Assessing the effects of changes in exploitation pattern using length composition data (with notes on VPA and cohort analysis)

Methods are described for using length composition data from commercial catches as a substitute for age-data in studying the dynamics of fish stocks. Particular attention is given to the extension of cohort and virtual population (VPA) techniques to length-data, and a brief description of the methods of application of VPA to age-data is also given. Given values for L infinity and M/K, length cohort analysis can be used to estimate the long term effect of a change in exploitation pattern. Alternatively, the results can be used to determine the current position on a yield isopleth diagram. Methods are also described for determining values of Z/K from the right hand part of a length composition, given a value for L infinity. If a value for M/K is also available, these results can also be used to determine the current position on a yield-isopleth diagram.