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Vol. 313: 261–270, 2006 MARINE ECOLOGY PROGRESS SERIES Published May 11
Mar Ecol Prog Ser

Use of stable isotopes to investigate dispersal of
waste from fish farms as a function of hydrodynamics

G. Sarà1,*, D. Scilipoti2, M. Milazzo1, A. Modica2

1Dipartimento di Biologia Animale dell’Università, Via Archirafi, 18, Palermo 90123, Italy
2Centro Oceanologico del Mediterraneo, Piazzale le del Fante, 40, Palermo 90146, Italy

ABSTRACT: Stable isotopes were used to examine differential effects of fish farm waste on the water
column and sediments. To achieve this objective, we chose 3 marine fish farms located along the
coast of Sicily (Mediterranean Sea) as point-source disturbances, and a control area. The hypothesis
that carbon and nitrogen isotope composition of particulate (POM) and sedimentary (SOM) organic
matter varied with increasing distance (from cages to 1000 m) was tested at 3 levels of hydrodynam-
ics: low (mean velocity of current [MVC] ~12 cm s–1), intermediate (MVC ~22 cm s–1), and high (MVC
~40 cm s–1). Different isotopic signals from allochthonous (fish waste) over natural (phytoplankton,
terrigenous, and sand microflora) inputs allowed identification of the ‘spatial effect regime’ of fish
farming. The increasing water current velocities seem to proportionally enlarge the relative area of
influence of the cages, particularly on sediments. At low hydrodynamics, an increasing contribution
of terrigenous signals was inferred: POM and SOM showing a depleted gradient of C (ranging from
–22.0 to –24.0 ‰) and N (from 5.0 to 2.0 ‰). At an intermediate hydrodynamic level, C and N showed
a slight increase in waste contribution, particularly in POM (δ15N from 2.6 to ~4.0 ‰). At high hydro-
dynamics, an enriching isotopic gradient (δ15NPOM-SOM from 1.8 to 4.6 ‰) suggested a notable
contribution of fish waste. Accordingly, the dispersal of waste from the cages seemed to be related
to movements at the bottom of the water column, confirming the recently identified role played by
resuspension movements.

KEY WORDS: Fish farming · Impact · Water column · Sediment · Stable isotopes · Hydrodynamics ·
Mediterranean

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INTRODUCTION (Mirto et al. 2000, 2002) and macrofauna composition
(Simenstad & Fresh 1995), and sediment profiles
In recent years, fish farming activity has increased (determined through imagery) (Karakassis et al. 2002).
tremendously along coasts worldwide (Naylor et al.
2000). Environmental impact assessment has been These studies revealed that impacts of fish farming
employed to examine the potential disturbance of fish may be detected from alterations in the structure and
farming on surrounding marine environments (Wu composition of natural communities. The extent of the
1995), and several biotic and abiotic descriptors have effect of fish farms (namely, Area Zone Effect or AZE)
been used for this. For example, aquaculture- seems to be limited in space (Pearson & Black 2000);
associated effects on the following parameters have however, the role of local hydrodynamics should be
been assessed: dynamics of dissolved substances in the taken into consideration (Cromey et al. 2002). In
water column (Alongi et al. 2003), structure of bacte- particular, a range of hydrodynamic levels may differ-
rio-plankton and phytoplankton communities in entially remove, distribute, and transfer fish farm
pelagic (Alongi et al. 2003) and benthic environments waste throughout the water column and underlying
(Findlay et al. 1995, Demir et al. 2001), meiofauna sediments (Alongi 1996). However, in only a few cases
has this been effectively investigated.

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