The term bycatch commonly refers to the part of a fishers catch that is not the target species.
Bycatch includes those fish captured by fishers (both commercial and recreational) that are undersized, prohibited, inedible or unsaleable. Different parts of the bycatch are sometimes called trash, discard and incidental catch.
The community is becoming more aware of issues associated with bycatch. In particular bycatch of juveniles of (commercially and recreationally) important fish species has become a major issue for industry and the recreational fishing community. In 1993, the Fisheries Working Group on Ecologically Sustainable Development identified three major issues in relation to bycatch:
i) the direct impact on the mortality of bycatch species from some fishing techniques
ii) the impact on ecological processes of discarding bycatch at sea.
iii) the cost to industry when bycatch consists of juveniles of commercially important species and the cost of catching fish with no commercial value.
Most recreational and commercial fishing techniques result in bycatch. For example, rod and line or handline methods, commonly used by both commercial and recreational fishers, capture a range of fish, some of which have no particular value to the fisher.NSW DPI scientists are constantly working with industry to improve gear selectivity.
Community concern over the level of bycatch in prawn trawl fisheries resulted in an observer based study (1988-1991). which quantified the bycatch throughout NSW prawn trawl fisheries. Recent research effort has been focused on excluding bycatch from prawn trawl gear by modifying the codend to incorporate bycatch reduction devices (BRDs).
Bycatch reduction devices have been used voluntarily by commercial fishers for many years and are now compulsory in all ocean and estuary prawn trawl nets.
One of the most common modifications has been the use of square-mesh panels in codends. Put simply they consist of a panel of square shaped mesh positioned in the upper side of the codend and work by exploiting the behavioural differences between prawns and fish.
As the fish are herded together they try to escape by heading towards the top and sides of the codend and can escape through the open square-shaped meshes. By contrast, prawns have a limited reaction to the trawl and tumble along the bottom of the net into the codend.
The size of square-mesh used in the panel directly determines the size of fish that are allowed to escape. Studies in the oceanic prawn trawl fishery have shown that codends with relatively small panels of square-mesh were effective in removing up to 40% of the total unwanted bycatch. Square-mesh panels have also been used in estuaries to exclude species such as mulloway and catfish. In the Hawkesbury River, square-mesh panels were effective in reducing the numbers of these fish by up to 40% and 50% respectively.
Fishers who seek to remove larger organisms from their trawls may use a separator panel, such as a solid metal or plastic grid located in the codend. In several experiments in the Clarence River, this device removed up to 90% of the total bycatch, and up to 67% of juveniles of commercially important species, with no significant reduction in prawns.
Future research may be conducted to allow scientists to quantitatively assess how well various designs reduce the impacts of prawn trawling.