Ocean highways in the Western Mediterranean: Which are the areas with increased exposure to maritime traffic for loggerhead turtles?

Abstract

Many marine megafauna taxa are tied to the sea surface for breathing which makes them vulnerable to vessel collisions. Sea turtles have developed efficient mechanisms to reduce surface time for breathing to a few seconds, but they can extend their surface periods to rest or to rewarm after diving into deep and colder waters. However, knowledge of collision occurrences is limited to data of turtles stranded along the coastline worldwide, whereas events occurring offshore go likely underestimated due to the sinking of carcasses. Here we performed a spatially explicit assessment to identify, for the first time, oceanic areas of higher exposure for sea turtles from maritime traffic in the Tyrrhenian Sea, Western Mediterranean. Satellite-tracking data were used to estimate utilization distributions of loggerhead turtles using Brownian bridge kernel density estimation. Maritime traffic density maps based on Automatic Identification System (AIS) data were extracted from open-access data layers, provided by the European Maritime Safety Agency, summarized, and used for the exposure analysis. Turtle occurrences were also investigated in response to vessel densities and seasonal patterns by fitting a generalized additive model to the data. Our results demonstrated that loggerhead turtles are potentially exposed to maritime traffic across the entire basin, especially in the easternmost part. The exposure varies among spring/summer and autumn/winter months. Highest turtle occurrences were found in regions primarily subjected to cargo, tanker, and passenger transportation. This study represents the first-ever effort to characterize the exposure of oceanic loggerhead turtles to maritime traffic and highlights oceanic areas of higher exposure where research and conservation efforts should be directed to understand the effective impact of this stressor on the species.

Spatiotemporal Analysis of Stranded Loggerhead Sea Turtles on the Croatian Adriatic Coast

“…Accounting for 33.33% of deaths, collision with vessels is the second major threat to loggerhead turtles in the Adriatic [ 23 , 27 , 29 , 34 ]. High tourist activities along the coast are likely to result in increased recreational boat traffic.…”

“…In order to breathe, sea turtles must surface or stay close to the surface. They may also stay at the surface for an extended period of time to rewarm and reoxygenate, which potentially exposes them to extensive marine traffic along migration routes [ 31 , 32 , 33 , 34 ]. Furthermore, sea pollution caused by plastic and chemical contaminants is undermining conservation efforts [ 35 , 36 , 37 , 38 , 39 ].…”