from the diagnostic files of the Institute of Aquaculture of the University of
Stirling and from unpublished research by the authors and their students in
the Institute.Parasitic DiseasesAlthough a vast array of parasites has been recorded from tilapias and
other cichlids most of them are from wild fish and no evidence of clinical
effect is given. The number of reports from cultured fish is much smaller but
clinical disease is much more frequently associated with infection in these
circumstances. It is with clinical infection in cultured fish that the aqua-
culture industry is concerned, but in many cases such infections are derived
from wild stock in the water supply and other hosts such as birds or inverte-
brates, or are introduced with fish brought in from wild sources for ongrow-
ing or breeding.a) Ichthyophthirius multifiliis. This parasite, which causes "Ich" or "white
spot" is a relatively large ciliate which causes heavy losses in a wide range of
cultured fish. It was first reported from tilapias by Paperna (1970) who
recorded it in wild tilapias from east African rift valley species. It was
believed to have been introduced to the area with mollies (MoNienesia
sphaenops) brought in for mosquito control. Brock and Takata (1955)
reported its introduction to the Hawaiian islands with imports of S. mos-
sambicus from Singapore. Thus any area where it is not presently found
would be advised to resist such importations very strenuously, unless in the
form of young fish which have been comprehensively treated with formalin
and securely quarantined before distribution. Ichthyophthirius has a closely
defined optimal temperature range for growth of 20 to 24°C (Meyer 1970)
and thus for tilapias it is unlikely that significant epizootics will occur in
tropical areas unless the fish are highly stressed. It is in the subtropical areas
when winter temperatures are both stressful to the tilapias and also pass
through the optimum range for the parasite as they rise in spring that severe
problems with this parasite will arise. It is a significant problem for fry since
they are particularly susceptible to infection and lesions may be found in the
nares, pharynx and gill as well as the skin. In intensive culture using recircu-
lation systems, it is a particular problem because of the difficulty of treating
the filter without destroying its bacterial flora. If the filter is isolated during
treatment the parasite is able to lodge, and survive within it for a considerable
period and thus infections can flare up when the circulation is restored.
(Richards pers. comm.). Efforts to induce infections experimentally produce
very variable results but natural infections can build up extremely rapidly,
with the entire body surface of fry being covered with trophonts and tomites
within 48 hr of their first being observed.
b) Trichodinids and Chilodonella spp. There are a number of ubiquitous
protozoan parasites which commonly cause disease in cultured fish including