|Other Abstract||Cyanobacterial blooms are causing severe problems in many lakes and reserviors due to increasing eutrophication in recent years. Bio-control of algae through introduction of filter-feeding fish (such as silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis) has been one of the most environmentally sound management propositions recently. This biomanipulation strategy is effective especially under eutrophic or hypertrophic conditions where phytoplankton community is dominated by colonial species and zooplankton community is dominated by microzooplankton. In China, silver and bighead carps are being used or tested in many lakes such as Lake Dianchi in Yunnan Province, Lake Chaohu in Anhui Province, and Lake Taihu in Jiangsu Province for the control of cyanobacterial blooms. In this thesis, we studied not only the feeding habits of silver and bighead carps which were stocked in large fish pens for the control of cyanobacterial blooms but also the impact of silver and bighead carp on the plankton communities and water quality. In addition, the controlling factors of spring-summer phytoplankton succession were also evaluated in Meiliang Bay, Lake Taihu. The main results and conclusions are as follows.
The pen-cultured silver and bighead carp all displayed fast growth. In 2005, the average growth rates of silver and bighead carps were 3.18g day-1 and 4.05g day-1, respectively. The maximum daily increments of the body weight of silver and bighead carps were 8.14g in June – July, and 10.5g in May – June, respectively. The survival rates of silver and bighead carp were estimated to be 26.5% and 41.6%, respectively. Silver carp fed mainly on phytoplankton but bighead carp mainly on zooplankton. Throughout the year, phytoplankton were predominant in the gut contents of silver carp, and the average biomass contribution was 68.5% with a range of 26.4% - 91.6%; however, zooplankton were predominant in the gut contents of bighead carp, and the average biomass contribution was 64.7% with a range of 15.4% - 98.4%, except in July and August when the gut contents of bighead carp were also dominated by phytoplankton.
There was an obvious feeding rhythm in silver and bighead carps. From July to October, feeding activity was most active between 14:00-18:00 h while the least between 06:00-10:00 h. However, in May, we observed a reversed feeding rhythm with a feeding peak at 06:00. Our results suggest that dominant plankton species in the lake water might be another important factor affecting feeding rhythm of silver and bighead carps. The feeding intensity of silver carp was significantly stronger than bighead carp in every month (P < 0.1). Silver carp had the highest mean gut fullness rate in August (8.12%), while bighead carp was in September (4.87%). Daily rations of silver and bighead carp were estimated by Egger’ model in the main growing season. The average daily ration was 13.62% for silver carp and 6.79% for bighead carp. Filtration rate was calculated from the daily ration and the density of plankton in the lake. During May - October, mean filtration rate of silver and bighead carp for phytoplankton were 0.73 L g-1 h-1 and 0.31 L g-1 h-1, respectively, and mean filtration rates for zooplankton were 0.32 L g-1 h-1 and 0.76 L g-1 h-1, respectively. Silver carp had a stronger ability of eliminating phytoplankton than bighead carp. To achieve a successful biomanipulation with a minimum effect of ichthyoeutrophication, the stocking proportion of bighead carp should be controlled in the future practice.
In 2005, the mean values of nutrient concentrations were generally higher in the fish pens than in the surrounding lake. However, no physicochemical parameters were significantly different between the fish pens and the surrounding lake (P > 0.1) except for pH (P = 0.08). Annual mean biomass of crustacean zooplankton was significantly higher in the surrounding lake (2.39mg L-1) than in the pens (1.83 mg L-1) (T-test, P < 0.05). Biomass of cladocerans was significantly higher in the surrounding lake (1.86 mg L-1) than in the fish pens (1.26 mg L-1) (T-test, P < 0.05). The copepods/cladocerans ratio was always higher in the fish pens than in the surrounding lake, but the difference was not significant (P > 0.1). Total phytoplankton biomass, Microcystis biomass and microcystin concentration were lower in the fish pens than in the surrounding lake water, but the difference was not statistically significant. Annual total biomass of phytoplankton was 8.39 mg L-1 in the fish pens and 9.74 mg L-1 in the surrounding lake. Annual mean chlorophyll a concentration was lower in the fish pens (43.73 µg L-1) than in the surrounding lake (48.06 µg L-1), but the difference was also not statistically significant. In the present study, the stocking density of silver plus bighead carp (about 40 g m-3 in July) was likely too low to achieve a significant difference in the above parameters between in the fish pens and in the surrounding lake.
In the main growth season (from April to November), we evaluated the diet niche breadth, diet overlap and growth of silver and bighead carp in 2004 with a low stocking density and in 2005 with a high stocking density. Both carps predated more zooplankton in 2004 than in 2005. Diet breadth index (Shannon-Weaver index) of silver and bighead carp was 1.38 and 1.13 in 2004, 1.40 and 1.25 in 2005, respectively. Silver carp had a broader diet breadth than bighead carp. Diet breadth of bighead carp was significantly correlated with abundance of crustacean zooplankton (p < 0.01), but such correlation was not significant for silver carp. Diet overlap estimated by Schoener’s similarity index was relatively high between silver and bighead carp. Their diet overlap obviously declined in 2005, with an average value of 0.7 in 2004 and 0.58 in 2005. Higher stocking density increased the diet breadth but decreased diet overlap in both carps. In addition, growth rates of silver and bighead carps were also significantly lower in 2005 than in 2004. It appears that silver and bighead carp were released from diet competition and shifted to feed on more zooplankton at low density, decreasing the efficiency of controlling cyanobacterial blooms.
In the present study, the average relative gut length was 10.30±2.31 for silver carp and 5.83±0.88 for bighead carp. There were significant correlations between gut length and standard body length for silver and bighead carp (P < 0.001). In the main growth seasons, silver carp significantly increased their relative gut length when feeding on more phytoplankton in 2005 (p < 0.01, 9.23 in 2004 and 10.77 in 2005, respectively); the relative gut length of bighead carp was 5.74 in 2004 and 6.03 in 2005, respectively. There was a nearly significant negative correlation between zooplankton proportion in the diet and the relative gut length when silver carp were stocked in a high density in 2005 (p = 0.112). Such resource polymorphisms in gut may be a good indication of temporal adaptation to resource conditions. During January to October in 2005, the hepato-somatic index of silver and bighead carp was 1.16% and 0.96%, respectively. There were significant correlations between the hepato-somatic index of silver and bighead carp and the zooplankton proportion in their gut contents (P = 0.073 for silver carp, P < 0.05 for bighead carp). Environmental variation can induce dramatic changes in the traits of organisms. Our results provided a field evidence for understanding the functional basis of resource polymorphisms and the evolution of phenotypic plasticity in planktivorous filter-feeding fishes.
During May to July in 2004 and 2005, the spring-summer successions of phytoplankton and crustacean zooplankton were weekly examined in Meiliang Bay, Lake Taihu. During the study period, the ecosystem of Meiliang Bay was characterized by (i) clearly declined nitrogen compounds (nitrate, TN and ammonium) and slowly increased phosphorus compounds (TP and SRP), (ii) increased total phytoplankton density and rapid replacement of chlorophyta (mainly Ulothrix) by cyanobacteria (mainly Microcystis), and (iii) rapid replacement of large-sized crustaceans (Daphnia and Moina) by small-sized ones (Bosmina, Limnoithona and Ceriodaphnia). Results from CCA and correlation analysis indicate that the spring-summer phytoplankton succession was primarily controlled by abiotic factors. Cyanobacteria were mainly promoted by increased temperature and decreased concentrations of nitrogen compounds. The pure contribution of crustacean was low for the variation of phytoplankton suggesting a weak top-down control by crustacean zooplankton in the subtropical Lake Taihu.|