Graeme Clive Hays

Understanding the relative abundance of breeding males and females is particularly important for taxa where skewed sex ratios threaten population viability, including for sea turtles for which climate warming increases risks of population feminisation. We report observations of mating pairs and “attendant” males, i.e. males within a few metres of the focal mating pair, for breeding green turtles ( Chelonia mydas ) around the world. At Isla de Aves (Venezuela, Caribbean), we observed a total of 665 mating pairs across 17 days of intense observations (June–July 2008), and on 50.2% of occasions pairs had attendant males. We mostly observed one or two attendant males per pair, but sometimes observed > 5. These observations suggest that breeding females were not struggling to locate breeding males. In the Chagos Archipelago (Indian Ocean), we made incidental observations from 2015 to 2024, and 6 of 31 observations of mating pairs (19.4%) had attendant males, with a maximum of 5 per pair, again implying there was often an abundance of breeding males, consistent with a previously estimated male-biased hatchling sex ratios at this site. Elsewhere in the world, attendant males have been observed with mating loggerhead turtles ( Caretta caretta ) in Greece and green turtles in the Southern Great Barrier Reef (Australia) and Ascension Island (South Atlantic), all sites where hatchling or juvenile sex ratio estimates again suggest males are not scarce. We suggest that observations of attendant males may be a useful component in the general toolkit for studying male sea turtles and their abundance in breeding areas.

Site fidelity describes the tendency to return to or reuse familiar sites, yet some animals may use alternative sites while maintaining fidelity to primary locations, with consequences for population estimates as well as resilience to changing environments. Here we assessed and compared nesting beach fidelity of hawksbill turtles ( Eretmochelys imbricata ) on Frégate Island, Seychelles using data from beach foot patrols (2017–2025; n = 575 turtles) and satellite telemetry (2023; n = 10 turtles) and then examined the implications of fidelity strategies for population estimates. We also reviewed published literature on nesting beach use across sea turtle species. Satellite tracking showed that 50% of equipped turtles were study‐beach faithful, laying clutches on one beach, and 50% were study‐beach unfaithful, nesting across 2–4 beaches on Frégate and on nearby islands. Consequently, clutch frequency estimated through tracking was 1.9 times higher than foot patrol estimates. Estimates of adult female abundance increased as the assumed proportion of clutches laid on other beaches increased, independent of clutch frequency. Across 51 studies at 43 sites around the world, study‐beach unfaithful individuals were more likely to be recorded nesting at mainland beaches (21.4% compared with 9.3% on island beaches), with mainland surveys covering greater distances (1.6–1000 km) than island surveys (1–15.9 km). Maximum inter‐nest distances varied across species: green turtles ( Chelonia mydas ; 2250 km), leatherbacks ( Dermochelys coriacea ; 1849 km), loggerheads ( Caretta caretta ; 800 km), olive ridleys ( Lepidochelys olivacea ; 320 km), and hawksbills (300 km). We demonstrate mixed fidelity strategies within an island hawksbill population, suggesting that fidelity reflects natal philopatry and the proximity of alternative sites. Limited monitoring coverage at remote sites likely contributes to under‐detection of nestings by study‐beach unfaithful individuals. Individuals using alternative sites signal behavioural plasticity that may contribute to resilience against habitat loss. We recommend characterising beach fidelity through regional data sharing and satellite tracking to inform reliable demographic inference and identify subpopulations vulnerable to climate change.

Changes in breeding periodicity in vertebrates can highlight important trends in population demographics that have conservation implications. Following widespread conservation measures, some previously endangered species have shown population recoveries across their range, with green turtles (Chelonia mydas) being an exemplar of global species recovery. At sites in the Indo-Pacific, green turtle nesting numbers have increased fourfold in 30 years. As sea turtles are generally non-annual breeders, we used a total of 1399 measurements of the breeding interval for marked individuals to reveal that the relative proportion of individuals breeding with a 4-year versus a 3-year interval has increased over time: 28% in 1997–2012 versus 37% in 2013–2024. Furthermore, this percentage of 4- versus 3-year nesting intervals was linked to annual nest numbers (R2 = 0.23, F1,23 = 6.92, p = 0.01). This lengthening of the breeding (remigration) interval may reflect density dependence, restricting foraging success so that individuals need longer to build body condition for migration and breeding. These results provide the first evidence for long-term lengthening of breeding intervals in this taxon and might have important global conservation implications in terms of reduced per capita reproductive output, a finding that echoes recent observations with cetaceans.

The mechanisms by which animals navigate during long ocean migrations to specific targets remain equivocal despite over a century of investigation. To address this question, we developed and deployed a new tag that allowed the compass heading of migrators to be remotely relayed via satellite. On transocean migrations (>1000 kilometers and mean duration 27.5 days) between nesting and foraging sites, green turtles ( Chelonia mydas ) tended to perform sections of travel with a consistent compass heading, even if that led them off course, before reorienting. That migrating turtles did not continuously fine-tune their heading but rather made occasional reorientations is consistent with the suggestion that they use geomagnetic signposts, or other crude maps, to facilitate occasional course corrections.

Offspring survival is a central component in understanding the life-history of species. With sea turtles, estimates of hatchling survival to adulthood can be traced back to theoretical considerations made over 50 years ago and before key parameters needed in the hatchling survival calculation had been quantified. Here, using up-to-date figures for adult survival rates and hence adult longevity and lifetime reproductive output, we show that the survival rate of hatchlings to adulthood will generally lie between 1 in 400 and 1 in 2000. We show that this survival rate probably varies across populations and species in line with variations in adult survival rates, remigration intervals and clutch frequency and will likely be impacted by factors such as local management, resource availability and bycatch pressures. Popular articles that tend to quote this survival rate as 1 in 1000 are broadly comparable with our estimates, although the survival rate likely varies appreciably across populations. We identify the key parameters that need to be measured for this value to be determined accurately for individual populations around the world. This approach of using values for adult reproductive output to estimate offspring survival rates to adulthood likely has broad utility across reptiles and other taxa.

While satellite tracking is widely used to identify areas of conservation importance, whether there is a need to continue tag deployments across many years is unclear. We show that destinations of migrating animals from the same breeding population can differ significantly across years, and hence we highlight the value of multi-year tracking studies. Between 2012 and 2024, we used Fastloc-GPS Argos and Iridium tags to track 58 green turtles (Chelonia mydas) from their nesting sites in the Chagos Archipelago. If tracking had taken place in a single year, the number of countries used as foraging destinations could have been hugely underestimated (n = 1 country in 2024 versus n = 7 countries across years). Overall, 47% of tracked individuals foraged in the Seychelles, which likely hosts hundreds of thousands of foraging turtles across age classes. Further, the importance of foraging in areas beyond national jurisdiction (ABNJs) was only revealed by tracking over multiple years. Across years, 9% of tracked individuals foraged on the Saya de Malha Bank, a remote ABNJ, equating to likely >1000 adult females and >10 000 green turtles using this foraging area. This cumulative insight from multi-year tracking likely applies broadly to capital breeders where there is environmental variability across the foraging range.

While many verte brates breed annually, there are a few that do not. Female sea turtles are a classic example of a generally non-annual nester, with often intervals between breeding seasons (termed the remigration interval) of 2–4 years. Non-annual nesting in sea turtles is believed to be due to long migration distances. While this hypothesis holds for populations where females migrate large distances (e.g. sometimes several 1000 km), it is harder to reconcile with cases where migration distances are short (e.g. sometimes <50 km). We show that even with very short breeding migrations, there are invariant time and energy costs to breeding. The time involved in the breeding process (migration, mating, nesting, internesting and returning to the foraging grounds) also comes with the opportunity cost of an individual being away from its foraging grounds and so lost foraging time. We show that this strategy of non-annual nesting is linked to high survival rates for adult sea turtles, a feature that is common across other non-annual breeders such as albatrosses and southern right whales.

Marine megafauna species are affected by a wide range of anthropogenic threats. To evaluate the risk of such threats, species’ vulnerability to each threat must first be determined. We build on the existing threats classification scheme and ranking system of the International Union for Conservation of Nature (IUCN) Red List of Threatened Species by assessing the vulnerability of 256 marine megafauna species to 23 at‐sea threats. The threats we considered included individual fishing gear types, climate‐change‐related subthreats not previously assessed, and threats associated with coastal impacts and maritime disturbances. Our ratings resulted in 70 species having high vulnerability ( v > 0.778 out of 1) to at least 1 threat, primarily drifting longlines, temperature extremes, or fixed gear. These 3 threats were also considered to have the most severe effects (i.e., steepest population declines). Overall, temperature extremes and plastics and other solid waste were rated as affecting the largest proportion of populations. Penguins, pinnipeds, and polar bears had the highest vulnerability to temperature extremes. Bony fishes had the highest vulnerability to drifting longlines and plastics and other solid waste; pelagic cetaceans to 4 maritime disturbance threats; elasmobranchs to 5 fishing threats; and flying birds to drifting longlines and 2 maritime disturbance threats. Sirenians and turtles had the highest vulnerability to at least one threat from all 4 categories. Despite not necessarily having severe effects for most taxonomic groups, temperature extremes were rated among the top threats for all taxa except bony fishes. The vulnerability scores we provide are an important first step in estimating the risk of threats to marine megafauna. Importantly, they help differentiate scope from severity, which is key to identifying threats that should be prioritized for mitigation.