Where giants roam: The importance of remote islands and seamount corridors to adult tiger sharks in the South Pacific Ocean

The movements of tiger sharks (Galeocerdo cuvier) across their global distribution are diverse and complex, and there remains a dearth of information about the cues that influence migrations of adults to and from offshore islands. We aimed to delineate broad-scale movements of a seasonally abundant tiger shark aggregation at Norfolk Island, a remote small island in the South Pacific Ocean, by identifying migratory pathways and important areas, as well as quantifying the association between space use and environmental factors. We satellite tracked 35 tiger sharks, consisting of some of the largest individuals ever monitored (median total length: 4.0 m), between February 2020 and April 2023. Tracking periods averaging 305 days (14 - 686 days) showed movements throughout large parts of the South Pacific Ocean including near New Caledonia, the Great Barrier Reef, Papua New Guinea, Chesterfield Islands, Vanuatu, Fiji, and New Zealand. The longest track was close to 17,000 km over 468 days. There was high seasonal fidelity to Norfolk Island with 88% of sharks tracked across multiple seasons returning at least once, mainly from New Caledonia. The median date of arrival and departure from Norfolk Island were in December and May, respectively. Coastal use of islands was the most important factor across monthly habitat suitability models, whereas sea surface temperature explained seasonal departures/arrivals from/to Norfolk Island. The findings of our study show diverse potential movement trajectories and cues used by tiger sharks, but importantly highlight the critical role of Norfolk Island and other nearshore areas in supporting large adult female tiger sharks.

Ecological and biogeographic features shaped the complex evolutionary history of an iconic apex predator (Galeocerdo cuvier)

BACKGROUND

The tiger shark (Galeocerdo cuvier) is a large iconic marine predator inhabiting worldwide tropical and subtropical waters. So far, only mitochondrial markers and microsatellites studies have investigated its worldwide historical demography with inconclusive outcomes. Here, we assessed for the first time the genomic variability of tiger shark based on RAD-seq data for 50 individuals from five sampling sites in the Indo-Pacific (IP) and one in the Atlantic Ocean (AO) to decipher the extent of the species' global connectivity and its demographic history.

RESULTS

Clustering algorithms (PCA and NMF), F_ST and an approximate Bayesian computation framework revealed the presence of two clusters corresponding to the two oceanic basins. By modelling the two-dimensional site frequency spectrum, we tested alternative isolation/migration scenarios between these two identified populations. We found the highest support for a divergence time between the two ocean basins of ~ 193,000 years before present (B.P) and an ongoing but limited asymmetric migration ~ 176 times larger from the IP to the AO (Nm ~ 3.9) than vice versa (Nm ~ 0.02).

CONCLUSIONS

The two oceanic regions are isolated by a strong barrier to dispersal more permeable from the IP to the AO through the Agulhas leakage. We finally emphasized contrasting recent demographic histories for the two regions, with the IP characterized by a recent bottleneck around 2000 years B.P. and the AO by an expansion starting 6000 years B.P. The large differentiation between the two oceanic regions and the absence of population structure within each ocean basin highlight the need for two large management units and call for future conservation programs at the oceanic rather than local scale, particularly in the Indo-Pacific where the population is declining.

Movement ecology of black marlin Istiompax indica in the Western Indian Ocean

The black marlin Istiompax indica is an apex marine predator and is susceptible to overfishing. The movement ecology of the species remains poorly known, particularly within the Indian Ocean, which has hampered assessment of their conservation status and fisheries management requirements. Here, we used pop-up archival satellite tags to track I. indica movement and examine their dispersal. Forty-nine tags were deployed off Kenya during both the north-east (November-April) and south-west (August-September) monsoon seasons, providing locations from every month of the year. Individual I. indica were highly mobile and track distance correlated with the duration of tag attachment. Mean track duration was 38 days and mean track distance was >1800 km. Individuals dispersed in several directions: north-east into Somalian waters and up to northern Oman, east towards the Seychelles, and south into the Mozambique Channel. Their core habitat shifted seasonally and overlapped with areas of high productivity off Kenya, Somalia and Oman during the first half of the year. A second annual aggregation off the Kenyan coast, during August and September, did not coincide with high chlorophyll-a (chl-a) concentrations or thermal fronts, and the drivers of the species' presence and movement from this second aggregation was unclear. We tested their habitat preferences by comparing environmental conditions at track locations to the conditions at locations along simulated tracks based on the empirical data. Observed I. indica preferred cooler water with higher chl-a concentrations and stayed closer to the coast than simulated tracks. The rapid and extensive dispersal of I. indica from Kenya suggests that there is likely a single stock in the Western Indian Ocean, with individuals swimming between areas of high commercial catches off northern Somalia and Oman, and artisanal and recreational fisheries catches throughout East Africa and Mozambique.

Global political responsibility for the conservation of albatrosses and large petrels

Migratory marine species cross political borders and enter the high seas, where the lack of an effective global management framework for biodiversity leaves them vulnerable to threats. Here, we combine 10,108 tracks from 5775 individual birds at 87 sites with data on breeding population sizes to estimate the relative year-round importance of national jurisdictions and high seas areas for 39 species of albatrosses and large petrels. Populations from every country made extensive use of the high seas, indicating the stake each country has in the management of biodiversity in international waters. We quantified the links among national populations of these threatened seabirds and the regional fisheries management organizations (RFMOs) which regulate fishing in the high seas. This work makes explicit the relative responsibilities that each country and RFMO has for the management of shared biodiversity, providing invaluable information for the conservation and management of migratory species in the marine realm.