Age composition,growth and body condition of the introduced Sarotherodon mossambicus (Cichlidae) in Plover Cove Reservoir,Hong Kong

Synopsis Age composition, growth and body condition of the Sarotherodon mossambicus (Peters) stock in the Plover Cove Reservoir were investigated. The fish in this reservoir have a relatively short life cycle; the stock consisted of mainly two to three age groups. Influx of recruits in September and October maintained a high proportion of O-age group fish in the stock. This percentage remained high until the following March or April when the I-age group attained dominance. Length-weight analyses indicated that S. mossembicus conform to the allometric formula W = aLⁿ with a and n exhibiting marked seasonal variations. Growth was most rapid during the first 20 months of life and decreased noticeably after 30 months. In general growth was limited to those months when the water temperature was above 20° C. Both body condition and visceral fat content exhibited seasonal variations which were not correlated with each other or with variations in temperature. However, body condition appears to be directly related to feeding intensity. Mean body condition decreased with increasing age and also decreased progressively during this study. This progressive decrease was probably related to the progressive decrease in salinity of the water and also the carrying capacity of the reservoir rather than to the nutrient content of the water. Mean values for the body condition of females were in general higher than for males, suggesting that females are more robust than males.Maximum accumulations of fat occurred in June and July, corresponding to the breeding period. Such accumulations are thought to be a prerequisite before spawning.