Growth and survival effects on maturation pattern in populations of grayling with recent common ancestors

Mortality and growth rates were shown to influence maturation patterns in five populations of grayling (Thymallus thymallus) in central Norway. The populations share recent common ancestors as they derive from introductions performed in 1910, and they inhabit lakes with different environmental conditions (i.e. length of growth season, lake area and fishing pressure). Mortality rate (range of Z-values: 0.36-0.77) and growth pattern varied strongly among the populations. Mortality rates were negatively associated with population mean age at maturity (r sp=-0.90), supporting life-history theory which predicts early maturation to be favoured under conditions with high adult mortalities. Maturation reaction norms differed significantly among the populations. Individuals from one population showed no maturation plasticity (all individuals matured at age three), whereas rapid growers were found to mature earlier than slow growers in the remaining four populations. Life-history theory is again supported as it predicts rapid growers to mature early due to high age-specific fecundity and short generation times. Given low mortality risks, slow growers are predicted to delay maturation in order to gain high first-time fecundity. In high-mortality systems all individuals are predicted to mature early. This theory is supported by the present data as populationwise maturation plasticity increased with decreasing mortality rates. In the population with no maturation plasticity the corresponding high mortality rates were probably due to high fishing pressures.