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Spatial ecology and conservation of the tiger shark, Galeocerdo cuvier, in the equatorial Atlantic Ocean

André S. Afonso1*, Fábio H. Hazin1 and Leonardo B. Veras2
  • 1 Federal Rural University of Pernambuco, Department of Fisheries and Aquiculture, Brazil
  • 2 Museu dos Tubarões de Fernando de Noronha, Brazil

Ensuring the conservation of marine top predators is essential to preserve ecosystem balance and structure. Yet, many shark populations are currently depleted due to human pressure including overfishing, finning, and habitat degradation. The tiger shark, Galeocerdo cuvier (Péron & Lesueur, 1822), is a near-threatened, wide-ranging carcharhinid occurring in both coastal and pelagic habitats at tropical and warm temperate latitudes. Tiger sharks are commonly caught in directed and bycatch fisheries for their fins, skin, flesh and liver oil (Simpfendorfer, 2009), inclusively off Brazil (Afonso and Hazin, 2014). This is a generalist, large-bodied species which acts as an apex predator in disparate food webs and which has been implicated in shark hazard scenarios off Brazil and elsewhere. Increasing the available knowledge of the bioecology of tiger sharks in the western equatorial Atlantic Ocean to assist with both shark hazard mitigation and shark conservation planning has hence been a most-pressing requirement. On that account, long-term, fisheries-independent research on tiger shark spatial ecology off northeastern Brazil was implemented in Recife in 2008 as a component of the local, non-lethal shark hazard mitigation program, which was based on the translocation of potentially aggressive species from the hazardous inshore area to offshore waters (Hazin and Afonso, 2013). In 2012, research efforts were widened towards the oceanic realm by including the marine protected area (MPA) of the Archipelago of Fernando de Noronha (FEN). The main aims were to assess patterns in tiger shark space use across different spatiotemporal scales and to ascertain potential underlying, intrinsic and extrinsic factors regulating movement behavior in this species. Tiger sharks were caught with longline and handline gear and carefully brought onboard, where they were measured, sexed and fitted with acoustic and satellite tracking devices. Acoustic tags (V16, Vemco) were surgically implanted into the coelomic cavity to enable the detection of tagged sharks by a stationary array of underwater acoustic receivers (VR2W, Vemco) strategically arranged off Recife and off FEN. Single-position satellite transmitting tags (SPOT5, Wildlife computers) were bolted to the first dorsal fin and rendered a geolocation estimate of the sharks each time they breached out of the water with a satellite in view. Pop-up satellite archival tags (mk10 and miniPAT, Wildlife computers) were attached to the first dorsal fin with an harness and collected depth, temperature and luminosity data for a preset amount of time, after when they detached from the harness to stream the recorded data through the ARGOS satellites at the sea surface. In this case, geolocation was estimated based on the chronology of crepuscular events using luminosity data. ARGOS-relayed and light-based geolocation estimates were computed with R software and processed with a set of available state-space movement models to obtain daily position estimates. Two-dimensional space utilization distributions were assessed with kernel methods to identify core habitats and high-use areas across tiger shark home ranges. Generalized linear and additive mixed models were used to determine the relationship between several candidate explanatory variables and tiger shark movements. Striking differences in tiger shark size structure between coastal and oceanic habitats were observed, with small (< 150 cm) juveniles prevailing off Recife (N = 56; range = 82 – 355 cm; mean = 158.2; s.d. = 58.4) and no sharks < 150 cm having been sampled in FEN (N = 57; range = 154 – 356 cm; mean = 241.9; s.d. = 40.7). In agreement, longline and telemetry data suggested tiger sharks to undergo an ontogenetic habitat shift from shallow habitats on the Brazilian continental shelf, which they likely use as pupping grounds (Driggers et al., 2008), to deep oceanic waters (Afonso and Hazin, 2015) within one year after birth (Afonso et al., 2012; Afonso et al., 2014). Considerable intraspecific variability in movement behavior was noticed, though. Most (≈ 85%) sharks tagged off Recife moved to the north along the Brazilian mainland or the oceanic province. Sharks tagged off FEN showed a greater diversity of large-scale (1,000s km) displacements, albeit westward movements were more frequent. Notwithstanding, one transoceanic movement towards Africa by a male juvenile was recorded (Afonso et al., 2017), corroborating the hypothesis that the equatorial Atlantic might be an important area for intercontinental connectivity by highly-migratory tiger sharks (Domingo et al., 2016). On the other hand, a high association of seemingly roaming tiger sharks with specific oceanic areas off the Brazilian shelf was evidenced. These included the Ceará Seamounts (Jovane et al., 2016) and the region influenced by the outflow of the Amazon River. Also, some sharks exhibited strong fidelity to the MPA of FEN and remained in this region for several months or even years, a trend which could relate with the local availability of high-quality prey following the implementation of conservation policies. Diel shifts in MPA space use by these sharks were conspicuous, with some areas being used mostly during daytime and others during the night, which could again reflect the spatiotemporal distribution of the prey assemblage. Further, possible evidence of intraspecific competition was detected as some sharks patrolled MPA coastal waters continuously for protracted periods of time whereas other sharks repeatedly performed brief incursions into these habitats followed by longer periods of absence. Great advances in unraveling the intricacies of tiger shark spatial ecology in the equatorial Atlantic Ocean have thus far been achieved, partially due to high-quality data provided by state-of-the-art tracking technology. Since the electronic transmitters used can operate during as much as 3 years and sampling is still in progress, a comprehensive analysis of tiger shark movements in this region is on hold. Yet, integrating movement information with concurrent sampling of reproductive and trophic features (e.g. blood analysis, stable isotopes) will expectedly clarify the ecological role of different geographical areas and habitats across tiger shark home ranges. This approach should also contribute to determine the influence of environmental, physiological and social drivers upon tiger shark movement behavior. By deriving spatially-explicit information about the bioecological processes underpinning tiger shark distribution dynamics, it will be possible to provide managers and stakeholders with more efficient tools for addressing species conservation and shark hazard mitigation simultaneously.

Acknowledgements

This research was funded by the Portuguese Foundation for Science and Technology, the Portuguese Association for the Study and Conservation of Elasmobranchs/Flying Sharks Research Fund, the Brazilian Coordination for the Improvement of Higher Education Personnel (CAPES), the State Government of Pernambuco, the National Geographic Society Conservation Trust, the Marine Conservation Action Fund of the New England Aquarium, and the Boticário Foundation.

References

Afonso AS, Andrade HA, Hazin FHV. 2014. Structure and Dynamics of the Shark Assemblage off Recife, Northeastern Brazil. PLoS ONE 9(7): e102369.
Afonso AS, Garla R, Hazin FHV. 2017. Tiger Sharks Can Connect Equatorial Habitats and Fisheries Across the Atlantic Ocean Basin. PLoS ONE 12(9): e0184763.
Afonso AS, Hazin FHV. 2015. Vertical movement patterns and ontogenetic niche expansion in the tiger shark, Galeocerdo cuvier. PLoS ONE 10(1): e116720.
Afonso AS, Hazin FHV. 2014. Post-release survival and behavior and exposure to fisheries in juvenile tiger sharks, Galeocerdo cuvier, from the South Atlantic. Journal of Experimental Marine Biology and Ecology, 454: 55-62.
Afonso AS, Hazin FHV, Barreto RR, Santana FM, Lessa RP. 2012. Extraordinary growth in tiger sharks Galeocerdo cuvier from the South Atlantic Ocean. Journal of Fish Biology, 81(6): 2080-2085.
Domingo A, Coelho R, Cortes E, Garcia-Cortes B, Mas F, Mejuto J, Miller P, Ramos-Cartelle A, Santos MN, Yokawa K. 2016. Is the tiger shark Galeocerdo cuvier a coastal species? Expanding its distribution range in the Atlantic Ocean using at-sea observer data. J Fish Biol, 88: 1123-1128.
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Hazin FHV, Afonso AS. 2013. A green strategy for shark attack mitigation off Recife, Brazil. Animal Conservation, 17(4): 287-296.
Jovane L, Figueiredo JJP, Alves DPV, Iacopini D, Giorgioni M, Vannucchi P, Moura DS, Bezerra FHR, Vital H, Rios ILA, Molina EC. 2016. Seismostratigraphy of the Ceará Plateau: Clues to Decipher the Cenozoic Evolution of Brazilian Equatorial Margin. Frontiers of Earth Science, 4: 90.
Simpfendorfer, C. 2009. Galeocerdo cuvier. The IUCN Red List of Threatened Species 2009: e.T39378A10220026. http://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T39378A10220026.en. Downloaded on 03 May 2018.

Keywords: acoustic telemetry, satellite telemetry, Longline survey, elasmobranch, Marine Protected Areas (MPA), Spatial Ecology, conservation, Tiger Shark, Galeocerdo cuvier, Migration

Conference: IMMR'18 | International Meeting on Marine Research 2018, Peniche, Portugal, 5 Jul - 6 Jul, 2018.

Presentation Type: Oral Presentation

Topic: Fisheries and Management

Citation: Afonso AS, Hazin FH and Veras LB (2019). Spatial ecology and conservation of the tiger shark, Galeocerdo cuvier, in the equatorial Atlantic Ocean. Front. Mar. Sci. Conference Abstract: IMMR'18 | International Meeting on Marine Research 2018. doi: 10.3389/conf.FMARS.2018.06.00021

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Received: 05 May 2018; Published Online: 07 Jan 2019.

* Correspondence: PhD. André S Afonso, Federal Rural University of Pernambuco, Department of Fisheries and Aquiculture, Recife, Brazil, afonso.andre@gmail.com