The Biology and Culture of Tilapias

Aflatoxins are toxic compounds produced by the mould Aspergillus
flavus. In salmonids they are now recognized as producing hepatomata at
very low levels in supplementary feeds. In tilapia culture, storage of feed in
poor conditions leads readily to growth of Aspergillus. Recent work has
shown that tilapias receiving feeds with a high level of aflatoxins B1 and B2
show a haemorrhagic syndrome, characterized by severe haemorrhage
into the branchial musculature below the dorsal commissure of the oper-
culum, a wide range of internal haemorrhages, distinct depression of haemo-
poiesis and massive accumulation of haemosiderin in both the splenic and
renal melanomacrophage centers (Haller and Roberts unpublished). Frank
hepatic neoplasia is not seen but pre-neoplastic islands of welldefined
hepatocytes, as described in rainbow trout (Salmo gairdneri) by Wales
(1970) are seen. Given the problems of food storage in those humid tropical
areas where intensive tilapia production has its greatest potential, this
condition seems likely to be recorded with increasing frequency.

Neoplasia

Mawdeslay-Thomas (1972) suggested, somewhat flippantly, that visceral

neoplasia of Sarotherodon nilotieus, which he diagnosed from Egyptian

tomb paintings, represented the earliest example of a documented fish

disease. However there has been very little in the way of descriptions of

neoplasia in tilapias since then. Haller and Roberts (1980) have described

dual neoplasia in the form of a lymphocytic lymphoma and a renal tubular

adenoma in the same specimen of S. spilurus but otherwise there have been

no descriptions of neoplasia in the group. Again this is almost certainly a

reflection of the lack of detailed observation rather than a true picture of

incidence.

Anomalies and Deformities

Although the incidence of congenital anomalies in cultured tilapias

would be expected to be similar to that in other genera, the following

two specific conditions occur with a frequency which makes them a partic-

ular problem.

Spinal malformations are not uncommon in small numbers in any inten-

sively reared species but in the tilapias a particular form of spinal deformity

has been studied by Tave (1980 pers. comm.) and by Pullin and Roberts

(unpublished). Although occasionally seen in S. spilurus, S. mossambicus and

S. niloticus, the anomaly seems most frequent in the Auburn strain of S.

aureus. Tave has defined two aberrant types, which he calls "saddle back"