River shark survey discovers significant population

One of the northern river sharks captured during the recent NERP East Alligator River Survey

Recently the NERP threatened euryhaline elasmobranch (sawfish and river sharks) project surveyed the East Alligator River for river sharks. From 26 September to 3 October 2013 these surveys documented a previously unrecognised significant population of the endangered northern river shark.

Seventeen sites were surveyed downstream of Cahill’s Crossing with northern river sharks recorded at all sites. Seventy-one individuals from young sharks to adult males and females were recorded and PIT tagged. Previous to this survey there were only two records of northern river sharks from the East Alligator.

A much lower number of the critically endangered speartooth shark were recorded (six) while bull sharks were present throughout the river (19 recorded). Bull sharks were also the only shark species recorded in limited fishing upstream of Cahill’s Crossing and in Magela Creek.

These surveys demonstrate that the East Alligator provides critical habitat for northern river sharks and together with the South Alligator River population (which the NERP team are also surveying), reflect the national and global importance of Kakadu National Park for this species.

In sum, the elasmobranch catch for this trip consisted of:

Northern river shark Glyphis garricki                    71

Speartooth shark Glyphis glyphis 6

Bull shark Carcharhinus leucas                              19

Many thanks to Mick Gorst for all of his great assistance, and to Anne O’Dea and the traditional owners of Kakadu.

This blog was originally published on the Charles Darwin Unviersity blog site and is reproduced with the kind permission of Dr Peter Kyne.

NERP is the National Environmental Research Program.