MSY

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.

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.