one of a number of potentially negative
interactions between intensive cage
culture and the coastal marine ecosystem.
The accumulation ofparticulate waste
together with changes in the physical
structure of the sediment, reduced levels
of oxygen and the presence of hydrogen
sulfide, result in significant changes in the
ecology and community structure of the
benthic macrofauna. In extreme cases the
macrofauna disappear altogether and in
many situations there is a reduction in the
biomass, abundance and species
composition, with only a few 'opportunistic'
species persisting. Most studies have
shown that the effects of enrichment are
limited to the immediate vicinity of the
farm. In Scotland, for example, Gowen et
al. (1988) found that the effects of
enrichment could not be detected beyond a
distance of 30 to 40 m from farms. The
workofWeston(1990), however,has shown
that in some locations more subtle effects
of enrichment can be detected at distances
of up to 100 m from the cages.
Enrichment of the
Water ColumnThe introduction of anthropogenic
nutrients into coastal waters can cause
hypernutrification (a substantial and
measurable increase in the concentration
of a nutrient). In turn, this could result in
eutrophication, that is, an increase in the
biological productivity of a water body (see
Jaworski 1979 and references cited
therein). The most likely first step in the
eutrophication process is an increase in
phytoplankton production and biomass
(Barlow et al. 1963; Caperon et al. 1971;
Eppley et al. 1972). In addition to this
direct effect on phytoplankton growth,
more subtle changes may occur in the
succession of phytoplankton species. For
example, where the growth of diatoms is
limited by the availability of silicate, other
species such as dinoflagellates which do
not require silicate may dominate the
phytoplankton (Officer and Ryther 1980).
The increase in phytoplankton
production can have undesirable
consequences. In the Baltic, enhanced
phytoplankton growth has contributed to
deoxygenation of deep Baltic water;
enrichment of the benthos (Baden et al.
1990); and changes in natural fisheries
(Hansson and Rudstam 1990).
Hypernutrification has been linked to an
increase in the frequency of algal blooms
in some coastal waters (for example, Lam
and Ho 1989; Maclean, this vol.) and also
an increase in the occurrence of blooms of
species which are toxic to other marine
organisms (and indirectly humans and
other animals which may feed on such
organisms) Lam et al. (1989).
The recognition that intensive cage
culture of salmonids generates substantial
quantities of dissolved nutrients together
with the rapid expansion of this industry
in oligotrophic coastal waters of countries
like Scotland and Norway has beenviewed
with concern by some government and
nongovernmental organizations (Anon.
1988; NCC 1989). Simple assessments of
the potential for large-scale
hypernutrification can be made (Gowen
and Ezzi 1992) and suggest that this is
unlikely at the current level of farming in
coastal waters of most countries.
Nevertheless, localized increases in the
concentration ofammoniain the immediate
vicinity of cage farms have been observed
by many researchers, and nutrient
enrichment of individual embayments
could occur. Gowen and Ezzi (1992) found
clear evidence of a fish farm having
increased the nutrient status of a Scottish
sea-loch basin and found that changes in
the concentration of ammonia were related
to the activity of the fish farm. During the
operation of the farm, ammonia
concentrations were significantly higher
(statistically) than concentrations in