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 f ive populations of grayling (Thymallus thymallus) in central Norway. The po pulations share recent common ancestors as they derive from introductions p erformed in 1910, and they inhabit lakes with different environmental condi tions (i.e. length of growth season, lake area and fishing pressure). Morta lity rate (range of Z-values: 0.36-0.77) and growth pattern varied strongly among the populations. Mortality rates were negatively associated with pap ulation mean age at maturity (r(sp) = - 0.90), supporting life-history theo ry which predicts early maturation to be favoured under conditions with hig h adult mortalities. Maturation reaction norms differed significantly among the populations. Individuals From one population showed no maturation plas ticity (all individuals matured at age three), whereas rapid growers were f ound to mature earlier than slow growers in the remaining four populations. Life-history theory is again supported as it predicts rapid growers to mat ure early due to high age-specific fecundity and short generation times. Gi ven low mortality risks, slow growers are predicted to delay maturation in order to gain high first-time fecundity. In high-mortality systems all indi viduals are predicted to mature early. This theory is supported by the pres ent data as populationwise maturation plasticity increased with decreasing mortality rates. In the population with no maturation plasticity the corres ponding high mortality rates were probably due to high fishing pressures.