The West-African euryhaline tilapia, Sarotherodon melanotheron heudelotii shift from visually feeding on
zooplankton when juveniles to mostly filter feeding on phytoplankton when adults. When reared using
an appropriate ration in intensive aquaculture systems, S. m. heudelotii also consume algal-based detritus,
and contribute to sediment mineralization, clean up their environment, and ultimately stimulate and
sustain algal growth. We analysed such practical advantages for phytoplankton-based recirculating
systems, using S. m. heudelotii and Chlorella sp. as biological material originating from the prototype of
such a system operated in Senegal. We performed a 24-h factorial design experiment in 36 tubs, crossclassifying
three levels of S. m. heudelotii (fishless control, unfed fish, and fed fish) with four levels of
Chlorella initial density.
Chlorella overall mean density increased significantly from fishless, to unfed fish, and fed fish
treatments, and with Chlorella initial density. S. m. heudelotii did not alter nitrogen nor phosphorus
concentrations, only affected by algal initial densities. Most ammonia excreted by fish was probably
uptaken by Chlorella. Bacteria-mediated diurnal nitrification was possibly an alternative ammonium loss
mechanism at highest oxygen concentrations. Algae were not limited by nitrogen or phosphorus but
most likely by low dissolved organic carbon availability. Chlorella differential responses with fed vs. unfed
Sarotherodon suggest that CO2 supplied by heterotrophic S. m. heudelotii respiration played a key role.
Observed Chlorella growth rates were similar to the highest rates obtained in algal mass cultures,
enriched with CO2, nitrate and phosphate, under artificial lighting.
Our results suggest the existence of a Sarotherodon-Chlorella mutualism in our systems, where S. m.
heudelotii provide CO2, the major limiting factor of Chlorella growth, whereas Chlorella oxygenate and
detoxify the watermedia from ammonia, promoting S. m. heudelotii production. This mutualism could be
used to optimize photosynthetic suspended-growth aquaculture systems, particularly in the Tropics
where light is abundant and temperature is continuously high.