Habitat use and site fidelity of neonate and juvenile green sawfish Pristis zijsron in a nursery area in Western Australia

Abstract

The largest of the sawfishes is the Critically Endangered green sawfish Pristis zijsron, a species believed to have undergone a major decline (38%) in extent of occurrence. Conservation efforts are hampered by the lack of information on the habitat requirements of this species. We used passive acoustic telemetry to document the movement patterns of 37 juvenile P. zijsron (<3000 mm total length) in a recently discovered nursery area in a remote estuary and adjacent mangrove creeks in the Pilbara region of Western Australia. All age classes had a high site fidelity near the mouth of the Ashburton River (<700 m upstream) or in the adjacent tidal mangrove creeks. Neonates stayed close to the river mouth for several months, with movement increasing with growth. For larger individuals, movement between the river mouth or creeks and nearshore coastal habitats was largely tidally driven, with nearshore coastal habitats used during low tide and protected tidal waters (mangrove creeks) used during high tide. Emigration from the river estuary appeared to be influenced by increases in freshwater discharge and high turbidity brought on by cyclonic rainfall events. The high relative abundance and site fidelity of multiple age classes of P. zijsron over multiple years confirms that the study area provides important nursery habitats for the species. Additionally, the localised movements of juveniles suggest that this population may be sensitive to disturbances within these habitats. Given the significance of this region for P. zijsron, it is imperative to focus conservation efforts here to ensure that the population remains in a relatively healthy condition.