6
P. Tett
in its seabed. By analogy with human health, we can say that an ecosystem is
healthy when all its parts are in good order and also when the interactions are in
balance with the needs of the biota. This is a topic to which I’ll return later – but
for now, please note a significant difference between the health of a human – for
whom the environment is something outside of the body and which is seen as a
factor conducive to good or bad health, depending on whether air or water is
clean or polluted – and the health of an ecosystem – which includes the state of
the non-living part. Suppose we add a fish farm – either fin fish or shellfish – to
an ecosystem such as Creran. Should we view the farm as bolted on to the outside
of the ecosystem – potentially able to perturb it through waste products and liable
to harm if some of this waste, for example, decays and consumes oxygen – or as
an addition to the loch’s ecosystem, participating in the exchange of materials?
And what about the humans who operate the farm and truck in fishmeal caught
in distant seas?
1.4 Aquacultural Pressures and Potential Impacts
on Ecosystems
Any fish farm is a site of concentrated food production. Shellfish such as mussels
take their food from the water flowing past them, and so one of their impacts
on the ecosystem is the removal of the phytoplankton that forms much of this food.
Depending on the extent of water movements, a mussel farm may harvest planktonic primary production from a wide area of sea – an area much greater than the
extent of the mussel farm itself.
In contrast, the feed given to farmed salmon is largely made from other fish,
caught in a different part of the ocean, but again harvesting the primary production
of much wider area of sea than the extent of the fish farm. Think of both types of
farm as the drain at the end of a bath, a vortex through which must flow large quantities of material. Both mussels and salmon draw oxygen from the water to support
their metabolism of this food, and, because of the vortex effect, can potentially cause
oxygen depletion – which would be fatal for the fish and shellfish. The way to avoid
this is to site a farm in a region of strong water flow – which will also carry
away the potentially toxic ammonia released by the animals’ metabolism, and
any other harmful dissolved substances such as those involved in ridding salmon
of sea-lice or preventing fouling on nets.
However, although the answer to pollution is dispersion and dilution, the dilution of fish farm wastes has to be sufficient for undesirable ecological consequences
to be avoided. It is, unfortunately, possible to site a farm in a region of flow sufficiently strong to avoid oxygen depletion or ammonia build-up around the farm, but
insufficiently flushed to avoid the accumulation of wastes on a larger scale. Bearing
this in mind, let us look at three types of potential ecological disturbance associated
with fish-farming. Figure 1.2 exemplifies these in a fjord, but most can occur anywhere in the sea.
P. Tett
in its seabed. By analogy with human health, we can say that an ecosystem is
healthy when all its parts are in good order and also when the interactions are in
balance with the needs of the biota. This is a topic to which I’ll return later – but
for now, please note a significant difference between the health of a human – for
whom the environment is something outside of the body and which is seen as a
factor conducive to good or bad health, depending on whether air or water is
clean or polluted – and the health of an ecosystem – which includes the state of
the non-living part. Suppose we add a fish farm – either fin fish or shellfish – to
an ecosystem such as Creran. Should we view the farm as bolted on to the outside
of the ecosystem – potentially able to perturb it through waste products and liable
to harm if some of this waste, for example, decays and consumes oxygen – or as
an addition to the loch’s ecosystem, participating in the exchange of materials?
And what about the humans who operate the farm and truck in fishmeal caught
in distant seas?
1.4 Aquacultural Pressures and Potential Impacts
on Ecosystems
Any fish farm is a site of concentrated food production. Shellfish such as mussels
take their food from the water flowing past them, and so one of their impacts
on the ecosystem is the removal of the phytoplankton that forms much of this food.
Depending on the extent of water movements, a mussel farm may harvest planktonic primary production from a wide area of sea – an area much greater than the
extent of the mussel farm itself.
In contrast, the feed given to farmed salmon is largely made from other fish,
caught in a different part of the ocean, but again harvesting the primary production
of much wider area of sea than the extent of the fish farm. Think of both types of
farm as the drain at the end of a bath, a vortex through which must flow large quantities of material. Both mussels and salmon draw oxygen from the water to support
their metabolism of this food, and, because of the vortex effect, can potentially cause
oxygen depletion – which would be fatal for the fish and shellfish. The way to avoid
this is to site a farm in a region of strong water flow – which will also carry
away the potentially toxic ammonia released by the animals’ metabolism, and
any other harmful dissolved substances such as those involved in ridding salmon
of sea-lice or preventing fouling on nets.
However, although the answer to pollution is dispersion and dilution, the dilution of fish farm wastes has to be sufficient for undesirable ecological consequences
to be avoided. It is, unfortunately, possible to site a farm in a region of flow sufficiently strong to avoid oxygen depletion or ammonia build-up around the farm, but
insufficiently flushed to avoid the accumulation of wastes on a larger scale. Bearing
this in mind, let us look at three types of potential ecological disturbance associated
with fish-farming. Figure 1.2 exemplifies these in a fjord, but most can occur anywhere in the sea.