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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