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

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et al., 1989), Shannon-Wiener H’ (diversity), Pielou’s (Evenness), etc. Each of these indices
has advantages and disadvantages. One advantage of the ABC method is that it does not
require a large number of samples to assess the status of the sediments, and can be very useful
in locations where suitable reference stations or historical background data are not available.
There are also more complex indices such as the AMBI (marine biotic index) and the BTI
(benthic trophic index). These indices require identification to species level and an
understanding of the trophic characteristics and ecological grouping (from undisturbed to
extremely disturbed communities) of each of the species.
Time required: sampling – depends on sea conditions and the method used, 1 – 2 days per
site; sorting and identification of species – depends on the species richness and taxonomic
level used, two weeks – several months per site.
Necessary equipment and infrastructure: SCUBA divers taking large diameter sediment
cores, or van Veen grab or box core; sieving & washing gear for large-volume sediment
samples, dissecting microscopes and keys for identification of the fauna. In the event that
meiofauna are studied, smaller-volume sediment samples are sufficient.
Level of expertise required: for sampling, basic technical knowledge and experience; for
sorting and identification to the family level, a moderate amount of experience with benthic
fauna; for taxonomic work to species level – very high level of expertise and experience are
necessary.
Water Quality
Changes in water quality around aquaculture facilities may be caused by a number of
processes. The farmed fish or shellfish, as well as associated biota (e.g. biofouling
communities or plankton) may severely reduce dissolved oxygen levels through respiration.
In addition, the release of metabolic waste products and their breakdown by-products may
lead to elevated nutrient levels, which may impact both planktonic and benthic biota.
In the case of filter-feeding bivalves that feed on suspended plankton and detritus, aquaculture
is considered "extractive" in that the cultured stocks actually remove (extract) organic
particles from the water column rather than add organic matter. In addition to quantifying the
environmental impacts of aquaculture, monitoring the ambient water quality around
aquaculture sites is essential in order to protect the cultured stock against toxins and
pollutants. In such cases, this reflects the impact of the environment on aquaculture rather
than that of aquaculture on the surroundings.
We generally find that the influence of aquaculture activities on surrounding water quality is
barely measurable. This is mainly due to the rapid dispersive (transport by currents) and
uptake (chemical and biological) processes. Thus, sampling should be scheduled around times
of peak farm production and warmest water conditions to capture maximal impact conditions.
Samples should also be taken along a diel or tidal cycle since these may reveal fluxes and
important fluctuations in key variables. Although the water residence time (or the flushing
rate) is a key variable that affects the sustainability of aquaculture facilities, it is interesting to
note that local trophic conditions have little effect on water quality. In well flushed
oligotrophic waters, nutrient uptake rates are extremely rapid and it is difficult to detect
nutrients outside the aquaculture zone. In mesotrophic and eutrophic waters, uptake rates may
be lower, but the flux of nutrients released from aquaculture is often very small in comparison
to the pool of ambient (background) nutrients and, ultimately, no dramatic effects on water
quality are detected. Determining water quality at a given site typically includes physical,
chemical and biological characterization.