78
M. Holmer et al.
The general state of seagrass beds and habitats, including those at popular dive
sites, is being assessed through mapping surveys and underwater videography, carried out by SCUBA divers. During the surveys, the divers record the state of health
and spatial extent of the main marine benthic habitats in the respective study area.
Potential changes in the state of the benthic habitats and their spatial distribution
are assessed by comparing maps of the situation recorded before initiation of the
tuna-penning activities with that after each monitoring session.
Monitoring of water quality consists of surveys of the same physico-chemical
and bacteriological attributes that have been monitored in the vicinity of sea bass
and sea bream farms. Samples of water for these surveys are being collected at
depths of 1 m and 5 m below the surface at several stations located in the immediate
vicinity of the tuna farms and at reference stations located at a distance from the
tuna cages.
Monitoring at the new offshore aquaculture zone (Fig. 2.2) commenced in June
2006 prior to the start of tuna penning activities there. The baseline survey for the
sediments and benthic diversity monitoring components was based on the same
design used at the other three tuna farms located closer to the coast, with samples
being collected remotely using a standard 0.1 m
2 Van Venn grab. However, because
of the deep waters that characterise the area, monitoring of the seabed using the
same underwater videography and SCUBA diving techniques that have been used
to date at the other tuna penning sites located in shallower waters, is not possible,
and it is planned to use remotely operated video cameras instead.
Overall, the results of the various monitoring components undertaken since 2000
for the three tuna farms located 1 km offshore indicated that, where detected, the
main adverse impacts resulted from accumulation of large amounts of feed-fish on
the bottom under and in the vicinity of the cages. The results from the video surveys
carried out near the tuna pens indicated that, towards the end of each penning season (in autumn), considerable amounts of dead uneaten feed-fish were present on
the seabed directly below the tuna pens, and this resulted in alterations in the physical
and biological characteristics of the seabed under the cages. The recorded changes
in biological characteristics included the disappearance of certain megafaunal
species (e.g., the irregular sea urchin Spatangus purpureus and the crinoid Antedon
mediterranea) that prior to the start of the penning operations were characteristic
of the soft sediment habitat where the tuna pens are located, and the appearance of
high population densities of detritus-feeding and scavenging macroinvertebrates
(e.g., the ophiuroid Ophiura texturata and the crab Inachus sp., and the fish Gobius
sp.). Gross changes in physical characteristics of the seabed included the presence
of large quantities of fish bones and a few anthropogenic items originating from the
tuna farms. The video surveys also showed that the amount of feed-fish present
varied considerably between different tuna farms and between cages within the
same farm, with some cages only having a few fish beneath them and others having
multiple layers. Overall, a consistent pattern was evident where a decrease in the
amount of uneaten fish occurred only when tuna were no longer present in the pens
during the fallowing period. The remaining uneaten fish decompose slowly and,
where the uneaten fish are present in large numbers, form a continuous layer of
M. Holmer et al.
The general state of seagrass beds and habitats, including those at popular dive
sites, is being assessed through mapping surveys and underwater videography, carried out by SCUBA divers. During the surveys, the divers record the state of health
and spatial extent of the main marine benthic habitats in the respective study area.
Potential changes in the state of the benthic habitats and their spatial distribution
are assessed by comparing maps of the situation recorded before initiation of the
tuna-penning activities with that after each monitoring session.
Monitoring of water quality consists of surveys of the same physico-chemical
and bacteriological attributes that have been monitored in the vicinity of sea bass
and sea bream farms. Samples of water for these surveys are being collected at
depths of 1 m and 5 m below the surface at several stations located in the immediate
vicinity of the tuna farms and at reference stations located at a distance from the
tuna cages.
Monitoring at the new offshore aquaculture zone (Fig. 2.2) commenced in June
2006 prior to the start of tuna penning activities there. The baseline survey for the
sediments and benthic diversity monitoring components was based on the same
design used at the other three tuna farms located closer to the coast, with samples
being collected remotely using a standard 0.1 m
2 Van Venn grab. However, because
of the deep waters that characterise the area, monitoring of the seabed using the
same underwater videography and SCUBA diving techniques that have been used
to date at the other tuna penning sites located in shallower waters, is not possible,
and it is planned to use remotely operated video cameras instead.
Overall, the results of the various monitoring components undertaken since 2000
for the three tuna farms located 1 km offshore indicated that, where detected, the
main adverse impacts resulted from accumulation of large amounts of feed-fish on
the bottom under and in the vicinity of the cages. The results from the video surveys
carried out near the tuna pens indicated that, towards the end of each penning season (in autumn), considerable amounts of dead uneaten feed-fish were present on
the seabed directly below the tuna pens, and this resulted in alterations in the physical
and biological characteristics of the seabed under the cages. The recorded changes
in biological characteristics included the disappearance of certain megafaunal
species (e.g., the irregular sea urchin Spatangus purpureus and the crinoid Antedon
mediterranea) that prior to the start of the penning operations were characteristic
of the soft sediment habitat where the tuna pens are located, and the appearance of
high population densities of detritus-feeding and scavenging macroinvertebrates
(e.g., the ophiuroid Ophiura texturata and the crab Inachus sp., and the fish Gobius
sp.). Gross changes in physical characteristics of the seabed included the presence
of large quantities of fish bones and a few anthropogenic items originating from the
tuna farms. The video surveys also showed that the amount of feed-fish present
varied considerably between different tuna farms and between cages within the
same farm, with some cages only having a few fish beneath them and others having
multiple layers. Overall, a consistent pattern was evident where a decrease in the
amount of uneaten fish occurred only when tuna were no longer present in the pens
during the fallowing period. The remaining uneaten fish decompose slowly and,
where the uneaten fish are present in large numbers, form a continuous layer of