Parental influences on egg quality, fry production and fry performance in Oreochromis niloticus (Linnaeus) and O. mossambicus (Peters)
Reproductive traits, age of female broodfish and aspects of parental behaviour influencing the production and quality of fry in the mouthbrooding tilapia species Oreochromis niloticus and O. mossambicus were investigated. Two incubation containers (conical and round-bottomed) and rearing temperature were first studied to ascertain their effects on egg and fry performance. Partial agitation of eggs in round-bottomed containers significantly (P < 0.05) improved hatchability and success rate of fry production, by 17% and 25%, respectively, compared with conical containers. For the temperature trials only O. niloticus were studied. Two egg acclimation conditions were tested; these influenced the temperature range of hatching, hatching success and the upper and lower median temperature tolerance limits of artificially reared eggs. Thermal tolerance of eggs and fry decreased with progressive development and optimum (> 90%) survival and growth of swim-up fry occurred at 28°-3QoC. Hatching times were inversely related to temperature (P < 0.01) and rates of development to hatching were best described by a curvilinear relationship (P < 0.01). Growth rates, gross yolk utilization efficiency to maximum body weight, age at maximal body weight, onset of exogenous feeding and 50% irreversible starvation (point-of-no- return, PNR) were temperature-dependent. At 24•, 28' and 30' c (xi) maximum body weight occurred on days 18, 9 and 6 post-hatching, respectively, four days earlier than fry at 24' C. Similarly, PNR occurred on days 23, 20 and 18 at 24•, 28• and 30•C, respectively. Reproductive traits of 0+, 1+ and 2+ age-classes of broodfish were investigated. In O. niloticus mean dry egg weight and clutch weight were significantly (P < 0.05) different between all three age-classes, and yearlings produced the smallest eggs, whereas for total and relative fecundity only yearling females were significantly (P < o. 05) different to older broodfish. In both species broodfish age-class had no effect (P > 0.05) on egg:body weight ratio. In both species all reproductive traits were significantly related (P< 0.01) to female age, length and weight. The strongest influences were maternal age on egg size and maternal length and weight on total fecundity and clutch weight. The influence of maternal age and hence egg size on hatching time, and growth, survival, onset of feeding and PNR of fry developing solely on their yolk reserves was investigated. Larger eggs produced longer (P < 0.001) and heavier (P < 0.001) fry which sustained starvation stress longer (P < 0.001). Initial advantages of egg size on growth persisted through to 60 days post-hatching (P <0.05). Feeding success was improved by using of yearlings. For fry from fry from 1+ and 2+ females instead 0+, 1+ and 2+ O. niloticus and O. mossambicus females, PNR was reached on days 9, 12 and 12, and 12, 15 and 18, respectively. Delaying initial feeding beyond six days post-hatching significantly (P < 0.05) reduced the growth of fry. (xii ) Overall survival (between 6-20 days post-hatching) was improved by using older females. The effects of parental breeding behaviour on fry production and quality were investigated. Egg fertilizing capacity of males was inversely related to their number of spawnings in a day. During oral rearing cumulative fry damage increased linearly during the first eight days after spawning and plateaued at 25%-29%. Possible reasons for fry damage are discussed. Naturally reared fry were lighter (P < 0.05) than artificially reared 'siblings'. The implications of broodfish age and size and parental breeding behaviour for mass production of high quality tilapia fry and the need, advantages and feasibility of artificially rearing Oreochromis eggs and fry are discussed.