Densities and drivers of sea turtle populations across Pacific coral reef ecosystems

Abstract

Sea turtle populations are often assessed at the regional to sub-basin scale from discrete indices of nesting abundance. While this may be practical and sometimes effective, widespread in-water surveys may enhance assessments by including additional demographics, locations, and revealing emerging population trends. Here, we describe sea turtle observations from 13 years of towed-diver surveys across 53 coral islands, atolls, and reefs in the Central, West, and South Pacific. These surveys covered more than 7,300 linear km, and observed more than 3,400 green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) sea turtles. From these data, we estimated sea turtle densities, described trends across space and time, and modelled the influence of environmental and anthropogenic drivers. Both species were patchily distributed across spatial scales, and green turtles were 11 times more abundant than hawksbills. The Pacific Remote Island Areas had the highest densities of greens (3.62 turtles km-1 , Jarvis Island), while American Samoa had the most hawksbills (0.12 turtles km-1 , Ta'u Island). The Hawaiian Islands had the lowest turtle densities (island ave = 0.07 turtles km-1) yet the highest annual population growth (μ = 0.08, σ = 0.22), suggesting extensive management protections can yield positive conservation results. Densities peaked at 27.5˚C SST, in areas of high productivity and low human impact, and were consistent with patterns of historic overexploitation. Though such intensive surveys have great value, they are logistically demanding and therefore have an uncertain budget and programmatic future. We hope the methods we described here may be applied to future comparatively low-cost surveys either with autonomous vehicles or with environmental DNA.

Contribution of green turtles Chelonia mydas to total herbivore biomass in shallow tropical reefs of oceanic islands

“…Nevertheless, complex interactions exist between body condition, forage quality and predation risk [22,53]. It is worth noting that green turtles >60 cm prevail in deeper habitats (~15 m) in the Hawaiian Archipelago [25], which is consistent with reduced predation risk at larger body size. Certainly, further research is required to better understand habitat selection by green turtles in tropical reef habitats in relation to predator avoidance.…”

“…Previous studies in oceanic islands across the Central and Western Pacific Ocean reported a patchy distribution of green turtles both at regional and local scales, with sea surface temperature, chlorophyll level and human disturbance as the main drivers [25]. Nutrient availability is one of the major determinants of algal cover and primary productivity in tropical reef systems [51,52] and hence a higher green turtle biomass was expected at sites with a higher algal productivity.…”

“…Green turtles in the Hawaiian Archipelago have one the lowest population densities reported from oceanic islands in the Central and Western Pacific Ocean, due to a combination of low sea surface temperature, low primary productivity and high human impact [25]. Nevertheless, the green turtle population in the Hawaiian Archipelago exhibits a high growth rate thanks to legal protection [16,17,25] and green turtles have been close to carrying capacity on the west coast of the island of Hawaii for more than two decades [59]. Ecosystem modeling indicates that the dynamics of subtidal vegetation at Kaloko-Honoköhau, on the west coast of the island of Hawaii, is ruled by sea urchin grazing, with green turtles playing a relevant role only at intertidal rocky habitats [30].…”

“…Finally, the abundance of green turtles was assessed in four 100 m x 10 m transects parallel to the shore [25,[41][42][43]. Turtle transects overlapped with those used for fish censuses.…”

“…Recent evidence shows that green turtle grazing is a major structuring force in seagrass meadows once populations are rebuilt [3,21,22] and the same could be true in other habitats. Tropical reefs are the main habitat of green turtles in the Western South Atlantic and most of the tropical Pacific [23][24][25][26], where extended sea grass meadows are scarce [27]. As a result, macroalgae and turf algae, but not seagrasses, represent the bulk of green turtle diet in those regions [24,28,29].…”