2 Monitoring of Marine Aquaculture
75
Since all sea bass and sea bream farms were located in the vicinity of Posidonia
oceanica meadows, monitoring of benthic habitats and assemblages centred on
assessing the spatial distribution, coverage and state of health of the seagrass within
the area occupied by the cages and their moorings. This essentially consisted of
mapping surveys of the seabed to assess potential changes in the spatial distribution
and coverage of seagrass habitat resulting from the fish-farming activities.
In an attempt to fill gaps in knowledge of the environmental impacts of sea bass
and sea bream farms, a number of studies were undertaken by the University of
Malta, most of which formed part of undergraduate and postgraduate research
projects (e.g., Cassar 1994; Dimech et al. 2002). Some of these studies included collecting data on environmental attributes that went beyond the minimum requirements
set by MEPA’s PDGF, for example, measurement of P. oceanica meadow and shoot
attributes (shoot density, mean number of leaves and leaf length per shoot, shoot
biomass and shoot epiphyte loading; see Cassar 1994; Dimech et al. 2002).
Overall, the results of environmental monitoring at sites used to farm sea bass
and sea bream indicated that seagrass meadows located directly below the fish
cages underwent severe regression or were completed decimated, and that the
effects of the aquaculture activities on the monitored seagrass attributes (shoot
density, mean leaf length, number of leaves per shoot, and epiphyte load) extended
a considerable distance (in the case of one farm, around 200 m; Dimech et al. 2002)
from the farm site. The results of a study aimed at assessing the impact of sea-based
fish cages (located in waters having a depth of 10 m) on the decapod, mollusc and
echinoderm fauna associated with P. oceanica beds indicated the presence of three
distinct zones in the vicinity of the farm (Dimech et al. 2002):
(i) Zone 1, comprising the area occupied by the cages and an additional band of 30 m
around the farm. The macrofaunal assemblages present within this zone were
characterised by a low species richness and the dominant trophic groups were grazers
and deposit feeders (decapods, polyplacophorans and gastropods), which exploit
the abundant epiphytes and deposited organic matter present close to the cages.
(ii) Zone 2, comprising the area located at a distance of between 30 m and 90 m
from the farm. This zone supported macrofaunal assemblages that had the
highest species richness and abundance, while the fauna was dominated by the
same trophic groups in Zone 1.
(iii) Zone 3, comprising the area located at distances exceeding 90 m from the farm.
This zone supported macrofaunal assemblages having species richness and
abundance values that were intermediate between those recorded from Zones 1
and 2, and in which the dominant trophic groups comprised grazers, deposit
feeders, suspension-feeders (mostly bivalves) and predators.
This “zoning” pattern, consisting of differences in the species composition and
structure of the benthic macrofaunal assemblages with increasing distance from the
farm site, is very similar to that recorded in the vicinity of offshore salmon farms
(e.g., Brown et al. 1987; Ye et al. 1991).
75
Since all sea bass and sea bream farms were located in the vicinity of Posidonia
oceanica meadows, monitoring of benthic habitats and assemblages centred on
assessing the spatial distribution, coverage and state of health of the seagrass within
the area occupied by the cages and their moorings. This essentially consisted of
mapping surveys of the seabed to assess potential changes in the spatial distribution
and coverage of seagrass habitat resulting from the fish-farming activities.
In an attempt to fill gaps in knowledge of the environmental impacts of sea bass
and sea bream farms, a number of studies were undertaken by the University of
Malta, most of which formed part of undergraduate and postgraduate research
projects (e.g., Cassar 1994; Dimech et al. 2002). Some of these studies included collecting data on environmental attributes that went beyond the minimum requirements
set by MEPA’s PDGF, for example, measurement of P. oceanica meadow and shoot
attributes (shoot density, mean number of leaves and leaf length per shoot, shoot
biomass and shoot epiphyte loading; see Cassar 1994; Dimech et al. 2002).
Overall, the results of environmental monitoring at sites used to farm sea bass
and sea bream indicated that seagrass meadows located directly below the fish
cages underwent severe regression or were completed decimated, and that the
effects of the aquaculture activities on the monitored seagrass attributes (shoot
density, mean leaf length, number of leaves per shoot, and epiphyte load) extended
a considerable distance (in the case of one farm, around 200 m; Dimech et al. 2002)
from the farm site. The results of a study aimed at assessing the impact of sea-based
fish cages (located in waters having a depth of 10 m) on the decapod, mollusc and
echinoderm fauna associated with P. oceanica beds indicated the presence of three
distinct zones in the vicinity of the farm (Dimech et al. 2002):
(i) Zone 1, comprising the area occupied by the cages and an additional band of 30 m
around the farm. The macrofaunal assemblages present within this zone were
characterised by a low species richness and the dominant trophic groups were grazers
and deposit feeders (decapods, polyplacophorans and gastropods), which exploit
the abundant epiphytes and deposited organic matter present close to the cages.
(ii) Zone 2, comprising the area located at a distance of between 30 m and 90 m
from the farm. This zone supported macrofaunal assemblages that had the
highest species richness and abundance, while the fauna was dominated by the
same trophic groups in Zone 1.
(iii) Zone 3, comprising the area located at distances exceeding 90 m from the farm.
This zone supported macrofaunal assemblages having species richness and
abundance values that were intermediate between those recorded from Zones 1
and 2, and in which the dominant trophic groups comprised grazers, deposit
feeders, suspension-feeders (mostly bivalves) and predators.
This “zoning” pattern, consisting of differences in the species composition and
structure of the benthic macrofaunal assemblages with increasing distance from the
farm site, is very similar to that recorded in the vicinity of offshore salmon farms
(e.g., Brown et al. 1987; Ye et al. 1991).