Studies On Feeding Of Exotic Fish And Plankton Ecology In Lake Erhai

This study carried out the investigation on the exotic introduced fish-Neosalanx taihuensis (icefish) and the artificial released fishes-Hypophthalmichthys molitrix (silver carp) and Aristichthys nobilis (bighead carp) in Lake Erhai.We mainly examined the diet composition, prey preferences, diurnal feeding activities, and food consumption of icefish. We studied the diet and the dynamic changes in food composition of silver carp and bighead carp.In addition, we also exploited the diel vertical migration (DVM) of plankton and the influencing factors under the strong predation pressure of the exotic fishes. The main results of this research are as follows:1. Lake Erhai underwent eutrophication from 2009 to 2010. The mean total nitrogen concentration was 0.60 mg/L, and the maximum value of 1.14 mg/L was recorded in July. The mean total phosphorus concentration was 0.03 mg/L, which decreased from November 2009 to February 2010 but increased from April to October. The mean chlorophyll a (Chi a) concentration was 12.41 μg/L, with the highest and lowest values recorded in September and February, respectively. The highest Chi a concentration occurred in Southern Erhai (13.26μg/L), followed by Northern Erhai (11.32μg/L), and then Middle Erhai (9.92 ug/L). The modified trophic state index in the whole lake was approximately 53, indicating that the trophic status of Lake Erhai transformed from mesotrophic to eutrophic. In particular, Southern and Middle Erhai were mesotrophic, whereas Southern Erhai was eutrophic.2. This study is the first to report on the diel feeding periodicity and daily food consumption of icefish in their invaded ecosystems to explore the potential impact of icefish grazing on Daphnia. Icefish was introduced into Lake Erhai in 1984. Results revealed a highly significant seasonal variation in the prey items in the guts of the sampled icefish. This finding suggests that a relationship exists between the fluctuations of available prey in the environment and the selective feeding of the fish. N. taihuensis preferred large zooplankton, such as Daphnia and calanoids. This preference may decline Daphnia density and weaken the inhibitory effect of large zooplanktonon on algal growth. The feeding activities of the fish were concentrated in the morning (8:00) and/or at dusk (20:00), except in September 2010. Moreover, the daily consumption of zooplankton by N. taihuensis significantly varied, both diurnally and among seasons. The daily consumption values ranged from 0.089 g to 0.237 g (wet weight) per 100 g wet fish weight at temperatures between 11.50℃ and 24.68℃.3. The weights of stocking H. molitrix and A. nobilis have been increased in recent years to control cyanobacterial blooms in Lake Erhai. Therefore, we studied the feeding characteristics of silver carp and bighead carp, exploited the regulatory abilities on algal growth and stocking ratios of the two fishes, and the diet competition with icefish. The annual investigation findings showed that H. molitrix and A. nobilis ingested 75 and 64 genera of preys, respectively. On the basis of percentage numerical abundance (%N), silver carp and bighead carp preyed on phytoplankton. Microcystis was the dominant prey in the guts of H. molitrix (64.85%), followed by Psephonema. Microcystis was also the most important prey in the guts of A. nobilis (68.51%), followed by Bosmina (12.22%). Meanwhile, zooplankton (mainly Bosmina and Daphnid) were predominant in the gut contents of the two fish species on the basis of percentage weight composition (%W). The proportions of average biomass reached 65.64% and 91.49%, respectively. Compared with other eutrophic lakes, the regulatory abilities of H. molitrix and A. nobilis on algal growth were not ideal, particularly the latter. Basing on our results, we discussed recommendations for the ratio of stocking H. molitrix and A. nobilis. In addition, the spatial variations of food composition of the two fishes were significant and closely related to the food availability in the area.4. During the investigation, we identified eight crustacean zooplankton species, namely, Daphnia hyaline, Bosmina longirostris, Ceriodaphnia quadrangular is, Diaphanosoma brachyurum, Chydorus sphaericus, Mesocyclops leuckarti, Phyllodiaptomus tunguidus, and Nauplii. All crustacean zooplankton exhibited strong and normal DVM patterns (down during the day, up at night) across sampling months. Correlation analysis with environmental factors showed that the vertical distribution of zooplankton was primarily affected by water transparency (P< 0.05). The weighted mean depth (WMD) of all zooplankton was not significantly correlated with the WMD of the dominant phytoplankton and Chl a. Moreover, a weak negative relationship was observed between zooplankton distribution and water temperature in January, March, and July 2010 (R2 between 0.1 and 0.4). Integrated analysis implied that the factors that induced DVM behavior between large and small zooplankton in Lake Erhai were predation avoidance and phototaxic behaviour. In particular, phototaxis contributed to the migratory behaviour of small species.5. During the sampling period, the highest (1.10±0.52×107 ind/L) and lowest (2.06± 1.05×105 ind/L) densities of Microcystis were recorded in November 2009 and March 2010, respectively. During water blooms (high density), Microcystis colonies in all months exhibited a strong DVM. The results of linear regression analysis showed that light illumination was the most important driving factor (P< 0.05) for the DVM of Microcystis colonies. Under the influence of light, Microcystis stayed in deeper strata during the daytime and migrated into the epilimnion at night and before dawn. Microcystis at a low density also showed a strong DVM (except for May 2010).The migration pattern in January was similar to that in water blooms (high density). However, the reverse migration pattern of Microcystis occurred in March because of the predation of Daphnia.