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

Abstract

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.

Manipulation of the energetic state of Atlantic salmon Salmo salar juveniles and the effect on migration speed

“…() did not see any effect of over winter feed restriction on smolting rate in S. salar either. In contrast, in the closely related S. trutta , feed restrictions in the autumn decrease the smolting rate in the spring (Näslund et al ., ).…”

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

“…Similarly, Jones, Bergman, and Greenberg (2015) demonstrated that reduced winter and spring feeding increased the tendency to smoltify for lake feeding brown trout from the River Klarälven, Sweden. In contrast, another experiment reported that trout facing food restriction in late autumn exhibited lower rate of silvering, which is indicative of a delay in smolting (Näslund, Sundström, & Johnsson, 2015). Thus, winter/beginning of spring appears to be a critical period when the decision is made about whether to smolt and migrate to a better feeding area.…”

Landscape features determine brown trout population structure and recruitment dynamics

“…However, the effect of resource availability on migration is not clear. For example, a reduction in food availability can result in lower body condition and fat content and increased smoltification (Jones, Bergman, & Greenberg, ) but has also been shown to reduce migration (Midwood, Larsen, Aarestrup, & Cooke, ) or delay smoltification (Näslund, Sundström, & Johnsson, ). Females are more likely to migrate, mature later, and at a larger body size (Klemetsen et al, ) than males, and the difference in fecundity between large and small female fish can result in differences in trout population dynamics (via egg and juvenile density; Elliot, ), even at small geographic scales.…”