returned during the flood tide. Such
recirculation can increase the residence
time of water within the basin and this has
implications for the dilution of soluble
waste from fish farms and the response of
the embayment ecosystem to the waste.
An understanding of the physical
characteristics of the coastal region in
which development is likely to take place
is critical for gauging the likelihood of the
accumulation or dispersal of waste from
intensive cage farming. In recognition of
this some workers (Weston 1986;
Hakanson et al. 1988; Lumb 1989) have
formulated simple principles, based on
coastal topography and bathymetric
features, for characterizing coastal areas
in terms of the potential for waste to
accumulate.
Ecological Change Associated with
Waste from Intensive
Cage CultureInrecent years a considerable amount
of research has been conducted into the
effects of fish farm waste on the coastal
marine ecosystem (see reviews by
Rosenthal et al. 1988 and Gowen et al.
1990). In this paper, the intention is to
present a brief overview of some of the
main findings of these studies.
Fish farm waste can bring about
enrichment of the coastal marine
ecosystem through the release of soluble
dissolved nutrients and particulate organic
waste. There have been a number of
attempts to quantify the output of waste
from marine salmonid farms (Ervik et al.
1985; Gowen et al. 1988; Hall et al. 1990;
Ackefors and Enell 1990; Holby and Hall
1991; Makinen 1991). From these studies
a reasonable estimate of the type
(particulate or soluble) and quantities of
waste released can be derived and used to
estimate the loading (amount per unit
volume) to the recipient waterbody.It is important to realize that data
from the studies quoted above are derived
from salmonid cage culture in north
temperate coastal waters and as suchmight
not be appropriate for cage culture of
warmwater fish. For example, the amount
of food wasted has an important bearing
on the severity of sedimentary enrichment
beneath the farm. Giventhe improvements
in feed composition and digestibility and
husbandry practice which are reflected in
improved food conversion ratios (in excess
of 2.0:l some six to eight years ago to
current ratios of about 1.83) the early
estimates of dry feed wastage of 20%
(Beveridge 1984) are probably too high
and a value of 12% might be considered
more realistic. With respect to some
warmwater fish, however, food conversion
ratios in the order of 3.03 or higher have
been reported. Clearly, adopting avalue of
12%for foodwastagein such circumstances
would be inappropriate.Enrichment of the Seabed
EcosystemEnrichment of the seabed ecosystem
resultingfrom the deposition ofparticulate
organic waste released from fish farms
has been studied by workers in a number
of countries: Brown et al. (1987) in Scotland;
Ritz et al. (1989) in Australia (Tasmania);
Weston (1990) in the USA (Washington
State). The results show that the changes
which take place are similar to and
consistent with other forms of organic
enrichment such as wood pulp and domestic
sewage sludge (Pearson and Rosenberg
1978).
The succession from undisturbed to
enriched sediment and the final
equilibrium conditions that result are
dependent on the quantity of material
deposited. Thelatter is, in turn, dependent
on the size of the farm, husbandry practice
and the topographic and hydrographic