Site selection and carrying capacity in Mediterranean ... - FAO Sipam

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seabed depends largely on the type of sediment. In the case sites characterised by coarse
sediments, impacts are generally low or even undetectable (Apostolaki et al. 2007), whereas
at muddy or silty sites the effects are more pronounced. This difference is due to the
considerably higher water movement over coarse sediments, which induces resuspension and
reduces anoxia of the seabed. The results of a study in the Mediterranean (Karakassis et al.
2000) showed that the organic carbon and nitrogen content of the sediment near the cages was
found to increase by a factor of 1.5 - 5, and microbial content by a factor of 4 - 28.
Sediment anoxia, patches with Beggiatoa and absence of macrofauna have been reported in
relation to salmon farming in the North Atlantic (Rosenthal and Rangeley, 1988; Hansen et
al. 1991) and in the Baltic Sea (Holmer and Kristensen, 1992). Notwithstanding the
microtidal regime of the Mediterranean, none of the studies carried out on fish farms located
in the region showed the presence of an azoic zone, in terms of macrofauna, in the vicinity of
or even directly below the cages (e.g. Karakassis et al. 2000, Tomassetti & Porrello 2005,
Klaoudatos et al 2006, Yucel-Gier et al. 2007, Dimitriadis & Koutsoubas 2008). Furthermore,
Maldonado et al. (2005) showed that in five semi-offshore farms in Spain, the effects on
benthic macroinvertebrate communities were even less detectable and found no substantial
differences between farm and control sites. In most cases, the effects of fish farming on
macrobenthic diversity and community structure were detectable and compatible with the
Pearson & Rosenberg (1978) model up to a distance of 10 m - 25m from the edge of the
cages. Kalantzi & Karakassis (2006) have shown from meta-analysis of data from different
parts of the world that a small number of variables (depth, distance and latitude) can be used
to predict changes in geochemical variables and biotic response of macrofauna. Regarding the
EU’s Water framework Directive, data from Aguado-Gimenez et al. (2006) from Spain and
Karakassis et al. (unpublished) from Greece indicate that the benthic quality directly beneath
fish farms cannot be considered “High” or “Good”, no matter the index used. Therefore, there
is a need to investigate the possibility of adopting the concept of the AZE (allowable zone of
effect), which is already used in the UK, to the Mediterranean as well.
A series of studies have attempted to use meiofauna as a means for detecting changes in
benthic habitats due to fish farms (Mirto et al. 1999, 2002; Mazzola et al. 1999; 2000; La
Rosa 2000, 2001; Sandouli et al. 2001, Kovac et al. 2001; Vezzuli et al. 2003; Lampadariou et
al. 2005; Najdek et al 2007). The results have been contradictory in terms of showing that the
abundance of benthic meiofauna was higher (Lampadariou et al. 2005) or slightly lower
(Najdek et al 2007) or significantly reduced (Mazzola et al. 1999; Mirto et al. 2002) as a
result of the fish farming activity. The diversity of meiofauna and functional groups seemed
to be reduced beneath the cages (Mirto et al. 2002). However, the lack of a general theory
regarding the response of meiofauna to organic enrichment, as well as problems related to
identification resulting from the limited number of taxonomic experts for this group, implies
that there is still a lot of research to be done before meiofauna can be used as a standard
monitoring tool.
The effects of tuna farming on the seabed are generally similar to those for sea bream and sea
bass but they seem to be more widely spread around the farms. Partly, this is due to the large
size of the farms but also to the very low Food Conversion Ratio (FCR) and consequently to
the large amount of wastes produced (Aguado-Gimenez et al. 2006). Vita & Marin (2007)
have shown that despite the water depth at the tuna farming site, the effects of tuna farming
on macrofauna was detectable up to 200m from the farm, although there was no anoxic zone
in this case. The six-month fallowing period was not sufficient for the system to fully recover.
A tuna farm in Croatian waters investigated by Matijevic et al. (2006) also showed
remarkable changes in redox conditions in the sediment, a two-fold increase in sediment
phosphorus, and a significant increase in nutrients in the bottom water layer. Recently
Vezzulli et al. (2008) investigated a tuna farm located at a site off the south-western Italian
coast that is exposed and has deep waters. Although they found little change for most of the