Reduction of by-catch has been an active area for research in Australia and around the world. The initial focus for research and management has been demersal trawl fishing, where the perception of waste and potential impact has been the greatest. However, the reasons for seeking to reduce unwanted catch apply to all fisheries, including those using demersal traps.
Demersal fish traps in NSW must be covered with mesh no smaller than 50 mm, and a galvanised hexagonal fencing wire is the most common material. However, many fishers use larger mesh and both 50 X 75 mm and 75 mm hexagonal mesh are in use. Those using and advocating larger mesh have done so to reduce the catch of undersize snapper and other small fish and to decrease sorting times. Other trappers are concerned about the effect of larger mesh dimensions on the catch of valuable species such as wrasses, sweep or bream which all have no minimum legal length (MLL) or have an MLL smaller than snapper.
An understanding of the differences in selectivity of different sizes of mesh on the species caught in fish traps will have a number of uses. This information is essential to determine the cost and benefit to fishers of changing mesh sizes. The selection probabilities for existing meshes can also be used to improve the assessments using age and length composition collected for snapper and bream in NSW. This is particularly important for snapper, where a very large proportion of the fishery is caught close to the MLL. Finally, because wire mesh can be made in a diversity of shapes and sizes, it may be possible to achieve desirable changes in selectivity for some species while not changing selectivity patterns for others.