Low growth rates at high population densities in sea turtles

Abstract Over 29 years (1996–2024) in a lagoon environment in the Chagos Archipelago (Indian Ocean) we conducted a mark-recapture growth rate study during which 135 juvenile hawksbill turtles (Eretmochelys imbricata) were each caught two or more times. Growth rate in straight carapace length (SCL) decreased in larger turtles with predicted mean growth rate for turtles with a starting SCL of 35 cm at 1.64 cm.y−1, decreasing to 0.90 cm.y−1 for a SCL of 60 cm: SCL(cm.y−1) = -0.03 x SCL + 2.68 (R2 = 0.15; F1,162 = 28.24, p < 0.001). After 2006, the size structure of the population changed, most likely because of an influx of small turtles (30–40 cm SCL). The resulting increase in population density may explain the decrease in size specific growth rates and body condition (calculated as mass/(SCL2 x SCW)) over time, with growth rates being faster at the start of the time series and slower at the end. For turtles in the SCL size classes 40–50 and 50–60 cm, when the initial measurement was taken prior to 1999 versus after 2018, the mean growth rates (SCL) decreased from 1.92 to 1.36 cm.y−1 and from 1.43 to 0.67 cm.y−1 in these two size classes respectively. A survey of 35 studies of hawksbill growth rates around the world showed that the size specific growth rates we recorded for immature hawksbills in the Chagos Archipelago are the lowest ever found for this species, likely due to density-dependent growth rates limited by food supply. Our findings point to the low growth rates that might have occurred more broadly prior to human exploitation of hawksbill turtles.