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

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5.2.9 Introduction of alien species
Aquaculture has been responsible for the introduction of many alien species during the last decades.
The environmental impact of introduced species is relatively unpredictable, but it could be often
critical and permanent. The Mediterranean Sea has received many species that are non-native and
hence originating from other seas. Human activity has increased the rate of alien species introduction,
affecting the marine biodiversity of the region. During the last 150 years, the number of exotic species
has increased, with the introduction of more than 100 alien species (Minchin, 2001). Additional to the
introduction of species, a main problem is when the exotic species becomes invasive, hence
increasing its population size at the expense of native species. A clear example of this is the
introduction of alien macrophytes by oyster farming. Transportation and transplantation of alien
molluscs into the Mediterranean has resulted in numerous unintentional introductions. In this sense,
oyster farms served as effective gateways for alien macrophytes into Mediterranean coastal waters.
Culture of the Japanese oyster Crassostrea gigas has been, compared with other major vectors such as
ballast waters, the main vector of introduction of exotic macrophytes (Verlaque et al., 2006).
5.3 Better management practices for site selection
An optimal site selection procedure can mitigate local environmental impacts. For example, from a
regional point of view, cumulative impact resulting from a concentration of several farms needs to be
assessed carefully. In these sense, even small management steps can minimize the negative influence
of aquaculture on coastal environment. Belle and Nash (2008) provide a summary of better
management practices for site selection with regard to cage fish farming in coastal areas. Their
recommendations are summarise as follows:
a) conduct baseline environmental site surveys;
b) avoid sites with frequent or extreme weather or sea-state conditions;
c) select sites with good water exchange that are not depositional environments;
d) select sites with water depth are least twice that of the net pen;
e) select sites with appropriate bottom type and profile;
f) do not site facilities in areas subject to harmful algal blooms;
g) avoid sites where culture activities will negatively impact sensitive populations of fish birds or
other wildlife;
h) select sites away from concentrations of predators and pests.
5.4 Global change and aquaculture
Fisheries, aquaculture and other uses of marine renewable resources are already affected by global
changes in the marine ecosystem. Adaptation of management measures, such as reallocation of coastal
facilities is, therefore, critical. Reallocation of aquaculture facilities may be required if ecological and
environmental aspects are affected by global change; e.g. carrying capacity of a body water,
sensitivity of a marine ecosystem, sea level or temperature changes etc. Adjustment of strategies
should aim at enhancing the resilience of marine renewable resources and their uses, and the current
capacity to respond to sudden changes. Already, uses of the natural environment are characterized by
massive over-capacities, excessive resources exploitation and pervasive conflicts within and between
uses. A variety of changes in climatic conditions following atmospheric CO2 enrichment have been
predicted. These predictions include increased air temperatures, regional shifts in precipitation, and
seasonal changes in the timing and duration of precipitation events and sea level rise.
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