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Spatial variations in feeding ecology of three Sparidae species – a stable isotope analysis

Stable isotope analysis is a useful way to investigate spatial differences in fish diets (e.g. Thomas and Cahoon 1993 ; Vander Zanden et al. 1997). Many littoral fishes have relatively high site fidelity, and typically forage within areas of less than 1 km2 (Jennings et al. 1997). Consequently, it can be expected that feeding ecology of juvenile fish is site specific and isotopic analysis of tissue can be used for identifying nursery habitats and estimating their contribution to adult populations (Beck et al., 2001 ; Herzka, 2005). In this study, stable carbon (δ13C) and nitrogen (δ15N) isotope signatures have been determined from muscle tissue of three juvenile fish species (Diplodus vulgaris, Diplodus puntazzo and Sarpa salpa) collected in coastal and transitional waters along the middle Adriatic Sea. The objective of the study was to determine if there are differences in the trophic position of these species with respect to species and habitat type (coastal or transitional waters). Isotopic values from the mussel (Mytilus galloprovincialis) were used for setting the baseline signal at each site. δ13C and δ15N showed variations between habitats. Overall, δ13C was highest in coastal waters whereas δ15N was highest in transitional waters. A similar pattern was also seen in the tissue from M. galloprovincialis suggesting that these differences could be due to differences in the isotopic baseline in these habitats. The isotope values of the three fish species did not cluster together, suggesting that these three closely related species feed on different prey. This pattern was seen in both the transitional and coastal habitats. D. vulgaris had higher δ13C than the two other species in both habitats. D. puntazzo had the highest values of δ15N among the three species in the transitional waters, whereas it had the lowest δ15N of the three in coastal waters. Littoral fishes tend to exhibit a wide range of isotope signatures (Deudero et al., 2004), dependent on the spectra of invertebrate and fish upon which they prey (Pinnegar and Polunin 2000). This is mainly because in complex littoral systems, fishes may be ultimately supported by a number of primary producers (e.g. phytoplankton, macroalgae, seagrasses) each with different values of δ13C (Bricout et al. 1990). Also, the diet and feeding behaviour of fishes are known to vary greatly in both time and space. There has been much debate concerning how consumption relates to prey abundance, and trophic positions of species within food webs are now known to be dynamic rather than fixed (Polis and Strong 1996). Thus, we made a new sampling design for more detailed stable isotope analysis on selected fish species within the Nursefish project in order to provide more robust conclusions regarding different trophic positions of these species in the different habitats.

Stable isotopes, Sparidae, Adriatic sea, feeding, spatial variations

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