maximum sustainable yield

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.

Escapement

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This spawned out Chinook Salmon contributes to the escapement estimate for its particular run.

Escapement is a term used in salmon management to mean, quite simply, how many salmon are able to “escape” premature death and complete their full life cycle.  These escapees have run the gauntlet of natural mortality, recreational fishing, commercial fishing, and other life-threatening episodes for the opportunity to spawn in freshwater.  Escapement is estimated by a number of different methods including fish counts (e.g., at a weir or dam viewing window) and carcass surveys (i.e., counting the dead salmon on spawning grounds).  For salmon managers, the amount of escapement for each salmon run informs population estimates and appropriate fishing quotas and other management strategies for future fishing seasons.

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