1 April 2015
Australia Zoo has been researching the critically endangered Speartooth Shark Glyphis glyphis in the Wenlock and Ducie Rivers since 2012, in collaboration with the University of Queensland and CSIRO.
These rivers, which bound the Steve Irwin Wildlife Reserve, are the only places in Queensland where this shark is known to occur and the Wenlock has been shown to support the largest populations. Speartooth Sharks also inhabit just a handful of rivers in the Northern Territory. So rare is the Spearie in fact, that no adults have ever been caught.
The shark's name comes from the distinctive spear shaped teeth in the lower jaw, and it also features nearly 700 electro-receptors around its head, more than twice the number Bullsharks possess. This suggests that the animal is very well adapted to hunting in dark and turbid water.
We are currently analysing the data of 85 juvenile to sub adult sharks - up to 1.45 metres long, 55 of which are being acoustically tracked in the same way crocodiles are being tracked in the Wenlock.
The data clearly shows that the Wenlock serves as crucial nursery habitat for juvenile, with these youngsters - still with umbilical scars, appearing in the river in October and November. It may be that the breeding females give birth to their pups at the mouth of the river, and the juveniles then swim upstream in the same manner as Bullsharks.
What is known is that these sharks use the tides to travel, and may swim 25 kilometres or more on the one tide, reversing direction as the tide turns. And significantly, Speartooth Sharks live in a narrow zone of the river where upstream fresh water first mixes with the salt, which is typically also very dirty. They also undertake very distinctive inter- seasonal movements. When the Wenlock first floods at the beginning of the wet, all the tagged sharks moved down to the river mouth following that brackish zone, some even entering Port Musgrave, the coastal embayment into which the river drains. Soon after the monsoon season finishes in April, the sharks begin ranging back to their upstream dry season home ranges, but they always stay within that migrating, brackish zone.
The fact that the Wenlock runs fresh all year appears to be crucial for this population of Speartooth Sharks. It is fed during the dry season from bauxite springs on the Steve Irwin Wildlife Reserve and nearby, and sandstone springs further upstream. This ecological function shows the amazing connections that occur across landscapes. The bauxite plateaus absorb monsoonal summer rain, which then flows out as springs into the Wenlock over the dry, creating just the right salinity levels for the sharks. Who would have thought that a bauxite plateau, many kilometres from a river, supported the existence of a critically endangered shark species!
Article by Barry Lyon, Steve Irwin Wildlife Reserve Conservation Manager