characteristics of the site. The
accumulation of particulate waste causes
an increase in the oxygen demand by the
sediment ecosystem, probably due to
increased chemical oxygen demand and
microbial activity. Increased oxygen
demand has been measured in the vicinity
of freshwater cage farms and in earthen
marine ponds. With respect to the former,
Enell and Lijf (1983) measured a rate of
between 34 and 41 mrn~l.O,.m"-day-~ and
Blackburn et al. (1988) measured an
oxygen consumption rate of between 42
and 60 rnm~l.rn-~.day-l in earthen ponds.
Similar rates have been measured i.n
sediments beneath salmon cage farms in
Norway (A. Ervik, pers. comm.).
Enhanced consumption of oxygen by
the sediment can result in depletion of
oxygen in the water overlying the sediment
(Tsutsumi and Kikuchi 1983; Gowen et al.
1988). In both studi.es, however, the periods
of oxygen depletion were short, only lasting
for a few months during the summer. In
general, intensive cage culture of fish is
unlikely to cause widespread
deoxygenation of bottom water in coastal
waters of northern Europe and North
America. The exceptions to this are some
low energy coastal marine environments,
particularly in those coastal embayments
in which bottom water remains trapped
for a period in excess of several months
and in which natural deoxygenation is
likely to take place (see, for example, Lazier
1963).
At those locations where particulate
waste accumulates on the seabed, the
amount of oxygen within the sediment
declines (and may fall to zero) due to an
imbalance between the supply and
consumption of oxygen. As a result, the
balance between oxidation and reduction
processes changes and the latter become
the dominant path way for the turnover of
organic material. Of these, sulfate
reductionislikely to be the mostimportant,
at Ieast initially, but since gas bubbles
released from enriched sediments beneath
fish farms contain methane in addition to
hydrogen sulfide (Samuelsen et al. 1988),
it seems probable that methanogenic
bacteria also play an important role in the
decomposition of particulate waste.
Bacterial activity within enriched
sediments beneath fish farms can be
sufficiently high to cause outgassing from
the sediment (Braaten et al. 1983). The
many anecdotal observations of the smell
of hydrogen sulfide in sediment samples
collected from beneath fish farms and the
presence ofhydrogen sulfide in gas bubbles
leaving the sediment surface has been
confirmed (Samuelsen et al. 1988). Since
hydrogen sulfide is toxic to fish there has
been considerable debate regarding the
potential for 'self pollution' and 'souring of
sites' to limit the production potential. It is
clear that problems have arisen in some
locations and fish farmers have beenforced
to abandon their sites.
Braaten et al. (1983) attributed damage
to the gills of farmed fish to hydrogen
sulfide released from the sediment and a
similar effect was suggested by Rosenthal
and Rangeley (1989). In addition, it is
known that continual use of some sites for
four or five years or more has caused a
deterioration in fish health and a decline
in productivity (Gowen, unpubl. data). At
those shallow water sites where there is a
substantial accumulation of waste, with
increased particulate loadingin the water,
short-term reduction in dissolved oxygen
in bottom water and vigorous outgassing,
it would be easy to link such changes with
a deterioration in fish health and reduced
production. However, the relationship
between the enrichment of sediments
(particularly hydrogen sulfide release) and
fish health remainsunclear. Furthermore,
experiencein British Columbia has shown
that the production potential has declined
at some locations where the water depth is
300 m (E.A. Black, pers. comm.), indicating
that perhaps benthic enrichment is only