Natural selection acting on body size, growth rate and compensatory growth: an empirical test in a wild trout population

Organisms usually grow slower than their maximum potential under a given set of conditions, suggesting that fast growth carries a corresponding fitness cost. A pattern of growth that might influence such costs is compensatory (or catch-up) growth, where individuals grow faster than expected. One form of compensatory growth occurs when small individuals grow faster for their size than do large individuals, thereby decreasing (or slowing the increase in) size disparity between themselves and larger conspecifics. We tested these ideas over several seasons in wild brown trout (Salmo trutta) by estimating selection acting on individual size, growth and compensatory growth. We then examined population-level growth patterns to determine whether they influence individual-level selection. Selection generally did not favour large individuals or those with slow growth; indeed, the opposite was more likely. Moreover, selection did not act against small/fast-growing individuals (i.e. those expressing compensatory growth). Populationlevel growth was proportional (i.e. mass increases were a constant percentage of initial mass across the range of body sizes) in all seasons except for the spring and, to a lesser degree, the early summer of the second year of life. In these two intervals, small fish showed greater proportional growth than large fish and small/fast-growing fish had higher (or at least not lower) survival. An intriguing explanation for these results is that fast (and compensatory) growth is only exhibited when the costs of fast growth are low. Our study introduces a novel approach for assessing growth at the population level, as well as the survival costs associated with individual size and growth.