Autumn food restriction reduces smoltification rate, but not over-winter survival, in juvenile brown trout Salmo trutta

The winter is often considered as a survival bottleneck for stream-living fish. Juvenile salmonids generally become less active during this period, and while food intake continues to some extent, growth rates are typically low. Here we present the results of an over-winter field experiment where energy levels were manipulated in late autumn. Three groups of juvenile (age 1+) brown trout, from an anadromous population, were monitored with respect to over-winter growth rate and survival (as indicated by recapture rates). Two groups were fed either high (HR), or low (LR) food rations in the laboratory for a month (October); the third group remained in the stream (STR). Over-winter growth rates were relatively low in all groups, and no growth compensation could be detected. Compared to HR and LR, STR fish had higher recapture rates after winter, indicating that laboratory housing may have affected the subsequent stream survival negatively. Comparing the two laboratory-housed groups, the LR group reached similar condition as the HR group in early spring, without indications of differences in survival. However, the initiation rate of body silvering (indicating initiation of smoltification) was lower in the LR group. Thus, it appears that food restriction during late autumn affect the onset of smoltification in juvenile brown trout. The results support previous laboratory studies indicating that salmonids modify their over-winter foraging behaviour to avoid too low energy levels at the end of winter. This modification appears to delay smoltification, but may not necessarily be costly in terms of over-winter mortality.