Our results highlight the value of a MPA to protect shark species with coastal habitats and strong site fidelity, yet they also show that MPA size is less effective in the case of migratory coastal-pelagic species. While blacktip sharks movements were totally encompassed by the GMR size, hammerhead shark movements were mostly protected by the reserve boundary only during the cold season.
Regardless the number of tagged individuals differed between species and between tagging locations, there was a significative difference of the travelled distance from the tagging location between hammerhead sharks (~ 221 Km) and blacktip sharks (~ 74 Km). Furthermore, all blacktip sharks (tagged in the north and south) moved within the GMR boundary all year round, while hammerhead sharks displayed broader movements with preference for oceanic waters, including the individual tagged around the southern islands. The lack of population structure in both hammerhead and blacktip sharks within the GMR 31, along with our findings suggest the tagging location should not have affected the obtained results.
The scalloped hammerhead shark is found all year round in the GMR, particularly in higher relative abundance during the cold season (June-October) at the islands of Darwin and Wolf. Klimley and Nelson 32 hypothesized this species uses the central refuging strategy 33, by using oceanic islands and seamounts as a refuge, from where they can take advantage of nearby foraging areas. Long-term passive acoustic tracking has shown hammerhead sharks to display strong site fidelity to the islands of Darwin and Wolf34,35. This study provides further support by showing an island-centred core UD around Wolf and Darwin Island. Results also suggest there is a strong seasonal component in the extension of the UD of hammerheads, with individuals progressively leaving the protection of the reserve with the advent of the warmer months. Hammerhead sharks’ long distance migrations between MPAs of the ETP have been reported to occur also during the warm season35, yet it is still unknown which factors trigger such behaviour. Pelagic species are known to change their geographic distribution between environmental seasons according to changing currents36,37, foraging38, reproductive39 or parturition needs. Given tracked hammerhead sharks were mostly males, reproductive needs are less likely to be responsible for the observed shift in UD. The marked seasonality in the oceanographic conditions of the GMR25 could be playing a major role in the spatial extension of the UD in hammerheads around the GMR by influencing the vertical and horizontal distribution of their preferred prey items. Bessudo, et al. 40 reported that hammerhead sharks around Malpelo and Cocos Islands undertake deep “yo-yo” dives (> 100 m) in waters away from the islands during the warm season, whilst the same individuals preferred surface waters (0-10m) near the islands during the colder season.
The blacktip shark is also a cosmopolitan species sharing a similar distribution to hammerhead sharks among tropical and subtropical oceans of the world41. There is a considerable amount of work available on the occurrence, demography and environmental preferences of juvenile blacktip sharks at nursery grounds in coastal and estuarine bays 42,43. Nevertheless, there is no available information regarding adult behavioural ecology and their habitat use. This study presents the first description on the UD of blacktip sharks and supports previous suggestions of strong site fidelity and residency for this species 44. Areas of shallow water (< 500 m deep) and low EKE in the centre of the south-central region of the GMR were preferred by both sexes of blacktip sharks. A possible explanation for the strong site fidelity may be philopatry of adults to their nursery grounds. Keeney, et al. 45 conducted genetic analysis and reported that female blacktip sharks display strong philopatry to their nursery grounds found around the Caribbean and North-western Atlantic region. The occurrence of important nursery grounds in the south central GMR46 further supports this for the GMR. The presence of feeding grounds in the area, either via prey availability (e.g. whitetip reef sharks47, sealions48), or food provisioning from ships or fishers49, could be also influencing the strong fidelity observed around Santa Cruz Island.
The reported habitat use in addition to the recently reported relative abundance increase27 and identified nursery grounds for blacktip sharks46 indicate that the GMR provides adequate protection for the different life history stages. By contrast, hammerhead sharks were reported to have declined by 50% within the GMR27, which in combination with our results suggest this species is not being effectively protected by this MPA. Although there are informal reports of juvenile hammerhead sharks, at present there is no scientifically assessed hammerhead nurseries within the GMR. This suggests only adult and sub-adults individuals are benefiting from the reserve’s protective coverage, particularly during the cold season. If fully functional nursery grounds were present, we’d expect UDs, especially of females, to show seasonal movement towards pupping grounds within the GMR. The apparent recovery of blacktip sharks and decline of hammerheads reflect these differing levels of protection in Galapagos. A similar scenario for both species has been reported for Cocos Island28, a smaller MPA also located the ETP.
The inadequacy of the reserve size to protect hammerhead sharks is of particular concern given this species’ current global endangered status30. A revision of the management of this species in the ETP suggested the creation of several small non-take MPAs (which include the existent MPAs of the region) enclosed in a large special marine managed area from Galapagos to Costa Rica50. Such a special managed area would allow a considerably reduced fishing effort and should be equal in size to the Exclusive Economic Zones of the countries with national jurisdiction in the region. Achieving this complex spatial zoning, however, would require high levels of national and international agreements and would have important economic implications for the industrial fishing operations in the area. Also, this will not stop interactions between fisheries and hammerhead sharks whenever they leave the small non-take MPAs.
Alternatively, dynamic spatial closures extending the current fixed GMR boundaries could reduce the susceptibility of hammerhead sharks to fishing operations. Examples of this are currently in place to manage the longline fisheries on the east coast of Australia. The near real time dynamic spatial allocation of take and non-take areas reduces the interactions between fishing fleets and pelagic by-catch species13,51. This management approach could be applied to create seasonal buffer zones and extend the boundaries of the GMR to reduce the capture susceptibility of hammerhead sharks. The reported seasonality in fisheries catches around the GMR by Martinez-Ortiz, et al. 52 supports the feasibility of this alternative, their study found that there was a seasonal difference in species caught, where larger fish including sharks made the majority of the catches in warmer seasons and considerably less catches in colder seasons.
A further in-depth evaluation of the habitat preferences of hammerhead sharks and commercial teleost fishes is recommended, particularly focused on increasing the number of tagged individuals and total tracked days. This evaluation will help to determine the level of habitat overlap between commercial fish species and hammerhead sharks, and the environmental factors managers could use to adopt seasonal spatial closures around the GMR and the neighbouring MPAs of Cocos and Malpelo Islands. Habitat preferences of hammerhead sharks should be evaluated with a larger number of individuals per season, as recommended when assessing the implementation of dynamic spatial closures37. The implementation of dynamic spatial closures will allow seasonal fishing (economic) and decreased shark mortalities (biological) out of the GMR, while maintaining the ecological balance of seamounts (ecological) and support to the valuable dive tourism industry (economic) within the GMR. If implemented, this scenario could provide global lessons for managing oceanic species, particularly those of high conservation concern such as the endangered hammerhead shark.