1 Fish Farm Wastes in the Ecosystem
3
a replacement source of marine protein, albeit sometimes by converting small fish
into larger ones. And, just as was the case during the early development of human
societies, this farming initially generated large amounts of waste, which accumulated in an environment hitherto thought to be pristine.
The metabolism of fin-fish is not dissimilar to that of humans, and, like people,
fish produce solid and dissolved wastes. Waste food and faeces voided into the
water tend to sink to the seabed. Many farmed fish are carnivores, and so must be
fed a protein rich diet, which they use inefficiently compared with the herbivores
and omnivores that are farmed on land. Consequently, they excrete dissolved compounds of nitrogen (especially, ammonia) and phosphorus (especially, phosphate)
by way, mainly, of their gills. These processes are natural; the problems due to
these wastes arise from intensive or semi-intensive farming, which takes in food
from an extensive region but concentrates the waste in a much smaller area around
a farm.
As an example, a farm stocked with 200,000 young salmon, and harvesting
about a thousand tonnes of fish towards the end of a 2-year production cycle, uses
about 1,200 t of feed made from 3,600 to 5,900 t of wild fish (according to conversion ratios in (Black 2001) ). The food supply represents a share of the primary
organic production of hundreds of square kilometres of sea. During the second year
of the cycle the farm releases an amount of nitrogen, phosphorus, and faecal matter
similar to that in the untreated sewage from several tens of thousands of humans.
But whereas these people would inhabit at least a few square kilometres even in the
most densely settled European cities, typical netpen farms of this size cover only a
fraction of a square kilometre. Furthermore, whereas the most human and industrial
wastes are now, in cities in the developed world, collected and treated before discharge, farm waste enters directly into the sea.
Although such wastes are in themselves natural, and so harmful only in excess,
some mariculture results in the production of a second category of wastes. These
are the man-made chemicals used to treat fish for disease, to make them grow
faster, or to prevent seaweeds, seasquirts and barnacles from growing on fish cages.
Speed-reducing fouling by these organisms has long been a problem for ships, and
the success of the British Navy during the Napoleonic wars was partly due to the
use of copper plating to prevent fouling of their wooden hulls (Rogers 2004).
Copper is expensive, however, and can cause problems due to electrolytic corrosion, and there was a search for other compounds that could be applied to hulls in
paint. The invention of the antifouling compound tributyl tin, or TBT, seemed to be
a break-through. After several decades of use, however, it was found to be harmful
to marine invertebrates, causing female dogwhelks to grow penises and farmed
oysters to become mis-shapen (Readman 2005). It is now banned from use by fish
farms and all small craft that anchor in coastal waters.
Thus, nutrients, organic matter and toxic pollutants have the potential to do harm
to marine organisms. Their actual impact depends, however, on the environment
into which these wastes are released. The next section looks at the properties of one
type of environment much used for aquaculture, and uses this example of a water
body to explain the idea of an ecosystem.
3
a replacement source of marine protein, albeit sometimes by converting small fish
into larger ones. And, just as was the case during the early development of human
societies, this farming initially generated large amounts of waste, which accumulated in an environment hitherto thought to be pristine.
The metabolism of fin-fish is not dissimilar to that of humans, and, like people,
fish produce solid and dissolved wastes. Waste food and faeces voided into the
water tend to sink to the seabed. Many farmed fish are carnivores, and so must be
fed a protein rich diet, which they use inefficiently compared with the herbivores
and omnivores that are farmed on land. Consequently, they excrete dissolved compounds of nitrogen (especially, ammonia) and phosphorus (especially, phosphate)
by way, mainly, of their gills. These processes are natural; the problems due to
these wastes arise from intensive or semi-intensive farming, which takes in food
from an extensive region but concentrates the waste in a much smaller area around
a farm.
As an example, a farm stocked with 200,000 young salmon, and harvesting
about a thousand tonnes of fish towards the end of a 2-year production cycle, uses
about 1,200 t of feed made from 3,600 to 5,900 t of wild fish (according to conversion ratios in (Black 2001) ). The food supply represents a share of the primary
organic production of hundreds of square kilometres of sea. During the second year
of the cycle the farm releases an amount of nitrogen, phosphorus, and faecal matter
similar to that in the untreated sewage from several tens of thousands of humans.
But whereas these people would inhabit at least a few square kilometres even in the
most densely settled European cities, typical netpen farms of this size cover only a
fraction of a square kilometre. Furthermore, whereas the most human and industrial
wastes are now, in cities in the developed world, collected and treated before discharge, farm waste enters directly into the sea.
Although such wastes are in themselves natural, and so harmful only in excess,
some mariculture results in the production of a second category of wastes. These
are the man-made chemicals used to treat fish for disease, to make them grow
faster, or to prevent seaweeds, seasquirts and barnacles from growing on fish cages.
Speed-reducing fouling by these organisms has long been a problem for ships, and
the success of the British Navy during the Napoleonic wars was partly due to the
use of copper plating to prevent fouling of their wooden hulls (Rogers 2004).
Copper is expensive, however, and can cause problems due to electrolytic corrosion, and there was a search for other compounds that could be applied to hulls in
paint. The invention of the antifouling compound tributyl tin, or TBT, seemed to be
a break-through. After several decades of use, however, it was found to be harmful
to marine invertebrates, causing female dogwhelks to grow penises and farmed
oysters to become mis-shapen (Readman 2005). It is now banned from use by fish
farms and all small craft that anchor in coastal waters.
Thus, nutrients, organic matter and toxic pollutants have the potential to do harm
to marine organisms. Their actual impact depends, however, on the environment
into which these wastes are released. The next section looks at the properties of one
type of environment much used for aquaculture, and uses this example of a water
body to explain the idea of an ecosystem.