Abstract
Coastal and estuarine systems provide critical shark habitats due to their relatively high productivity and shallow, protected waters. The young (neonates, young‐of‐the‐year, and juveniles) of many coastal shark species occupy a diverse range of habitats and areas where they experience environmental variability, including acute and seasonal shifts in local salinities and temperatures. Although the location and functioning of essential shark habitats has been a focus in recent shark research, there is a paucity of data from the South Pacific. In this study, we document the temporal and spatial distribution, age class composition, and environmental parameters of young bull sharks (Carcharhinus leucas) in the Rewa, Sigatoka, and Navua Rivers, Fiji's three largest riverine systems. One hundred and seventy‐two young bull sharks were captured in fisheries‐independent surveys from January 2016 to April 2018. The vast majority of the captures were neonates. Seasonality in patterns of occurrence of neonate individuals suggests a defined parturition period during summer. Environmental parameters between the Rewa and the Sigatoka River differed significantly, as did the recorded young bull sharks abundance. According to the surveys, young bull sharks occur in all three rivers with the Rewa River likely representing essential habitat for newly born bull sharks. These results enhance the understanding of bull shark ecology in Fiji and provide a scientific basis for the implementation of local conservation strategies that contribute to the protection of critical habitats.
Highlights
Essential fish habitats (EFH) are “those waters and substrate nec‐ essary to fish for spawning, breeding, feeding or growing to matu‐ rity” (Rosenberg, Bigford, Leathery, Hill, & Bickers, 2000)
Ne‐ onate and YOY bull sharks experience environmental variability including acute and seasonal shifts in local salinities and water tem‐ peratures which can expose them to a range from 0 to 40 Practical Salinity Units (PSU) and 14.4–32.4°C, respectively (Froeschke et al, 2010; Heupel & Simpfendorfer, 2008)
Our results confirm the presence of young bull sharks in all three surveyed rivers
Summary
Essential fish habitats (EFH) are “those waters and substrate nec‐ essary to fish for spawning, breeding, feeding or growing to matu‐ rity” (Rosenberg, Bigford, Leathery, Hill, & Bickers, 2000). For exam‐ ple, within subtropical regions, age‐associated habitat transitions have been documented with YOY bull sharks occupying locations with lower mean salinities than juveniles (Heithaus et al, 2009; Simpfendorfer, Freitas, Wiley, & Heupel, 2005), while subadults and adults were more abundant in nearshore marine areas (Werry, 2010; Werry, Lee, Otway, Hu, & Sumpton, 2011). Juvenile bull sharks were confirmed in the Navua River close to the Shark Reef Marine Reserve (SRMR) where large adult bull sharks are abundant (Brunnschweiler, Abrantes, & Barnett, 2014; Cardeñosa, Glaus, & Brunnschweiler, 2017) In addition to these observed occurrences, results from an interview‐based survey documented small sharks in all of Fiji's major rivers (Rasalato, Maginnity, & Brunnschweiler, 2010) suggesting that in particular Fiji's largest riverine systems, the Rewa and Sigatoka Rivers on the southern coast of Viti Levu represent EFH for young bull sharks. We include fishery‐dependent data on the size, umbilical scar condition, and sex of young bull sharks captured opportunistically by local fishermen in the Navua River
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