Predicted distributions and abundances of the sea turtle ‘lost years’ in the western North Atlantic Ocean

Abstract

Oceanic dispersal characterizes the early juvenile life‐stages of numerous marine species of conservation concern. This early stage may be a ‘critical period’ for many species, playing an overriding role in population dynamics. Often, relatively little information is available on their distribution during this period, limiting the effectiveness of efforts to understand environmental and anthropogenic impacts on these species. Here we present a simple model to predict annual variation in the distribution and abundance of oceanic‐stage juvenile sea turtles based on species’ reproductive output, movement and mortality. We simulated dispersal of 25 cohorts (1993–2017) of oceanic‐stage juveniles by tracking the movements of virtual hatchling sea turtles released in a hindcast ocean circulation model. We then used estimates of annual hatchling production from Kemp's ridley Lepidochelys kempii (n = 3), green Chelonia mydas (n = 8) and loggerhead Caretta caretta (n = 5) nesting areas in the northwestern Atlantic (inclusive of the Gulf of Mexico, Caribbean Sea and eastern seaboard of the U.S.) and their stage‐specific mortality rates to weight dispersal predictions. The model's predictions indicate spatial heterogeneity in turtle distribution across their marine range, identify locations of increasing turtle abundance (notably along the U.S. coast), and provide valuable context for temporal variation in the stranding of young sea turtles across the Gulf of Mexico. Further effort to collect demographic, distribution and behavioral data that refine, complement and extend the utility of this modeling approach for sea turtles and other dispersive marine taxa is warranted. Finally, generating these spatially‐explicit predictions of turtle abundance required extensive international collaboration among scientists; our findings indicate that continued conservation of these sea turtle populations and the management of the numerous anthropogenic activities that operate in the northwestern Atlantic Ocean will require similar international coordination.

Managing fisheries in a world with more sea turtles

“…Developing a framework to conduct annual stock assessments on sea turtles will require considerable interjurisdictional coordination to obtain data on anthropogenic activities that impact sea turtles and an investment in basic research on sea turtle ecology [16,37]. To obtain the needed information, the direct involvement of other government agencies, fisheries, industries, conservation groups, academia and the private sector should be encouraged.…”

“…While some sea turtle strandings might be linked to gillnet interactions or other fishing activities, they are also shaped by environmental conditions that influence mortality, the probability of washing ashore and the probability of being reported [30]. Our aim was not to ascertain cause, but to use these data as an index of abundance across the state, assuming that the more turtles that are present in an area, the more likely it is that one will wash ashore (for whatever reason) and be reported [16]. This index of turtle abundance may be biased high, because the dataset includes sea turtle that strand outside of estuarine waters, where turtles are not susceptible to gillnet interactions.…”

“…In addition to observed indices of turtle abundance, we used a recently developed model that predicts the distribution and abundance of juvenile Kemp's ridley, green and loggerhead sea turtles during their oceanic stage [16]. The approach has been described in detail and appears to accurately depict environmental drivers that contribute to annual variation in turtle distributions [31].…”

“…The approach has been described in detail and appears to accurately depict environmental drivers that contribute to annual variation in turtle distributions [31]. Moreover, the model shows good agreement in predicting spatiotemporal variation in the recruitment of oceanic-stage Kemp's ridley and green turtles to coastal waters in the Gulf of Mexico [16]. For this analysis, we computed the number of 0.5-to 3.5-year-old green and loggerhead turtles, and 0.5-to 2.5-yearold Kemp's ridley that crossed shoreward of the 75 m isobath near the coast of North Carolina (33.8-36.5 N; figure 1; electronic supplementary material, table S4).…”

Seascape Genetics and the Spatial Ecology of Juvenile Green Turtles

“…Dispersal patterns can be inferred by analyzing large numbers of simulated trajectories with computed trajectories, based on the assumption that hatchlings (and then juveniles) released from natal beaches drift passively with ocean currents. This simple approach has been, and is still widely used, to obtain a first-order estimate of the pelagic juveniles' spatial and temporal distributions (e.g., [26][27][28][29]). More elaborate models have recently been developed [30,31] that simulate dispersal under the combined effect of ocean currents and active swimming movements motivated by the need to find suitable habitats (i.e., adequate water temperatures and food).…”