The functional grazing response of a phytoplanktivorous fish Oreochromis mloticus to mixtures of toxic and non‐toxicstrains of the cyanobacterium Microcystis aeruginosa

Small (10 g) tilapia (Oreochromis niloticus) were exposed to pure and mixed populations of toxic and non‐toxic strains of the cyanobacterium Microcystis aeruginosa (100% toxic, 50% toxic, 25% toxic, 0% toxic) at two particle concentrations (1 × 106 and 5 × 10sparticles ml−1). At both concentrations there was a progressive decrease in grazing rate as the percentage of toxic cells increased. Differences in opercular beat rates, and hence the volumes of water passed over the gills, were also recorded among treatments, opercular beat rates decreasing as the percentage of toxic cells increased. Although in all treatment groups with toxic cells present, the medium had detectable levels (>250 ng I−1) of extracellular microcystin‐LR toxin present, grazing was correlated with particle‐bound rather than extracellular levels.