AGE STRUCTURE IN SPAWNING SHOALS OF CHONDROSTOMA-NASUS

The age structure was examined of ten samples of spawning Chondrostoma nasus taken from a number of Moravian streams (Czech Republic). Most of the samples analysed exhibited an age structure with a single peake d curve, however, females from the River Jihlava (Mohelno) and both ma les and females from the River Oslava, (Namest n. O.) showed age struc ture consisting of a double peaked curve. The results showed that the spawning shoal from the Trebuvka stream was significantly affected by local fisheries management, the removal of mature brood fish for artif icial spawning resulting In a population biased very much towards youn ger fish (i.e. 8 years). Many male specimens from the River Oslava (Na mest n. O.), sampled in 1975, were found to have matured earlier than the expected 5 or 6 years old i.e, at 3 years, though most specimens w ere mature at 4 years of age. At all the other sites such early matura tion was rare. In all the spawning shoals sampled, the average age of females was higher than that of males due to a higher rate of survival and later maturation.