Abstract
The isotopic concentrations of carapace scutes, skin, muscle and blood of loggerhead sea turtles (Caretta caretta) from the Balearic Archipelago were analysed to investigate the pattern of variation between tissues and to assess the position of this species in the trophic webs of the Algerian Basin. Skin showed higher d13C values than muscle or carapace scutes and these showed higher values than blood. Conversely, muscle showed higher d15N values than skin, skin showed higher values than blood and blood showed higher values than carapace scutes. Dead and live sea turtles from the same habitat did not differ in the concentration of stable isotopes. However, some of the tissues of the turtles caught in drifting long-lines in the oceanic realm showed higher d13C values than those from the turtles caught by hand or in trammel nets over the continental shelf, although they did not differ in the d15N. Comparison of the concentration of stable isotopes in the turtles with that of other species from several areas of the Algerian Basin revealed that they consumed planktonic prey and that the trophic level of the sea turtles was higher than that of carnivorous cnidarians but lower than that of zooplanktophagous fish and crustaceans.
Highlights
The diet of marine vertebrates has been investigated by stomach content analysis.results are sometimes inconsistent, which may be due to at least two factors
The aims of this paper are (1) to describe how nitrogen and carbon isotopic concentration varies among tissues in loggerhead sea turtles (Caretta caretta) and (2) to assess the position of loggerhead sea turtles in food webs of the Mediterranean on the basis of their isotopic concentration
FIG. 1. – Isotopic concentration of carapace scutes ( ), muscle ( ), skin (L) and blood (×) of turtles caught by drifting long-line, trammel net and hand-caught
Summary
The diet of marine vertebrates has been investigated by stomach content analysis. Results are sometimes inconsistent, which may be due to at least two factors. The prevalence of prey with hard parts is likely to be overestimated, because it is difficult to detect soft prey, 88 M. Access to live, free-ranging specimens of scarce marine species such as sea turtles and some marine mammals is limited, which has forced most studies to rely on stranded animals. In diseased individuals, feeding behaviour in the period immediately preceding stranding may have been altered by weakness and include unusual prey that can bias the results provided by stomach content studies. As many stranded specimens are killed by fishing activities, diet may spuriously appear to be dominated by bait species which would otherwise only be marginal to the normal diet
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