Recruitment of a critically endangered sawfish into a riverine nursery depends on natural flow regimes

Abstract

The freshwater sawfish (Pristis pristis) was recently listed as the most Evolutionarily Distinct and Globally Endangered (EDGE) animal. The Fitzroy River in the remote Kimberley region of north-western Australia represents a significant stronghold for the species, which uses the freshwater reaches of the river as a nursery. There is also mounting pressure to develop the water resources of the region for agriculture that may substantially affect life history dynamics of sawfish in this system. However, the relationship between hydrology and population dynamics of freshwater sawfish was unknown. We used standardized catch data collected over 17 years to determine how wet season volume influences recruitment of freshwater sawfish into their riverine nursery. Negligible recruitment occurred in years with few days of high flood levels (above 98th percentile of cease-to-flow stage height), and relatively high recruitment occurred in years with 14 or more days of high flood levels. This relationship is indicative of a distinct boom-or-bust cycle, whereby freshwater sawfish rely almost entirely on the few years with large wet season floods, and the brief periods of highest water levels within these years, to replenish juvenile populations in the Fitzroy River nursery. This has direct implications for sustainable water resource management for the Fitzroy River basin in order to preserve one of the last known intact nursery habitats for this globally threatened species.

Niche partitioning between river shark species is driven by seasonal fluctuations in environmental salinity

“…In tropical rivers and estuaries, anthropogenic threats such as habitat degradation, water extraction, pollution and fisheries continue to threaten freshwater and euryhaline elasmobranch communities (Dwyer et al., 2019; Field et al., 2013; Grant et al., 2019; Knip et al., 2010; Lear et al., 2019). While some elasmobranch species such as C. leucas are thought to adapt to more urbanized rivers and estuaries quite readily (Heupel & Simpfendorfer, 2011), other species have been shown to more susceptible to extensive coastal development (Grant et al., 2019; Jennings, Gruber, Franks, Kessel, & Robertson, 2008).…”

“…sharks, rays and sawfish) are among the most poorly understood and threatened groups of vertebrates (Compagno & Cook, 1995; Grant et al., 2019), with the majority of species listed classified as Data Deficient by the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species, or threatened with extinction (IUCN Red List categories CR, EN, VU; IUCN, 2018). The greatest diversity of euryhaline elasmobranchs is found in tropical regions (Grant et al., 2019), where neonate and juvenile life stages typically segregate from adults into low salinity nursery habitats that are thought to promote the survivorship of young through the exclusion of large conspecifics and other predators (Heupel & Simpfendorfer, 2011; Lear et al., 2019; Matich, Heithaus, & Layman, 2010). However, tropical rivers and estuaries are highly dynamic environments where species must continually adjust to daily and seasonal fluctuations in salinity, flow, temperature and turbidity that can not only impact the maintenance of physiological homoeostasis but also higher order processes such as patterns of animal behaviour and space use.…”

“…throughout the tide, across seasons and between years), and what the proximate and ultimate mechanisms are that drive these spatial patterns. This understanding is important given that tropical rivers and estuaries are increasingly under threat from land clearing, pollution and flow modification (Grant et al., 2019; Lear et al., 2019; Vörösmarty et al., 2010) and by occupying nearshore and inshore habitats, euryhaline elasmobranchs are confined by physical boundaries that limit their ability to evade human‐induced threats (Compagno & Cook, 1995). Given the high susceptibility of euryhaline elasmobranchs and their nursery areas to habitat degradation and modification (Dwyer et al., 2019; Lear et al., 2019), combined with low fecundity, late sexual maturation, naturally low population densities and highly threatened status of all sharks of the genus Glyphis (Ballantyne & Robinson, 2010; Grant et al., 2019; Martin, 2005), there is an urgent need to understand the environmental and physiological factors influencing the occurrence of C. leucas and G. glyphis to predict how these species will respond to environmental change.…”

New insights into the food web of an Australian tropical river to inform water resource management

“…Water managers are tasked with setting water allocation policies that protect the ecological and cultural values of the river, particularly those of the river’s lower reaches that are most likely to be impacted by water extraction. However, there are scant ecological data from the Fitzroy River to support decision making 13 , but see 14 for an exception.…”

“…However, there are scant ecological data from the Fitzroy River to support decision making 13 , but see 14 for an exception.…”