Microbial, Cellular and Molecular Biology

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Browsing Microbial, Cellular and Molecular Biology by Author "Admassu Demeke"

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    Age and growth determination of tilapia, Oreochromis niloticus L. (Pisces: Cichlidae) in some lakes in Ethiopia
    (Addis Ababa University, 1998-12) Admassu Demeke; Mengistou Seyoum (PhD)
    Tilapia, Oreochromis niloticus L. is the most important fish in Ethiopia. Knowledge on age and growth of the fish is essential for fisheries management, and for further detailed studies. In this study, age and growth of O. niloticus in Lakes Zwa\, Langeno and Chamo was studied from microzones and macrozones in otoliths. The usefulness of otoliths to determine age was established by examining the existence of regularly formed translucent and opaque macrozones. The effect of subtle changes in temperature on body growth and on otolith macrozone formation was also experimentally tested. The growth of the fish in various lakes was also compared. Two translucent macrozones, each associated with biannulus (a type of annulus formed twice a year), were fOlmed each year in otoliths of O. niloticus in Lake Zwai. Biannulus formation was connected to fluctuations in temperature, quality and quantity of food, and to intensive spawning activity. Translucent macrozone was fOlmed in otoliths, and somatic tissue growth was slow, in tilapia that were transferred from 240C to slightly lower temperature treatments. Fish that were transferred from 240 C to higher temperatures grew rapidly, and more opaque material was deposited in their otoliths. At the high test temperatures, translucent macrozone was formed in otoliths of fish that had dpening or dpe gonads. O. niloticus in Lake Zwai has two spawning peaks, hence two major annual recruitment cohorts. Fish from the two cohorts were discriminated and age was determined from the number of biannulus in otoliths by relating median hatch-dates with the dates of capture. Ages determined in this manner (macrozonal ages) were highly correlated (r2 = 0.85) with microzonal ages. Growth of juvenile O. niloticus in Lakes Chamo, Zwai and Langeno was described by Gompertz model fitted to length and microzonal age data. Growth was most rapid in Lake Chamo, intennediate in Lake Zwai and slowest in Lake Langeno. Rapid growth of juvenile O. niloticus in Lake Chamo was attributed mainly to warm temperature and better food quality in this lake. The growth of adult O. niloticus in Lakes Zwai, Awassa and Haiq was described by von Bertalanffy model fitted to mean length and macrozonal age data. Growth rate was most rapid in Lake Haiq, intermediate in Awassa and least in Zwai. Rapid growth in Haiq was attdbuted to better quality food in this lake.