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

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column). There is fear that some large species, such as tuna, birds and mammals are at risk
due to various interactions with fish farms and fish farmers. However, it is not known whether
these negative interactions can have a significant effect on populations of such species.
2.4 Effects on the water column and nutrients
The Mediterranean is the largest oligotrophic water body on Earth (Krom et al. 2005) and,
unlike most marine areas it is, at least in part, P-limited (Krom et al. 1991). A recent
lagrangian experiment carried out in the Eastern Mediterranean (Thingstad et al. 2005)
showed no increase in primary production or chlorophyll a (Chla) content after an episodic
addition of phosphorous (P). This was interpreted as being due to rapid grazing of the
increased phytoplankton production. Comparative experiments with nutrient addition in
mesocosms studied within the framework of the COMWEB project (Olsen et al. 2001, 2006)
showed that the nutrients added to Mediterranean, Baltic and Norwegian coastal mesocosms
were effectively removed from the water via photosynthesis, but only in the case of the
Mediterranean experiment did the autotrophic biomass not increase. Olsen et al (2006)
provided several explanations for this Mediterranean peculiarity, such as a high
phytoplankton lysis rate (Agusti & Duarte 2000), or efficient grazing by mesozooplankton
(mainly dominated by doliolids in their experiment). Karakassis et al. (2005) have shown that
there is little risk of hypernutrification at the large spatial scale in the Mediterranean due to
nutrient input from fish farms and concluded that water quality effects are likely to occur at
small spatial scales. In a recent series of investigations on the water column in the vicinity of
fish farms located adjacent to the Mediterranean coast, there was little observed increase in
Chla content (Pitta et al. 1999, La Rosa et al. 2002, Pitta et al. 2005) which was also the case
in other surveys made in the vicinity of fish farms in other parts of the world (Nordvarg &
Jahansson 2002; Soto & Norambuena 2004). This was despite the continuous nutrient supply
which is known to be discharged from fish farming activities (Holby & Hall 1991, Hall et al.
1992; Karakassis et al. 2005 and references therein). A recent study by Dalsgaard & Krause-
Jensen (2006) using macroalgal and phytoplankton bioassays revealed high primary
productivity near fish cages, which decreased rapidly with distance from the farm. The
experiment with dialysis bags was repeated using filtered and unfiltered seawater (Pitta et al.,
2009) and showed that grazing played an important role in the regulation of phytoplankton
communities, which is also compatible with the findings of Thingstad et al. (2005) regarding
addition of P, as well as with the findings of Machias et al. (2004, 2005, 2006) regarding
rapid transfer of nutrients up the food web. Recent studies using meta-analysis of a large
number of papers dealing with the ecological effects of aquaculture (e.g. Sara 2007) have
shown that the effects of nutrient addition on the water column in marine ecosystems are
much less than in freshwater or estuarine ones.
2.5 Organic enrichment of the seabed
Nutrient enrichment of the seabed (i.e. increased organic content of the sediment) is the most
widely known effect of fish farming globally. Such effects have been reported from various
parts of the world, including Scotland (Brown et al. 1987), the East coast of Canada
(Hargrave et al. 1993), as well as for a farm sited on a sandy bottom in Puget Bay in the
north-eastern Pacific (Weston, 1990). Increased levels of organic material in sediments by a
factor of two were reported for a soft bottom by Brown et al. (1987) and Hargrave et al.
(1993), as well as by Holmer and Kristensen (1992) for non-specified sediment types.
Considerably higher levels of increase (by a factor of four) were reported by Weston (1990)
for a salmon farm located over a sandy seabed. The loose and flocculent black sediment
beneath the farms, commonly named “fish farm sediment” (Holmer, 1991), is characterised
by low values of redox potential, a high content of organic material, and accumulation of
nitrogenous and phosphorous compounds (Hall et al. 1992). The severity of the impact on the