A commercial dry diet and both live and frozen zooplankton were compared as food for cultured larvae of Lake Hallwil whitefish Coregonus suidteri during the first 3 weeks after hatching. Whitefish larvae fed live zooplankton grew considerably larger (16.4 mm final length) than those fed frozen zooplankton (14.4 mm), but mortality did not differ significantly (0% versus 3%). Larvae fed the dry diet reached nearly the same length (13.6 mm) as the fish fed frozen zooplankton. However, mortality of fish fed the dry diet was significantly higher (34%) than that of fish fed live or frozen zooplankton. We conclude that diet acceptance is a key factor for whitefish larvae, accounting for the highest growth rates by larvae fed live zooplankton. We also tested the effect of two additional factors on larval mortality: food particle size and water circulation in the rearing tanks. The size of food pellets was reduced from 200–400 μm to 100–200 μm. The water inflow to the tanks was placed below the water surface to increase the time the dry diet was afloat. Our results showed that neither particle size nor inflow placement affected mortality when fish were fed ad libitum. Nevertheless, feed floating times were prolonged significantly when the inflow was placed below the water surface, making it possible to decrease the feeding frequency, reduce food losses, and, thus decrease the amount of daily ration fed.
In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April-May/June), Bythotrephes longimanus occurs below 0-20 m, while Leptodora is absent. In summer and early autumn (July-September/October), when Leptodora dominates during daytime in the 0-20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic. In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphnia decreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.
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