ESTUARINE FISH FARMING
143
source of energy for bacteria, which in turn give the particles a high
nutritive value.
Another remarkable instance of how organic material may become
enriched in an estuarine environment (in this case the estuary of the
Thames) is supplied by Newell (1965). He found that the amount of
nitrogen in the mud deposits shows a regular increase with decrease
in particle size. Since the total surface area of a fine-grained deposit
is far greater than that of an equal volume of coarse-grained deposit,
the amount of nitrogen must be in some way influenced by the surface
area of the deposit. Further, the ratio of carbon to nitrogen was
highest ( 2 5 :1) in coarse-grained deposits, but approaches 7 :1 in finegrained deposits. The organic matter in fine-grained deposits supports
a €ar richer bacterial flora and its associated micro-fauna than in coarse
ones, even where the amount of organic matter as potential bacterial
food is the same.
Newell found that the percentage of carbon is high in the freshlydischarged faecal pellets of the snail Hydrobia ulvae (Pennant) (about
9.5% of dry weight), while the level of nitrogen is low (about 0.02%
of dry weight). These pellets were cultured, and it was found that a
population of non-photosynthetic micro-organisms developed, which
oxidized the carbon in the faecal pellets to obtain energy for synthesizing
proteins. The percentage of nitrogen in the cultured faecal pellets rose
rapidly. These enriched pellets were fed back to Hydrobia, and during
their passage through the gut the percentage of nitrogen again diminished. From this it could be concluded that the nitrogen in the pellets
represents a population of bacteria and associated micro-organisms
which fix atmospheric nitrogen at the expense of some of the carbon.
Newell considered that the organic matter (detritus) had little nutritive
value for the molluscs studied other than its content of bacteria and
micro-organisms. He suggested that, since a population of microorganisms is attached to the surface of the fine soil particles, it might
be just as profitable for a deposit feeder to ingest these as to ingest
organic debris. It may be said in passing that many of the grains of
soil present in the gut of the grey mullet, an estuarine fish, are found
to have a coating of micro-organisms, including small naviculoid
diatoms.
Since blue-green algae, many of which are also nitrogen fixers,
flourish in environments rich in organic matter, the presence of abundant detritus in estuarine soils must result in the fixation of much
atmospheric nitrogen. Indeed, it is otherwise difficult to account for
the production, year after year for decades, of fish which may have
20% protein in their flesh, as has, for instance, Chanos (Schuster, 1952),
143
source of energy for bacteria, which in turn give the particles a high
nutritive value.
Another remarkable instance of how organic material may become
enriched in an estuarine environment (in this case the estuary of the
Thames) is supplied by Newell (1965). He found that the amount of
nitrogen in the mud deposits shows a regular increase with decrease
in particle size. Since the total surface area of a fine-grained deposit
is far greater than that of an equal volume of coarse-grained deposit,
the amount of nitrogen must be in some way influenced by the surface
area of the deposit. Further, the ratio of carbon to nitrogen was
highest ( 2 5 :1) in coarse-grained deposits, but approaches 7 :1 in finegrained deposits. The organic matter in fine-grained deposits supports
a €ar richer bacterial flora and its associated micro-fauna than in coarse
ones, even where the amount of organic matter as potential bacterial
food is the same.
Newell found that the percentage of carbon is high in the freshlydischarged faecal pellets of the snail Hydrobia ulvae (Pennant) (about
9.5% of dry weight), while the level of nitrogen is low (about 0.02%
of dry weight). These pellets were cultured, and it was found that a
population of non-photosynthetic micro-organisms developed, which
oxidized the carbon in the faecal pellets to obtain energy for synthesizing
proteins. The percentage of nitrogen in the cultured faecal pellets rose
rapidly. These enriched pellets were fed back to Hydrobia, and during
their passage through the gut the percentage of nitrogen again diminished. From this it could be concluded that the nitrogen in the pellets
represents a population of bacteria and associated micro-organisms
which fix atmospheric nitrogen at the expense of some of the carbon.
Newell considered that the organic matter (detritus) had little nutritive
value for the molluscs studied other than its content of bacteria and
micro-organisms. He suggested that, since a population of microorganisms is attached to the surface of the fine soil particles, it might
be just as profitable for a deposit feeder to ingest these as to ingest
organic debris. It may be said in passing that many of the grains of
soil present in the gut of the grey mullet, an estuarine fish, are found
to have a coating of micro-organisms, including small naviculoid
diatoms.
Since blue-green algae, many of which are also nitrogen fixers,
flourish in environments rich in organic matter, the presence of abundant detritus in estuarine soils must result in the fixation of much
atmospheric nitrogen. Indeed, it is otherwise difficult to account for
the production, year after year for decades, of fish which may have
20% protein in their flesh, as has, for instance, Chanos (Schuster, 1952),