Growth and morphology of Critically Endangered green sawfish Pristis zijsron in globally important nursery habitats

Abstract

Understanding growth rates and other basic life-history information of imperilled species is essential to assessing the extent of threats to a population, but often difficult due to limited access to study subjects. Here we used mark-recapture data to estimate growth rates of juvenile Critically Endangered green sawfish (Pristis zijsron) in a globally important nursery in the eastern Indian Ocean (Western Australia). Our results suggest that growth of juvenile sawfish in this part of the central Western Australian coast is significantly slower compared to populations on the north-eastern coast of Australia. Additionally, growth rates differed between nearby areas within the nursery region, potentially due to differential productivity or anthropogenic effects. Morphological relationships between total length, rostral length, mouth gape, and clasper length are presented, which will allow for greater accuracy in estimating biological parameters in this species, while updated information on size at maturity (> 3200 mm) and size at birth (approximately 750–900 mm) will help to clarify life-history parameters for this data-poor species. Furthermore, there were distinct differences in the number of rostral teeth of green sawfish between this eastern Indian Ocean population and other populations throughout their current distribution, indicating substantial genetic differentiation in this species globally. These results will help to accurately assess growth trajectories and potential impacts of fisheries and other threats to green sawfish. Additionally, results highlight the importance of assessing population-specific growth rates in threatened species and of considering potential long-term life-history impacts of anthropogenic developments and activities.

Kinship assessment and insights into reproductive behaviour of the Critically Endangered green sawfish Pristis zijsron in Western Australia

“…A total of 90 half sibling dyads were identified amongst all sampled individuals (Figure 2), involving 67 genotyped P. zijsron Catch data also indicated that one paternal half sibling dyad was pupped at roughly the same time; 2011 #06 was sampled six months prior to its half siblings 2011 #28 (Figure 2b), with the difference in TL (209 mm) consistent with local growth rates (Lear et al, 2023). It was estimated that the remaining paternal half siblings were pupped at least 5 months and up to 12 years apart, and were conceived by up to five dams for any single group.…”

“…Captured individuals were then released at the site of capture. All sawfish sampled were either young of the year (YOY or 0+) or juveniles older than one year of age (referred to as 'juveniles'), with age classes assigned based on regional growth rate information reported for P. zijsron by Lear et al (2023). Samples were collected from a total of 114 immature P. zijsron: 40 between 2011 and 2014 (21 YOY; 19 juveniles), and 74 between 2020-2022 (56 YOY; 18 juveniles) (Table 1).…”

“…It is unsurprising that no individual sampled in 2011-2014 was assigned as a parent to P. zijsron sampled in 2020-2022; a time span of 11 years. Pristis zijsron in the Ashburton River and adjacent tidal creeks have an estimated maturation time of at least 9-10 years (Lear et al, 2023). It therefore seems unlikely that young of the year (YOY) sawfish sampled between 2011 and 2014 would be reproducing prior to 2020-2022, particularly in Hooley Lagoon or Hooley and Four Mile Creeks, where maturation time is potentially delayed (see Lear et al, 2023).…”

Collaborative methods identify a remote global diversity hotspot of threatened, large‐bodied rhino rays

“…The high occurrence of green sawfish within the study region and presence of all size classes highlights the value of the area for this threatened species. It is established that green sawfish are born annually in and around the Ashburton River estuary (Onslow region), which they utilize as a nursery for extended periods (Morgan et al, 2015;Lear et al, 2023) and subsequently expand their range into other nearby creeks (Morgan et al, 2017). However, small numbers of neonates/juveniles were also observed along coastal areas of the Pilbara and the Exmouth Gulf, and juveniles have also been reported to the north and south of the study area (Morgan et al, 2011;Whitty et al, 2014; Department of Primary Industries and Regional Development, 2020).…”

Evolution of the Critically Endangered Green Sawfish Pristis zijsron (Rhinopristiformes, Pristidae), Inferred from the Whole Mitochondrial Genome

“…And it is the largest sawfish species of family Pristidae, reaching an impressive length of up to 24 feet. In Australia, the distribution of green sawfish is from the east coast of the central Pacific Ocean, around the northern continent, to Shark Bay in the south-west to the eastern Indian Ocean, but the population in north-eastern Australia has now declined dramatically, and may even be extinct along much of the east coast of Australia [12][13][14]. On the contrary, the species is still regularly found in western Australia.…”