Overview

Loggerhead turtles (Caretta caretta) are a globally vulnerable species of marine turtle. The Mediterranean Sea subpopulation, which ranges throughout the entire Mediterranean basin, is listed as least concern by the International Union for the Conservation of Nature (IUCN), but experiences significant threats in the region including bycatch in fisheries, climate change, coastal development, and marine pollution. Broad-scale patterns of distribution and abundance can provide regional managers a tool to effectively conserve and manage this species at basin and sub-basin scales.

In 2020, a spatial density model estimating loggerhead density, abundance, and distribution across the Mediterranean Sea was generated as a long-term annual average. The model was adjusted for availability bias using dive data from loggerhead turtles tagged with time depth recorders from multiple regions within the Mediterranean Sea. Geographic extrapolation in areas near surveys was undertaken with caution. Mean abundance for the long-term average model was estimated as 1,201,845 (CV 0.22). This estimate represents the first basin-wide estimate of abundance for this species in the Mediterranean not based on demographic models. This work was funded by the U.S. Navy to assist with assessing risk to protected marine species resulting from military readiness activities.

Mediterranean Loggerhead Turtle Density Model

The data are an annual estimated density of loggerhead turtles, expressed as the number of individuals per square kilometer, within the Mediterranean Sea. The study area was gridded into 5x5 kilometer blocks, the resolution at which density modeling was performed. The study area encompasses all major subregions of the Mediterranean Sea, except the Sea of Marmara, which loggerhead turtles do not frequent. The DENSITY attribute contains the estimated density (individuals per square kilometer) and the UNCERTAINTY attribute contains the estimated coefficient of variation (CV) for the density estimate. The density estimates represent the mean throughout year, averaged for the period 2003-2018. For most of the study area, density was estimated using a habitat-based density model that correlated local abundance observed on systematic line transect surveys with environmental conditions observed at that same location and time. For unsurveyed areas and times, density was estimated by extrapolation. Density was not estimated for nearshore locations and inshore waters as they were often not covered by surveys or lacked reliable remoted sensed covariate coverage. See Sparks and DiMatteo (2020) for more information about the modeling process. The metadata documentation has more information regarding individual attributes.

The shoreline and other geographic features were delinated by NAVFAC from a database provided by the National Geospatial-Intelligence Agency (NGA).

Download: NUWC_Mediterranean_Loggerhead_v1.zip

Citations

Sparks, Laura M. DiMatteo, Andrew D (2020) Loggerhead Sea Turtle Density in the Mediterranean Sea. Technical Report prepared for Naval Undersea Warfare Center Division.

DiMatteo et al. (2022). Basin-wide estimates of loggerhead turtle abundance in the Mediterranean Sea derived from line transect surveys. Frontiers in Marince Science, 28 September 2022 Sec. Marine Megafauna https://doi.org/10.3389/fmars.2022.930412

This dataset and the density models used to produce it were developed by the Environmental Planning Lab at Naval Undersea Warfare Center, Newport RI.

Acknowledgements

The authors would like to thank all the data providers and organizations that contributed line transect surveys and dive data to this project: Blue World Institute, The University of Valencia, Tethys Institute, ISPRA, IFAW, The University of Exeter, ACCOBAMS, PELAGIS, Alnitak, Stazione Zoologica Anton Dorne, Julie Belmont, Simone Panigada, Olivier Boisseau, Giancarlo Lauriano, Drasko Holcer, Caterina Fortuna, Vincent Ridoux, Helene Peltier, Toni Raga, David March, Robin Snape, Sandra Hochscheid, Annette Broderick, Brendan Godley, Robin Snape, and Julia Haywood. Jason Roberts and Ana Canadas provided analytical support and cleaned and delivered much of the line transect survey data. Elizabeth Becker and David Miller provided R code and advice on including environmental variability in the uncertainty estimate. Danielle Jones of the Naval Facilities Engineering Command Atlantic assisted with project management.