Assessing population changes in bycatch species using fishery-dependent catch rate data

Managing the sustainability of bycatch species is important for many fisheries worldwide. However, high survey costs, rarity of vulnerable bycatch species, and widespread use of bycatch reduction devices, increase the difficulty of assessing sustainability. We developed a sample size estimation framework using zero-inflated count models that incorporated the mean catch, catch variability, and the impact of Turtle Excluder Devices (TEDs) and Bycatch Reduction Devices (BRDs) on bycatch sampling for a subset of bycatch species in Australia's Northern Prawn Fishery (NPF). Minimum sample sizes required to detect small to moderate declines (10–25%) in the mean catch were too high for many species in the short term (5 y) and above the capacity of the fishery (28,080 trawls per year), because of the high exclusion rates from TEDs and BRDs. For sampling periods beyond 5 y, detection of either a 25% or a 50% decline was possible for some more common species of elasmobranchs, sea snakes and all species of fish. In conclusion, sampling on commercial trawl vessels cannot evaluate changes in sea turtle abundance and some of the larger elasmobranchs. However, the monitoring of sea snakes and fishes is viable using these methods if catch rates are well correlated with effort and the target species.