MATURITY, SPAWNING AND SEX-RATIO OF PIKE PERCH, STIZOSTEDION-LUCIOPERCA (L.), IN 2 BULGARIAN RESERVOIRS AS COMPARED TO OTHER EUROPEAN HABITATS

The reproductive biology of pike perch, Stizostedion lucioperca (L.), was studied in two Bulgarian reservoirs; results were compared with da ta from other water bodies within its natural European range. Age at f irst maturity, onset of spawning, and sex ratio were determined and fo und to be different between the populations of the two reservoirs. Fis h from the cooling reservoir of a power station matured earlier at sma ller sizes because of elevated temperatures; this also resulted in an increased proportion of older males. It is suggested that age at first maturity is determined by growth rate and temperature. Temperatures a lone seem not to limit the distribution or onset of spawning within th e European range. The reasons for a shift in sex ratio among older fis h in some populations are discussed.