There is worldwide concern over the potential effects of by-catch and discarding of fish in commercial fisheries, particularly trawling. Although mortalities of discards are highly variable, it is likely that a large proportion of fish discarded at sea by trawlers do not survive. Consequently, discards at sea represent real losses from fish populations. Therefore, stock assessments that ignore the discarded component of catch are biased by an unknown amount, resulting in biomass and yield estimates that may be incorrect.

In Australia, the issue of primary concern is the direct mortality resulting from the capture and discard of commercially and recreationally important species by trawlers. This may result in negative impacts on: (i) stocks of fish targeted by the fishery concerned and/or (ii) other commercial or recreational fisheries (interacting fisheries) which catch the species discarded.

Fish trawling occurs off the coast of NSW between Crowdy Head and Eden and components of this fishery are managed by NSW Fisheries (north of Barrenjoey headland and less than 3 nm offshore to the south of Barrenjoey) and by the Commonwealth - the South East Fishery (> 3 nm offshore south of Barrenjoey).

Prior to the commencement of this project (in 1992): (i) except for anecdotal reports, nothing was known about quantities and sizes of fish discarded by fish trawlers; (ii) no reliable information existed about the quantities and sizes of fish in retained catches for fish trawlers north of Barrenjoey; (iii) quantities and sizes of non-quota species retained by fish trawlers in the SEF were not generally known.

Consequently, there was a need to (i) quantify magnitudes and size-compositions of retained and discarded catches of fish trawlers; (ii) facilitate assessment of the impact of by­catch and discards on the fish trawl fisheries in NSW and on interacting fisheries.