https://nzobisipt.niwa.co.nz/resource?r=ba_estuary_sharks Tom Vierus Leibniz Center for Tropical Marine Ecology Researcher

Bremen DE

Stefan Gehig Leibniz Center for Tropical Marine Ecology Researcher

Bremen DE

Juerg M. Brunnschweiler University of the South Pacific Researcher

PO Box 1168 Suva FJ

Kerstin Glaus University of the South Pacific Researcher

PO Box 1168 Suva FJ

Martin Zimmer University of Bremen Researcher

Bremen DE

Amandine D. Marie University of the South Pacific Researcher

PO Box 1168 Suva FJ

Ciro Rico University of the South Pacific Researcher

PO Box 1168 Suva FJ

Ciro Rico School of Marine Studies, Faculty of Science, Technology and Environment, The University of the South Pacific Researcher

PO Box 1168 Suva FJ

2024-08-02 eng Population declines in shark species have been reported on local and global scales, with overfishing, habitat destruction and climate change posing severe threats. The lack of species-specific baseline data on ecology and distribution of many sharks, however, makes conservation measures challenging. Here we present a fisheries-independent shark survey from the Fiji Islands, where scientific knowledge on locally occurring elasmobranchs is largely still lacking despite the location’s role as a shark hotspot in the Pacific. Juvenile shark abundance in the fishing grounds of the Ba Estuary (north-western Viti Levu) was assessed with a gillnet- and longline-based survey from December 2015 to April 2016. A total of 103 juvenile sharks identified as blacktip Carcharhinus limbatus (n = 57), scalloped hammerhead Sphyrna lewini (n = 35), and great hammerhead Sphyrna mokarran (n = 11) sharks were captured, tagged, and released. The condition of umbilical scars (68 % open or semi-healed), mean sizes of individuals (± SD) (C. limbatus: 66.5 ± 3.8 cm, S. lewini: 51.8 ± 4.8 cm, S. mokarran 77.4 ± 2.8 cm), and the presence of these species over recent years (based on fishermen interviews), suggest that the Ba Estuary area is a critical habitat for multiple species that are classified as “Near Threatened” or “Endangered”. Specifically, the area likely acts as a parturition ground over the studied period, and potentially as a subsequent nursery area. We identified subareas of high abundance and found that temperature, salinity and depth acted as small-scale environmental drivers of shark abundance. The data suggests a tendency for species-specific spatial use, both horizontally (i.e. between sampling areas) and vertically (i.e. across the water column). These results enhance the understanding of shark ecology in Fiji and provide a scientific basis for the implementation of local conservation strategies that contribute to the protection of these threatened species. Samplingevent GBIF Dataset Type Vocabulary: http://rs.gbif.org/vocabulary/gbif/dataset_type_2015-07-10.xml Observation GBIF Dataset Subtype Vocabulary: http://rs.gbif.org/vocabulary/gbif/dataset_subtype.xml marine, harvested by iOBIS This work is licensed under a Creative Commons Attribution (CC-BY 4.0) License. Ba Estuary, Viti Levu, Fiji 177.594 177.694 -17.42 -17.465 2015-12-02 2016-04-18 Sharks species Carcharhinus limbatus Blacktip shark species Sphyrna lewini Scalloped hammerhead shark species Sphyrna mokarran Great hammerhead shark notPlanned Tom Vierus School of Marine Studies, Faculty of Science, Technology and Environment, The University of the South Pacific Researcher

Suva FJ

The main shark-fishing survey was conducted on 26 days from December 2015 to April 2016. Bottom-set gillnets and longlines were set at depths ranging from 1 to 15 m in the seven sampling areas, with a total of 73 and 30 deployments, respectively. All deployments were carried out between 18:00 and 02:00 from a 7 m fiberglass boat with a 40 HP engine. Two assistants and a captain were present at all times. Bait used on longlines consisted predominantly of Indian mackerel (Rastrelliger kanagurta), and occasionally of red snapper (Lutjanus argentimaculatus), squid (Loligo sp.), and mullet (Mugil cephalus). Up to two gillnets (100 m length and 3 m width, ~10 cm mesh size) were deployed simultaneously with a soak time of 1–6 hr. To minimize animal stress and mortality, gillnets were regularly checked in intervals of 15–25 min. When feasible, a longline (75 m) with 27 hooks was additionally deployed at the same sampling sites to assess potential catch differences attributed to gear selectivity. Distance between gangions attached to the floater line varied from 2.4 to 2.8 m. Gangion length ranged between 0.6 and 3 m, with the last 0.5 m consisting of 1.5 mm steel wire and a baited 13″ circle hook. In total, fishing effort ranged from 6 to 10.36 hr (longline) and 15 to 23.08 hr (gillnet) per sampling area. Total soak time of gillnet and longline deployments varied from 30 min to 6 hr, and from 45 min to 3 hr, respectively. Sampling effort was intended to be uniformly distributed among the seven sampling areas and ranged from 24.5 to 33.3 hr/area (mean: 28 ± 3 hr/area). The study was conducted in a shallow bay environment (depth < 15 m) in north-western Viti Levu, the main island of the Republic of Fiji. The sampled area around the Ba River mouth is part of a larger bay that is sheltered from the open sea by patches of fringing reefs and from the mainland by mangroves. The sea bottom predominantly consists of muddy substrate and seagrass beds. The area is under strong tidal influence, with a tidal range of approximately 2 m (www.tide-forecast.com, 2016). There is activity by artisanal and subsistence fishermen from surrounding villages in the estuary. While sharks are by tradition not explicitly targeted in fishing operations, they regularly occur as low-value bycatch. Based on the results of a pilot survey, seven 1.13 km2 circular sampling areas in the immediate vicinity of the river mouth were selected. Each sampling area featured contrasting environmental features (e.g., depth, distance to mangroves, turbidity) and overlapped with areas that local fishermen identified as having higher shark abundances. Sampling areas 1–6 contained 10 sites each, and area 7 contained nine sampling sites. 2024-08-02T08:01:26.802+12:00 dataset Vierus T, Gehig S, Brunnschweiler J M, Glaus K, Zimmer M, Marie A D, Rico C (2024). Data from: Discovery of a multi-species shark aggregation and parturition area in the Ba Estuary, Fiji Islands. Version 1.0. Southwestern Pacific Ocean Biogeographic Information System (OBIS) Node. Samplingevent dataset. https://nzobisipt.niwa.co.nz/resource?r=ba_estuary_sharks&v=1.0 Vierus, Tom et al. (2019). Data from: Discovery of a multi-species shark aggregation and parturition area in the Ba Estuary, Fiji Islands [Dataset]. Dryad. Vierus, T., Gehrig, S., Brunnschweiler, J.M., Glaus, K., Zimmer, M., Marie, A.D. and Rico, C., 2018. Discovery of a multispecies shark aggregation and parturition area in the Ba Estuary, Fiji Islands. Ecology and Evolution, 8(14), pp.7079-7093.