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

Karissa O Lear,Dean C Thorburn,Nathan Green,Brendan C Ebner,Travis Fazeldean,David L Morgan,J R Albert,Adrian C Gleiss,Stephen J Beatty,Jeff M Whitty

doi:10.1038/s41598-019-53511-9

Karissa O Lear, Dean C Thorburn + Show 8 more

Open Access

https://doi.org/10.1038/s41598-019-53511-9

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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.

Highlights

The freshwater sawfish (Pristis pristis) was recently listed as the most Evolutionarily Distinct and Globally Endangered (EDGE) animal
Including all size classes and capture methods, over 500 distinct individual freshwater sawfish were caught in the Fitzroy River between 2002 and 2018 in over 5,600 20-m net hours sampled
Annual catch per unit effort (CPUE) for all freshwater sawfish ranged from 0.008–0.45 sawfish 20-m net−1 h−1 in freshwater environments, and 0–0.28 sawfish 20-m net−1 h−1 in estuarine environments

Summary

Introduction

The freshwater sawfish (Pristis pristis) was recently listed as the most Evolutionarily Distinct and Globally Endangered (EDGE) animal. 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. Such dynamic environments show substantial inter-annual variation in environmental conditions and resource availability, and typically host species that have evolved strategies to deal with the www.nature.com/scientificreports dynamics of the water and energy supply regime[14,15] Determining how these species interact with their ecosystem and cope with the inherent extreme environmental variation requires ecological studies spanning long time periods in order to reveal the full extent of important ecological processes and population dynamics[16,17]. The freshwater sawfish has been identified as a promising ‘flagship species,’ that could be harnessed to enhance awareness and engender conservation efforts in freshwater environments[26]

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