The juvenile life-stage is a major bottleneck for survival in many fish species. Therefore, identifying individual traits affecting survival in this life-stage is crucial to understand the natural selection within populations. Recently the interactions between behavioural traits and life-history traits have gained increased attention. Surprisingly, there is a lack of studies in natural conditions investigating what individual traits, and how their interactions, can affect fitness. 

Body size is one of the most important morphological traits affecting fitness in a wide variety of species. A large body size often gives an individual an advantage in competition for territory, resources and mating. Thus, it could be expected that individuals grow as fast as physiologically possible to gain fitness advantages against conspecifics. However, in many organisms high growth rates have been found to come at a cost to fitness. To examine the costs of high growth rates, situation where exceptionally high growth rate is present needs to be found. One of these situatíons is when an individual tries to retain normal growth trajectories after a period of growth depression, a so called compensatory growth. If compensatory growth is a response to growth depression during the first year of life it could be one factor affecting survival for the brown trout fry in our experiments.   

One behavioural trait that may change as a response to higher growth rates is activity. If activity is changed due to increased foraging rate (i.e. hyperphagia) it could be expected that these changes alters the growth-mortality trade-off. But there is an ongoing discussion among biologists whether an individual’s behavioural traits are plastic and responds quickly to changes in ecological factors, or if they are intrinsic and highly canalized to resist such changes. Recently it has also been found that the interindividual variation in activity can be repeatable and that high activity increased the probability of survival within a population. These results questions one of the assumptions in the growth-mortality-trade-off-theory. Therefore, it is important to further investigate how interindividual variation in activity in relation to other fitness related traits affects survival in fish fry.

The aim of this study was to investigate if brown trout fry

1. were able to compensate in length, weight and condition after period of growth depression induced by limited food resources. We also investigate if
2. the fish changed their individual activity level in a short term perspective as a result of changes in resource availability
3. if a period of starvation followed by resumed feeding affected the probability of survival during a subsequent period in natural conditions
4. and if the behavioural trait; activity, affected probability of survival under natural conditions.