Effects of environmental change and plastic ingestion on the energy budget of loggerheads

Loggerhead turtles are vulnerable to natural and anthropogenic environmental pressures due to their longevity, global distribution, and their migratory way of life. Two significant environmental conditions, temperature and available food, have been correlated to growth, reproduction, and maturation of loggerhead turtles, often accounting for most of the observed variability. With the current and expected environmental changes, patterns in the environmental conditions could change as well, resulting in different food availability and temperature in habitats used by loggerhead turtles. Another important pressure is the plastic pollution (i.e. anthropogenic debris) present in all the oceans. Over 267 marine species have been reported to have ingested plastic. Sea turtles are considered particularly prone to ingestion of anthropogenic debris, and ingestion of plastic has been reported in as many as 76% of the studied loggerhead turtles. The problem has been recognized, but the effects of plastic ingestion on the energy budget and the life cycle has not yet been studied in detail. In addition, genetic variability present between loggerhead turtle populations might affect the metabolic responses to the environmental pressures. We used the previously developed energy budget (DEB) model for loggerhead turtles of the North Atlantic and Mediterranean populations to study multiple aspects of environmental conditions independently, and to understand how each condition affects the relevant process and life history traits. The results suggest that the ultimate size (length and weight) is primarily affected by food availability, and that growth and maturation are primarily affected by temperature, whilst also showing correlation with available food. Reproduction is affected by both food and temperature, with the former influencing the size of the reproduction buffer, and the latter influencing the rate of the related processes (such as vitallogenesis). Length at puberty varied between scenarios, but only by a small proportion, suggesting that interindividual variability plays a larger role for length at puberty than the environmental factors do.