Altitudinal variation in the relationship between growth and maturation rate in salmon parr

1. It is well established that the likelihood of an individual animal becoming mature is influenced strongly by its previous growth. However, with few exceptions, it is not known whether common relationships between the two parameters apply within and among populations. 2. Using data collected from tributaries of a single river, we investigate the relationship between size and maturation in Atlantic salmon parr, and whether spatial variation in the incidence of parr maturation can be explained by variation in growth rates. 3. There was spatial variation in the size of parr, a large proportion of which was attributable to altitude. However, this did not explain the variation between sites in the maturation rate of parr. In contrast to earlier findings, there was no relationship between the mean size of parr at a site and the percentage of males that matured there for the age class in which most mature males were found. 4. Within a site size at age was a good predictor of maturation, but across sites this relationship was modified by altitude. For a given age and size a parr was more likely to become sexually mature if it came from a high-altitude site. 5. Spatial variation in the incidence of male parr maturity is not simply the result of variation in growth rates. The size threshold above which parr male parr matured depended on altitude, which could serve as a proxy for growth opportunity. 6. We suggest adaptive explanations for variation in the relationship between growth and maturation, particularly the importance of relative, as opposed to absolute body size. 7. Our study suggests a high level of complexity in the proximate mechanisms governing life-history strategies in order to bring about adaptive strategy choice.