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Feeding ecology of "pelagic" loggerhead turtles, Caretta caretta, in the northern Adriatic Sea : proof of an early ontogenetic habitat shift

The life history of loggerhead sea turtles may be seen as a series of ecological and geographic shifts. The ontogenetic habitat shift from oceanic to the neritic zone is certainly the most important one, affecting in the population’ s demography (Bolten 2003). The size-at-recruitment of neritic zone differs between loggerhead populations worldwide (Bjorndal 1997). Although size class analysis of several studies suggested an early recruitment of northern Adriatic by small juveniles (Lazar and Tvrtković 2003, Casale et al. 2004), size at ontogenetic habitat transition and diet of early juvenile stages remained unknown. In order to gain an insight into the ontogenetic habitat shift from perspective of feeding ecology of these early stages, we focused our study on the smallest loggerheads that recruit northern Adriatic, with CCLn-t < 40 cm. We analyzed diet composition of 17 small juvenile loggerheads (CCLn-t: 25.0 – 39.2 cm, mean: 33.9 cm, SD: 4.5), captured dead by coastal gill net fishery in Slovenian waters, northern Adriatic, in 2002-2004. Food items were classified into the 15 taxa, plus unidentified digested remains, anthropogenic debris and pebble/stone fragments, and wet weighted (w.w.). In order to quantify the abundance of pelagic vs. benthic prey in the diet, we classified these taxa into the two groups: the benthic prey and the pelagic prey. The benthic group included 12 taxa (Foraminifera, Porifera, Anthozoa, Mollusca, Polychaeta, Crustacea, Bryozoa, Brachiopoda, Ophiuroidea, Echinoidea, Algae, Angiospermae) and pebbles, whilst in the pelagic group we classified Cephalopoda, Osteichthyes and Insecta ; decomposed unidentifiable organic remains and anthropogenic debris were also conservatively classified into the pelagic group. Three highest ranked prey groups include anemones (39.9%), crustaceans (23.4) and molluscs (14.3). Anthropogenic debris (remains of plastic and rope fragments) were found in 58.8% specimens. Though according the size, loggerheads in this study should still be in the pelagic (oceanic) stage (Bjorndal 1997), the benthic taxa were dominant in the diet. They were found in all samples, with in average of 87.6% w.w., and the range of values for individual turtles between 19.2 and 97.2%. The results showed that loggerheads with CCLn-t as small as 25 cm already recruit N Adriatic neritic zone, suggesting that these waters host transitional habitats for juveniles, most likely belonging to Greek management unit (Lazar et al. 2004). A shallow continental shelf with easy-available benthic communities seem to cause the ontogenetic habitat shift in loggerheads smaller in size than in most of other populations worldwide.

Sea turtles; feeding ecology; onogenetic habitat shift; life history

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