Both forms of aquaculture, finfish and shellfish,
take place in shallow inlets, bays and lagoons.
In the UK, aquaculture has become most highly developed
in Scotland, and in particular in the sea lochs
and voes of mainland west coast, Western and Northern
Isles. There are a variety of potential environmental
impacts from aquaculture (NCC, 1989) however for
the purposes of this review discussion below is
limited to impacts on the benthos.
Finfish Culture - Atlantic salmon (Salmo
salar) is the most commonly farmed species although
there are farms for halibut (Hippoglossus hippoglossus)
and turbot (Scophthalmus maxima). The farming
of cod is currently being investigated. The overwhelming
majority of farms consist of floating cages at sea
although there are some land-based farms utilising
To date, studies have shown that the most obvious
benthic impacts of finfish culture relate to the
depositition of organic material (faeces and uneaten
food) and dispersion of nitrogenous wastes in solution.
Benthic impact has been well documented and tends
to be restricted to the immediate vicinity of the
cage group, with the extent and severity of impact
being most pronounced at low energy locations where
water exchange and/or wave action is limited. Monitoring
practices at farm sites are closely related to the
extent of this "footprint" on the seabed. As well as impacting
on the benthos, the release of hydrogen sulphide
from anoxic sediments below cages has implications
for the health of the farmed fish82 Over
the past few years a trend has developed in the
salmon industry away from the most sheltered sites
to those with greater tidal exchange which helps
to ameliorate direct impact on the benthos. Studies
on the recovery of the benthos following organic
enrichment from salmon farming indicate varying
periods of recovery depending on prevailing hydrographic
conditions, with the majority of sites studied showing
some recovery within two years. Clearly, pump-ashore
farms offer the potential for treatment of effluent
prior to discharge.
A further potential impact on the benthos within
shallow inlets and bays arises from the use of chemicals
and medicines. A variety of compounds are employed
ranging from anti-fouling treatments to antibiotics
and treatments for sea lice infestation of salmon.
Anti-biotics are of concern due, for example, to
their potential to impact on microbial processes
and through the development of drug resistance in
There are concerns about the potential impacts
on benthic communities in proximity to salmon farms
from the discharge of medicines used to control
sea lice infestations as active compounds are effectively
being discharged directly into the environment.
These medicines can be split into two broad categories:
those delivered orally in medicated feed, and bath
treatments which are added to the cages in solution.
The most commonly used sea lice treatment has been
the organophosphate dichlorvos, a bath treatment,
although use has declined due to "Red
List" status and the development of resistance
in sea lice. Studies on environmental effects of
dichlorvos demonstrated sublethal effects in intertidal
invertebrate communities. Other bath treatments
are currently in varying stages of development with
cypermethrin and azamethiphos being recently authorised
for use. In general, it is probably true to say
that the greater dispersive characteristics of high
energy sites are beneficial in ameliorating the
impact of bath treatments. Sites with restricted
exchange (lagoons) can be considered most vulnerable.
In-feed treatments have a direct route to the benthos
via any uneaten food. Recent studies of one such
compound, ivermectin, demonstrated mortality in
sediment dwelling worms with potential consequences
for the recovery of the seabed 82.
A major research study has recently been started
to investigate the environmental impact of sea lice
treatments but no results are yet available with
which to inform this review.
Shellfish - A number of different methods of shellfish
cultivation are used in UK waters with issues for
consideration at the seed collection, on-growing
and harvesting stages of the process64.
Depending on the species, molluscs may be suspended
in lantern nets, laid in trays or poches (large
meshed sacks) on the shore, attached to ropes suspended
in midwater or relaid in more suitable areas for
re-growing. Collection of seed mussels is not considered
to have an environmental impact in the UK as it
is not extensive and only licensed from unstable
beds64. In the Wadden Sea however, massive
mortalities of eider ducks have been associated
with greatly reduced mussel stocks as a consequence
of harvesting spat for aquaculture82.
Intertidal collection may result in some effects
such as from trampling and disturbance of foraging
There has also been concern about the inadvertent
introduction of alien species (such as the seaweed
Sargassum muticum) on shellfish which are
imported as seed stock for cultivation.
The effects of on-growing depend on the habitat,
type and scale of cultivation. Changes in sediment
composition and benthic community structure have
been observed under long-lived cultures of Mytilus
edulis for example. A three year study showed
that faecal matter and detached mussels increased
sedimentation under the lines at a rate of 10 cm/yr.
The effects on the sediment under the culture were
reduced grain size, high organic content and a negative
Redox potential. Benthic fauna were replaced by
opportunistic polychaetes and only limited recovery
was observed when the site was re-sampled 6 months
after harvesting89. In these respects
the effects are similar to those beneath finfish
Examination of the sediment structure and the infauna
beneath Manila clam lays revealed no significant
differences in particle size, organic content or
photosynthetic pigment between control areas and
the lays while the clams were growing20.
There were also no significant differences in the
faunal diversity beneath the lays when compared
to control sites, but there was a greater density
of benthic species under the lays. The infauna were
dominated by deposit feeding worms, Lanice conchilega,
and the bivalve, Mysella bidentata, compared
to the white ragworm, Nephtys hombergii,
in the control area. In another study, species effects
were seen in the first 6 months with the infauna
dominated by opportunistic species92.
The nets used to contain the clams and provide protection
from predation, increased sedimentation and settlement
of green macroalgae and are likely to have had a
major influence on some of the infauna92.
Effects on benthic communities of small scale culture
may be limited and localised. If the area covered
is large there is potential for conflict with bird
feeding or roosting sites64.
The harvesting stage of cultivation has also raised
various concerns relating to physical disturbance.
Harvesting of clams by hand raking has been reported
as causing a 50 % reduction in diversity and abundance
of infauna97. Suction dredging may be
another method which is used. In one study this
caused an 80-90 % reduction in non-target fauna
and left a trench 10 cm deep20. A sediment
plume was created but reduced to background levels
within 40 days. Regeneration of species diversity
and abundance, after harvesting in the winter, was
completed by the summer - a period of 7 months.
Natural sedimentation had nearly restored the sediment
structure to pre-harvesting conditions after 4 months
suggesting that there may be minimal long term effects
if sites are left to recover. In Scotland Manila
clam has only been trialed; no commercial production
has taken place. Restricting harvesting to early
winter could ameliorate site restoration if the
main mechanisms for recolonisation is by larval