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Management - Stock Assessments

Stock assessments are conducted periodically to determine the status of a fish stock relative to its biological reference points.   An assessment includes a vast array of scientific information and provides fisheries managers with the data necessary to make informed decisions that ensure the health and sustainability of the resource.   The data collected generally describe the life history characteristics of the species, such as age, growth, sexual maturity, reproductive capacity, mortality, range, stock boundaries, and feeding and habitat preference.   Data provided by commercial and recreational fishermen is termed fishery-dependent data while data provided by researchers conducting standardized surveys is termed fishery-independent data.   Fishery dependent data informs decision makers about the harvest of a species while fishery independent data informs decision makers about the abundance and recruitment of a fish stock. 

The life history, harvest, abundance, and recruitment data are used to quantitatively assess the fish stock and determine whether it is overfished or undergoing overfishing.   In order to do this, fisheries managers must set biological reference points.  Maximum sustainable yield (MSY) is a term used to describe the largest catch that can be continuously removed from a stock without endangering its long-term sustainability.   A stock is undergoing overfishing when the catch exceeds the ability of the fishery to replenish itself.  Spawning stock biomass (SSB) is the total weight of the stock that is of reproductive age.  A fish stock is overfished if the spawning stock biomass is reduced to a level which can no longer produce the maximum sustainable yield.   When a fish stock is discovered to be overfished or undergoing overfishing, fisheries managers are required by law to develop regulations that will rebuild the stock.

Once all of the data is collected, scientists use mathematical and statistical models to assess the condition of the stock.  There are three basic types of models used depending upon the quality of the data available: surplus production, statistical catch-at-age, and virtual population analysis.  A surplus production model is the most basic and is used when there is very limited data on the stock.  This model requires only total catch and effort data.  The statistical catch-at-age models are more complex and require that fishery data be separated into age-classes because fish of different ages will experience different levels of natural and fishing mortality.   These models estimate the abundance of each age-class and allow for managers to make predictions about stock abundance in the future.   Virtual population analysis requires the most complete and accurate data sets.   These models look backwards at historical catch and can calculate stock size and mortality rates for each age class in the past, revealing trends in the fish stock and the impacts of previous harvest levels. 

After assessment modelers have applied the chosen model to the data, they must evaluate the goodness of fit.  Goodness of fit describes how well the model fits the data.  Modelers can then adjust parameters such as survival rate and number of recruits to better fit the model to the observed data.   Modelers need to know how sensitive the model is to change.  Sensitivity analysis is the last step in the modeling process.  It examines changes to the output of the model based on different values for each of the parameters and provides modelers with a vitally important estimate of uncertainty.
The final step in the assessment process is recommendations.  Stock assessments are designed to provide managers with the necessary information to best manage the stock by revealing how the stock might respond to specific management actions.   Management options are rated based on how likely they are to achieve the stated management objectives.  In addition, research recommendations are made to help improve the data and reduce uncertainty for the next stock assessment.   Ultimately, the completed stock assessment is presented to a state or regional fisheries management council.  The council then makes the final decision on which management options to undertake and include in the fishery management plan for that species with the goal of maximizing the biological, social, and economic benefits of the resource. 

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