Sawfish

Abstract: Environmental DNA (eDNA) is a relatively new tool for the detection of rare, threatened and invasive species in water bodies. In this study we investigated the utility of an eDNA approach in detecting the Critically Endangered largetooth sawfish Pristis pristis in freshwater habitats in northern Australia. Water samples were collected from large aquaria mesocosms containing largetooth sawfish and other aquatic species, and floodplain waterholes and the main river channel of the Daly River, Northern Territory. Water samples were filtered using a 20 mu m nylon filter. DNA was extracted from filters and analysed with PCR using species-specific mitochondrial cytochrome c oxidase subunit I (COI) primers designed to amplify only largetooth sawfish DNA. PCR products were cleaned and the COI gene sequenced to confirm the species identity. Using 3 aquaria, with one containing a largetooth sawfish, this method positively identified sawfish only in the correct aquarium. In the field water samples, 7 of 8 floodplain waterholes produced a sawfish eDNA PCR product, while eDNA was not detected in the main river channel. Based on gillnet sampling and traditional ecological knowledge, largetooth sawfish were known to occur at half of the waterhole and floodplain sites that tested positive for sawfish eDNA. These results demonstrated that an eDNA approach to detecting largetooth sawfish can produce reliable outcomes and can be used as a survey tool to help with conservation efforts for this and other threatened elasmobranchs.

Abstract: The freshwater sawfish (Pristis microdon) is a critically endangered elasmobranch. Ontogenetic changes in the habitat use of juvenile P. microdon were studied using acoustic tracking in the Fitzroy River, Western Australia. Habitat partitioning was significant between 0+ (2007 year class) and larger 1+ (2006 year class) P. microdon. Smaller 0+ fish generally occupied shallower water ( 0.6 m. Significant differences in hourly depth use were also revealed. The depth that 1+ P. microdon occupied was significantly influenced by lunar phase with these animals utilising a shallower and narrower depth range during the full moon compared with the new moon. This was not observed in 0+ individuals. Habitat partitioning was likely to be related to predator avoidance, foraging behaviours, and temperature and/or light regimes. The occurrence of 1+ P. microdon in deeper water may also result from a need for greater depths in which to manoeuvre. The present study demonstrates the utility of acoustic telemetry in monitoring P. microdon in a riverine environment. These results demonstrate the need to consider the habitat requirements of different P. microdon cohorts in the strategic planning of natural resources and will aid in the development of management strategies for this species.