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Impacts of Shellfish Farming
Compared to finfish farming, the reported environmental effects
of commercial shellfish farming are generally perceived as minimal.
It is widely acknowledged that shellfish aquaculture requires
clean water and natural food supply (i.e. it does not use introduced
food or chemicals), and produces little waste, all of which mitigate
potential impacts on the marine environment.
On occasion, the shellfish farming industry has been the foremost
critic of Ireland's poor record of coastal water quality in shellfish
production areas, and has previously brought a case against the
State to the European Commission and the European Court of Justice
(which ruled in 2003) regarding the absence of pollution-reduction
programmes in such areas.
However, as the shellfish industry intensifies,
there is cause for concern. Questions are being raised as to the
health of our marine ecosystems, while our knowledge of those
ecosystems is limited and incomplete. Potential negative effects
of shellfish aquaculture and other marine industries must be acknowledged
and the precautionary principle (the concept of minimizing impacts
by proceeding with caution in the absence of knowledge) must be
applied.
Potential negative impacts of shellfish aquaculture include:
• alteration of marine bird nesting, feeding and migrating
habitats,
• disruption of intertidal water and substrate movement,
• depletion of microorganisms in the water column,
• decreased biodiversity brought about by cultivating
single species,
• introduction of non-native species, and
• unknown cumulative effect of all these potential
impacts.
Sustainable
Shellfish: Recommendations for responsible aquaculture (David
Suzuki Foundation 2005)
The localised (immediate farm area) and regional (bay or geographical
area) impacts of shellfish cultivation will depend not only on
the species being cultured but the characteristics of the site
and on farm management practices (Heffernan 1999).
Bottom cultivation of mussels
Bottom cultivation of blue mussels (Mytilus
edulis) involves the location, collection and transplantation
of wild mussel spat into richer, shallower waters using a dredger.
Successful ongrowing of re-laid spat requires sandy shallow beds.
When the mussels reach commercial size (9-18 months later), they
are harvested by dredger (Heffernan 1999). The impacts associated
with bottom cultivation include:
- Benthic enrichment due to biodeposition [1]
of organic waste, which may result in a change in community
structure beneath the farm.
- Cultured shellfish may compete for space resulting in the
original habitats being suffocated as they attain full size.
- Shellfish beds can compete for and deplete
phytoplankton biomass (representing a net loss of energy from
the ecosystem) resulting in food shortage for other filter feeders
and ultimately for predators near the top of the food chain.
The effect of this depletion will vary hugely depending on nutrient
inputs, flushing times, season and dependent biomass. This potentially
large impact of mussel culture on the ecosystem could conflict
with other important functions of the system, such as being
a breeding area for birds and a nursery area for fish [2].
- A mature shellfish bed may add to the biodiversity
of an area [3]. However, this advantage may
be counterbalanced by the loss of such a community at harvest.
- Shellfish cultivation will usually have an
effect on water chemistry [4]. How persistent
this is will depend on the size of the culture area and the
flushing time.
- Shellfish can play an important role in nutrient cycling via
biodeposition, thus contributing to the nutrients available
in the ecosystem.
- Cultured shellfish recycle a significant amount of nutrients
making more nutrients available for primary production and potentially
counterbalancing the extraction of phytoplankton from the system.
However, to what degree this occurs is unknown.
- Predator numbers are likely to suffer as they are routinely
removed from the beds by the shellfish farmers. Scottish mussel
farmers use a combination of netting and scaring devices to
protect beds from eider ducks and other bird predators, with
varying degrees of success. In Ireland, eiders are relatively
rare and so there is little threat from this source to mussel
farms.
One of the major impacts associated with bottom cultivation of
mussels, and all other bottom cultured species, is harvesting
by dredging (see below).
Suspended mussel cultivation
The impacts of suspended mussel cultivation are
very similar in many ways to that of bottom culture. The local
impacts tend to be different whereas the regional effects tend
to be similar. This is because local impacts depend more on cultivation
method and regional effects depend more on species type.
Heffernan 1999
The additional impacts associated with suspended
(rope) culture include:
- Biodeposits generally accumulate in sites that are not well
flushed [5].
- Accumulated biodeposits can result in a change
in the sediment characteristics with sediments becoming finer,
enriched and in some cases anoxic (oxygen depleted) [6].
- Benthic enrichment due to biodeposition can change the community
structure under the farms dramatically. Recovery can take years.
The severity of such an impact will depend on the size of the
area under cultivation and the hydrology of the area [7].
- Suspended shellfish farms lose a proportion of the crop to
the seabed, thus enhancing the area for predators [8].
Unlike bottom cultivation, the suspended crop can be protected
from diving birds using scaring devices or underwater anti-predator
nets. A large amount of the losses are due to mussels being
knocked off, rather than consumed, by birds.
In some areas with poor flushing characteristics,
the deposition of organic detritus beneath suspended mussels has
resulted in benthic enrichment. In Bantry Bay mussel longlines
have caused significant benthic enrichment in sections of the
bay subject to poor water exchange. The impacts include increased
organic content of sediments, decreased faunal diversity and the
prominence of opportunistic polychaetes. In Glengarriff Harbour,
deposits of pseudo-faeces beneath longlines are also prevalent
and it is likely that continued expansion of mussel farming in
any part of Bantry Bay will lead to further deterioration of the
benthic environment. In Killary Harbour (a fjordic inlet), benthic
enrichment under mussel rafts is minimal, due to the depth of
the site allied to good dispersion, but a large increase in the
numbers of starfish has been recorded. This is probably due to
accumulations of detached mussels upon which this species feeds.
OSPAR QSR 2000 (Region III Celtic Seas)
Scallop cultivation
The principal scallop species cultured in Ireland at present
is the native scallop (Pecten maximus), although there
is growing interest in the queen scallop (Chlamys opercularis).
Given the fact that scallops are filter-feeders, they may be expected
to exhibit many of the same impacts as those described for mussel
cultivation (above).
The ecological impacts of scallop cultivation
will depend on which culture method is chosen. The two options
practised in Ireland are:
a. Suspended culture in lantern nets, followed by ear-hanging
or continuation in lantern nets to market size.
b. Suspended culture followed by ongrowing from 40mm to market
size on the seabed.
Heffernan 1999
Specific impacts identified as being associated with scallop
cultivation include:
- Predators may benefit from an increased food supply. For ongrowing
of scallops only 50% survival is expected.
- Crabs and starfish are removed from the scallop beds by the
farmers. If such predators are removed from the scallop beds
this will impact on their numbers.
- Collecting wild spat may have a negative affect on the populations
of organisms that depend on it.
- Aquaculture may introduce genetically different stocks to
an area thus diluting the uniqueness of local populations.
One of the major impacts associated with scallop cultivation
is harvesting by dredging (see below).
Native oyster cultivation
The native oyster (Ostrea edulis) is cultured in relatively
small amounts in Ireland. A firm gravel bottom is the preferred
substratum and it grows best with minimum exposure to air and
minimum overcrowding (Heffernan 1999). The usual culture method
employed for the native oyster is extensive culture. This involves
collection of wild spat and relaying in a more productive area.
The material on which the oyster larvae will
settle is called cultch. This cultch (usually mussel shells) is
laid down on the seabed in spring. A layer of algae grows on the
cultch, making it a suitable surface for the oyster larvae to
settle on. The spat are then collected by dredging and relaid
in a more productive area. Dredging is also the method of harvesting
the native oyster. Oyster fisheries require some maintenance which
involves removal of predators, e.g. crabs and starfish.
Heffernan 1999
Given the fact that oysters are filter-feeders, they will exhibit
many of the same impacts as those described for mussel cultivation
(above). Specific impacts identified as being associated with
oyster cultivation include:
- Predators may benefit from an increased food supply.
- Starfish are physically removed from the oyster beds which
impacts on their numbers.
- Imports of oysters may introduce non-target species with disastrous
effects. Introduction of the organism Bonamia ostreae,
responsible for causing Bonamiasis disease in Ostrea
edulis, into Ireland from France has resulted in outbreaks
and caused serious mortalities.
The impacts of harvesting bottom cultured oysters by dredging
are discussed below.
Impacts of dredging
Both pre-cultivation substrate preparation and the harvesting
of bottom cultured mussels, scallops and oysters are carried out
by dredging. As shellfish are buried in the seabed sediments,
quite a significant amount of substrate must be disturbed in order
to remove them. Clearly, co-occurring non-target species will
also be removed, buried or otherwise disturbed. In Ireland, simple
conventional boat-towed dredges are used — typically they
have a toothed bar to dig into the sediment and a steel mesh to
retain the catch.
Dredging is fairly indiscriminate in nature
and the impact depends upon a large number of factors. These factors
include the weight of the gear on the seabed, the type of towing
gear, the nature of the bottom sediments and the strengths of
tides and currents (Jones 1992)…
Dredging also affects the geography and thereby hydrology of the
seabed. It many ways it is a ploughing of the sediments and as
such can cause massive disruption to the fauna as well as the
sediment structure, type and appearance. Dredges shear off high
hummocks, fill in low spots, changing the configuration of the
bottom, removing areas more exposed to or protected from the current,
exposing shellfish, worms and other shellfish dwelling species
to predation (Mc Allister and Spiller, 1994).
Heffernan 1999 (incl. citations)
The impacts of shellfish dredging include:
- Dredging generally has a much greater effect on soft mud than
on hard sandy sediments.
- On sandy sediment the trend seems to be loss of the older
molluscs and sedentary organisms.
- On soft muddy sediment dredging changes the nature of the
sediment to become more gravelly as the finer sediments get
washed away in the sediment plume.
- The impact will depend on the type of dredge used.
- The habitat may take months, if not years, to recover.
- Dredging may destroy habitat and reduce juvenile recruitment.
- The impact will depend on the importance of the site, e.g.
if there are Zostera beds of importance to birds/fish.
- The community structure can be affected.
- Dredging can leave fauna open to predation as their environment
is drastically changed, as they are exposed immediately after
dredging.
- Overfishing can lead to population crashes in predators and
ultimately in the target species itself.
Dredging can cause high mortality in non-target species. This
in turn can provide food for other organisms.
Eleftheriou and Robertson (1992) observed that
during their experimental scallop dredging very large concentrations
of the burrowing sand-eel Ammodytes were destroyed. This
species is the staple diet of many seabirds such as arctic terns,
kittiwakes, puffins, great skuas and red-throated divers (O’Connor
et al. 1992). Hislop and Webb (1992) also reported Ammodytes
as a component of the diet of salmon. So, dredging in an area
with birds who are dependent on these species could have serious
ecological impacts…
Mc Loughlin et al. (1991) showed that 4 to 5 times as many scallops
were crushed or damaged as were caught and landed by the scallop
gear. This is a totally unsustainable use of the resource and
will have long-term implications for recruitment and on other
organisms which depend on scallops as their food source…
Dredging may also have an adverse knock-on effect on other marine
biota. One concern is that dredging may destroy the amount of
productive fish habitat. Associated flora and fauna are recognised
as providing much more that just a food source for the target
fishery. Seagrass (Zostera) habitat is being lost the world
over and mechanical damage certainly plays its role (Kurland 1994;
Short 1996). Seagrass not only provides food for overwintering
waterfowl such as brent geese and wigeon, fish such as pollack
and winter flounder feed on sand shrimp and other small organisms
associated with eelgrass beds (Kurland 1994). In addition, their
dense underwater canopy has a nursery function. For example, herring
and other fish lay their eggs on the surface of seagrass leaves;
newly shed lobsters seek refuge there while their shell hardens;
scallops settle there to avoid predation; and starfish, snails,
mussels and other creatures attach themselves to the seagrass
leaves. In short, seagrass beds perform critical roles as foraging
shelter and nursery habitat for marine life, contributing markedly
to the overall productivity of shallow coastal embayments (Castel
et al. 1989; Kurland 1994; Short et al. 1996). Furthermore, seagrasses
also play a role in stabilising sediments, controlling erosion
and maintaining water quality (Kurland 1994).
Heffernan 1999 (incl. citations)
Pacific oyster cultivation
In Ireland, Pacific oysters (Crassostrea gigas) are usually
cultured intertidally on steel trestle supporting structures.
A firm substratum is necessary to support the weight of a tractor,
and to facilitate planting, maintenance and harvesting. Hatchery-supplied
spat is on-grown to market size in mesh bags or plastic trays,
supported by the trestles — the function of which is to keep
the oysters off the sea bottom and to prevent grit getting inside
the animal. The mesh bag facilitates ease of handling and also
reduces predation by crabs, starfish and birds. The mesh size
of the bags is increased as the oyster grows (Heffernan 1999).
In summary, the probable and actual impacts of intertidal Pacific
oyster cultivation are:
- Increased ammonia concentration in the water column.
- Organic enrichment beneath the trestles.
- Reduced current velocity and thus increased sedimentation
due to the presence of trestles.
- Disturbance or compression of the sediment due to the movement
of tractors.
- All the above contribute to a change in the fauna beneath
or beside the farm.
- Disturbance of the fauna (notably birds) of the area at low
spring tides thus reducing the time available to them for feeding
in winter.
- Occupation of a valuable feeding ground for birds thus having
an adverse effect on their survival and reproduction.
- Occupation of a valuable roosting ground for birds.
- Competition for phytoplankton with the indigenous inhabitants
who are dependent on the same resource.
- Adverse effects on several species of birds if the shore is
cleared of rocks and seaweed.
- Adverse effects on a number of species if large amounts of
seaweed are collected for packing the shellfish.
- There are serious ecological implications in the import of
live Pacific oysters and their shell contents into the country.
- It is possible that the presence of trestles will so alter
sedimentation patterns as to change the entire nature of the
ecotope.
- Pacific oysters have spawned in Irish waters and could eventually
establish a viable population. As they are generally a more
tolerant species than the native oyster and they may well compete
favourably thus displacing the native species.
See Heffernan (1999) for a more detailed discussion.
Manila clam cultivation
The Manila clam (Tapes semidecussatus or Ruditapes
phillipinarum), a non-indigenous species, requires more wave
sheltered conditions than mussels and oysters. Clams, which need
to be buried in sand in order to develop a normal shell, are harvesting
by ploughing using a modified potato harvester.
The hatchery-supplied seed is grown off-bottom between mesh in
flat wooden frames raised on wheels so that they can be manoeuvred
up and down the shore. After about a year, the clams are on-grown
using the "plot system", which involves rotovating the
site to remove predators (e.g. crabs), ploughing the sand and
sowing the juvenile clams. The clams are then protected from predators
by incorporating a strip of netting over them. Clams are usually
grown further up the shore than oysters and they are commonly
grown on the same shore. This arrangement is possible as the oysters
are grown about 0.5 m off the ground on trestles, whereas the
clams are dug into the sand (Heffernan 1999).
The impact of Manila clam cultivation has little to do with the
species being cultivated and much to do with the method of cultivation.
The potential and actual impacts include:
- The anti-predator netting covering the plots of Manila clams
are responsible for most of the impacts on the benthos as they
increase sedimentation and % organic material.
- The netting results in an increase in the numbers of deposit-feeding
worms.
- Removal of rocks, seaweed, crabs and local mussel beds can
have serious knock-on effects on the ecology.
- Phytoplankton depletion is likely, given sufficient numbers
of clams.
- There is a small, but unlikely, possibility that the Manila
clam could become established in Ireland.
- Loss of habitat to birds feeding and roosting on sand is a
high probability.
- Tractors on the shore can result in the churning up of sediments
with losses in flora and fauna.
- Harvesting is likely to lead to losses of non-target species.
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