overfishing

Population dynamics

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Fish population dynamics (image credit: Katherine Nicholson, slideplayer.com)

Fish population dynamics is the study of change in a fish population over time.  Fundamentally, a population size after some time interval equates to the population size before that interval plus births (i.e., recruitment) and immigration and minus mortality and emigration.  Many fish populations follow a logistic pattern of density-dependent growth.  Beginning with a population size where space and food are not limiting, the population grows rapidly in an exponential pattern; at a certain population density, population growth slows and stabilizes at a given carrying capacity.

Density-dependent factors governing fish population dynamics include competition, predation, disease, and parasitism.  Fish population dynamics can also be driven by density-independent abiotic factors such as temperature, dissolved oxygen, and water chemistry.

In fisheries, these basic population dynamics principles can be applied to harvest strategies to estimate maximum sustainable yield (MSY) with the goal to prevent overfishing.

Exclusive Economic Zone (EEZ)

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Exclusive economic zone of the United States (image credit: NOAA).

An Exclusive Economic Zone (EEZ), as defined by the United Nations Convention on the Law of the Sea, is a 200 nautical mile jurisdiction granted to coastal nations for “sovereign rights for the purpose of exploring and exploiting, conserving and managing the natural resources, whether living or non-living, of the waters superjacent to the seabed and of the seabed and its subsoil, and with regard to other activities for the economic exploitation and exploration of the zone.”  Within an EEZ, a country claims exclusive rights to manage fisheries and prevent overfishing.  While the outer 200 mile boundary determines a country’s sovereign limits, within the U.S., most coastal states claim an inner three nautical mile zone from the shore as state waters (some places claim nine nautical miles).  For fisheries, such as defined under the Magnuson-Stevens Fishery Conservation and Management Act, management in the inner zone belongs to the states and the remainder of the EEZ is under purview of the federal government (e.g., the National Oceanic and Atmospheric Administration).

Maxium sustainable yield (MSY)

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Optimizing fishing effort (Diagram credit: http://ib.bioninja.com.au).

Maximum sustainable yield (MSY) is a classic concept from fish population dynamics.  It operates under the assumption that populations have a carrying capacity, or equilibrium size, where the the resources available match the population requirements and if individuals are removed from the population, they will be replaced (i.e., are renewable).  When harvest removes a portion of the population, it  lowers pressures on the population’s associated resources, such as food and habitat, allowing the remainder of the population to benefit from less competition and balance the population back to the carrying capacity.  By this theory, the surplus biomass can be harvested sustainably, without long-term impacts to the population.

MSY is a commonly applied fisheries management tool, and even a guiding principle of the Magnuson-Stevens Fishery Conservation and Management Act, the primary legislation governing marine fisheries management in United States federal waters.  However, it is important to note that there are many situations in which the assumptions of MSY are not met which result in overfishing and unsustainable practices.

Overfishing, overfished

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The terms overfishing and overfished are confusing because they address a similar subject.  The difference between the two terms is subtle but significant.

Status of overfishing and overfished species monitored by the National Oceanic and Atmospheric Administration (NOAA)

Status of overfishing and overfished species monitored by the National Oceanic and Atmospheric Administration (NOAA)

Overfishing refers to the current fishing rate which results in a higher harvest, or fishing mortality rate, than maximum sustainable yield (the maximum harvest level without negatively impacting the sustainability of the stock).  Overfishing is generally divided into two classes:

  • growth overfishing: where fish are harvested at a size smaller than would produce maximum yield per fish.
  • recruitment overfishing: where the reproductive capacity of a stock is diminished to a point where the spawning stock biomass is not sufficient to maintain the sustainability of a stock.

Overfished, on the other hand, is the state of a stock upon which overfishing has occurred.  The stock is no longer able to produce at a maximum sustainable yield.  It is important to note that a stock may be overfished, but overfishing may not be occurring.  Stocks that are overfished can be managed for fishing pressure that is low enough to allow the stock to rebuild to a level to support maximum sustainable yield.

For more information, please visit:

  • http://www.fishwatch.gov/features/overfishing_overfished_same_thing.htm
  • http://www.nmfs.noaa.gov/stories/2013/05/05_02_13status_of_stocks_2012.html