Spatial distribution and habitat utilisation of the speartooth shark Glyphis sp. A in relation to fishing in Northern Australia

About the report

A three week field trip to deploy listening stations and capture and tag Glyphis sp. A in the Wenlock River was undertaken in July/August 2007. Eight listening stations were deployed in the Wenlock River and covered a range of habitats from the mouth of the river (salinity = 27 ‰) to the freshwater reaches approximately 40 km upstream (salinity = 1.4 ‰). A total of 29 Glyphis sp. A were captured, however animals were only captured in salinities below 13 ‰ despite considerable fishing effort in salinities between 13-26 ‰. We only captured neonate and juvenile Glyphis sp. A below 80 cm total length (TL) during our survey. These data suggest that larger sub-adult sharks are occupying a different habitat to neonate and juvenile sharks which appear to be using the upper reaches of the river as a nursery area. The presence of larger sub-adult Glyphis sp. A in previous surveys downstream of the August 2007 capture sites indicates that these animals are utilising more saline habitats towards the mouth of the River. Although the current survey did sample these areas, no animals were captured, possibly due to seasonal shifts in the species habitat utilisation.

Three Glyphis sp. A were tracked continuously for periods of 24-27 hours and displayed similar movement patterns highlighted by limited movement and repeated use of the same habitat. All three sharks moved between 4-12 km up stream during the flood tide and similar distances downstream during the ebb tide.

We attached coded tags to 11 Glyphis sp. A within the listening station array between 27 July-6 August 2007. Data downloaded from the acoustic receivers on the 31 August 2007 provided information on the habitat utilisation of five animals showing that Glyphis sp. A in the Wenlock have an extremely small critical habitat. All five animals spent more than 80 % of their time in a limited region of the river with a total area of less than 10km2.

In addition to their limited habitat utilisation, the narrow distribution of Glyphis sp. A within the Wenlock River makes them highly vulnerable to over fishing and localised extinction. Data on their population size and trends in population size over time are urgently required. The limited habitat utilisation of neonate and juvenile sharks makes Marine Protected Areas (MPAs) a viable management tool to reduce fishing mortality; however more data are required on the movement patterns of sub-adult and adult sharks to inform on the management needs of these animals.