Differential geographical trends for loggerhead turtles stranding dead or alive along the Andalusian coast, southern Spain

Abstract

Every year, an undetermined number of loggerhead turtles cross the Gibraltar Strait entering and going out of the Mediterranean Sea. An unknown percentage of them strand on the Andalusian beaches, alive or dead, with an unequal distribution along the coast. We found a geographical pattern in the density of strandings, as well as in the proportion of them that were alive and dead. Atlantic areas receive a higher number of strandings, although most of them correspond to dead individuals, especially on the west coast (province of Huelva), whereas on the Mediterranean coast there is less difference between the number of alive and dead turtles stranded. The causes of stranding also presented a spatial segregation along these coasts: net fisheries were concentrated in Huelva, cold stunning was more frequent in Atlantic Cádiz, and debilitated turtle syndrome and longline were biased to the Mediterranean coast. The Atlantic areas might be an important accumulation zone for turtles, but where they endure a high human-induced stress and mortality. In the Mediterranean area, different causes, such as the narrowness of the Alborán basin, the ocean currents, human activity, or the number of turtles crossing, may increase the number of turtles stranding alive on the coast.

The North Atlantic Oscillation and sea surface temperature affect loggerhead abundance around the Strait of Gibraltar

“…This hypothesis is supported by the results provided by Bellido et al (2010), who found that loggerheads stranded in the Gulf of Cadiz were larger than those stranded in the Alboran Sea, which suggests that adults and larger juveniles preparing to migrate to the breeding grounds concentrate in the Gulf of Cadiz. Using Argos satellite-tag data, Eckert et al (2008) analysed the movement pathways of 15 juvenile and subadult loggerhead turtles tagged in the western Mediterranean Sea, and demonstrated that around the Alboran Sea individual variations in loggerhead distribution were partially size dependent.…”

“…Seventy-eight per cent of the turtles were dead at the time of stranding, and 22% of turtles were found alive. More information on loggerhead stranding data can be found in Bellido et al (2010).…”

“…The causes of stranding were diverse and most strandings were due to buoyancy, cold stunning, longline fisheries, net fisheries, trauma and debilitated turtle syndrome (DTS) (Bellido et al, 2010). We followed the definition of DTS provided by Day et al (2010): "debilitated turtles are characterized by emaciation and heavy burdens of external and internal parasites, and bacterial infections, but the underlying cause of their condition is unknown".…”

“…Many authors have found an association between the number of strandings recorded on the coast and the abundance of loggerhead turtles in the sea (Camiñas and de la Serna, 1995;Epperly et al, 1996;Tomás et al, 2008;Bellido et al, 2010). In this context, the local time series of turtle strandings may be a useful proxy for estimating trends in the abundance of marine turtles (MacLeod et al, 2004;Witt et al, 2007;Chaloupka et al, 2008b).…”

“…In this context, the local time series of turtle strandings may be a useful proxy for estimating trends in the abundance of marine turtles (MacLeod et al, 2004;Witt et al, 2007;Chaloupka et al, 2008b). Annually, hundreds of loggerheads, born on beaches in the North Atlantic Ocean (Eckert et al, 2008;Monzon-Arguello et al, 2010), strand along the southern Iberian Peninsula on their way to their feeding grounds in the western Mediterranean Sea or while they are returning to their natal regions (Bellido et al, 2010).…”