2 Monitoring of Marine Aquaculture
79
decomposing organic material that continues to decay gradually. Sometimes, following storms and possibly due to strong bottom currents, this layer is admixed
with the underlying mobile sediment. In places where the decomposition process is
complete, the only remains are fish bones that eventually disperse in the sediment
leaving little or no trace of the original uneaten fish on the surface. Once the source
of the impact (periodic addition of new uneaten food) is removed, the slow recovery
to the original state is signalled by the reappearance of some of the megafaunal
species that formed part of the original benthic assemblage characterising the bare
muddy sand bottom over which the tuna pens are located (Borg and Schembri,
unpublished data).
The results of benthic diversity monitoring indicated that, at times, a significant decrease in species richness, and in the abundance of the indicator macrobenthic species, occurred in the vicinity of particular tuna farms, but this effect
was mainly restricted to the area directly below the cages. Similarly, significantly
higher levels of organic carbon and/or organic nitrogen and/or significant changes
in mean sediment grain size were recorded in some of the monitoring sessions,
but the observed changes were again mainly restricted to the seabed area directly
below the cages.
The mapping and videographic surveys of important habitats and dive sites
located in the vicinity of the tuna farms did not detect any changes in the physical
and biological characteristics of the monitored sites. Likewise, the water quality
studies did not show any consistent trend in the levels of the monitored variables
that could be attributed to the tuna penning activities (Schembri et al. 2002). Lower
levels of oxygen, reduced water transparency, and elevated nutrient levels were at
times recorded at the tuna penning sites relative to the reference sites during the
farming season (July – December), however, the observed changes in the monitored
variables were sporadic and not statistically significant. Data collected in June 2006
from the new offshore aquaculture zone are still being analysed and consequently,
results from the monitoring programme for tuna farms located within this area are
not yet available.
2.3.4 Conclusions and Recommendations in Malta
Guidelines for environmental monitoring of aquaculture activities in Malta were
issued by the responsible local agencies relatively early during the period of initiation and expansion of local fish-farming involving culture of sea bream and sea
bass. However, most fish farms failed to adhere to the environmental monitoring
requirements, at least on a regular basis, while it appears that enforcement was not
effective (Schembri et al. 2002). As a result, few monitoring data on the impact of
sea bream and sea bass aquaculture activities on the marine environment exist.
Where data are available, the results of benthic environmental monitoring indicated
an overall adverse impact on seagrass beds in the vicinity of sea bream and sea bass
cages. However, site characteristics such as the current regime, water depth and
79
decomposing organic material that continues to decay gradually. Sometimes, following storms and possibly due to strong bottom currents, this layer is admixed
with the underlying mobile sediment. In places where the decomposition process is
complete, the only remains are fish bones that eventually disperse in the sediment
leaving little or no trace of the original uneaten fish on the surface. Once the source
of the impact (periodic addition of new uneaten food) is removed, the slow recovery
to the original state is signalled by the reappearance of some of the megafaunal
species that formed part of the original benthic assemblage characterising the bare
muddy sand bottom over which the tuna pens are located (Borg and Schembri,
unpublished data).
The results of benthic diversity monitoring indicated that, at times, a significant decrease in species richness, and in the abundance of the indicator macrobenthic species, occurred in the vicinity of particular tuna farms, but this effect
was mainly restricted to the area directly below the cages. Similarly, significantly
higher levels of organic carbon and/or organic nitrogen and/or significant changes
in mean sediment grain size were recorded in some of the monitoring sessions,
but the observed changes were again mainly restricted to the seabed area directly
below the cages.
The mapping and videographic surveys of important habitats and dive sites
located in the vicinity of the tuna farms did not detect any changes in the physical
and biological characteristics of the monitored sites. Likewise, the water quality
studies did not show any consistent trend in the levels of the monitored variables
that could be attributed to the tuna penning activities (Schembri et al. 2002). Lower
levels of oxygen, reduced water transparency, and elevated nutrient levels were at
times recorded at the tuna penning sites relative to the reference sites during the
farming season (July – December), however, the observed changes in the monitored
variables were sporadic and not statistically significant. Data collected in June 2006
from the new offshore aquaculture zone are still being analysed and consequently,
results from the monitoring programme for tuna farms located within this area are
not yet available.
2.3.4 Conclusions and Recommendations in Malta
Guidelines for environmental monitoring of aquaculture activities in Malta were
issued by the responsible local agencies relatively early during the period of initiation and expansion of local fish-farming involving culture of sea bream and sea
bass. However, most fish farms failed to adhere to the environmental monitoring
requirements, at least on a regular basis, while it appears that enforcement was not
effective (Schembri et al. 2002). As a result, few monitoring data on the impact of
sea bream and sea bass aquaculture activities on the marine environment exist.
Where data are available, the results of benthic environmental monitoring indicated
an overall adverse impact on seagrass beds in the vicinity of sea bream and sea bass
cages. However, site characteristics such as the current regime, water depth and