Effects of Silver Carp Density on Zooplankton and Water Quality: Implications for Eutrophic Lakes in China

Abstract

We examined the responses of zooplankton community, water transparency, chlorophyll a-and nutrients to manipulation of density of silver carp (Hypophthyalmichthys rnolitrix) in an one-way factorial experiment using enclosures placed in Donghu (East Lake, 30" 33' N, 114" 23' E), located in Wuhan, P. R. China. Enclosures (18.75 m3) were treated with four silver carp densities, 0,81,225,485 g/m2. Total zooplankton abundance (excluding nauplii and rotifers except for Asplanchna sp.) and the mean size of dominant cladoceran species were significantly greater in enclosures with 0 and 81 fish densities than those in enclosures with 225 and 485 fish densities. Water transparency also improved significantly when silver carp densities were 0 or 81 g/m2. We did not find significant effects of silver carp density on chlorophyll a, total phosphorus, or total nitrogen concentrations.We conclude that by reducing planktivorous fish to below the current density (190 g/m'), the zooplankton community can be shifted from the dominance of small-bodied Moina sp. to dominance of large-bodied Daphnia sp. Further, the water clarity can be increased. INTRODUCTIONA recent survey reported that the proportion of lakes that can be classified as eutrophic (total phosphorus ranging from 0.01 8 to 0.97 mg/L) in China has increased from 5% to 55% in the past decade (Jin 1994). Water transparency of less than 1 m and reduced

Studies on the Effects of Silver Carp (Hypophthalmichthys molitrix) on the Phytoplankton in a Shallow Hypereutrophic Lake through an Enclosure Experiment

“…In the absence of grazing by planktivorous fish, enhanced crustacean grazing suppressed the small algae that are vulnerable to crustaceans, and inedible algae like large colony, toxic cyanobacteria species or those with gelatinous sheaths flourished (LYNCH andSHAPIO, 1981 andVANNI, 1987). In our experiment as well as in the experiments carried out by FUKUSHIMA et al (1999, EXP2) andWU et al (1997), the initial dominant phytoplankton were nanophytoplankton. Increased crustaceans in the fishless enclosures caused reduction of small phytoplankton, but did not cause an outburst of inedible algae like cyanobacteria.…”

“…Experimental design and descriptions of the enclosures are similar to those given by WU et al (1997). Eight polyethylene enclosures (each 2.5 × 2.5 m by 2 m depth) were hung from steel pipes which were mounted at the bottom.…”

“…Although many studies were made to clarify the zooplankton-fish interaction in the lake (XIE et al, 2000;YANG et al, 1999 andWU et al, 1997), information on the effects of silver carp on phytoplankton is limited. The aim of the present study is to clarify the phytoplankton response to a gradient of silver carp biomass densities using enclosure methods together with a comparative study of silver carp impact on phytoplankton under different conditions.…”

Biomanipulation in mesocosms using silver carp in two Chinese lakes with distinct trophic states

“…Increased fish density may increase the possibility of fish coming in contact with sediment on the lake bed and consequently promote the release of nutrients from the sediment. The results from the Lake Taihu enclosures are consistent with the study conducted in eutrophic Lake Donghu enclosures by Wu et al (1997).…”

“…In the Lake Taihu enclosures, the removal of algae by silver carp at 35 and 70 g m −3 resulted in a particularly noticeable reduction in phosphorus, primarily in particulate form, whereas dissolved phosphorus was evident at similar levels in different treatments. Wu et al (1997) also found that water had the lowest TP values when silver carp biomass was 81 g m −3 (medium biomass), whereas TN did not differ between fish enclosures and non-fish enclosures. Cui et al (2004) have also found that silver carp are more effective at removing phosphorus than nitrogen.…”