Spring inflow effects on a rare shark
Words and photo: Barry Lyon
Recent studies have shown how spring inflow into the Wenlock River provides vital nursery habitat for one of the world’s rarest sharks.
The speartooth shark Glyphis glyphis, was named from a single, stuffed museum specimen way back in 1839. The place of capture was not known, so this shark was a real scientific mystery. It remained so until eventually discovered in southern New Guinea in the 1960s, and in Australia in the Bizant River on eastern Cape York in 1982. It has not been seen there since.
Later surveys established that the speartooth shark occurs in just a handful of rivers in northern Australia.
Until 2015, all records of speartooth sharks were of immature sharks living in highly turbid, fast flowing estuaries. In Queensland, the critically endangered speartooth shark is currently known only from the Wenlock and Ducie Rivers on north west Cape York.
This shark looks superficially similar to the bull shark. Two distinguishing features are the spearhead shaped teeth in the lower jaw (hence its name), and more than twice the number or electro-receptors around the head.
These electro-receptors enable sharks to detect the electrical currents of nearby animals, helping them to locate prey in dark or murky water.
The movements of 40 acoustically tagged speartooth sharks were researched in a collaborative study between the University of Queensland, CSIRO, and Australia Zoo in the Wenlock and Ducie Rivers.
In a nutshell, the sharks were found to occupy brackish, highly turbid (murky) estuarine and inshore coastal habitats, with salinity levels between one and 18 parts per thousand.
However, they moved seasonally in response to sustained changes in freshwater inflow. During the dry season, ‘spearies’ inhabited the upper estuary, then migrated to the lower estuary and adjacent coastal waters during the wet season when strong freshwater inflow occurred. As the new dry season developed, the sharks moved back upstream. All these movements allowed them to follow their preferred brackish water zone.
The Wenlock River features perennial dry season freshwater flow. As rainfall run off is extremely rare during the dry, it is a combination of sandstone and bauxite springs running into the river that provides that flow. The freshwater mixing with marine water in the estuary provides the particular brackish dry season habitats that speartooth sharks so vitally rely on.
This remarkable story of natural connections between land, water and rare river wildlife goes even wider. The spring water has to come from somewhere. In this case the sandstone and bauxite plateaus act as catchments for the spring aquifers, in the manner of a roof for a rainwater tank.
However, in these cases, the sandstone and bauxite layers are very porous – like huge geological sponges. So, much of the rain, instead of running off, seeps deep down into the ground, eventually reaching and topping up the spring aquifer.
Disturbing or removing the geological sponges on a large scale would readily upset the spring hydrology and thus the critical dry season habitats that the threatened speartooth sharks occupy. This would be like taking away the roof.
The spring inflow into the Wenlock River may also benefit the breeding activities of barramundi, allowing them to move downstream to breed when conditions are right, while land locked barra in other rivers have to wait for wet season floods.
There is still much to learn, however these examples highlight the need to care for catchments and eco-hydrological processes for the health and diversity of our valuable river systems.