Fisheries-independent surveys identify critical habitats for young scalloped hammerhead sharks (Sphyrna lewini) in the Rewa Delta, Fiji

Abstract

Sharp declines in numerous shark populations around the world have generated considerable interest in better understanding and characterising their biology, ecology and critical habitats. The scalloped hammerhead shark (SHS, Sphyrna lewini) is subject to a multitude of natural and anthropogenic threats that are often exacerbated within the coastal embayments and estuaries used during SHS early life stages. In this study, we describe the temporal and spatial distribution, age class composition, and reproductive biology of SHS in the Rewa Delta (RD), Fiji. A total of 1054 SHS (including 796 tagged individuals; 101 of which were recaptured) were captured from September 2014 to March 2016 in the RD. A majority of the captures in this area were neonates and young-of-the-year (YOY) (99.8%). Significant seasonality in patterns of occurrence of both neonates and YOY individuals suggests a defined parturition period during the austral summer. Between the seven sampling sites in the RD we also found significant differences in SHS neonate catch per unit of effort, and average total length of individuals. According to the data, the RD is likely to represent an important nursery area for SHS up to one year of age.

Essential waters: Young bull sharks in Fiji's largest riverine system

“…These results suggest that the parturition period in Fiji's southern riverine systems occurs during the wet austral summer season, with a possible peak between December and January. This is in line with Cardeñosa et al (), who reported that bull shark sightings or catches by fishers in Fiji mainly occur during summer, and also largely overlap with the characterization of other shark species' parturition season in Fiji (Marie et al, ; Vierus et al, ). Furthermore, parturition occurring in the austral summer corresponds with direct observations made in the SRMR where pregnant bull sharks leave the feeding site in late October, returning after parturition at the beginning of the year (Brunnschweiler et al, ; Brunnschweiler & Baensch, ).…”

“…To date, information on parturition sites and nursery areas is known for only a few species. Marie, Miller, Cawich, Piovano, and Rico () confirmed the Rewa Delta as important habitat for juvenile scalloped hammerhead sharks ( Sphyrna lewini ), and Vierus et al () discovered a multispecies shark aggregation and parturition area in the Ba Estuary on the northern coast of Viti Levu. The latter study documented three juvenile bull sharks caught several kilometers upstream in the Ba River.…”

“…To date, information on parturition sites and nursery areas is known for only a few species. Marie, Miller, Cawich, Piovano, and Rico (2017) (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.…”

Discovery of a multispecies shark aggregation and parturition area in the Ba Estuary, Fiji Islands

“…As exclusively juvenile sharks were encountered, it is likely that the studied nearshore environment constitutes another parturition ground in Fiji (Brown et al., ; Cardeñosa et al., ; Marie et al., ). Size ranges of C. limbatus and S. lewini (66 ± 4 and 52 ± 5 cm, respectively) were in accordance with size ranges of neonate and young‐of‐the‐year sharks from previously published studies (Castro, ; Castillo‐Géniz, Márquez‐Farias, Rodriguez de la Cruz, Cortés, & Cid del Prado, ; Brown et al., for S. lewini ).…”

“…Using data collected from dive operators, citizen scientists and local fishermen, an increasingly clear picture of shark species abundance throughout Fiji is emerging (Brunnschweiler, Abrantes, & Barnett, 2014; Glaus, Adrian-Kalchhauser, Burkhardt-Holm, White, & Brunnschweiler, 2015;Rasalato, Maginnity, & Brunnschweiler, 2010;Ward-Paige, 2014). Specific locations with confirmed species occurrence in the scientific literature are only available for Viti Levu (Brown, Seeto, Lal, & Miller, 2016;Brunnschweiler & Earle, 2006;Cardeñosa, Glaus, & Brunnschweiler, 2017;Marie, Miller, Cawich, Piovano, & Rico, 2017) and Vanua Levu (Goetze & Fullwood, 2013), the two largest islands of Fiji. In the former case, this has led to the establishment of the Shark Reef Marine Reserve, Fiji's first national marine park, and the Fiji Shark Corridor which comprises approximately 30 miles of coastline (Brunnschweiler, 2010).…”