Somatic growth dynamics of West Atlantic hawksbill sea turtles: a spatio‐temporal perspective

Abstract

Somatic growth dynamics are an integrated response to environmental conditions. Hawksbill sea turtles (Eretmochelys imbricata) are long‐lived, major consumers in coral reef habitats that move over broad geographic areas (hundreds to thousands of kilometers). We evaluated spatio‐temporal effects on hawksbill growth dynamics over a 33‐yr period and 24 study sites throughout the West Atlantic and explored relationships between growth dynamics and climate indices. We compiled the largest ever data set on somatic growth rates for hawksbills – 3541 growth increments from 1980 to 2013. Using generalized additive mixed model analyses, we evaluated 10 covariates, including spatial and temporal variation, that could affect growth rates. Growth rates throughout the region responded similarly over space and time. The lack of a spatial effect or spatio‐temporal interaction and the very strong temporal effect reveal that growth rates in West Atlantic hawksbills are likely driven by region‐wide forces. Between 1997 and 2013, mean growth rates declined significantly and steadily by 18%. Regional climate indices have significant relationships with annual growth rates with 0‐ or 1‐yr lags: positive with the Multivariate El Niño Southern Oscillation Index (correlation = 0.99) and negative with Caribbean sea surface temperature (correlation = −0.85). Declines in growth rates between 1997 and 2013 throughout the West Atlantic most likely resulted from warming waters through indirect negative effects on foraging resources of hawksbills. These climatic influences are complex. With increasing temperatures, trajectories of decline of coral cover and availability in reef habitats of major prey species of hawksbills are not parallel. Knowledge of how choice of foraging habitats, prey selection, and prey abundance are affected by warming water temperatures is needed to understand how climate change will affect productivity of consumers that live in association with coral reefs.

Distribution, size range and growth rates of hawksbill turtles at a major foraging ground in the eastern Pacific Ocean

“…This is greater than the mean growth rate we documented of 2.8 cm year -1 for hawksbills in CNP. However, individual growth rates in CNP ranged from -0.78 to 7.08 cm year -1 , which is within the range found in the west Atlantic population (-2.1 to 22.6 cm year -1 ; Bjorndal et al, 2016), and the smaller annual mean may be a result of the comparatively short timeframe of our study. However, growth rates can also differ between populations of conspecifics as a result of habitat and food availability within foraging areas.…”

Ecological regime shift drives declining growth rates of sea turtles throughout the West Atlantic

“…Similar declines occur in annualized mean growth rates in two carnivorous species—West Atlantic hawksbills (Figure d) and North Atlantic loggerheads (Figure e,f)—following the highest growth rates in 1997. The growth functions for hawksbills (Figure d) and loggerheads (Figure e) were based on studies using capture–mark–recapture data and analyses similar to those in the present study (Bjorndal et al., , ). The second loggerhead function (Figure f) was generated based on a very different approach using skeletochronology, different analyses, and a different loggerhead dataset (Avens et al., ) that reinforces the observed decline presented here.…”

“…Annualized mean growth rates (standardized) for green turtles (a); annualized sea surface temperature ( SST , °C) (b); annualized Multivariate El Niño Southern Oscillation Index ( MEI ) (c); annualized mean growth rates for hawksbills (standardized), modified from Bjorndal et al. () (d); annualized mean growth rates (standardized) for loggerheads, modified from Bjorndal et al. () (e); and loggerhead growth rates with centered smoothed GAMM function scale on the y ‐axis, modified from Avens et al.…”

“…The decline in hawksbill growth rates was also strongly correlated with warming SST in the Caribbean and declining MEI values, with a better fit with the latter (Bjorndal et al., ). The MEI and SST effects were attributed to indirect negative effects of rising temperatures on foraging habitats (primarily coral reefs) and prey organisms.…”

“…Many of the authors of this study collaborated on earlier studies of somatic growth dynamics in West Atlantic hawksbills (Bjorndal et al., ) and Northwest Atlantic loggerheads (Bjorndal et al., ) based on capture–mark–recapture data and using a similar modeling approach. These studies revealed that growth rates for hawksbills and loggerheads exhibited similar, continuing declines beginning in 1997.…”