The developmental biogeography of hawksbill sea turtles in the North Pacific

Abstract

High seas oceanic ecosystems are considered important habitat for juvenile sea turtles, yet much remains cryptic about this important life‐history period. Recent progress on climate and fishery impacts in these so‐called lost years is promising, but the developmental biogeography of hawksbill sea turtles (Eretmochelys imbricata) has not been widely described in the Pacific Ocean. This knowledge gap limits the effectiveness of conservation management for this globally endangered species. We address this with 30 years of stranding observations, 20 years of bycatch records, and recent simulations of natal dispersal trajectories in the Hawaiian Archipelago. We synthesize the analyses of these data in the context of direct empirical observations, anecdotal sightings, and historical commercial harvests from the insular Pacific. We find hawksbills 0–4 years of age, measuring 8–34 cm straight carapace length, are found predominantly in the coastal pelagic waters of Hawaii. Unlike other species, we find no direct evidence of a prolonged presence in oceanic habitats, yet satellite tracks of passive drifters (simulating natal dispersal) and our small sample sizes suggest that an oceanic phase for hawksbills cannot be dismissed. Importantly, despite over 600 million hooks deployed and nearly 6000 turtle interactions, longline fisheries have never recorded a single hawksbill take. We address whether the patterns we observe are due to population size and gear selectivity. Although most sea turtle species demonstrate clear patterns of oceanic development, hawksbills in the North Pacific may by contrast occupy a variety of ecosystems including coastal pelagic waters and shallow reefs in remote atolls. This focuses attention on hazards in these ecosystems – entanglement and ingestion of marine debris – and perhaps away from longline bycatch and decadal climate regimes that affect sea turtle development in oceanic regions.

Active dispersal in loggerhead sea turtles ( Caretta caretta ) during the ‘lost years’

“…Because the ocean is a complex and dynamic environment, inferences about the directed movement of an individual animal within the open ocean require attention to the underlying physical processes that shape migratory pathways [8,12,13]. This is especially true for ocean currents, which have been shown to play key roles in sea turtle ecology [7,8].…”

Time in tortoiseshell: a bomb radiocarbon-validated chronology in sea turtle scutes

“…Few historical records exist for Hawaii, but recent analyses indicate the population was historically more abundant and geographically widespread [3]. Today, fewer than 20 females nest each year [4,5] suggesting that Hawaii hawksbills are the smallest sea turtle population on the Earth. Like other populations, Hawaii hawksbills inhabit coral reefs and estuaries [5,6], and forage on sponge, algae and macroinvertebrates [7].…”

“…Today, fewer than 20 females nest each year [4,5] suggesting that Hawaii hawksbills are the smallest sea turtle population on the Earth. Like other populations, Hawaii hawksbills inhabit coral reefs and estuaries [5,6], and forage on sponge, algae and macroinvertebrates [7]. However, much remains unknown about the basic life history of the population, which if understood, could aid long-term conservation and restoration efforts.…”

“…Often, these metrics are inferred from empirical proxies. Such methods may involve stable isotopes, skeletochronology, morphometrics, biotelemetry and modelling [5,[9][10][11][12][13]. However, for their value to be understood, it is critically important that such proxies are independently calibrated and validated [13 -15].…”

“…We fit von Bertalanffy growth function (VBGF) models to the results, producing a range of age estimates for each turtle. We find Hawaii hawksbills deposit eight growth lines annually (range [5][6][7][8][9][10][11][12][13][14], with model ensembles producing a somatic growth parameter (k) of 0.13 (range 0.1-0.2) and first breeding at 29 years (range [23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Recent bomb radiocarbon values also suggest declining trophic status.…”