Estimation of effective mesh sizes and their utilization in assessment

A method to estimate the effective mesh sizes used by different fleets fishing on the same stock is de
scribed. The model of the fisheries makes use of the von Bertalanffy and the traditional Beverton &
Holt equations to describe the dynamics of the stock. Logistic curves are used to describe the recruit
ment of the fish to grounds, the selective properties of the gears and the discard practice. The effective
mesh sizes are estimated by minimizing the sum of squares between the estimated and observed relative
length or age composition of the landings. The model can be used to estimate the effects on the
different fleet’s fishing mortalities and catches of changes in the mesh sizes and effort in one or more fleets etc.