Assignment tests, telemetry and tag-recapture data converge to identify natal origins of leatherback turtles foraging in Atlantic Canadian waters

Disentanglement network data to characterize leatherback sea turtle Dermochelys coriacea bycatch in fixed-gear fisheries

To characterize sea turtle bycatch in fixed-gear fisheries in Massachusetts, USA, we analyzed a 15 yr dataset of entanglement reports and detailed documentation from disentanglement operations. Almost all (272) of the 280 confirmed entanglements involved leatherback turtles Dermochelys coriacea. The majority of turtles were entangled in actively fished (96%), commercial (94%) pot/trap gear with unbroken/untriggered weak links, specifically the buoy lines marking lobster, whelk, and fish traps. Most reports came from recreational boaters (62%) and other sources (26%), rather than commercial fishers (12%). Leatherback entanglements occurred from May to November, with peak reporting in August, and included adult males, adult females, and subadults. All entanglements involved the turtle’s neck and/or front flippers, with varying degrees of visible injuries; 47 entangled leatherbacks were dead in gear, 224 were alive at first sighting, and 1 case was unknown. Post-release monitoring suggested turtles can survive for days to years after disentanglement, but data were limited. While the observed entanglements in our study are low relative to global bycatch, these numbers should be considered a minimum. Our findings are comparable to observed numbers of leatherbacks taken in Canadian fixed-gear fisheries, and represent just one of multiple, cumulative threats in the North Atlantic. Managers should focus on strategies to reduce the co-occurrence of sea turtles and fixed-fishing gear, including reductions in the number of buoy lines allowed (e.g. replace single sets with trawls), seasonal and area closures targeted to reduce sea turtle-gear interaction, and encourage the development of emerging technologies such as ‘ropeless’ fishing.

Discrete, high-latitude foraging areas are important to energy budgets and population dynamics of migratory leatherback turtles

Many broadly distributed migratory species exhibit fidelity to fine-scale areas that support vital life history requirements (e.g., resource acquisition, reproduction). Thus, such areas are critical for population dynamics and are of high conservation priority. Leatherback sea turtles are among the world's most widely distributed species, and their breeding and feeding areas are typically separated by thousands of kilometres. In this study, we analysed turtle-borne video data on daytime feeding rates and energy acquisition in Nova Scotia, Canada, to quantify the importance of this discrete, seasonal foraging area for leatherback energy requirements. Based on daytime foraging only, we estimate that a single foraging season in Nova Scotia could support 59% of a non-breeding leatherback's annual energy budget, and 29% of energetic requirements for a female on a typical 2-year reproductive cycle. However, maximum energy intake rates for leatherbacks are nearly four times lower than those of mammals and birds due the low energy content of leatherbacks' gelatinous zooplankton prey. These results illustrate that high quality, local-scale foraging areas such as Nova Scotia are critically important to the stability and future growth of the leatherback population in the Northwest Atlantic Ocean. Thus, as with other migratory species, efforts to reduce threats and maintain habitat quality in such areas should be high conservation priorities.

Linking loggerhead locations: using multiple methods to determine the origin of sea turtles in feeding grounds

Many marine megavertebrate taxa, including sea turtles, disperse widely from their hatching or birthing locations but display natal homing as adults. We used flipper tagging, satellite tracking and genetics to identify the origin of loggerhead turtles living in Amvrakikos Gulf, Greece. This location has been identified as hosting regionally important numbers of large-juvenile to adult sized turtles that display long-term residency and/or association to the area, and also presents a male biased sex ratio for adults. A total of 20 individuals were linked to nesting areas in Greece through flipper tagging and satellite telemetry, with the majority (16) associated with Zakynthos Island. One additional female was tracked from Amvrakikos Gulf to Turkey where she likely nested. Mitochondrial DNA mixed stock analyses of turtles captured in Amvrakikos Gulf (n = 95) indicated 82% of individuals originated from Greek nesting stocks, mainly from Zakynthos Island (63%), with lesser contributions from central Turkey, Cyprus and Libya. These results suggest that the male-biased sex ratio found in Amvrakikos Gulf may be driven by the fact that males breed twice as frequently on Zakynthos, resulting in their using foraging grounds of greater proximity to the breeding site. Conservation measures in localised foraging habitats for the protection of marine vertebrates, such as sea turtles, may have positive impacts on several disparate breeding stocks and the use of multiple methods to determine source populations can indicate the relative effectiveness of these measures.

Mercury and selenium ingestion rates of Atlantic leatherback sea turtles (Dermochelys coriacea): a cause for concern in this species?

Bodily accumulation of certain toxic elements can cause physiologic harm to marine organisms and be detrimental to their health and survival. The leatherback sea turtle (Dermochelys coriacea) is a broadly distributed marine reptile capable of consuming hundreds of kilograms of gelatinous zooplankton each day. Little is known about toxicants present in these prey items. Specifically, mercury is a known neurotoxin with no known essential function, while selenium detoxifies bodily mercury, but can be toxic at elevated concentrations. I collected 121 leatherback prey items (i.e., gelatinous zooplankton) from known leatherback foraging grounds and sampled the esophagus and stomach contents of stranded turtles. All samples were analyzed for total mercury and selenium. Additionally, two prey items and three liver samples were analyzed for methylmercury, the most toxic form of the element. Total mercury concentrations in prey items ranged from 0.2 to 17 ppb, while selenium concentrations ranged from <10 to 616 ppb; methylmercury concentrations in liver ranged from 25 to 236 ppb. Prey items had methylmercury concentrations below the limits of detection (<0.4 ppb). Hazard quotients and exposure rates indicate that leatherbacks of all life stages may be at risk for selenium toxicity. For endangered species like the leatherback, continued anthropogenic deposition of mercury and selenium into the environment is concerning, especially since bodily mercury and selenium concentrations increase as organisms age. Because leatherbacks are long-lived and have large daily prey consumption rates, mercury and selenium loads may increase to physiologically harmful levels in this imperiled species.

Migration, foraging, and residency patterns for Northern Gulf loggerheads: implications of local threats and international movements

Northern Gulf of Mexico (NGoM) loggerheads (Caretta caretta) make up one of the smallest subpopulations of this threatened species and have declining nest numbers. We used satellite telemetry and a switching state-space model to identify distinct foraging areas used by 59 NGoM loggerheads tagged during 2010–2013. We tagged turtles after nesting at three sites, 1 in Alabama (Gulf Shores; n = 37) and 2 in Florida (St. Joseph Peninsula; n = 20 and Eglin Air Force Base; n = 2). Peak migration time was 22 July to 9 August during which >40% of turtles were in migration mode; the mean post-nesting migration period was 23.0 d (±13.8 d SD). After displacement from nesting beaches, 44 turtles traveled to foraging sites where they remained resident throughout tracking durations. Selected foraging locations were variable distances from tagging sites, and in 5 geographic regions; no turtles selected foraging sites outside the Gulf of Mexico (GoM). Foraging sites delineated using 50% kernel density estimation were located a mean distance of 47.6 km from land and in water with mean depth of −32.5 m; other foraging sites, delineated using minimum convex polygons, were located a mean distance of 43.0 km from land and in water with a mean depth of −24.9 m. Foraging sites overlapped with known trawling activities, oil and gas extraction activities, and the footprint of surface oiling during the 2010 Deepwater Horizon oil spill (n = 10). Our results highlight the year-round use of habitats in the GoM by loggerheads that nest in the NGoM. Our findings indicate that protection of females in this subpopulation requires both international collaborations and management of threats that spatially overlap with distinct foraging habitats.

Pan-atlantic analysis of the overlap of a highly migratory species, the leatherback turtle, with pelagic longline fisheries

Large oceanic migrants play important roles in ecosystems, yet many species are of conservation concern as a result of anthropogenic threats, of which incidental capture by fisheries is frequently identified. The last large populations of the leatherback turtle, Dermochelys coriacea, occur in the Atlantic Ocean, but interactions with industrial fisheries could jeopardize recent positive population trends, making bycatch mitigation a priority. Here, we perform the first pan-Atlantic analysis of spatio-temporal distribution of the leatherback turtle and ascertain overlap with longline fishing effort. Data suggest that the Atlantic probably consists of two regional management units: northern and southern (the latter including turtles breeding in South Africa). Although turtles and fisheries show highly diverse distributions, we highlight nine areas of high susceptibility to potential bycatch (four in the northern Atlantic and five in the southern/equatorial Atlantic) that are worthy of further targeted investigation and mitigation. These are reinforced by reports of leatherback bycatch at eight of these sites. International collaborative efforts are needed, especially from nations hosting regions where susceptibility to bycatch is likely to be high within their exclusive economic zone (northern Atlantic: Cape Verde, Gambia, Guinea Bissau, Mauritania, Senegal, Spain, USA and Western Sahara; southern Atlantic: Angola, Brazil, Namibia and UK) and from nations fishing in these high-susceptibility areas, including those located in international waters.

Assessing migratory connectivity for a long-distance migratory bird using multiple intrinsic markers

Patterns of migratory connectivity are a vital yet poorly understood component of the ecology and evolution of migratory birds. Our ability to accurately characterize patterns of migratory connectivity is often limited by the spatial resolution of the data, but recent advances in probabilistic assignment approaches have begun pairing stable isotopes with other sources of data (e.g., genetic and mark-recapture) to improve the accuracy and precision of inferences based on a single marker. Here, we combine stable isotopes and geographic variation in morphology (wing length) to probabilistically assign Wood Thrush (Hylocichla mustilena) captured on the wintering grounds to breeding locations. In addition, we use known-origin samples to validate our model and assess potentially important impacts of isotopic and morphological covariates (age, sex, and breeding location). Our results show that despite relatively high levels of mixing across their breeding and nonbreeding ranges, moderate levels of migratory connectivity exist along an east-west gradient. In addition, combining stable isotopes with geographic variation in wing length improved the precision of breeding assignments by 10% and 37% compared to assignments based on isotopes alone or wing length alone, respectively. These results demonstrate that geographical variation in morphological traits can greatly improve estimates of migratory connectivity when combined with other intrinsic markers (e.g., stable isotopes or genetic data). The wealth of morphological data available from museum specimens across the world represents a tremendously valuable, but largely untapped, resource that is widely applicable for quantifying patterns of migratory connectivity.

Geographic patterns of genetic variation in a broadly distributed marine vertebrate: new insights into loggerhead turtle stock structure from expanded mitochondrial DNA sequences

Previous genetic studies have demonstrated that natal homing shapes the stock structure of marine turtle nesting populations. However, widespread sharing of common haplotypes based on short segments of the mitochondrial control region often limits resolution of the demographic connectivity of populations. Recent studies employing longer control region sequences to resolve haplotype sharing have focused on regional assessments of genetic structure and phylogeography. Here we synthesize available control region sequences for loggerhead turtles from the Mediterranean Sea, Atlantic, and western Indian Ocean basins. These data represent six of the nine globally significant regional management units (RMUs) for the species and include novel sequence data from Brazil, Cape Verde, South Africa and Oman. Genetic tests of differentiation among 42 rookeries represented by short sequences (380 bp haplotypes from 3,486 samples) and 40 rookeries represented by long sequences (∼800 bp haplotypes from 3,434 samples) supported the distinction of the six RMUs analyzed as well as recognition of at least 18 demographically independent management units (MUs) with respect to female natal homing. A total of 59 haplotypes were resolved. These haplotypes belonged to two highly divergent global lineages, with haplogroup I represented primarily by CC-A1, CC-A4, and CC-A11 variants and haplogroup II represented by CC-A2 and derived variants. Geographic distribution patterns of haplogroup II haplotypes and the nested position of CC-A11.6 from Oman among the Atlantic haplotypes invoke recent colonization of the Indian Ocean from the Atlantic for both global lineages. The haplotypes we confirmed for western Indian Ocean RMUs allow reinterpretation of previous mixed stock analysis and further suggest that contemporary migratory connectivity between the Indian and Atlantic Oceans occurs on a broader scale than previously hypothesized. This study represents a valuable model for conducting comprehensive international cooperative data management and research in marine ecology.