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

Karen A. Bjorndal, Alan B. Bolten, Milani Chaloupka, Vincent S. Saba, Cláudio Bellini, Maria A. G. Marcovaldi, Armando J. B. Santos, Luis Felipe Wurdig Bortolon, Anne B. Meylan, Peter A. Meylan, Jennifer Gray, Robert Hardy, Beth Brost, Michael Bresette, Jonathan C. Gorham, Stephen Connett, Barbara Van Sciver Crouchley, Mike Dawson, Deborah Hayes, Carlos E. Diez, Robert P. van Dam, Sue Willis, Mabel Nava, Kristen M. Hart, Michael S. Cherkiss, Andrew G. Crowder, Clayton Pollock, Zandy Hillis‐Starr, Fernando A. Muñoz Tenería, Roberto Herrera‐Pavón, Vanessa Labrada‐Martagón, Armando Lorences, Ana Negrete‐Philippe, Margaret M. Lamont, Allen M. Foley, Rhonda Bailey, Raymond R. Carthy, Russell Scarpino, Erin McMichael, Jane A. Provancha, Annabelle Brooks, Adriana Jardim, Milagros López‐Mendilaharsu, Daniel González‐Paredes, Andrés Estrades, Alejandro Fallabrino, Gustavo Martínez‐Souza, Gabriela M. Vélez‐Rubio, Ralf H. Boulon, Jaime A. Collazo, Robert Wershoven, Vicente Guzmán Hernández, Thomas B. Stringell, Amdeep Sanghera, Peter B. Richardson, Annette C. Broderick, Quinton Phillips, Marta Calosso, John A. B. Claydon, Tasha L. Metz, Amanda L. Gordon, Andre M. Landry, Donna J. Shaver, Janice Blumenthal, Lucy Collyer, Brendan J. Godley, Andrew McGowan, Matthew J. Witt, Cathi L. Campbell, Cynthia J. Lagueux, Thomas L. Bethel, Lory Kenyon

Somatic growth is an integrated, individual-based response to environmental conditions, especially in ectotherms. Growth dynamics of large, mobile animals are particularly useful as bio-indicators of environmental change at regional scales. We assembled growth rate data from throughout the West Atlantic for green turtles, Chelonia mydas, which are long-lived, highly migratory, primarily herbivorous mega-consumers that may migrate over hundreds to thousands of kilometers. Our dataset, the largest ever compiled for sea turtles, has 9690 growth increments from 30 sites from Bermuda to Uruguay from 1973 to 2015. Using generalized additive mixed models, we evaluated covariates that could affect growth rates; body size, diet, and year have significant effects on growth. Growth increases in early years until 1999, then declines by 26% to 2015. The temporal (year) effect is of particular interest because two carnivorous species of sea turtles - hawksbills, Eretmochelys imbricata, and loggerheads, Caretta caretta - exhibited similar significant declines in growth rates starting in 1997 in the West Atlantic, based on previous studies. These synchronous declines in productivity among three sea turtle species across a trophic spectrum provide strong evidence that an ecological regime shift (ERS) in the Atlantic is driving growth dynamics. The ERS resulted from a synergy of the 1997/1998 El Niño Southern Oscillation (ENSO) - the strongest on record - combined with an unprecedented warming rate over the last two to three decades. Further support is provided by the strong correlations between annualized mean growth rates of green turtles and both sea surface temperatures (SST) in the West Atlantic for years of declining growth rates (r = -0.94) and the Multivariate ENSO Index (MEI) for all years (r = 0.74). Granger-causality analysis also supports the latter finding. We discuss multiple stressors that could reinforce and prolong the effect of the ERS. This study demonstrates the importance of region-wide collaborations. This article is protected by copyright. All rights reserved.