Green turtle somatic growth dynamics distributional regression reveals effects of differential emigration

• Published in: Marine ecology. Progress series (Halstenbek) Vol. 616; pp. 185 - 195
• Main Authors: Bjorndal, Karen A., Bolten, Alan B., Chaloupka, Milani
• Format: Journal Article
• Language: English
• Published: Oldendorf Inter-Research Science Center 09-05-2019
• Subjects: Aquatic reptiles; Body size; Capture-recapture studies; Chelonia mydas; Demographic variables; Demographics; Ecological aggregations; Ecosystems; Emigration; Environment models; Environmental changes; Foraging; Growth rate; Inspection; Model testing; Population density; Population growth; Regression analysis; Regression models; Reptiles & amphibians; Resource management; Sea turtles; Sustainability; Turtles; Variance; Variance analysis
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps12946

Because somatic growth is an integrated response to variables on individual, population and ecosystem levels, growth rates have been used as indicators of change in a wide range of demographic and environmental variables. Models of sea turtle growth that relate changes in productivity to environmental changes have all focused on the mean or expected growth rate. We used distributional regression analysis for the first time in sea turtle growth studies to evaluate both the mean and variance of the growth response as a function of covariates. We analyzed a 39 yr (1979–2017) data set with 1176 growth increments for 727 green turtles Chelonia mydas from a capture-mark-recapture study in the southern Bahamas. The distributional regression model had an improved fit to the growth data in comparison to the mean-response-only model. By testing predictions based on inspection of variance plots, we reveal novel insights on the importance of differential growth rates as cues for emigration. Variance in growth rates decreases with increasing body size and increasing population density because turtles that grow slowly at small sizes or at high densities exhibit differential emigration. Drivers of differential emigration have received little attention in demographic studies of sea turtle foraging aggregations and will be more important as sea turtle populations recover and population densities on foraging grounds increase. Future conservation and management of increasing green turtle populations depend on improved understanding of resource use, sustainability of grazing, and cues for emigration. Distributional regression is a powerful analytical approach for these studies.