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262 Mar Ecol Prog Ser 313: 261–270, 2006

Stable isotopes appear to be a promising tool for sediments around fish cages were mostly unvegetated.

assessing patterns in dispersal of waste from fish At each location, current meters (RD Instruments)

farms (McGhie et al. 2000, Jones et al. 2001, Burford were positioned at intervals of 0.5 m to continuously

et al. 2003, Sarà et al. 2004, Vizzini & Mazzola 2004). measure current velocities and directions from surface

Hobson (1999) argued that stable isotopes are the to bottom at time intervals of 1 h, from autumn to

most powerful tools available to trace organic matter spring. The flow at each site was basically unidirec-

for several fields of ecological research, and others tional, according to a general scheme of water mass

have recommended their use in pollution studies circulation in the Tyrrhenian Sea and the Strait of

(e.g. Kidd 1998). In this study, the spatial effect of Sicily (Millot 1999, Onken & Sellschopp 2001). Data on

organic enrichment from fish farming was investi- the local hydrodynamic regime of the 3 cage locations

gated on a regional scale. Carbon and nitrogen sta- were characterised by different mean velocities that

ble isotopes were adopted to test: (1) the extension of increased from surface to bottom (Table 1; CEOM

AZE at different hydrodynamic levels; and (2) differ- 2002). Water columns and sediments in all locations

ential signatures in the water column (as particulate  were oligotrophic, according to Mediterranean values
organic matter [POM] and sedimentary organic mat-      (suspended and sedimentary chlorophyll a: ~1 µg l–1
ter [SOM]) as a ‘memory tracer’ of degree of distur-   and ~1.2 µg g–1, respectively) (Sarà et al. 1998, Sarà et

bance.                                                 al. 1999, CEOM 2002).

                                                       At each of the 3 fish farm locations, 6 sampling sites

                                                       were positioned downstream along the main axis of

        MATERIALS AND METHODS                          the water current according to 3 distance categories

                                                       (Fig. 1): (1) two sites were placed within a distance of

Study area and sample collection. Our study was 50 m downstream from the centre of the fish farm

conducted from May to June 2001 in 3 areas (Fig. 1) off (hereafter referred to as ‘cages’ or 0 m sites), (2) two

the coast of Sicily (37° 45’ N, 13° 45’ E). The 3 areas are sites were placed ~500 m downstream (hereafter

influenced by constant, seasonally fluctuating, terrige- referred to as 500 m sites), and (3) the final 2 sites were

nous inputs, which originate from nearby streams.      placed at a distance of ~1000 m downstream from the

In each area, a marine fish farm (and with a mean centre of the fish farm (hereafter referred to as 1000 m

biomass production of ~40 tons of Dicentrarchus labrax sites). To compare results and to investigate the influ-

and Sparus aurata) (CEOM 2002) was positioned ence of natural variability on the cage systems, data

within a couple of km from the coast and moored on were also collected at an external control location. At

the bottom at a depth of about 26 to 30 m. In all areas, this control location off the northwest coast of Sicily

                                                                            (Egadi Island Strait), 6 sites were posi-

                                                                            tioned at a depth of 15 to 20 m, each

                                                                            positioned along the main axis of the

                                                                            water current (mean velocity of current
                                                                            [MVC] ~20 cm s–1), and at not more than

                                                                            500 m from each other in order to simu-

                                                                            late the same spatial pattern of the cage

                                                                            locations (Fig. 1). Inputs other than pri-

                                                                            mary and terrigenous production were

                                                                            not present in this location (Sarà et al.

                                                                            1999).

                                                                            Laboratory analysis. Water samples

                                                                            were collected at a depth of about 14 m

                                                                            (1 m below the end-edge of the cage

                                                                            net), while sediments were taken from
                                                                            2 quadrats (400 cm2 surface area) using

                                                                            hand cores (collected by SCUBA

                                                                            divers). In the laboratory, water samples

                                                                            (~4 l) were screened through a 200 µm

                                                                            mesh net in order to remove larger zoo-

Fig. 1. Study areas (not to scale), including the control area ( : Egadi    plankton and debris, and filtered
Islands). Scheme of fish farming systems also shown, depicting location of  through pre-combusted fibreglass fil-
cages, distances of sampling sites from cages, type of comparisons among    ters (Whatman GF/F) (at 450°C for 4 h).
                                                                            The top layer (0 to 1.5 cm) of each core
                         sites, and direction of main water current
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