedy of the Commons ==
The "Tragedy of the Commons" refers to a paper which was written by biologist Garret Hardin in 1968 <ref name=H68>Hardin, G. 1968. The Tragedy of the Commons. Science, 162, 1243-1248</ref>. He used the expression as a metaphor for the problems of overuse and degradation of natural resources including the destruction of fisheries, the over harvesting of timber, and the degradation of water resources<ref>Ostrom, E., Burger, J., Field, C.B., Norgaard, R.B. and Policansky, D. 1999. Revisiting the Commons: Local Lessons, Global Challenges. Science 284 (5412): 278</ref>. The word tragedy applies in this article to the depletion of the common fish resources. The term commons does not imply the absence of property rights, as a crucial distinction must be made between common property and open access. Common property systems have recognized users, agreed rules, and institutions for monitoring and enforcement, whereas open-access systems lack effective control over access and use. Hardin’s original model describes primarily an open-access regime rather than a true commons governed by collective rules<ref name=O90>Ostrom, E. 1990. Governing the Commons: The Evolution of Institutions for Collective Action. Cambridge University Press.</ref><ref name=MA>McCay, B.J. and Acheson, J.M. (eds.) 1987. The Question of the Commons: The Culture and Ecology of Communal Resources. University of Arizona Press.</ref>.

Hardin's analysis focused on a pasture that herders use in common for grazing their cattle. There are no problems with the common usage of the pasture until the number of animals reaches the [[carrying capacity]] of the pasture. In order to gain extra profits, herders add additional animals to the common pasture. The problem is that each additional animal means more grazing in the pasture, and the continual addition of animals eventually leads to overgrazing of the pasture. The end result is the destruction of the pasture. In the words of Hardin, "each man is locked into a system that compels him to increase his herd without limit-in a world that is limited"<ref name=H68/>.

==Fishery in the Tragedy of the Commons scenario==
Fisheries are similar to Hardin’s pasture in that increased fishing pressure has caused certain stocks of fish to become overfished to a point that threatens the survival of the fishery. All the conditions described by Hardin are met in this case: an unrestricted number of users, unfettered by any limits on their access, extract an increasing share of a resource until natural resources are severely depleted. Fishers tend to have little incentive to practice conservation, for they know that if they do not catch the available fish, someone else probably will<ref> Harris, J. and Codur, A-M. (Lead Authors) 2008. Economics of fisheries. In: Encyclopedia of Earth. (Eds. Cutler J.) Cleveland Global Development and Environment Institute.</ref> Overfishing is often not only the result of individual free-riding behavior, but also of broader political and economic drivers such as fishing subsidies, pressure to maintain employment in coastal communities, international seafood markets, and lobbying by industrial fishing interests. These factors can encourage overcapacity and delay necessary management measures even when risks are well known<ref>Sumaila, U.R., Khan, A., Dyck, A., Watson, R., Munro, G., Tydemers, P. and Pauly, D. 2010. A Bottom-Up Re-estimation of Global Fisheries Subsidies. Journal of Bioeconomics 12: 201–225</ref><ref>Hilborn, R., Orensanz, J.M. and Parma, A.M. 2005. Institutions, Incentives and the Future of Fisheries. Philosophical Transactions of the Royal Society B 360: 47–57</ref>.

According to Shepherd (2003<ref>Shepherd, J.G 2003. Economic Aspects of Fisheries Management. In: Sustainable Fisheries: Myth or Mirage School of Ocean & Earth Science, University of Southampton</ref>), fish resources all over the world are in danger of extinction, the major risks being:
* An excessive fishing fleet capacity and fishing effort
* Depleted fish stock
* Low profitability (operating surpluses near zero)
* High inter-annual variability of stock size and catches
* Excessive risk of collapse of fish stocks
It should be noted that alongside these factors others such as water pollution in particular with heavy metals, persistent organic pollutants, nutrients from agriculture and oil, in marine and coastal areas have also played a decisive role in the reducing fish stocks<ref>UNEP 2000. The State of Environment- Europe and Central Asia, Marine and coastal area. UNEP Global Environment Outlook 2000</ref>. The oceans have been called a common heritage resource – they belong to everyone and no one. But under the 1982 Law of the Sea, agreed to under United Nations auspices, nation can claim territorial rights to many important offshore fisheries. They can then limit access these fisheries by requiring fishing licenses.

==Critique of the Tragedy of the Commons Model==

The tragedy of the commons is a simplistic and fairly abstract model. Factors such as regulating mechanisms and community sanctions are not accounted for in the theory, as the assumption is made that common property means open access to all<ref name=MA/>. Many assert that what the theory describes is not a true commons, but an open access regime or free-for-all situation, where no authority has any control<ref name=M87>Marchak, P. 1987. Uncommon property. In P. Marchak, N. Guppy, & J. McMullan (Eds.), Uncommon property: The fishing and fish-processing industries in British Columbia. Toronto, Ontario, Canada: Methuen Publications.</ref>. One would be hard pressed to find a scenario where a natural resource is being used and a true free-for-all situation exists, particularly in the more populated areas of the world. In most instances of resource exploitation, the state plays a regulatory role, to maximize the capital gains from the resource and ensure conservation<ref name=M87/>. The inevitability of the tragedy of the commons theory then becomes questionable, as these regulatory mechanisms are not accounted for. In the case of the Northwest Atlantic fishery, the Canadian Federal government was heavily involved in the regulation and management of the fishery, primarily through the federal Department of Fisheries and Oceans (DFO). The role of government management/mismanagement is crucial to the discussion of the Newfoundland fishery and its collapse. Appendix B illustrates that resource depletion may result not only from lack of regulation, but also from inadequate governance. Scientific warnings were underestimated, quotas remained too high, and political pressure to protect employment delayed restrictive measures. This suggests that the problem was not simply a tragedy of the commons, but also a failure of centralized management.

Other environmental factors that have been implicated include greater predation of cod by seals because of the decreased seal hunt, and an increase in the mortality of capelin, one of the cod's main food sources<ref>Steele, D. H., Andersen, R., & Green, J.M. 1992. The managed commercial annihilation of northern cod. Newfoundland Studies 8: 34-68</ref>. Ecosystem changes such as predator-prey interactions, climate variability, habitat degradation, and pollution can strongly influence fish stocks and may interact with fishing pressure. Effective fisheries management therefore requires an ecosystem-based approach that considers both human exploitation and environmental change rather than treating overfishing as an isolated problem. Many of these environmental factors had impacts on the Cod stocks, but one must still look back to government mismanagement and overfishing as primary causes. This is not to discount environmental factors, but to suggest that they probably factored in at the most inopportune time<ref name=M2>Mason, F. 2002. The Newfoundland Cod Stock Collapse: A Review and Analysis of Social Factors. Electronic Green Journal, 17</ref>.

==Avoiding the Tragedy of the Commons==

Elinor Ostrom (1990<ref name=O90/>) identified several governance principles for the sustainable management of common-pool resources:

The resource boundaries and the legitimate users must be clearly defined. Rules for resource use should be adapted to local ecological and social conditions and be perceived as fair. Users should be able to participate in setting and modifying these rules.

Effective monitoring of both the resource and user behavior is essential, together with graduated sanctions for rule violations and accessible mechanisms for conflict resolution.

Local users must have the recognized right to organize and manage the resource system. When the resource is part of a larger system, governance should be organized in multiple connected levels, linking local management to broader regional and national institutions.

Ostrom shows that the tragedy of the commons is not inevitable. Many local communities around the world have successfully managed forests, fisheries, irrigation systems, and grazing lands for long periods without privatization or strict central control. Sustainable outcomes depend largely on the presence of legitimate institutions, shared rules, trust, monitoring, and collective responsibility.

Two fishery examples of the Tragedy of the Commons are discussed in the appendices A and B,

==Appendix A: The Tuna Example ==

[[Image:The_Bluefin_Tuna.jpg|thumb|left|Figure 1:The Bluefin Tuna (Thunnus thynnus), Source: NOAA's National Marine Fisheries Service]]

Tuna are several species of ocean-dwelling fish in the family Scombridae, mostly in the genus ''Thunnus''. Tunas are fast swimmers—they have been clocked at 70 km/h (43 mph)—and include several species that are warm-blooded. Tunas are sub-classified into five genera (''Thunnus'', ''Euthynnus'', ''Katsuwonus'', ''Auxis'' and ''Allothunnus'') with fifteen species altogether. They are all oceanic, capable of long migrations or movements<ref>Allen, R. L. 2002. Global tuna resources: limits to growth and sustainability. In S. Subasinghe & P. Sudari eds. Global tuna industry situation and outlook: resources, production & marketing trends and technological issues. Proceedings of the Tuna 2002 Kuala Lumpur, 7th INFOFISH World Tuna Trade Conference, pp. 3-12</ref>. Total catch of the five main tuna species expanded quite strongly between 1985- 2005: from 2.16 million MT to over 4.2 million MT, The main tuna catching nations are concentrated in Asia, with Japan and Taiwan (Province of China) as the main producers. Other important tuna catching nations in Asia are Indonesia, the Republic of Korea and the Philippines<ref>Josupeit, H. 2004. Global World Tuna Markets” INFOFISH TUNA CONFERENCE 3-5 June 2004, Bangkok, Thailand, Food and Agriculture Organisation</ref>.

[[Image:Tuna_catches_by_species.jpg|thumb|right|210px|Figure 2: Review of Global Tuna Trade and Major Markets Source: David James Consultant FAO and Helga Josupeit, Fish Utilization and Marketing Service FAO, Power Point Presentation 2007]]

Growing demand for tuna products has been stimulating increases in the catches. At the same time, demand for tuna has been keeping prices at levels that have ensured adequate income for all stakeholders. However, in the case of some species such as skipjack continuous high exploitation has created an excess of supply, causing prices, and therefore income of operators, to decline.

===Management of tuna===
There are five regional bodies responsible for managing tuna stocks:
* [http://www.ccsbt.org/ Commission for the Conservation of Southern Bluefin Tuna] 
* [http://www.iattc.org/ Inter-American Tropical Tuna Commission] 
* [http://www.iccat.int/ International Commission for the Conservation of Atlantic Tunas] 
* [http://www.iotc.org/ Indian Ocean Tuna Commission] 
* [http://www.wcpfc.int/ Western and Central Pacific Fisheries Commission] 
The five met together for the first time in Kobe, Japan in January 2007. The meeting concluded with an action plan drafted by some 60 countries or areas. Concrete steps include issuing certificates of origin to prevent illegal fishing and greater transparency in the setting of regional fishing quotas.

===The case of the Atlantic bluefin tuna (''Thunnus thynnus'')===
Atlantic, Pacific and southern bluefin contribute relatively little in terms of volume to the total catches of principal market tunas, but their individual value is high due to their use for sashimi and sushi (Japanese delicacy). These fish are migratory and are known to traverse the Atlantic Ocean in a few months. Bluefin tuna are among the largest bony fish in the ocean, reaching over 3.05 meters in length and over 500 kilograms in weight. Their lifespan can exceed 30 years, making them long lived among fish species <ref>Committee to Review Atlantic Bluefin Tuna 1994. An Assessment of Atlantic Bluefin Tuna. Ocean Studies Board, Commission on Geosciences, Environment and Resources (CGER), National Academy Press, p. 5</ref>.

[[Image:Distribution_of_the_Atlantic_and_Pacific_bluefin_tunas.jpg|thumb|left|220px|Figure 3: Distribution of the Atlantic and Pacific bluefin tunas and their respective fisheries operation area Source: Allen, R. L. 2002. Global tuna resources: limits to growth and sustainability, page 164]]

Catches of Atlantic bluefin followed a generally declining trend from the early 1950s to the early 1970s <ref name=A2>Allen, R. L. 2002. Global tuna resources: limits to growth and sustainability. In S. Subasinghe & P. Sudari, eds. Global tuna industry situation and outlook: resources, production & marketing trends and technological issues, p. 168</ref>. The Atlantic population of the species has declined by nearly 90 percent since the 1970s <ref>Safina 1996. Thunnus thynnus. 2006 IUCN Red List of Threatened Species</ref>. Atlantic bluefin tuna take eight years to mature to large-medium sized fish. Scientists believe that the decline in the numbers of larger sized bluefin tuna can be attributed to the high volume of juvenile bluefin tuna caught. The problem is that fishers catch so many juvenile bluefin tunas that there are none left to mature <ref name=N99>Nickler, P. 1999. A tragedy of the commons in coastal fisheries: Contending prescriptions for conservation, and the case of the Atlantic bluefin tuna, p. 3</ref>.

There are two ways to save the Atlantic bluefin tuna stock- protect them in their breeding grounds and in their feeding grounds. This will require immediate action in both the central Atlantic, to reduce the mortality of the giant bluefin while foraging, and in the Gulf of Mexico and Mediterranean, where bluefin breed as discrete populations <ref>Radford, T. 2005. Scientists call for urgent action to save Atlantic Tuna, The Guardian, April 28 2005</ref>.

The International Commission for the Conservation of Atlantic Tuna (ICCAT) has recommended, in light of severe stress on the Atlantic Ocean’s population of bluefin tuna that, for the indefinite future, no commercial fishing for juvenile fish or this species is allowed<ref name=N99/>.

[[Image:Bluefin_tuna_stock.jpg|thumb|right|Figure 4:The Bluefin Tuna (Thunnus thynnus), Bluefin tuna stock in the Mediterranean is threatened by overfishing and illegal fishing Source: Brian Skerry / National Geographic Image Collection]]

===Conclusion===
It is difficult to be prescriptive regarding what is an appropriate extractive policy for a fishery; the policy will differ depending on the individual characteristics of the fishery, the fishers and the objectives of the property right holder. Some form of regulation is needed in a fishery to prevent the “Tragedy of the Commons” – where individual fishers are motivated to operate beyond the maximum sustainable yield, often leading to biological and economic over-exploitation. Without a clearly defined set of policies, the consequent institutions may not achieve a desired result<ref>Peterson, E.H. 2006. The Case of Pacific Tuna. Institutional Economics and Fisheries Management, p. 40</ref>.

==Appendix B: the Newfoundland's cod crisis==
The 1992 moratorium on fishing for Northern Cod (''Gadus morhua''), announced by the Canadian Federal Minister of Fisheries and Oceans, marked a symbolic end to the way of life that had sustained Newfoundland’s outports for hundreds of years. It also marked the completion of an ecological regime shift, from an ocean ecosystem dominated by cod and other predatory groundfish, to one in which such fish are comparatively scarce, and lower-trophic-level invertebrates more common. Around 19,000 fishers and workers related to fishing activities were directly affected and up to 20,000 other jobs were lost or harmed<ref>Steele, D. H., Andersen, R. and Green, J.M. 1992. The managed commercial annihilation of northern cod. Newfoundland Studies 8: 34-68</ref>.

[[Image:newfoundland-and-labrador-map.gif|thumb|right|Map of the province of Newfoundland and Labrador, Canada.]]
[[Image:Gadus morhua.jpg|thumb|left|The Atlantic Cod, ''Gadus morhua'' Linnaeus,1758. Source: Craasmann, A.]]

===Overfishing===
Many authors have cited overfishing as the cause of the cod stock collapse<ref>Sinclair, P. R. 1996. Sustainable development in fisheries dependant regions? Reflections on Newfoundland Cod fisheries. Sociologia Ruralis 36: 225-235.</ref><ref>Hannesson, R. 1996. Fisheries (mis)management: The case of the north Atlantic cod. Oxford, England: Fishing News Books</ref><ref>Finlayson, A. C. 1994. Fishing for truth: A sociological analysis of northern cod stock assessments from 1977-1990. St. John's, Newfoundland, Canada: Memorial University of Newfoundland, Institute of Social and Economic Research.</ref>. In the case of the Newfoundland cod, there were three distinct groups involved in harvesting the resource: local inshore fishermen, Canadian draggers and trawlers, and deep-sea foreign fishing vessels.

[[Image:figure3-4.jpg|thumb|right|Collapse of Atlantic Cod Stocks Off the East Coast of Newfoundland in 1992.]]

===Government Management===
The government's involvement in the fishery is best explained by a statement made by Sinclair (1992<ref>Sinclair, P. R. 1992. Atlantic Canada's fishing communities: The impact of change. In D. A. Hay, & G. S. Basran (Eds.), Rural sociology in Canada. Don Mills, Ontario, Canada: Oxford University Press.</ref>). Since 1977, the Government of Canada has been the manager of the fisheries. Instead of fish being a resource available to anyone with the means to catch them [i.e. a commons], they became state property, the rights to which were delegated in the management plans. Therefore, the management policy of the Canadian state has become a major factor in the condition of the industry since this time" (p. 93). The Federal government, through the DFO, controlled the number of fishermen through licensing systems, set quotas for different types of vessels, and, acting upon information from its own scientists, set the Total Allowable Catch (TAC) for the industry each year<ref>McGraw, D. 1996. Course manual, Sociology/Anthropology 3322 (7th ed.). St. John's, Newfoundland, Canada: Memorial University of Newfoundland, Division of Continuing Education</ref><ref>Palmer, C. and Sinclair, P. 1997. When the fish are gone: Ecological disaster and fishers in Northwest Newfoundland. Halifax, Nova Scotia, Canada: Fernwood Publishing</ref>. So quite obviously, a lot of the blame for the overfishing has to be placed there, as the DFO told the fishermen what they could go out and get. Acting upon faulty data , the DFO licensed too many fishermen and set TAC's that were too high<ref name=M2>Mason, F. 2002. The Newfoundland Cod Stock Collapse: A Review and Analysis of Social Factors. Electronic Green Journal, 17.</ref>.

===Fishery in the tragedy of the commons scenario===
Following Ophuls' model of a fishery in the tragedy of the commons, once the critical point was reached, either privatization or collapse would occur. It is easy to see how the collapse of the Newfoundland fishery could have been a tragedy of the commons, with the many groups and countries that were taking from the resource base. Given the conditions necessary for a tragedy of the commons, and its ultimate outcome, the Newfoundland cod stock collapse would then have to be considered inevitable and unavoidable. However, the proposition that the Newfoundland's cod crisis fits in the tragedy of the commons is not accepted by many environmental specialists, due to its simplistic arguments<ref name=M2/>.

===The Tragedy of State Mismanagement===

As the exploitation of the Newfoundland fishery was so predominantly guided by the government, the (mis)management of the fishery and its subsequent collapse match well with ideas put forth by Marchak (1987)<ref name=M87/>. In her book Uncommon Property. Marchak calls this "the tragedy of state mismanagement" theory (unit 4, p. 3). She argues that a fishery is not a true commons, as the fisher lacks management rights normally associated with property and common property. The state has appropriated the property, and makes all of the management decisions. Fishermen get told who can fish, what they can fish, and essentially, what to do with the fish once it is caught. In this regard then, when a resource such as the Newfoundland fishery collapses, it is more a tragedy of state mismanagement than a tragedy of the commons.

===Related articles===
:[[Carrying capacity analysis]]
:[[Effects of fisheries on marine biodiversity]]
:[[Legislation for the sea]]
:[[EU Common Fisheries Policy (CFP)]]

===External links===
:[http://en.wikipedia.org/wiki/Tragedy_of_commons Tragedy of the commons]
:[https://editors.eol.org/eoearth/wiki/Economics_of_fisheries Economics of fisheries- Encyclopedia of Earth]

===Further reading===
Robert J. Smith 1981. Resolving the Tragedy of the Commons by Creating Private Property Rights in Wildlife. Cato Journal, Vol. 1, No. 2, pp. 439-468.

== References ==
<references/>

{{author
|AuthorID=
|AuthorFullName=Caroline Krause
|AuthorName=Ckrause}}

{{author
|AuthorID=16488
|AuthorFullName=Alvaro Miranda Oliveira
|AuthorName=AOliveira}}

[[Category:Coastal and marine fisheries]]
[[Category:Coastal and marine ecosystems]]
[[Category:Integrated coastal zone management]]

Heterozygosity

2026-04-13T19:33:44Z

Dronkers J: Created page with " {{Definition|title=Heterozygosity |definition= the possession of two different alleles of a particular gene or genes by an individual. The degree of heterozygosity is def..."

{{Definition|title=Heterozygosity
|definition= the possession of two different [[allele]]s of a particular gene or genes by an individual. The degree of heterozygosity is defined as the proportion of sites on the chromosome at which two randomly chosen copies differ in DNA sequence<ref>Altshuler, D. 2012. The Inherited Basis of Common Diseases. Goldman's Cecil Medicine 1: 195-198</ref>.}}

The opposite of heterozygosity is homozygosity, in which a harmful mutation can cause disease because both gene alleles are defective.

==References==
<references/>