Sustainability of fishery bycatch: a process for assessing highly diverse and numerous bycatch

In tropical prawn (shrimp) trawl fisheries it is daunting to assess the sustainability of bycatch species because they are diverse and there is little historical and biological information for quantitative stock assessments. We developed a process to examine the likely impact of prawn trawling on the sustainability of bycatch species and applied this to fish bycatch in the Australian Northern Prawn Fishery. The 411 fish bycatch species were ranked with respect to biological and ecological criteria that contributed to two overriding characteristics, namely first, their susceptibility to capture and mortality due to prawn trawling, and second the population's capacity to recover after depletion. The rank of each species on these two characteristics determined its relative capacity to sustain trawling, and therefore its priority for research and management. Species that were the least likely to be sustainable came from the families Apogonidae, Ariidae, Bathysauridae, Callionymidae, Congridae, Diodontidae, Labridae, Opisthognathidae, Plotosidae, Synodontidae and Tetraodontidae. These species are highly susceptible to capture by trawls, they are benthic or demersal, their primary habitat is soft sediments, and their diet may include prawns. The recovery capacity of these species is also low, with the estimated removal rate by trawling high. The species that were the most likely to be sustainable came from the families Carangidae, Clupeidae, Ephippidae, Scombridae, Sphyraenidae and Terapontidae. They are less susceptible to capture by trawls, they are generally pelagic, their primary habitat is not in trawl grounds, and they have a broad depth distribution and range in the fishery. These species also have a greater capacity to recover, as most individuals have bred before capture, and a low estimated removal rate by trawling. The final ranking of the species must be used with caution because of the assumptions made in the process. However, the process is a valuable first step towards ensuring the sustainability of the bycatch species. Because of the simplicity of the process, it can be readily used in fisheries, particularly those with diverse bycatch, to manage the sustainability of their bycatch.