Growth of a grazing phytoplanktivorous fish and growth enhancement of the grazed alga.

The blue tilapia, Tilapia aurea, consumes the green alga, Ankistrodesmus falcatus, with some algal cells passing through the gut in viable condition. Grazing fish grew and the grazed algal populations had enhanced densities compared to ungrazed populations. Algal growth enhancement was not produced by either bacteria or fish excreting an algal limiting nutrient. Possible ingestion and digestion of bacterial cells by the fish might release nutrients to the algal cells that were otherwise unavailable. Blue tilapia may be affecting algal communities in ways other than by passive size selection. Enhancement of algal growth by the blue tilapia could have important implications for understanding the phytoplankton dynamics in waters containing blue tilapia.