Natural meanders in the middle and lower flow of the stream Trotina were replaced by man-made straightened and deepened artificial stretches in 1957. Since 1957 the man-made bed has obtained a barbel zone character, and brown trout and grayling were stocked An overfall sluice 1 m high situated 11 km from the headspring divides the upper trout stream in intensive management and the lower barrierless flow, in which the migration of fish is possible between the sluice and the estuary into a branch of the Labe river. Our localities are situated downstream from the sluice at the end of trout stream in extensive management which continues with the lowest coarse fish water: Trotina A, a man-made artifical sandy-clay stream bed is 2 km long, 4 - 4.5 m wide and 0.1 - 0.6 m deep. The neighbouring, upstream situated, totally channelized bed (crude stones in cement mortar) of Trotina B is 0.5 km long, 4 m wide and 0.2 - 0.3 m deep. In 1991 a 220-volt shocker was used to reduce coarse fish in these localities. Population density and biomass of fish, both per hectare, based on this catch, are calculated. According to the published data, the first catch with electricity results in about 70 % of the population. This is the reason for a possible difference in the actual and calculated density of about 30 %. The difference in actual and calculated biomass is smaller due to higher catchability of larger fish In Trotina A among 7 species the dominating chub (density 94 %, biomass 96.5 %) was caught. Calculated total density of 302 fish and total biomass of 58,91 kg, both per hectare, are the lowest according the published data. Among 4 species caught in the channel of Trotina B the chub (density 63.2 %, biomass 18.5 %) is also dominating. Calculated total density of 19 fish and total biomass of 2.628 kg, both per hectare, are very small. In both localities coarse fish is prevailing, in Trotina A reaching 99.1 % and in Trotina B 94.8 %, both in total density. In biomass the coarse fish in Trotina A is reaching 97.8 %, in Trotina B 81.5 %, both in total. In the channel of Trotina B the average weight of one fish is lower than in the sandy-clay stream bed of Trotina A. High weight of the only one brown trout caught in channel (w = 485 g) reduces the percentage of coarse fish biomass. In this channel of Trotina B the calculated total density and total biomass, both per hectare, are very small: the density is reaching 6.3 %, the biomass only 4.5 % of the values calculated in Trotina A.
