Impact of feeding activity of silver carp on plankton removal from a high-rate pond effluent

The effect of silver carp (Hypophthalmichthys molitrix Val.) feeding activity on the plankton communities in a high-rate pond technology system (HRPTS) effluent was investigated over a period of 100 days. The experiment was conducted at the experimental wastewater treatment plant of the Agronomic and Veterinary Medicine Institute (AVI) of Rabat, Morocco, using a HRPTS in a fish pond receiving the plant effluent. The effluent was highly dominated by phytoplankton (99.95%). Silver carp could survive and grow in the fish pond. Production was 37 kg with a very low mortality rate (12%). The high specific intestine weight (7%) and intake rates of biomass and phytoplankton by silver carp (616 g kg(-1) of fish day(-1) and 1.6 x 10(11) cell kg(-1) of fish day(-1), respectively) demonstrated the importance of the feeding activity of the fish. Zooplankton intake rates were lower (2 x 10(7) bodies kg(-1) of fish day(-1)). The high intestine index (3 to 4.3 for fish sizes of 14 to 22 cm) and the dominance of phytoplankton in the gut contents (99.95%) confirmed an omnivorous/phytoplanctivorous diet. Silver carp were efficient in removing plankton from the HRPTS effluent. The net removal yields of biomass were 285 g m(-3) day(-1) and 322 g kg(-1) of fish day(-1), 7 x 10(10) algal cells kg(-1) of fish day(-1) and 8.7 x 10(7) zooplankton bodies kg(-1) of fish day-', with net removal rates of 47, 64 and 62%, respectively. The total suspended solids concentration decreased from 211 in the inflow to 112 mg L-1 in the fish pond.