CORRECTING ANNUAL CATCHES FROM SEASONAL FISHERIES FOR USE IN VIRTUAL POPULATION ANALYSIS

The catch equation used in virtual population analysis (VPA), and most annual age-structured methods, assumes a constant fishing mortality r ate (F) throughout the year even though many, if not most, fisheries a re seasonal. Breaking this assumption of a constant F creates a bias i n the resulting population-size estimates when the observed catch is u sed as input in VPA The bias can be reduced by changing the time step in the analysis to quarters or months, as has been suggested in the pa st, but this change is not always easy or practical. This paper presen ts an alternative method for reducing the bias: correction of the catc h values to meet the assumption of a constant fishing mortality rate. A simple algorithm is presented that gives the number of fish that wou ld have been caught from a given population if the observed fishing mo rtality rate had been spread evenly throughout the year. An iterative process improves the required guess for the population size such that the bias is eliminated.