To overcome the problems of tilapia overcrowding in ponds, several methods of controlling reproduction in tilapia culture systems have been developed: polyculture with a police fish eliminating the fry by predation and/or culture of monosex male fish (the males having a better growth rate than females).
The methods adopted for all-male stocks production include hand sexing, hybridization and hormonal sex reversion.
Hormonal sex reversal is now considered as the most efficient technique for all-male progenies production, at a commercial point of view, but is not without problems such as the level of technicity required to obtain 100% male populations and the possible impact of large-scale use of synthetic artificial steroid hormones to the aquatic environment.
In terms of feeding regime, Oreochromis tilapias, and particularly Oreochromis niloticus, are usually considered as phytoplanktivorous fish, able to ingest and digest large quantities of phytoplanktonic algae as well as blue green algae.
According to several authors, tilapias would be the only real herbivorous fish considering their gut structure (length of 14 times the total length of fish and pH of stomach very acid, around 1.5 allowing the destruction of cells walls).
Under culture conditions, in fertilized ponds, Oreochromis niloticus feeds on rotifers, copepods, Cladocera, chironomid larvae, diatoms, green algae, blue green algae, and decayed organic matter from macrophytes, nanoplankton as well as mud, clay, and sand.
Oreochromis niloticus appears then be an opportunistic fish in terms of feeding habit that is closer to omnivorous-detritivorous fish than strictly microphytophagous.
The detritus fraction of feed ingested by Oreochromis niloticus appears to be most important and has been underestimated for a long time.
The fish is able to adapt his morphology and his feeding behavior to digest the detritus component of feed: selection of organic fraction of feed and hydrolysis by using several small pharyngeal teeth, digestion thanks the low pH of stomach and, at least, the amino acids are assimilated all along the gut.
Young Oreochromis niloticus tilapias feed mainly on micro-zooplankton and micro-crustaceans while larger fish feed on a wider range of food.
Oreochromis feed on the water column, on the bottom and on sub-strata following three main ways of ingestion: suction by creating a water flow directed into its mouth, filtration through its gill rakers, and grazing on sub-strata and on the bottom of the pond. Recent studies showed that grazing appears to be energetically the most efficient feeding strategy for Oreochromis tilapias.
Authors:
Lionel Dabbadie and Jerome Lazard
