Recruitment of perch (Perca fluviatilis L) from Baltic coastal waters

Growth and abundance of perch (Perca fluviatilis L.) larvae and juveniles were studied in Baltic coastal waters during the period 1971-1985 and related to data on subsequent year-class strength calculated from 4-7 year olds. To simulate winter conditions, experiments on size-related weight-loss during starvation and its effects on mortality were also carried out. Statistical analyses demonstrated a positive correlation between)lear-class strength and autumn abundances of juveniles of the first year as well as between these abundances and weight in the same season. Thus, it was demonstrated that in the Baltic coastal waters variations in perch year-class strength are positively related to growth during the first year of life. The faster growth rate and thus greater size attained probably enhanced survival, since the susceptibility to mortality from predation is lowered. Starvation due to food shortage was not found to be influential, since growth mostly was close to maximum at the prevailing temperature. High condition factors of young-of-the-year fish supported this conclusion. Conditions during larval development were also demonstrated to affect perch recruitment. Thus, in temperatures outside the interval 10 degrees to 20-25 degrees C, larval mortality was high. Except at such adverse temperatures, mortalities were low. Winter starvation was shown under field conditions, and in laboratory experiments starvation rates, and thus mortality, were demonstrated to increase exponentially with decreasing size. Consequently, starvation during this season should be an important stock regulator under natural conditions, during which food consumption is prohibited for long periods. The proportions of mortality will depend upon the size of the juveniles attained at the end of the first growth season and the severity of the winter period. Based on the results, a model predicting relative year-class strength is proposed.