Ontogenetic depth partitioning by juvenile freshwater sawfish (Pristis microdon: Pristidae) in a riverine environment

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) compared with 1+ individuals, which mainly occurred in depths >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.

What Is the Fate of Amputee Sawfish?

“…These behaviors have likely evolved in conjunction with the evolution of the elongated rostrum, because both P. pristis and their close relatives, rhinobatid shovelnose rays, respond to electric fields on the bottom in a similar fashion by biting them (Wringer et al 2012). Though scant information exists on predator-prey interactions of sawfishes in the wild, some species exhibit ontogenetic changes in habitat selection that are hypothesized to be driven by dietary shifts and/or predator avoidance (e.g., Whitty et al 2009). to readily source more than 1,000 rostra that had been removed from sawfishes in northern Australia, indicating that the practice of saw removal was at least historically commonplace.…”

Critically endangered Pristis microdon (Elasmobranchii), as a host for the Indian parasitic copepod, Caligus furcisetifer Redkar, Rangnekar et Murti, 1949 (Siphonostomatoida): New records from northern Australia

“…At the time of capture in the Fitzroy River site, mean salinities ranged from 10 to 15 ppt and water temperatures from ~25 to 31°C were recorded (see Whitty et al 2009). Similarly the Leichhardt River in September has mean salinities of 24 to 29 ppt and water temperatures from 25 to 28°C were recorded.…”

“…<1%) may simply be an artefact of the sampling bias towards juveniles of P. microdon. In northern Australia P. microdon are believed to be pupped near the mouths of rivers during the monsoonal wet season, then migrate upstream into freshwater habitats (Peverell 2005, Thorburn et al 2007, Whitty et al 2009) where they are often found large distances inland (Morgan et al 2004, Peverell 2008. The upstream movement of juvenile P. microdon from saline to fresh waters has been identified by the above authors as a possible behavioural mechanism to cleanse the body of unwanted parasites; although new born pups have been found This behaviour may in part lead to a loss of C. furcisetifer, which belongs to an almost exclusively marine group and has not been recorded on juvenile P. microdon in fresh waters of the Fitzroy and Leichhardt Rivers nor the Daly River, Northern Territory, Australia, and Sepik River and Lake Murray, Papua New Guinea (Ogawa 1991, present study).…”

“…Both sites are tidal and moderately saline, but receive high volumes of freshwater discharge particularly between January and June in each year from monsoonal weather patterns (see Whitty et al 2009). At the time of capture in the Fitzroy River site, mean salinities ranged from 10 to 15 ppt and water temperatures from ~25 to 31°C were recorded (see Whitty et al 2009).…”

Habitat Use and Foraging Ecology of a Batoid Community in Shark Bay, Western Australia

“…Because both sets of behaviors can benefit poikilothermic organisms, but depending on the thermal environment may predict similar or different habitat choices, it is necessary that studies of habitat use of mesoconsumers in thermally heterogeneous environments consider both temperature and predation risk, especially considering ongoing environmental change such as large predator decline and climate change. Movement patterns of several elasmobranchs have been examined using a variety of tagging and telemetry methods, but studies have tended to focus on large predators (e.g., Andrews et al 2010, Jorgensen et al 2010, commercially important species (e.g., Conrath and Musick 2010, King and McFarlane 2010, Sulikowski et al 2010, species with conservation concerns (e.g., Skomal et al 2009, Whitty et al 2009, Simpfendorfer et al 2010, or in easily accessible coastal shark nurseries (e.g., Heupel and Hueter 2002, Heupel et al 2004, Carlson et al 2008. Little attention has focused on the movements of batoids (e.g., rays) -(but see , Collins et al 2007) -even though they have the potential to be ecologically and economically important (VanBlaricom 1982, Thrush et al 1991).…”