1 Fish Farm Wastes in the Ecosystem
9
between the first and second parts of this natural cycle is weakened because of
excess primary production and the formation, in the absence of sufficient grazing
by planktonic or benthic consumers, of excess phytoplankton or seaweed
biomass.
Thus, the harmful consequences that may result from nutrient enrichment
include increasing frequencies and intensities of Harmful Algal Blooms (HABs),
including Red Tides, nuisance blooms causing foaming, toxic blooms that can kill
farmed fish, and increased occurrences of incidents of shellfish-vectored toxins,
such as those causing paralytic shellfish poisoning (Anderson and Garrison 1997).
If blooms sink into deeper water, the decay of their biomass can cause oxygen
depletion. Increased amounts of phytoplankton attenuate light more strongly, with
the consequence that the growth of seaweeds and seagrasses may be retarded.
Opportunistic green or brown seaweeds spread over seagrass meadows or over the
slower-growing brown fucoid and laminarian seaweeds that are the natural flora of
temperate seashores and the shallow sublittoral. Although green seaweed growth
can be stimulated close to cages, eutrophication is a phenomenon that is more typical of water bodies, such as lochs or coastal seas, as a whole. It is thus distinct from
the local impacts of particulate waste, although the change in the balance of pelagic
organisms associated with eutrophication (Fig. 1.3(b) ) can be likened to the
changes caused by organic input to the benthos (Fig. 1.3(a) ).
The third type of potential disturbance is that from chemicals that are used to
prevent or treat fish illnesses or parasitical infections, to improve fish growth, or to
prevent fouling of nets or farm structure. Let us look at two groups of such chemicals, starting with the compounds azamethiphos and emamictin benzoate, used to
rid farmed salmon of parasitic sea-lice.
These lice are crustaceans that burrow under the scales of the fish, causing sores
that irritate the salmon and offer a route for infection by pathogenic micro-organisms.
Young lice are planktonic, and so can infect other farmed or wild salmon. For all
these reasons, fish-farmers in Scotland are required to treat their fish to keep lice
infestation to a minimum. The two chemicals are arthropocides – that is, they are
intended to kill lice, which are members of the arthropod phylum, but not salmon,
which are vertebrates.
The problem is that many members of the plankton are also arthropods, the
group that includes insects, spiders and crustaceans. To be precise, the sea-lice
are copepod crustaceans, as their planktonic larvae show, and so chemicals that
kill sea-lice are also at risk of killing planktonic copepods and thus of damaging
an important link in marine food webs. Azamethiphos, which is applied externally, is a greater hazard than emamectin, which is given to salmon in their
food and reaches the lice by way of the fish bloodstream. However, some
emamectin reaches the sediment in fish faeces and uneaten food, and here it
may harm benthic crustaceans. Both the chemicals are degraded by light and
oxygen, and can also be removed by adsorption on particles; and these processes
augment dilution and dispersion in bringing concentrations below levels at
which harm might result.
9
between the first and second parts of this natural cycle is weakened because of
excess primary production and the formation, in the absence of sufficient grazing
by planktonic or benthic consumers, of excess phytoplankton or seaweed
biomass.
Thus, the harmful consequences that may result from nutrient enrichment
include increasing frequencies and intensities of Harmful Algal Blooms (HABs),
including Red Tides, nuisance blooms causing foaming, toxic blooms that can kill
farmed fish, and increased occurrences of incidents of shellfish-vectored toxins,
such as those causing paralytic shellfish poisoning (Anderson and Garrison 1997).
If blooms sink into deeper water, the decay of their biomass can cause oxygen
depletion. Increased amounts of phytoplankton attenuate light more strongly, with
the consequence that the growth of seaweeds and seagrasses may be retarded.
Opportunistic green or brown seaweeds spread over seagrass meadows or over the
slower-growing brown fucoid and laminarian seaweeds that are the natural flora of
temperate seashores and the shallow sublittoral. Although green seaweed growth
can be stimulated close to cages, eutrophication is a phenomenon that is more typical of water bodies, such as lochs or coastal seas, as a whole. It is thus distinct from
the local impacts of particulate waste, although the change in the balance of pelagic
organisms associated with eutrophication (Fig. 1.3(b) ) can be likened to the
changes caused by organic input to the benthos (Fig. 1.3(a) ).
The third type of potential disturbance is that from chemicals that are used to
prevent or treat fish illnesses or parasitical infections, to improve fish growth, or to
prevent fouling of nets or farm structure. Let us look at two groups of such chemicals, starting with the compounds azamethiphos and emamictin benzoate, used to
rid farmed salmon of parasitic sea-lice.
These lice are crustaceans that burrow under the scales of the fish, causing sores
that irritate the salmon and offer a route for infection by pathogenic micro-organisms.
Young lice are planktonic, and so can infect other farmed or wild salmon. For all
these reasons, fish-farmers in Scotland are required to treat their fish to keep lice
infestation to a minimum. The two chemicals are arthropocides – that is, they are
intended to kill lice, which are members of the arthropod phylum, but not salmon,
which are vertebrates.
The problem is that many members of the plankton are also arthropods, the
group that includes insects, spiders and crustaceans. To be precise, the sea-lice
are copepod crustaceans, as their planktonic larvae show, and so chemicals that
kill sea-lice are also at risk of killing planktonic copepods and thus of damaging
an important link in marine food webs. Azamethiphos, which is applied externally, is a greater hazard than emamectin, which is given to salmon in their
food and reaches the lice by way of the fish bloodstream. However, some
emamectin reaches the sediment in fish faeces and uneaten food, and here it
may harm benthic crustaceans. Both the chemicals are degraded by light and
oxygen, and can also be removed by adsorption on particles; and these processes
augment dilution and dispersion in bringing concentrations below levels at
which harm might result.