Abundance estimates of perch fry (Perca fluviatilis), complicated by grouped behaviour

Schnabel's mark and recapture abundance estimates of perch fry in Kli?ava Reservoir (Czech Republic), differentially marked in autumn and in spring, partly failed due to the existence of groups of individuals which behaved differently. There were three groupings. 1) a group of fry caught by shore seining from autumn of the first year (O+) to summer of the second year of life (1+), the only group in the littoral during late autumn and early spring and the fry with the slowest growth rate; 2) a group of perch fry that consisted of males maturing at 1 year and that dominated the trap catches in reservoir; 3) a group of fry that avoided the littoral in late autumn and during March-May 1983, which appeared in the littoral in May and June 1983, thus diluting the previously marked littoral fry. These 3 groupings of fry were relatively stable and separable throughout 1–2 years, apart for the mixing of groups 1 and 3 that took place in the littoral during the summer. The density of perch fry of 1982 year class in spring 1983 was estimated to be about 0.094 individuals per m².