Trends in catch rates of sawfish on the Australian North West Shelf

Abstract

Northwestern Australia is thought to have some of the world’s last remaining viable sawfish populations, although little quantitative data exists on their status or trends. This study examined 17 years of logbook bycatch records (n = 815) for green sawfish Pristis zijsron and narrow sawfish Anoxypristis cuspidata from a trawl fishery operating on the Australian North West Shelf. Incidental sawfish captures by the fishery are rare, occurring approximately once every 75 trawls (~199 trawl hours). To standardize catch rates and account for excess zeros in the data, we employed generalized additive models for location, scale, and shape (GAMLSSs) using a zero-inflated Poisson distribution. For green sawfish, catch rates approximately doubled over the study period, while an oscillating trend was observed for narrow sawfish catch rates. Reported captures occurred throughout the management boundaries of the fishery, which operates in mid-shelf waters from 48 to 121 m. A weak seasonal signal in catch rates was detected for both species, with the highest capture numbers occurring during autumn-winter, consistent with an expected inshore migration for parturition during spring-summer. Logbook trends were partly corroborated by independently verified data collected in a subset of years, which also showed an increasing proportion of green sawfish in the catch. Our findings emphasize the importance of sawfish populations in northwestern Australia within the context of global conservation efforts for this taxon.