Figure 7. Map showing Australian distribution of green sawfish

Figure 8. Green sawfish catch per unit effort data for the Queensland Gulf of Carpentaria fisheries (Peverell, 2005).

Population genetic diversity of green sawfish from northern Australia was also investigated. The levels of mtDNA and nDNA diversity in green sawfish were moderate to low and high, respectively, and within the range of values reported for other elasmobranchs, including other species of Pristis (Phillips et al., 2011; Phillips, 2012). However, the levels of mtDNA diversity in green sawfish in the Gulf of Carpentaria are reduced compared to those for the west and east coasts and are amongst the lowest reported for elasmobranchs.

The pattern of moderate to low levels of mtDNA and high levels of nDNA diversity suggest that the Australian population of green sawfish was founded by small numbers of individuals followed by population expansion and growth (Phillips, 2012). The signature of the genetic bottleneck/founder effect is very pronounced in the green sawfish, largetooth sawfish and dwarf sawfish and for all three species is stronger than those reported to date for any other elasmobranch. For both the green sawfish and dwarf sawfish, this signature is particularly strong in the Gulf of Carpentaria. This suggests that in addition to the founder effect, there have been contemporary declines in abundance and the continual pressure has prevented any recovery of alleles, especially in the Gulf of Carpentaria where levels of MtDNA diversity are reduced and the signature of the founder effect/genetic bottleneck is very strong (Phillips, 2012).

Important populations: The genetic analysis suggests that green sawfish form regional assemblages with little maternal or paternal movement between populations. Although the boundaries of these regions remain unknown, the implication is that local extinction will not be replenished in the short to medium term by outside migration. This means that the individual assemblages, or regions, should be identified and managed as independent populations (Phillips, 2012).

One of the more important assemblages identified for green sawfish include the Cape Keraudren region in northern Western Australia (Stevens et al., 2008). In 2008 several green sawfish were captured in gillnets at Cape Keraudren and large numbers of individuals were seen swimming in shallow water along a beach. This high abundance of animals potentially represents one of the highest densities of green sawfish in Australia. This assemblage is probably the southern stronghold of this species in Western Australia.

The Gulf of Carpentaria assemblage may also warrant special status as the levels of genetic diversity appear to be reduced compared to those for the west coast of Australia (Phillips, 2012) and green sawfish are considered rare in the Gulf (Peverell, 2005, 2009).

Also, any remaining populations on the east coast should be considered “near the edge of the species range” and are important in order to maintain genetic diversity.