Question 49
Parent spawners used for
the collection of garnetes are
known as
(a) Brooders
(b)Parr
(c) Mouth brooders
(d) Cirhinus mirigal
Choose correct option
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Answers
Explanation:
The tilapias are members of the Cichlidae family and are amongst the most widely produced food fishes in the world, second only in importance in aquaculture terms to the carps. Although the Cichlidae family is widely distributed throughout the tropics, tilapias are an exclusively African and Levantine association, and including the genera Tilapia, Sarotherodon and Oreochromis as well as many lesser known genera.
The subdivision into genera is largely made on the basis of breeding behaviour. Members of the genus Tilapia are substrate spawners and nest guarders, whilst members of the genus Sarotherodon are generally paternal or biparental mouth-brooders. Oreochromis species are arena spawners and maternal mouth-brooders. Mouth-brooding is generally recognized as having evolved from the more primitive substrate-spawning behaviour.
Of the 90 or so known species of tilapia, there are around 30 belonging to the genus Tilapia, 18 are paternal/biparental, and 42 belong to the genus Oreochromis.
10.2 GENERAL BIOLOGY
Geographical distribution
The natural distribution of tilapias reflects both historico-geographical factors which lead first to isolation and then speciation, and the environmental requirements/preferences of different species.
The genus Tilapia is widely distributed throughout western Africa with the exception of two species, T. zillii and T. rendalli, which have extended their ranges into central and eastern parts of the continent. By contrast, members of the genus Sarotherodon tend to occur in small, specialized, locally abundant populations with limited geographic distribution, and are common in the Rift lakes, rare in West Africa and absent from the Zaire basin. Oreochromis species are widely distributed throughout Central and East Africa and several of the lakes in the Levant.
The natural distribution of the tilapias has radically changed over the past fifty years by introductions which have been widely documented (Figure 10.1). Species have been stocked into lakes and reservoirs in order to try and increase fishery yields or control weeds and for aquaculture purposes into ricefields, fishponds and cage systems in lakes. Accidental introductions have also occurred, with the result that the tilapias are now widespread thoughout the tropics and subtropics. They are even used in several heated-water systems in temperate countries.
1 Based on manuscripts prepared by Drs K.J. Rana and M.C.M. Beveridge, Institute of Aquaculture, University of Stirling, Scotland
Fig. 10.1
Fig. 10.1 Introductions of tilapias (six principal species) outside Africa (modified from Philippart and Ruwet, 1982)
General biology of tilapias
The tilapia body shape is typical of the Cichlidae: very deep and laterally compressed. They have large, usually cycloid scales and a double lateral line system, one of which runs from the posterior of the operculum to a point level with the posterior end of the dorsal fin, whilst a second lateral line begins immediately below this point and continues to the tail. The anterior half of the characteristically long dorsal fin is spiny whilst the posterior half has soft, branched rays. At the base of the posterior section is a black patch, the “tilapia spot”, which is particularly apparent in juveniles and is more pronounced in some species than in others.
The tilapias are all principally herbivorous and this is reflected in anatomical adaptations. There are both bicuspid and tricuspid jaw teeth and small, sharp, pharyngeal teeth which vary in size and coarseness depending upon fish size and dietary preferences, and which are used for shredding coarser material and partially disrupting plant cell walls. The gill rakers are surprisingly small and stubby and not particularly numerous (a standard count of only 8–12 in some species) belying the ability of many species to filter-feed on small algae and other planktonic material. The intestine is long and coiled (up to 14 times the body length) in order to help in the absorption of nutrients from a largely plant-based diet.