Jeff M. Whitty

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

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.

Discovery of a pupping site and nursery for critically endangered green sawfish Pristis zijsron

A pilot study targeting sawfishes in the southern Pilbara region of Western Australia, which is undergoing a major expansion in human activity, was conducted using gillnets during April and October 2011 in the Ashburton Estuary and adjacent mangrove creeks. Catch per unit effort was greatest in the Ashburton Estuary in October, due to an influx of green sawfish Pristis zijsron pups, and was orders of magnitude higher than previously reported for any Pristidae; the study sites contained P. zijsron up to almost 3 m total length. This study identified the first pupping site for P. zijsron in Western Australia, and the most southerly known nursery area for the species in Australian waters, and is potentially the most important globally.

Mechanical challenges to freshwater residency in sharks and rays

ABSTRACT Major transitions between marine and freshwater habitats are relatively infrequent, primarily as a result of major physiological and ecological challenges. Few species of cartilaginous fish have evolved to occupy freshwater habitats. Current thought suggests that the metabolic physiology of sharks has remained a barrier to the diversification of this taxon in freshwater ecosystems. Here, we demonstrate that the physical properties of water provide an additional constraint for this species-rich group to occupy freshwater systems. Using hydromechanical modeling, we show that occurrence in fresh water results in a two- to three-fold increase in negative buoyancy for sharks and rays. This carries the energetic cost of lift production and results in increased buoyancy-dependent mechanical power requirements for swimming and increased optimal swim speeds. The primary source of buoyancy, the lipid-rich liver, offers only limited compensation for increased negative buoyancy as a result of decreasing water density; maintaining the same submerged weight would involve increasing the liver volume by very large amounts: 3- to 4-fold in scenarios where liver density is also reduced to currently observed minimal levels and 8-fold without any changes in liver density. The first data on body density from two species of elasmobranch occurring in freshwater (the bull shark Carcharhinus leucas, Müller and Henle 1839, and the largetooth sawfish Pristis pristis, Linnaeus 1758) support this hypothesis, showing similar liver sizes as marine forms but lower liver densities, but the greatest negative buoyancies of any elasmobranch studied to date. Our data suggest that the mechanical challenges associated with buoyancy control may have hampered the invasion of freshwater habitats in elasmobranchs, highlighting an additional key factor that may govern the predisposition of marine organisms to successfully establish in freshwater habitats. Summary: The high negative buoyancy of elasmobranchs increases the cost of locomotion and may be responsible for the scarcity of sharks in fresh water.

Are vertical migrations driven by circadian behaviour? Decoupling of activity and depth use in a large riverine elasmobranch, the freshwater sawfish (Pristis pristis)

Circadian rhythms occur widely amongst living organisms, often in response to diel changes in environmental conditions. In aquatic animals, circadian activity is often synchronised with diel changes in the depths individuals occupy and may be related to predator–prey interactions, where the circadian rhythm is determined by ambient light levels, or have a thermoregulatory purpose, where the circadian rhythm is governed by temperature. Here, these two hypotheses are examined using animal-attached accelerometers in juvenile freshwater sawfish occupying a riverine environment displaying seasonal changes in thermal stratification. Across seasons, diel patterns of depth use (shallow at night and deep in the day) tended to occur only in the late dry seasons when the water was stratified, whereas individuals were primarily shallow in the early dry season which featured no thermal stratification. Activity was elevated during crepuscular and nocturnal periods compared to daytime, regardless of the thermal environment. Our observation of resting at cooler depths is consistent with behavioural thermoregulation to reduce energy expenditure, whereas activity appears linked to ambient light levels and predator–prey interactions. This suggests that circadian rhythms in activity and vertical migrations are decoupled in this species and respond to independent environmental drivers.

An upstream migration fought with danger: freshwater sawfish fending off sharks and crocodiles

Studying predation on rare aquatic taxa is difficult particularly when dealing with legally protected, large predatory species. Moreover, observing predation on species found in low numbers is difficult in remote, turbid rivers, particularly where the species is nocturnal (Gleiss et al. 2017). Fig. 1, showing an Australian freshwater crocodile (Crocodylus johnstoni) preying upon a freshwater sawfish (Pristis pristis), represents a rare example with considerable implications for conservation. For a fish that is pupped at around 800 mm total length (TL) with formidable weaponry, one would assume that rates of natural predation would be low; however, sawfish are at risk from other large predators in the tropical freshwaters of northern Australia, including the bull shark (Carcharhinus leucas), estuarine crocodile (Crocodylus porosus), and the Australian freshwater crocodile. In 2011, we examined scarring on 39 freshwater sawfish ranging from 0.96 to 1.58 m TL at the Fitzroy River, and found evidence of bite marks on 23 (or 58.97%) individuals. Based on the appearance of the bite marks, the predators responsible for the attacks were crocodiles (21 occasions; most likely from Australian freshwater crocodiles) or bull sharks (three occasions), with one sawfish having marks attributed to both predators (see Figs. 1 and 2). Some had multiple bite marks, ranging from recent to healed scars; one having crocodile bite marks on its rostrum, head, caudal peduncle, and caudal fin. We have observed shark bites out of the fins of larger freshwater sawfish (Fig. 2). However, all appeared to be in otherwise relatively healthy condition. The study site is 160 km upstream from the river mouth, and each of the sawfish examined had migrated upstream from their birth place in the estuary (Morgan et al. 2011). Throughout the river, estuarine, and freshwater crocodiles as well as bull sharks threaten the freshwater sawfish with predation. Freshwater sawfish attain lengths >2.4 m by age 5 and grow faster than freshwater crocodiles (snout–vent length of ~0.8 m at age 20 [Tucker Ecology, 0(0), 2017, pp. 1–3

Do they stay or do they go? Acoustic monitoring of whale sharks at Ningaloo Marine Park, Western Australia

Whale sharks Rhincodon typus were monitored via acoustic transmitters at the northern end of Western Australias Ningaloo Marine Park to establish the extent to which the species inhabits the region beyond the whale-shark ecotourism industry season, which usually extends from March to August in each year. Despite the vast majority (c. 98%) of photographic submissions of R. typus from Ningaloo Reef being between March and August, acoustic detections from the tagged R. typus at Ningaloo were recorded in all months of the year, but do not preclude the occurrence of extended absences. It is concluded that as a species, R. typus occurs year round at Ningaloo, where it generally remains in close proximity to the reef edge, but that some individuals move outside of the detection range of the array for extended periods.