Quantitative estimation of the daily ingestion of phytoplankton by Tilapia nilotica and Haplochromis nigripinnis in Lake George, Uganda

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Abstract
A direct method has been used to estimate the amount of phytoplankton ingested per day by herbivorous fish in Lake George, Uganda. The quantities ingested are linearly related to the weight of the fish, as given by the following regression equations: for Tilapia nilotica: y= 271+13.3 x and for Haplochromis nigripinnis: y=–29.6+21.9 x, where y is the dry weight of phytoplankton (mg) ingested per day and x is the wet weight of the fish (g).Summary: Tilapia nilotica and Haplochromis nigripinnis have a diurnal feeding cycle. They start feeding shortly before dawn and feed continually until about dusk. They do not feed during the night. Although there is some individual variation, a linear increase in the average dry weight of stomach contents from dawn to dusk, and a linear decrease from dusk until about midnight is generally observed. The dry weight of algae passing into the intestine increases steadily until the first algae eaten in the morning reach the end of the intestine. The rates of change in these weights are used to estimate the total amount of food ingested over 24 hours. It was found that the rate of passage of algae from the stomach to intestine was greater during feeding than after feeding had stopped. The quantities ingested are linearly related to the weight of the fish, as given by the following regression equations: T. nilotica y= 271+13.3 x H. nigripinnis y=–29.6+21.9 xwhere y is the dry weight of phytoplankton (mg) ingested per day and x is the wet weight of the fish (g).

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