Gill net mesh selectivities for gummy shark, Mustelus antarcticus Günther, taken in south-eastern Australian Waters

Abstract

Experiments designed to estimate length-specific selectivities for gill nets used to catch gummy shark, M. antarcticus, were carried out in south-eastern Australian waters between 1973 and 1976. Eight nets were used, with mesh sizes ranging from 5.08 to 22.78 cm (2-9 in.) in steps of 2.54 cm (1 in.). Existing methods for estimating mesh selectivities are reviewed, and an alternative method, which overcomes some problems in the existing methods, is described. In this method, an assumed selectivity function is fitted directly to catch data for a number of different mesh sizes, with the parameters of the selectivity function being estimated simultaneously across mesh sizes and length-classes.

The new method is used to estimate selectivities for the gummy shark data, on the assumption that the selectivity function takes the shape of a gamma distribution. Data from the 2- and 3-in. mesh nets did not satisfy the assumptions of the procedure, but estimates of mesh selectivities were successfully obtained for the 4-9-in. mesh nets.