Sawfish of the Kimberley

NS Branch March Meeting

Guest speaker Karissa Lear ( PhD Candidate, Murdoch University) gave a presentation that was very timely, given that only five days before, the State Government had released a statement regarding the Fitzroy River Management Plan—a water allocation plan and a proposed national park, all of which will impact on the survival of the river’s sawfish.

While most of the audience knew the difference between sawfish and swordfish, the difference between sawfish and sawsharks was less well known. There are probably numerous differences but the most obvious is the position of the gills—on the underside for the former, like rays, and laterally for the latter.

Sawshark

The north-west is home to four of the world’s five species of sawfish but most research has been done on the Freshwater Sawfish (Pristis pristis) and the Dwarf Sawfish (Pristis clavata). Karissa and the team from Murdoch University have concentrated their research on the Freshwater Sawfish. This work has also involved the local Indigenous communities and rangers, who have provided both anecdotal information and practical help with the field work.

All the northwest sawfish are listed as either Critically Endangered or Endangered by the IUCN. Of all the sharks and rays, sawfish are considered the most likely to become extinct. At least 20 countries have lost all their sawfish and 43 countries have lost at least one species.

The lifecycle of the Freshwater Sawfish, Australia’s largest freshwater fish, is only partially understood. Very little is known about the adults, which can grow to 7 metres, other than the pregnant females move in from the ocean to the mouths of rivers along the Kimberley coastline—particularly the Fitzroy—during the Wet season. They then give birth to live young (pups), which swim upstream and spend the next 4-6 years in the river, tributaries and pools before returning to the sea. Juveniles have been found as far as 500 km (by stream length) from the sea.

Freshwater Sawfish

Studies have shown the number of newborn is higher in wet years and that sawfish lose weight during the Dry season when their habitat is reduced to isolated pools.

Researchers are using both acoustic tagging and “smart” tagging (using accelerometers, the same technology as used in Fitbits) to record the activities of the sawfish. Karissa is using smart tagging to answer questions such as: when are sawfish active? where do they rest? where do they forage? and how do these parameters change with size or river conditions?

Her studies have also shown that, at least in captive tagged fish, their resting metabolic rate increases with water temperature. This has serious implications for the species as both climate changes (warming) and water extraction will lead to an increase in water temperature. The consequence could be an increase in the mortality rate of young sawfish due to starvation.


Recommended viewing

Don Poynton