Papua New Guinea: A Potential Refuge for Threatened Indo–Pacific River Sharks and Sawfishes

Abstract

The conservation of threatened elasmobranchs in tropical regions is challenging due to high local reliance on aquatic and marine resources. Due primarily to fishing pressure, river sharks (Glyphis) and sawfishes (Pristidae) have experienced large population declines in the Indo-Pacific. Papua New Guinea (PNG) may offer a refuge for these species, as human population density is low, and river shark and sawfish populations are thought to persist. However, few data are available on these species in PNG, and risk posed by small-scale fishers is poorly understood. This study observed elasmobranch catches in small-scale fisheries in riverine and coastal environments in the East Sepik (northern region), Gulf, and Western Provinces (southern region) of PNG. Surveys were conducted over a period of weeks to months in each region, during the dry season across seven field trips from 2017 to 2020. We observed a total of 783 elasmobranchs encompassing 38 species from 10 families. River sharks made up 29.4% of observations in the southern region, while sawfishes made up 14.8 and 20.3% in the northern and southern regions, respectively. River sharks were commonly caught by small-scale fishers in lower riverine environments in southern PNG, while sawfishes were generally less common and mainly observed through dried rostra. The primary threat to river shark and sawfish populations is their capture by small-scale fishers targeting teleosts for swim bladder. Persisting populations of river sharks and sawfishes indicate that PNG is the second known nation with viable populations of multiple species in the Indo-Pacific. However, populations are declining or at high risk of decline, and fisheries management and conservation are required to realize the potential of PNG as a long-term refuge.

Elemental analysis of vertebrae discerns diadromous movements of threatened non‐marine elasmobranchs

“…Across Australia and PNG there is very limited evidence that A. cuspidata penetrates into estuaries within rivers (e.g., lack of riverine records in study areas of Grant, White, Amepou, Appleyard et al, 2021;Field et al, 2013;Morgan et al, 2011;Peverell, 2005;Thorburn et al, 2003). All A. cuspidata specimens used in the present study were obtained from a fishery operating mainly within, and closely adjacent to the mouth of, the Sepik River (Grant, White, Amepou, Appleyard et al, 2021). However, the salinityindicating elements suggest that these individuals were generally not using these low-salinity environments at the river mouth for prolonged periods, with the exception of two individuals (it should be noted that the Sepik River lacks a defined estuarine zone within the river, with water being practically fresh at the mouth; Coates, 1987).…”

“…In the Indo‐Pacific, river sharks ( Glyphis spp.) and sawfishes (Pristidae) underline the imperiled conservation state of non‐marine elasmobranchs, with viable populations now largely restricted to regions of low human population density (e.g., Grant, White, Amepou, Appleyard et al, 2021; Morgan et al, 2011). Presently, our limited understanding of life‐history‐specific habitat use patterns of river sharks and sawfishes may be impeding their effective conservation management (Dulvy et al, 2016; Stevens et al, 2008; White et al, 2015).…”

“…Diadromous environmental use patterns were explored for two river sharks (the northern river shark Glyphis garricki and the speartooth shark Glyphis glyphis ) and two species of sawfish (largetooth sawfish Pristis pristis and narrow sawfish Anoxypristis cuspidata ). Neonates and juveniles of both river shark species are known to occur in large macro‐tidal rivers of northern Australia (Dwyer et al, 2020; Lyon et al, 2017; Pillans et al, 2009) and Papua New Guinea (PNG, Grant, White, Amepou, Appleyard et al, 2021). However, it is unclear how long they remain in riverine environments and whether these species have segregated or communal nurseries in rivers they co‐occur in.…”

An anecdotal observation of an amputee sawfish recapture and development of safe release guide for Papua New Guinea gillnet fisheries

“…Sawfishes (Pristidae) are highly threatened, with four species listed as Critically Endangered, and one as Endangered on the International Union for the Conservation of Nature Red List of Threatened Species. Papua New Guinea (PNG) has recently been identified as a sawfish 'stronghold' within the Indo-West Pacific (Grant et al 2021a). However, sawfishes in PNG are vulnerable to small-scale fishers using gillnets to target barramundi (Lates calcarifer) and scaly craoker (local name 'stone fish', Nibea squamosa) for the fish maw (teleost swim bladder) trade (Grant et al 2021a(Grant et al , 2021b.…”

“…Papua New Guinea (PNG) has recently been identified as a sawfish 'stronghold' within the Indo-West Pacific (Grant et al 2021a). However, sawfishes in PNG are vulnerable to small-scale fishers using gillnets to target barramundi (Lates calcarifer) and scaly craoker (local name 'stone fish', Nibea squamosa) for the fish maw (teleost swim bladder) trade (Grant et al 2021a(Grant et al , 2021b. When captured, fishers often amputate the rostrum of sawfish to assist with dis-entanglement, regardless of whether the animal is being retained for consumption, sale, or being released (Grant et al 2021b).…”