Water Qual. Res. J. Canada, 2005 • Volume 40, No. 2, 191–201 Copyright © 2005, CAWQ Impact of Feeding Activity of Silver Carp on Plankton Removal from a High-Rate Pond Effluent Nadia Berday,1* Driss Zaoui,1 Abdeljaouad Lamrini2 and Mustapha Abi3 1Department of Biology, Faculty of Sciences, University of Chouaib Doukkali, P.B. 20, El Jadida, Morocco 2Department of Fisheries, Hassan II Agronomic and Veterinary Medicine Institute, P.B. 6202, Rabat Institutes, 10101, Morocco 3National Center of Hydrobiology and Fish-Culture of Azrou, P.B. 11, Azrou, Morocco 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 1011 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 107 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 1010 algal cells kg-1 of fish day-1 and 8.7 x 107 zooplankton bodies kg-1 of fish day-1, 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. Key words: silver carp, high-rate pond technology system effluent, plankton removal, food consumption, gut contents Introduction digestive tracts are adapted to a phytoplanctivorous diet (Cremer and Smitherman 1980; Li 1991; Hampl et al. The wastewater treatment plant of AVI is an experimen- 1983; Bitterlich 1985a; Domaison and Devaux 1999) so tal pilot plant using the high-rate pond technology sys- silver carp could be used to reduce phytoplankton and tem (HRPTS) (El Hafiane et al. 2003). It was built in zooplankton concentration in the HRPTS effluent. 1997 with the goal of obtaining results that could be The aim of this work was to determine the diet of used to design large-scale HRPT plants and that efflu- silver carp in a fish pond receiving a HRPTS effluent and ents could be exploited efficiently for irrigation. The El its impact on plankton removal from the effluent by fol- Attaouia HRPTS project, supported by the USAID and lowing the plankton progress in the fish pond and in the El Attaouia Municipality, treats a mean flow rate of digestive tract of the fish. 700 m3 day-1 of domestic sewage and was carried out in 2002 based on the AVI prototype. Materials and Methods The HRPTS is a very efficient wastewater treatment system (the removal rate of fecal coliforms is 4 logarith- Wastewater Treatment Plant of Agronomic mic units) (El Hafiane 2003). The high phytoplankton and Veterinary Medicine Institute (AVI) production that occurs in this system enhances degrada- tion of organic matter, but the high algal biomass con- The wastewater treatment plant of AVI treats waste- centration in the effluent causes plugging problems when water collected from the AVI campus with a mean flow the effluent is reused in drip irrigation systems. rate of 63 m3 day-1. The system consists of anaerobic Silver carp is the species most used for plankton con- treatment followed by aerobic treatment. The anaero- trol in eutrophic lakes (Leventer and Teltsch 1990; Barry bic system is constituted of two units with two covered and Costa-Pierce 1992). This fish is a versatile omnivo- ponds in series, of a total capacity of 126 m3 and a rous species (Bitterlich and Gnaiger 1984), so it is able to retention time of 1 day. The aerobic system is made up significantly reduce phytoplankton and zooplankton of an algal high-rate pond (AHRP) followed by two (Kajak et al. 1975). The specialized gill apparatus and maturation ponds in series. The AHRP has an area of 960 m2 , a depth of 0.5 m, and a retention time of * Corresponding author; [email protected] 4.6 days. The maturation ponds that receive the AHRP 191 Downloaded from http://iwaponline.com/wqrj/article-pdf/40/2/191/233613/wqrjc0400191.pdf by guest on 28 September 2021 192 Berday et al. effluent have a total area of 170 m2, a depth of 1 m, and at the inflow and at the downstream part of the fish a total retention time of 1.4 days. The fish pond is situ- pond, at six depths (0, 20, 40, 60, 80 and 100 cm). The ated downstream of the plant and has an area of 26 m2 fish pond bottom samples were collected at the and a depth of 1.2 m. upstream, the downstream and the middle of the fish pond. Sampling of the aforementioned depths and the Fish culture. Fingerlings of silver carp (Hypophthalmich- bottom was done using an aquarium water pump. At the thys molitrix Val.) (582) and grass carp (Ctenopharyn- end of the sampling operation, all samples of the inflow godon idella Val.) (145) of a mean weight of 1 to 3 g were were mixed to obtain an “inflow composite sample” brought from the Carp Culture Plant of Deroua of Beni (inf). All samples collected in the fish pond at different Mellal and introduced in the fish pond in March 2001. depths were mixed to obtain a fish pond composite sam- Grass carp were introduced in the fish pond with the aim ple (p) and all samples of the pond bottom were mixed of reducing development of filamentous Cyanophycae on to obtain a “bottom composite sample.” the pond sides. The fish pond received freshwater during The physical and biological parameters analyses the first 15-day acclimation period, to permit the fish to were completed on the three composite samples. The fol- overcome the stress caused during transport. The pond lowing water parameters were measured: total suspended was treated with green malachite (0.1 g m-3) because the solids (TSS), total biomass, phytoplankton and zooplank- grass carp were infected with Saprolenia during trans- ton. The TSS were determined using a filtration method port. The Saproleniose caused 47% mortality of grass at 105°C using Wathman GF/C filters. The wet weight carp during the first days following fish introduction, (w.w.) of the total biomass (B) was calculated according while the silver carp mortality rate remained very low to the formula B = TSS.100(100-H)-1 where H was the (6%). Then, to adapt the fish to the HRPTS effluent, well relative humidity of algae (H = 93% was determined water was progressively replaced by this effluent during a according to Rodier [1996]). Plankton was identified and second 15-day acclimation period. During the whole counted on an optical microscope (Olympus). The plank- acclimation period, silver carp grew to 13 g while grass ton was determined using the manuals of Bourrely (1966, carp grew to only 2.3 g. 1968, 1970) and Cox (1981). The cells and body counts The experiment was carried out from April to July of phytoplankton and zooplankton were completed using 2001, using 548 and 76 silver and grass carp fingerlings, Thoma and Rosenthal cells, respectively. respectively. The inflow rate of the fish pond (HRPTS Before introducing the fish into the pond, the same effluent) was 4.3 m 3 day-1 from April to June and 9 m 3 parameters were followed fortnightly in the pond without day-1 in July. At night, the fish pond was mechanically fish from January to July 2000 (control pond). The pond aerated to avoid anoxy. The physicochemical character- flow rates were 4.3 m 3 day-1 from January to June and istics of the fish pond are summarized in Table 1. No 9m3 day-1 in July. Sampling and physicobiological analy- supplementary food was applied to the fish pond during ses were completed in the same method as in the fish pond. the study period. Fish sampling. Fish were sampled four times during the Physical and biological parameters analysis of the pond study period one day after water sampling. One hundred water. The fish pond was sampled four times during the silver carp and 40 grass carp were caught in the morning study period from April to July 2000. Water sampling using a fishing net, weighed and measured. Six silver was conducted every two hours between 8:00 and 18:00, carp were retained for digestive tract analysis and the remaining fish were put back in the fish pond. Silver carp gut content and feces analysis. The fish TABLE 1. Physical, chemical and biological water parameters of the fish pond (mean ± standard error), (number of data = 8) were sacrificed, digestive tracts were removed, weighed and the length of the intestines were measured. The Parameters Influent Fish pond intestines were gutted and their contents were weighed T (°C) 26.2 ± 1.3 26 ± 1 and homogenized in 100 mL of 1% formaldehyde solu- DO (mg/L) 14 ± 3 8.1 ± 1 tion to immobilize the plankton.