1999 Greater Amberjack Stock Assessment

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Alternative stock assessment methods applied to
greater amberjack, Seriola dumerili, in Florida

Gary A. Nelson
Florida Fish and Wildlife Conservation Commission
Florida Marine Research Institute
St. Petersburg, FL
December 13, 1999

As one alternative to the age-structure sequential population analysis model used by (Legault and Turner, 1999), a Delury depletion model (Hilborn and Walters, 1992) was used to estimate fishing mortality for greater amberjack. The Delury model treats a population as a homogenous assemblage of individuals that are equally exposed to fishing and natural mortality events. With the Delury model, the objective is to estimate the recruitment and population sizes that must have occurred to have produced the observed pattern in catches. A mathematical equation (also known as the process model) is used to describe change in population size.

The "recruitment" values do not necessarily reflect the input of "new" individuals into the population through reproduction or growth; rather, it reflects individuals from multiple age classes that had to enter the exploitable portion of the population at the beginning of the year to produce the observed pattern in catches. Estimation of numbers by year is done by minimizing the sum of squares between the observed catches and predicted catches estimated from the model.

Predicted catch is estimated from the relationship between catch and effort. Given estimates of average population size and observed catches, fishing mortality (F) from fishery (i) at time (t) is then estimated.

For other information:
Stock assessments for finfish and invertebrate
Greater Amberjack Species Account