Foraging ecology of loggerhead sea turtles Caretta caretta in the central Mediterranean Sea: evidence for a relaxed life history model

Abstract

In the central Mediterranean Sea, gut contents and feces of 95 turtles captured by bottom trawlers and pelagic longliners fishing in neritic and oceanic areas, respectively, were examined. Benthic prey were more abundant than pelagic, a probable bias due to the higher digestibility of the latter. Animal and plant taxa belonging to 12 Phyla and 20 Classes were observed, with 63 new records of prey species, and Malacostraca, Gastropoda, and Echinoidea were the most commonly occurring animal classes. Results showed a highly opportunistic foraging behavior by the turtles on both live and dead material in the epipelagic zone as well as on all types of seafloor. Benthic taxa were found in turtles as small as 26 cm curved carapace length (CCL), indicating an early use of benthic resources, and also among turtles over a wide size range caught by pelagic longliners. The lack of evidence of a strict oceanic/ pelagic stage and of a clear habitat shift in the observed size range (25 to 80.3 cm CCL), when considered together with other recent findings, challenges the current ontogenetic model of life history for the species. A relaxed model is proposed, with an early short obligate epipelagic stage due to limited diving capacity, followed by the main opportunistic amphi-habitat stage, with a tendency to prefer benthic prey as turtles grow and their benthic foraging efficiency improves. Under this model, temporary or permanent association or fidelity to specific oceanic or neritic zones would vary among individuals or populations according to food availability and oceanographic features in the foraging or migratory areas. KEY WORDS: Sea turtle · Caretta caretta · Diet · Life history · Mediterranean Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 372: [265][266][267][268][269][270][271][272][273][274][275][276] 2008 Mediterranean basin (Margaritoulis et al. 2003). In particular, the area between Sicily (Italy) and Africa is considered among the most important areas for loggerheads and this is where pelagic longliners and bottom trawlers capture high numbers of turtles (Casale et al. 2007a).Diet data from different neritic areas of the basin show markedly different compositions of benthic taxa, an indication of opportunistic feeding (Laurent & Lescure 1994, Godley et al. 1997, Tomas et al. 2001. However, the ecology of loggerhead turtles in the basin is still poorly known. Laurent et al. (1998) hypothesized a transitional stage between the strictly oceanic and neritic stages, in which loggerhead turtles would feed mainly on pelagic prey but also on benthic prey, but this has not yet been adequately investigated through diet analyses. This is not a simple task, since loggerhead turtles in the neritic stage are known to feed throughthe whole water column (Bolten 2003), so that finding pelagic and benthic prey in the same sample (e.g. Tomas et al. 2001) is not proof of a transitional stage.Small loggerhead turtles (minimum size: 29.5 and 22 cm respectively; Casale et al....

Movement patterns of marine turtles in the Mediterranean Sea: a review

“…The duration of the first strictly epipelagic period varies according to population and oceanographic features, being most extended in those turtles that disperse over wide oceanic regions (Musick & Limpus 1997). In the Mediterranean, where neritic (on the continental shelf) and oceanic areas (out of the continental shelf) are not widely separated, loggerhead turtles start feeding upon benthic prey rather early: an analysis of gut and faecal content has shown that benthic feeding occur in turtles as small as 26 cm of curved carapace length (CCL) (Casale et al 2008a).…”

A critical review of the Mediterranean sea turtle rescue network: a web looking for a weaver

“…In the central Mediterranean between Italy, Tunisia, and Libya, potential neritic and pelagic foraging habitats are close to each other (Casale et al 2008b) (Fig. 1).…”

“…Loggerheads frequent different habitats during different life stages. As juveniles grow, they are increasingly found in neritic habitats, switching from epipelagic prey caught in oceanic areas to benthic prey (Schroeder et al 2003, Casale et al 2008b. Adults show extended fidelity to their neritic feeding (2010) and Echwikhi et al (2012).…”

Sea Turtles in Florida's Atlantic Waters

“…Marine turtles undergo multiple ontogenetic habitat shifts (Bolten, 2003b;McClellan and Read, 2007;Snover, 2008) as they migrate across ocean basins and along coastal zones (Dodd and Byles, 2003;Eckert et al, 2006;Foley et al, 2008;Girard et al, 2009). Various pelagic or benthic communities serve as temporary homes at different stages and within a single stage of their life cycle (Witzell, 2002;Bolten, 2003a;Polovina et al, 2006;Casale et al, 2008;Witherington et al, 2012). Documentation of where and when marine turtles move among habitats is scattered across both reviewed and gray literature.…”

“…Such habitat shifts refl ect dietary shifts. However, the direction of the cause-effect relationship remains a point of discussion (Witzell, 2002;McClellan and Read, 2007;Casale et al, 2008).…”