cage, you can get more dead pockets in the center as the fish can only move the water
around so much. Also the cage area which can be made to face the water flow is limited.
With an elongated cage, you can put the long side across the water flow and so maintain
much better conditions, although putting it across a strong flow can give you additional
structural problems.ROBERTS: As Dr. Nash has indicated, we have had considerable experience of cage
culture, particularly with salmonids in Scotland. Because of the very rapid development
of freshwater cage culture in Scotland, we now have several 2 to 300 t units in relatively
small lochs. We have a project at the Institute of Aquaculture measuring the environ-
mental impact of cage culture in Scottish lochs. We are already finding that there is a very
significant effect of having a 300 t unit in a loch. The effect seems principally to be not
only in terms of the chemical water quality, but a more general effect on the fish fauna of
the loch, for example, enlarged eel populations. Of course, the entrepreneur who has the
fishing rights on the loch can say, "That's fine; I will put eels in the cages as well because
the price of eels is so much higher than the price of trout!" But there are limits, certainly,
as to safe stocking levels.
We have also studied samples from cage culture mortalities in the Philippines where the
obvious cause was anoxia associated with a very highly eutrophic system. As biologists
interested in cage culture we should hold back the entrepreneur as he will double and
triple and quadruple his production without giving thought for tomorrow and tomorrow
may be a very long day of misfortune if we are not careful.
One other thing which Dr. Coche raised in terms of cage culture was the possible
differing risks of cage culture and other systems. We are not completely convinced
that there is a significantly higher stress in cages compared with other high density
culture systems. Stress of course would increase disease risks. But two particular group
of parasitic copepods, Argulus species in fresh water and salmon lice in salt water can hide
out in the cage structures. They are not obligatorily living on the fish. As soon as you
stress the fish, the adrenalin surge seems to give the parasites a message that they had
better leave their hosts and they all disappear off the fish on to the cage-netting; so that if
you pull fish out to treat them, you only get a small proportion of the parasites. When
you put the fish back in, the parasites come back and reinfest the fish. Obviously, their
hiding places are multiplied on fouled netting.
We have also experienced problems of wild fish infecting the cultured fish in the
cages. In Scotland, the parasitic burden of the wild fish in the lakes with cages may
increase to a significant extent and this annoys the anglers or capture fishermen. Part
of the problem is that birds fishing in the cages facilitate the cycle of the fish parasites
and spread them. Cages in our view have a very big future in aquaculture terms, but they
have some specific disease problems, and must be monitored very carefully in terms of
their environmental impact.