Can filter‐feeding fishes improve water quality in lakes?

1. In this paper we examine the potential of a cichlid fish species ( Sarotherodon galilaeus ) to both maintain positive growth rates through filter‐feeding on phytoplankton and improve water quality in Lake Kinneret through suppression of dinoflagellate ( Peridinium gatunense ) blooms. 2. Seasonal plankton consumption by S. galilaeus from Lake Kinneret was examined experimentally by monitoring changes in plankton assemblages during 24 h in 5‐m 3 mesocosms containing varying densities of fish. Taxon‐specific grazing rates ranged from 0 to 17 mg g fish  day –1 , with mean total consumption of 1.6% fish body weight per day. During the spring bloom of P. gatunense , S. galilaeus consumed mostly (94%) netphytoplankton (≥20 μm). The remaining 6% consisted mostly of nanophytoplankton (<20 μm). During the summer and fall, net‐ and nanophytoplankton accounted for 54 and 42%, respectively, of the diet of S. galilaeus . Zooplankton and flagellated and ciliated protozoans made up the remaining 4%. 3. Simulations using a fish bioenergetics model indicated that consumption rates ( C ) were near maximum in spring (90% C max ), while consumption was reduced in summer‐fall (59% C max ). Sarotherodon galilaeus obtains sufficient energy through filter‐feeding year‐round, although most growth (≥60%) occurs during the spring P. gatunense bloom. 4. Despite efficient feeding on P. gatunense and nanophytoplankton by S. galilaeus , estimates of instantaneous plankton mortality caused by ingestion were two orders of magnitude lower than maximum potential plankton growth rates. Thus the potential for the S. galilaeus population in Lake Kinneret to positively affect water quality through algal suppression is low.