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The age sructure of the roach (Rutilus rutilus) population and the chevin (Leuciscus cephalus) population in the Blanice river
BLÁHA, Josef
The Blanice river has its spring in the Šumava Protected Landscape Reserve near Zlatá at an altitude of 972 m a.s.l. and empties in the Otava river near Putim at an altitude of 362 a.s.l. The size of its catchment area is 860,5 km2, the watercourse is 93,3 km and the flow rate at the confluence is 4,23 m3.s-1. The upper reaches of the Blanice has a typical submontane, trout character, which is disturbed by the Husinec reservoir (the water area is 68 ha, maximal depth 25,5 m). The lower two thirds of the river run through an array of ponds in the region of Vodňany and Protivín. This area is intensively cultivated by agricultural activities. These two factors play the major role the fact that the lower part of the river is strongly eutrophicated. This is also supported by the abundance survey, according to which the most abundant species are roach (Rutilus rutilus) and chub (Leuciscus cephalus). The survey was made at 9 localities encompassing the main biotope types in the longitudinal profile of the lower part of the Blanice in the region of Vodňany and Protivín. The harvests were carried out within a 50-meter section, always in the upstream direction towards the place that kept the fish from escape (a small weir, a knickpoint). The most abundant species {--} Rutilus rutilus and Leuciscus cephalus {--} were evaluated from the viewpoint of the age distribution of the population.

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