The Biology and Culture of Tilapias

and unpublished) and it is likely that this would also apply to tilapias in

culture.

Although only one virus infection has been recorded in tilapias it is

inconceivable that a similar range of commensal and pathogenic viruses

as found in the salmonids and other intensively cultured species is absent

from the tilapias. It would seem to be only a matter of time before these are

manifested in intensive culture given the rate of the development of the

industry and the improvement of diagnostic facilities.

Toxic Conditions

Toxic conditions may arise from toxic substances in the diet or in the

water. They range from high levels of metabolic wastes such as ammonia

to gaseous supersaturation of the medium. The range of potential toxic

agents is very wide but only small numbers have been definitely associated

with losses.

In intensive culture systems, pumping or piping faults can result in super-

saturation of the water with dissolved gases. When absorbed into the fish

during respiration the change in partial pressure can result in the gas coming

out of solution and blocking or rupturing blood capillaries. Bubbles of gas

then accumulate at sites which vary depending on the age of the fish. In

tilapia fry these supersaturation bubbles can form anywhere but are found

particularly in the area of the yolk sac or, in older fish, in the gill and skin.

In adults the scale and skin structure is such that normally the only clinical

location of gas bubbles is on the gill although it is likely that small occlusive

emboli are also distributed unnoticed, throughout the tissues, since affected

stocks rarely grow particularly well thereafter.

Although some freshwater algae such as Prymnesium parvum are toxigenic
per se the main deleterious effect of algae in tilapia culture is death from
anoxia following an algal bloom. Generally the microorganisms responsible
are of the genera Microcystis, Anabaena, Oscillatoria and Spirulina. Accord-
ing to Swingle (1967) the anoxia results from a sudden dominance of one
particular species which multiplies phenomenally under favorable conditions
to form a thick scum on the surface. This prevents light penetration, causes
death of all submerged vegetation and restricts oxygenation to the upper 6
cm of the pond. Although fish can live in this upper layer any sudden drop
of temperature, or other weather changes such as wind or rain, can induce
overturn or mixing of the layers with resultant anaerobiosis of the entire
pond and mass mortality of the fish. These usually die with the characteristic
features of anoxia: a wide open mouth and dilated branchial chambers.