The Biology and Culture of Tilapias

similar syndromes in tilapias and there seems no reason to suppose that

further studies will not also reveal its presence in cultured tilapias as a

significant pathogen.

Infection with Mycobacterium fortuitum, commonly known as fish tuber-

culosis, is well known in all cichlids kept in aquaria (Nigrelli and Vogel

1963). In the wild it has been recorded only infrequently in tilapias, e.g.,

Roberts and Matthiesen (1979) recorded it in S. andersonii and T. sparnnanii

from the Okavangu swamp in Botswana. In intensive culture it has only been

recorded once in S. niloticus in intensive culture in Kenya (Roberts and

Haller unpublished). Affected fish showed small focal granulomata in the

liver, spleen and kidney. Mortalities were limited but again it seems likely

that under certain circumstances, particularly if trash fish are fed, this

condition could be a potential source of severe losses.

Mycotic Infections

Only two fungi are recognized as serious pathogens of tilapias although

various others, such as demateacious moulds, are suspected of causing

occasional mortalities. Phycomycetes of the genus Saprolegniu can be a

cause of severe losses as with other cultured fish: again in association with

traumatic damage from handling, from sexual aggression or at temper-

atures approaching the minimum range for the tilapias. Thus in Israel hand-

ling is avoided between October and May (Sarig 1971) and prophylactic

spraying of ponds with malachite green is often carried out. Saprolegniu

fungal growths. are usually disposed as grey mats arranged in whorls over

the surface of affected fish. Such infections are usually seen much more

easily while the fish are in the water than when they are removed.

Infection with Bmnchiomyces spp. can be a significant cause of mortality

when fish are reared in poor quality water with a very high level of decaying

vegetable or other organic material. The fungus may invade the gill via the

branchial vessels or the epithelium and cause massive destruction of the

respiratory surfaces. Liming of the pond has been suggested as means of

reducing losses but improvement of the water quality is a much more rational

approach.

Viral Diseases

The only validated report of a viral disease of tilapias is the report by
Paperna (1974) of lymphocystie infection of wild tilapias in east African
lakes. This condition has caused severe problems in marine fish culture
in Europe and in tropical aquaria and it represents a potential hazard for
intensive culture systems. However, experimental vaccination studies have
shown that susceptible marine fish can be readily protected (Roberts 1975