The relationship between emergence from spawning gravel and growth in farmed rainbow trout Oncorhynchus mykiss.

The relationship between the timing of emergence from spawning gravel and growth after emergence was investigated in farmed Oncorhynchus mykiss. A relationship between the time of emergence and growth became evident after 6 months of rearing, where individuals with an intermediate emergence time had grown larger compared with early and late emerging individuals.

Effects of maternal stress coping style on offspring characteristics in rainbow trout (Oncorhynchus mykiss).

Maternal size, age, and allostatic load influence offspring size, development, and survival. Some of these effects have been attributed to the release of glucocorticoids, and individual variation in these stress hormones is related to a number of traits. Correlated traits are often clustered and used to define the proactive and reactive stress coping styles. Although stress coping styles have been identified in a number of animal groups, little is known about the coupling between stress coping style and offspring characteristics. In the present study, plasma cortisol levels in ovulated mothers and cortisol levels in non-fertilized eggs from two rainbow trout (Oncorhynchus mykiss) strains selected for high (HR) and low (LR) post-stress plasma cortisol levels were compared. Offspring characteristics such as egg size, larval growth, and energy reserves also were compared between the two strains. Maternal plasma and egg cortisol levels were correlated, but no difference between the HR and LR strains was detected in either parameter. LR females produced larger eggs, and larvae with larger yolk sacs compared to HR females, however no differences in larval body size (excluding the yolk) was detected between strains. Considering that the HR and LR strains have a number of correlated behavioral and physiological traits that resemble the reactive and proactive stress coping styles, respectively, the results suggest that proactive mothers invest more energy into their offspring, producing larvae with larger energy reserves. It is possible that larger energy reserves in proactive larvae support the energy requirement for establishing and defending territory in salmonid fish. Furthermore, in the present study we found a positive relationship between mother plasma cortisol and egg cortisol; however neither mother plasma cortisol nor egg cortisol differed between strains. These results indicate that cortisol endowment from the mother to the offspring plays a minor role in the transfer of the behavioral and physiological traits which separates these strains.