In the early morning light, the team of scientists quietly maneuvered their boat through the serene waters of the Ashburton River estuary. PhD candidate Jack Ingelbrecht, leading the expedition, scanned the surface, his eyes keen for any sign of movement. “Keep an eye out, everyone,” he murmured, his voice barely above a whisper. “We’re nearing the prime nursery area.” As they approached a shallow inlet, they cast their nets, the monofilament gillnets gently settling into the water, a technique refined from years of experience. And then the waiting began. One thing they don’t tell you about wildlife science is how much waiting has to happen before anything else happens.
“Careful now,” someone cautioned as they slowly reeled in the net. A young green sawfish (Pristis zijsron) emerged from the water, its distinctive saw-like snout visible as it thrashed slightly in the net. “Got one!”
Together, the team carefully guided the sawfish onto its back, submerging its gills to induce a state of tonic immobility—a technique crucial for minimizing stress during measurements. One pair of hands measured the sawfish’s total length, while another carefully inspected the sex organs to determine its gender and yet another jotted down notes on a waterproof clipboard. They made way for the geneticists on the tram, who took tissue samples from the sawfish’s fins, using clippings and hole punches, and preserved them meticulously in small vials of 100% ethanol.
“Samples secured,” a voice announced, their voice tinged with satisfaction as the team gently released the sawfish back into the water, watching as it swam away with powerful strokes of its tail. “Let’s mark this location and move on to the next site.”
Throughout the day, the team repeated this process, navigating the labyrinth of tidal creeks and river bends, encountering more young green sawfish along the way. Each capture, each measurement, and each tissue sample brought them closer to understanding the intricate social and genetic dynamics of this critically endangered animal in their vital nursery habitats. It’s a groundbreaking study set in Western Australia, where researchers have utilized single nucleotide polymorphisms (SNPs) to delve into the species’ kinship dynamics and philopatry.
Today, over one-third of elasmobranch species face extinction threats, and many believe this is a call for scientists to further integrate genetic data into conservation strategies. Among the most imperiled are sawfishes (Pristidae), known for its distinctive elongated saw-like rostrum, whose decline over the past century is largely attributed to entanglement in fishing gear. Historically, they played significant ecological roles as apex predators, influencing prey populations and maintaining ecosystem balance. But sawfish have seen a significant reduction in its range, and for green sawfish it’s estimated a decline of 38% across the Indo-West Pacific with viable populations mostly confined to northern Australia. Found in tropical and subtropical waters of the Indo-West Pacific, including Australia, the green sawfish is known to inhabit shallow coastal and estuarine habitats, and are vulnerable to anthropogenic threats such as habitat degradation, fishing pressures, and incidental capture in fishing gear. Both male and female P. zijsron are believed to exhibit regional philopatry, reinforcing the need for localized conservation efforts and protection of critical habitats.
The study, spanning from 2011 to 2022, involved the genetic analysis of 104 green sawfish individuals. It provides compelling evidence of localized philopatry in both male and female green sawfish within this specific region, a behavior highlighted by the presence of numerous maternal and paternal half-sibling pairs born in different years, suggesting that several female sawfish have returned repeatedly to these waters to give birth. Notably, the analysis revealed a significant level of relatedness across the study area, with 88% of the sampled P. zijsron individuals having at least one genetic link up to the third degree. Over the study period, researchers identified 52 full sibling dyads, 90 half sibling dyads, and 72 third-degree dyads among the sampled individuals. This pattern spans over a decade, indicating a remarkable fidelity to the Ashburton River and its surrounding creeks as critical parturition sites.
Interestingly, while philopatry is evident among these sawfish, the degree to which it occurs can vary among individuals and populations. Some may disperse to alternative sites, and male-biased dispersal has been noted in related species like largetooth (Pristis pristis) and narrow sawfish (Anoxypristis cuspidata). This hints at a complexity of migration behaviors within the sawfish family, one that we have yet to unravel or understand but points to evolutionary adaptations in these animals shaped by environmental and genetic factors.
This new research also sheds light on the genetic diversity and potential risks of inbreeding within remaining green sawfish populations. The identification of multiple paternity within maternal half sibling groups has led to the team wanting to carry out further investigation into genetic diversity levels here and look into the occurrence of inbreeding, which could threaten the long-term viability of these already endangered populations. Further research, they believe, is needed to explore the extent of multiple paternity across different populations of this species and its implications for population dynamics and conservation strategies.
From a conservation perspective, protecting key habitats like the Ashburton River and the adjacent creeks is paramount. These areas not only serve as crucial breeding grounds but seem to be crucial in the long-term sustainability of P. zijsron populations. Preserving nursery areas and ensuring connectivity between populations are crucial steps in safeguarding the genetic health and resilience of the green sawfish in the face of anthropogenic threats such as fishing pressures and habitat loss. By understanding the species’ kinship patterns and philopatric behavior, conservationists can better prioritize and manage these animals. Moving forward, continued research and targeted conservation efforts will be essential in ensuring a sustainable future for the green sawfish in Western Australia and beyond.
“We’re making progress,” one of the scientists said, as the team’s gaze lingering on the estuary. With a collective nod, they headed back to the shore, eager to analyze another day’s findings and help safeguard the future of the critically endangered predator.
