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

15 GFCM:XXXV/2011/Dma.9
2.2 Effects of different types of aquaculture
Fish farming in cages affects the marine environment through several pathways, particularly through
the release of nutrients, particulate material and chemicals. Aquaculture practiced using land-based
closed systems is more likely to avoid most of these adverse problems by using treatment methods to
control the quality of effluents, although this will increase the production costs to potential
unsustainable levels.
The effects of molluscan farming on the environment are usually simpler and in most cases they go
towards the opposite direction of finfish farming, at least in terms of effects on the water column. In
particular, mussel farming results in intense filtration of seawater and removal of particulate material
from the water column. In this context, it has been suggested that coupling fish with mussel farming is
likely to reduce the overall effect of aquaculture. Along these lines there have also been suggestions
for polyculture systems of various types. However, there is still the need for information on the
economic viability of such systems, as well as on their efficiency in improving the environmental
conditions in areas of deployment. For example, mussels are likely to take up considerable amounts of
small-size particles from the water column but they cannot remove large-size food or faecal pellets
which are the very ones causing anoxic conditions on the seabed below fish cages. Furthermore,
mussels produce large amounts of pseudofaeces (filtered and rejected particles) settling on the seabed,
thereby inducing benthic effects similar to those caused by fish cages.
Tuna farming has been introduced recently in the Mediterranean but up to 2007 there were more than
60 farms registered in the Mediterranean, particularly in Croatia, Cyprus, Greece, Italy, Malta, Spain,
Tunisia and Turkey (CIESM 2007). There are serious reservations regarding the sustainability of this
industry and the direct effects it has on wild tuna stocks, as well as the indirect effects on other fish
species used to feed the tuna in captivity, and the potential for causing mass mortalities through
translocation of pathogens when using imported fish feed (see review in CIESM 2007). However,
from the local environmental point of view it would be expected that the fact that these farms are
located in offshore waters with considerable depth, and their potential to use state-of-the-art
equipment, could minimize their direct impact on the surrounding environment. Furthermore, as the
farming/fattening season is approximately 6 months, there is a period during which the environment is
allowed to recover, and this is expected to result in better environmental performance for these farms.
Despite the above, the information available to date on interactions of tuna-farming with the
environment is not very encouraging.