This is a preliminary report of an ongoing Shark Conservation Research Program by Amandine Marie, Celso Cawich, Tom Vierus, Cara Miller, Susanna Piovano and Ciro Rico.
The Scalloped Hammerhead Shark (SHS, Sphyrna lewini; Figure 1) is among the most globally threatened shark species and has recently being included in the Appendix II of CITES. This study presents empirical evidence for the existence of a SHS nursery in the Rewa Delta in Fiji (Figure 2). To the best of our knowledge, it is the largest and probably the most important nursery ground for the species ever documented in the THROUGHT SCIENTIFIC RESEARCH in the world.
A total of 1217 SHS captures (including 107 recaptures) were made in the Rewa Delta from September 2014 to March 2016 by using gill nets. A total 952 individual SHS (+ 102 recaptures) were captured during the field surveys and an additional total of 163 SHS were captured by local fishermen as by-catch (five of these 163 individuals were previously tagged during our study). Of the 952 captured individuals, 797 individuals were tagged and released. Ninety three out of 952 of the individuals we captured during our survey died (9.8%) as a consequence of the netting. Of these 93 individuals 20 were tagged.
According to the definition of a shark nursery area currently accepted by the scientific community our results unambiguously confirmed that the Rewa Delta is a nursery ground for SHS because the following conditions were met:
A) Sharks are more commonly encountered in nursery the area than in other surrounding areas
A significant spatial difference in the number of individuals caught was found among the sites over 18 months of the study. The highest Catch per Unit Effort (CPUE) was found right in the middle of the Rewa Delta at about 3 km from the mouth of the river.
B) Sharks have a tendency to remain or return for extended periods
Site fidelity of SHS neonate and young of the year (YOY) was unambiguously demonstrated for the Rewa Delta. Firstly, the presence of neonate and YOY sharks was observed in the Rewa Delta throughout the year and from the length distribution of the capture there was evidence of the presence of at least two age classes, neonates, YOY and 1+ years. There were also significant differences in the number of each age class utilizing the area throughout the year. Opened (O) and semi-healed (SH) umbilical scars characterise the neonate period which is complete upon scar closure, which takes approximately 15 days. During this period, SHS can reach a maximum of 60 cm. Our results showed that 8.5% and 27.2% of the captures displayed opened and semi-healed scars, respectively. These suggest that the parturition period in Fiji has the same seasonality of other Pacific areas, including Hawaii and Australia. YOY identified by the healed or well-healed umbilical scars composed the majority of the SHS sampled during the study period (64.4%). These individuals measured on average 55cm ± 7.4cm and their estimated ages were between 15 and 300 days. They were present in all months, and this class was the only one present from July and September 2015. Based on the reported growth estimates, the older male caught was at least one year old (length of 86 cm) and older female was between 2 and 3 years old (length of 116.5 cm). Secondly, site fidelity to the Rewa Delta by gravid females for parturition and for neonates and YOY was inferred because of the 797 tagged sharks, 88 individuals (11.0%) were recaptured during the field surveys, and some of them were recaptured more than once and remained in the Rewa Delta for a period ranging between 1 and 174 days.
C) The nursery area or habitat is repeatedly used across years.
The long term presence of SHS in the Rewa Delta was observed throughout the study period. Firstly, during the study period, we regularly caught SHS with some temporal variation in terms of number of sharks caught per month. A total of 311 gillnet survey representing 667.1 standardized fishing hours were carried out during the study period. CPUE per gillnet survey ranged from 0 to 15 SHS/hour (1.81 ± 2.6), with the highest mean CPUE in February 2015 and March 2016 and, the lowest in November 2014 and December 2015. Secondly, using the mark-recapture data and the length distribution of SHS, we were able to estimate the growth rate of SHS. Thus, males (N = 26) and females (N = 11) recaptured between one and five months after tagging grew an average total length of 5.1 ± 3.9 cm and 4.7 ± 4.8 cm respectively. Consequently, our results on mark-recapture confirmed a long term presence of SHS in the Rewa Delta as well as the use of this area as nursery area as sharks fed and grew up during the period that they stay in the delta. Thus, with the empirical evidence found through this study, a previous study by Brown et al. (2016) and anecdotal reports by local Fijian fishermen, this nursery seems to have existed for at least 30 years.
Technical advice to protect the Rewa Delta
The Republic of Fiji is part of CITES and the Convention on the Conservation of Migratory Species. Because our study results showed clear evidence that the Rewa Delta is a SHS nursery, we would like to offer a number of recommendations to the Ministry of Fisheries and Forest in order to protect this ecologically important area for SHS. We would like to open a discussion with the stakeholders to achieve an agreement that fulfils the resources requirement of the people whose livelihood depend on the Rewa Delta while minimising the risk of severely compromising the long term survival of this nursery area. Several measure and their respective consequences should be considered. This includes the following options, listed in order of importance for the conservation of the species and its habitat.
- – Complete ban of gillnet fishing in the Rewa Delta throughout the year;
- – Partial ban of gillnet fishing in the Rewa Delta from sunset to sunrise throughout the year;
- – Partial ban of gillnet fishing in the Rewa Delta from sunset to sunrise during the parturition period (October-April);
- – Partial ban of gillnet fishing in the Rewa Delta from sunset to sunrise during the peak of the parturition period (December-March).
Brown K.T., Seeto J., Lal M.M. & Miller C.E. (2016) Discovery of an important aggregation area for endangered scalloped hammerhead sharks, Sphyrna lewini, in the Rewa River estuary, Fiji Islands. Pacific Conservation Biology.