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

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GFCM:XXXV/2011/Dma.9
Perez et al. (2008) have shown a significant effect of fish farming on P. oceanica physiology,
particularly of the total nitrogen content and of δ 15 N in epiphytes. Invasion by sulphide in the
seagrass habitat has resulted in higher plant mortalities close to fish farms (Frederiksen et al.
2007). Holmer & Frederiksen (2007) have shown that despite the low accumulation of the
organic matter (which is expected due to the highly dispersive characteristics at such sites),
the sulphate reduction rates remain at very high levels, inducing significant habitat
degradation. Recovery of the meadows after the cessation of fish farming which has been
referred to above is very slow (Delgado et al. 1999).
A synthesis paper from the MedVeg project (Holmer at al. 2008) has examined a series of
drivers of seagrass decline due to fish farming effects and identified sedimentation of waste
particles in the vicinity of fish farms as the main driver of benthic habitat deterioration.
Holmer et al. (2008) have recommended a safety distance of 400m from fish farms for the
conservation management of P. oceanica, together with establishment of permanent seagrass
sampling plots for monitoring the health of the meadows.
The health of P. oceanica beds in marine protected areas has been found to be not much
better that of beds in unprotected areas in the western Mediterranean (Montefalcone et al.
2009). On the other hand, according to Boudouresque et al. (2006), the most degraded P.
oceanica beds are found in the vicinity of urban areas and/or close to harbour zones, implying
that local multiple stressors are behind seagrass declines (Orth et al. 2006); this is also
compatible with the positive dynamics of P. oceanica beds found in Mediterranean marine
protected areas (MPAs) by González-Correa et al. (2007).
Maerl habitat
Maerl beds are formed by an accumulation of unattached calcareous red algae (Rhodophyta),
growing as a superficial living layer on sediments within the photic zone. They are are
spatially complex habitats with a high degree of species and trophic group diversity (Barbera
et al. 2003). Mediterranean maerl beds are to be considered for inclusion in national
inventories of sites of conservation interest, as required by the SPABIM Protocol of the
Barcelona Convention. In spite of their importance, and the requirement for their conservation
management, European maerl grounds suffer a variety of anthropogenic perturbations,
including direct exploitation through extraction, fishing impacts and chemical pollution by
organic matter and excess nutrients. Hall-Spencer et al. (2006) carried out a survey in 3
shallow sites (10 m – 15 m) in Scotland and found that fish farming affects negatively maerl
beds. A similar survey by Sanz Lázaro (2009) in the Mediterranean also showed significant
negative effects, verifying the predictions of laboratory experiments by Wilson et al. (2004).
Maerl has a very low growth rate; approximately 0.1 mm - 1 mm per year, and the production
and accumulation rates are similar to the lower end of growth rates for tropical coral reef
environments (Bosence and Wilson, 2003). Although rapid on a geological time-scale, these
accumulation rates are far too low for the maerl to be regarded as a sustainable resource for
extraction for agricultural and industrial use. In a recent study by Georgiadis et al. (2009)
undertaken in the Aegean Sea (Eastern Mediterranean), coralligène formations were recorded
at water depths ranging from 60 m to 114.0 m, with 81% of the total surface being restricted
to water depths between 70 m and 90 m. Within 50 m – 120 m depth range, maerl habitats
occupied 4.3% of the seabed area. Therefore, a problem with maerl conservation becomes
important when considering moving fish farming offshore. In this case, the site selection
process should ensure that there are no maerl beds in the close vicinity of the proposed farm
site.
Marine protected areas
There is particular concern on the effects of aquaculture on sensitive areas such as Marine
Protected Areas (MPAs). In the Mediterranean, some fisheries (mainly small pelagic
planktivore species) have increased due to an increase in nutrient supply, whereas some