Young Brown Trout Research Articles

Summer stream habitat partitioning by sympatric Arctic charr, Atlantic salmon and brown trout in two sub‐arctic rivers

Summer habitat use by sympatric Arctic charr Salvelinus alpinus, young Atlantic salmon Salmo salar and brown trout Salmo trutta was studied by two methods, direct underwater observation and electrofishing, across a range of habitats in two sub‐arctic rivers. More Arctic charr and fewer Atlantic salmon parr were observed by electrofishing in comparison to direct underwater observation, perhaps suggesting a more cryptic behaviour by Arctic charr. The three species segregated in habitat use. Arctic charr, as found by direct underwater observation, most frequently used slow (mean ±s.d. water velocity 7·2 ± 16·6 cm s−1) or often stillwater and deep habitats (mean ±s.d. depth 170·1 ± 72·1 cm). The most frequently used mesohabitat type was a pool. Young Atlantic salmon favoured the faster flowing areas (mean ±s.d. water velocity 44·0 ± 16·8 cm s−1 and depth 57·1 ± 19·0 cm), while brown trout occupied intermediate habitats (mean ±s.d. water velocity 33·1 ± 18·6 cm s−1 and depth 50·2 ± 18·0 cm). Niche overlap was considerable. The Arctic charr observed were on average larger (total length) than Atlantic salmon and brown trout (mean ±s.d. 21·9 ± 8·0, 10·2 ± 3·1 and 13·4 ± 4·5 cm). Similar habitat segregation between Atlantic salmon and brown trout was found by electrofishing, but more fishes were observed in shallower habitats. Electrofishing suggested that Arctic charr occupied habitats similar to brown trout. These results, however, are biased because electrofishing was inefficient in the slow‐deep habitat favoured by Arctic charr. Habitat use changed between day and night in a similar way for all three species. At night, fishes held positions closer to the bottom than in the day and were more often observed in shallower stream areas mostly with lower water velocities and finer substrata. The observed habitat segregation is probably the result of interference competition, but the influence of innate selective differences needs more study.

Downstream displacement of post‐emergent brown trout: effects of development stage and water velocity

Brown troutSalmo truttawere introduced at hatching into distinct sections of two parallel artificial channels, one with a constant low velocity (control) and one with velocity changes (experimental), at such times as to produce 12, 3 and 0 day old fish (age after emergences) when the velocity was changed in the experimental channel. This experimental design was repeated in 2002 and 2003 at comparable dates. Young brown trout were sensitive to an increased water velocity for 5 to 6 days after emergence. Water velocity modified the displacement patterns qualitatively but not quantitatively. Eighty per cent of fish moved downstream at all water velocities. Velocity changes, however, advanced the time by which 80% of the fish had displaced downstream.

Niche overlap between young brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) in a subalpine Norwegian river

Niche overlap between young brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) in a subalpine Norwegian river

Age and growth of Siberian sculpin (Cottus poecilopus) and young brown trout (Salmo trutta) in a subalpine Norwegian river

Age and growth of Siberian sculpin (Cottus poecilopus) and young brown trout (Salmo trutta) in a subalpine Norwegian river

Competitive habitat displacement of brown trout by Siberian sculpin: the role of size and density

Habitat competition in brown trout Salmo trutta and Siberian sculpin Cottus poecilopus was investigated by varying density, fish size, and species composition in stream channels providing areas of different substratum particle sizes. In allopatry, both small (52 ± 4 mm LT) and large (86 ± 6 mm LT) brown trout exhibited strong preference for the intermediate (8–11 cm diameter) and large (17–21 cm) gravel substrata. There was a tendency for more brown trout to occupy finer (2–4 cm) substrata with increasing density, in particular for large brown trout. Also, more small brown trout were observed on finer substrata when tested with large brown trout, suggesting interspecific competition for restricted space. Both small (56 ± 6 mm LT) and large (88 ± 10 mm LT) Siberian sculpin preferred the large gravel in all tests, and did not change their substratum preferences much with increasing densities, suggesting higher tolerance for ‘crowding’. The large Siberian sculpin preferred the coarser substratum, and the largest individuals were consistently found on it. In sympatry with large Siberian sculpin, habitat displacement of brown trout occurred, indicative of interspecific competition. A higher proportion of small and large brown trout occupied the finer substrata than in allopatry. Habitat selection by large Siberian sculpin appeared to be unaffected by species composition and density. Small Siberian sculpin were displaced to finer substrata when tested with large Siberian sculpin, suggesting intraspecific competition. The results indicate that Siberian sculpin are potential habitat competitors for young brown trout.

Can laboratory studies on dominance predict fitness of young brown trout in the wild?

Laboratory studies suggest that dominance and aggression increase fitness, but this hypothesis has rarely been tested under natural conditions. We therefore designed a combined laboratory–field experiment to detect how social status and aggression relate to growth rate, movement and habitat choice in a natural stream. In 1998 and 1999, juvenile brown trout were caught in the wild and paired in staged dyadic contests in the laboratory where relative dominance rank was determined. Three categories of fish could be distinguished: dominants, subordinates and non-aggressive individuals of indeterminate status. All tested fish were released back into the stream and recaptured after 3 and 8 weeks. Dominant fish grew faster than subordinates, but non-aggressive fish grew as fast as dominants. Social status had no significant effect on recapture rates. Movement was not significantly related to status, but smaller individuals were more mobile and preferred faster-flowing habitats closer to the shore than larger fish. The utilisation of pool and riffle habitats varied among status categories, but this relationship was not consistent between years. These results support the hypothesis that dominance increases fitness in the wild. However, our findings also indicate that less aggressive individuals can be successful in heterogeneous natural habitats. Thus, studies performed under laboratory conditions may overestimate the fitness advantage of aggressive behaviour.

Residence duration influences the outcome of territorial conflicts in brown trout ( Salmo trutta )

From the value asymmetry hypothesis, we predicted that increasing residence duration should increase perceived territory value, which should (1) increase the motivation of a replacement owner to defend the territory, (2) decrease the probability that the original owner regains the territory and (3) increase contest duration and aggression between the original and replacement owner. These predictions were tested on young brown trout. First, individual fish were allowed to establish territories for 3 days. These original owners were then moved to an adjacent territory, whereupon replacement owners were allowed to take up their territories for either 2 or 4 days. After 4 days, the original owners were returned to their old territories, now containing a new size-matched resident. The resulting conflict was observed and the initiator, duration, aggression levels and the winner of the contest were determined. Replacement trout that had been resident for 4 days initiated and won more contests against original owners than did 2-day replacements. This suggests that the motivation to defend the territory increases with residence time, which will determine the outcome of territorial contests between opponents of similar resource-holding potential (i.e. size). Moreover, contests with 4-day replacements were longer and aggression levels were higher than in contests with 2-day replacement trout. These results are consistent with the value asymmetry hypothesis, extending its validity for explaining the behaviour of territorial animals.

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro-habitat use by 1838 young Atlantic salmon Salmo salar [mean LT 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (LT 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro-habitat variables snout water velocity and height above bottom, but with differences between size-classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size-classes. On a meso-habitat scale, brown trout were more frequently observed in slow pool-glide habitats, while young Atlantic salmon favoured the faster high-gradient meso-habitats. Small juveniles <7 cm of both species were observed most frequently in riffle-chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species-specific habitat overlap between streams may be complete. However, between-species habitat partitioning remains similar.

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south‐west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean LT 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (LT 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes &lt;7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles &lt;7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes &lt;7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.

Contrasting temperatures, waterflows, and light: seasonal habitat selection by young Atlantic salmon and brown trout in a boreonemoral river

AbstractHabitat use and habitat selection by young Atlantic salmon and brown trout were investigated by direct underwater observation. We sampled during winter and summer water temperatures (low: 3–7°C; high: 9–12°C) coinciding with low and high waterflows (12–20 and 60–80 m3s−1), and during day and night in winter, and on six selected stations in the river. Observations of 396 salmon and 120 trout indicated a distinct seasonal pattern in behaviours and habitat selection. Feeding was the dominant behaviour at high water temperatures during summer. In winter, there was a diurnal pattern in behaviour; both species sheltered in interstitial spaces in the substrate during daylight, but during night held positions on or close to the substrate in slower flowing stream areas. Coarse substrate providing cover was therefore an important habitat factor during daylight at low water temperatures, while slow‐flowing water was important during night. Although spatial niche overlap was considerable both in summer and winter, salmon and trout segregated with respect to meso‐ and microhabitat selection, and relatively more at low temperatures. Both species changed their use of mesohabitats towards more slow‐flowing glide/flat habitats in winter. Irrespective of season, trout preferred in general more slow‐flowing water than salmon did, but the difference was more pronounced in winter. Salmon used a wider range of water depths and in particular water velocities, than did trout. Both species were less tolerant of high water velocities at low water temperatures. The seasonal and diurnal pattern in habitat selection reported have important implications for habitat research and habitat‐hydraulic modelling. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Experience and social environment influence the ability of young brown trout to forage on live novel prey

Efficient feeding is crucial for the growth, survival and reproductive success of most animals. In artificial-rearing environments, however, animals are deprived of many stimuli normally experienced in the wild, which may alter feeding behaviour, and thus influence their survival and reproductive success upon release in nature. In a laboratory experiment, we investigated the effect of hatchery rearing on the ability of brown trout, Salmo trutta, to capture and consume a novel live prey item. Hatchery-reared and wild-caught trout, originating from the same river, were fed single black crickets, either in isolation or in visual and olfactory contact with another hatchery-reared or wild-caught fish. Total consumption, time to first bite and feeding efficiency were monitored. Wild-caught trout ate more, were quicker to attack, and consumed attacked prey more efficiently than hatchery-reared fish. Food consumption and efficiency increased in both wild and hatchery-reared trout during the experiment. We propose that the differences in feeding ability between wild-caught and hatchery-reared brown trout were mainly due to differences in previous experience of feeding on live prey. Wild-caught trout tended to eat more and sooner when in visual contact with another fish than when in isolation. This trend was not seen for the hatchery-reared fish, which may be due to environmental differences between the hatchery and the natural stream. The initial inability of hatchery-reared fish to forage on live prey may reduce their success when released in the wild, especially when in competition with resident wild fish.

Latitudinal variation in growth of young brown trout Salmo trutta

Latitudinal variation in growth of young brown trout Salmo trutta

Latitudinal variation in growth of young brown trout Salmo trutta

Summary 1. A new laboratory‐based growth model for brown trout (Salmo trutta) was used to explore latitudinal variation in growth among natural populations. The model included the effects of differences in ambient temperatures and fish size among populations. Annual growth rates of anadromous brown trout parr from 22 Norwegian populations at 61–70 °N were compared with predictions from the growth model. Published field data from one Spanish, 15 British and four Danish populations at 44–58 °N were included in the analysis to increase the latitudinal range. 2. Among the Norwegian populations, the ratio between observed and predicted growth rates was not significantly different from 1·00 in eight rivers, but was significantly higher in eight, and was significantly lower in six. Observed growth was highest, relative to predicted growth, in the coldest rivers. In Spanish, British and Danish rivers, observed growth did not exceed predicted growth. 3. The ratio between observed and predicted annual growth rate decreased significantly with increasing annual mean temperature. Observed annual growth was higher than predicted growth only in rivers with an annual mean temperature lower than 5·1 °C, and this indicates that some kind of thermal adaptation may occur in trout populations in the coldest rivers. 4. Regression analyses showed that besides the direct effects of temperature and body size predicted by the growth model, annual growth rates were significantly related to annual mean temperature, densities of juvenile salmonids, duration of twilight (average for May–August) and latitude. Adding these variables to the original model increased the explanatory power from 73·9 to 80·6%.

Habitat preference increases territorial defence in brown trout ( Salmo trutta )

The striking ability of territory owners to repel intruders has generated a number of theoretical explanations as well as experimental studies in many animal species. However, effects of individual habitat preferences on territorial defence have rarely been studied. From the territory value hypothesis, we predicted that owners of preferred habitats should invest more resources in defence than owners of non-preferred habitats. We tested this prediction with young territorial brown trout in a two-stage experiment. First, trout were allowed to choose individually between gravel and a uniform bright substrate. As expected, they showed a significant (79%) preference for gravel. However, there was considerable variation between individuals in substrate preference, with a few fish preferring the bright substrate. Half of the tested fish were then transferred to a gravel substrate and the rest to a bright substrate, manipulating habitat type in relation to preference. Territory owners were then staged against size-matched intruders whereupon contest aggression was observed and the winner of each contest determined. Overall, owners won most of the contests. Satisfied owners won 86% and owners of less preferred territories, 74% of the contests. Furthermore, more satisfied owners attacked sooner and were more aggressive relative to the intruders. We conclude that brown trout show individual variation in habitat preference, which appears to be linked with their investment in territorial defence. These results suggest that understanding and modelling of animal contests could benefit from considering how territorial defence is influenced by individual habitat preference and specialisation.

Seasonal and Diel Changes in Behaviour, Microhabitat use and Preferences by Young Pool-dwelling Atlantic Salmon, Salmo salar, and Brown Trout, Salmo trutta

There was a pronounced decline in activity of young pool-dwelling Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, as the water temperatures dropped in the autumn and early winter, and the fish switched from a predominantly diurnal towards a nocturnal activity pattern. Such a switch in activity pattern has previously been observed in young brown trout, but the present study is the first documentation for juvenile Atlantic salmon under natural conditions. Juvenile fish fed actively even when water temperatures were below 0°C, although foraging behaviour at near-freezing temperatures was recorded exclusively during night surveys. This indicates that other proximate factors, in addition to water temperature, affect the activity of young salmon and trout in rivers. Trout kept feeding positions significantly higher above bottom than salmon in August and September, but both species reduced the height above bottom at the onset of winter, possibly due to reduced swimming performance and lowered food availability in the upper part of the water column.

Predator–prey interactions between brown trout Salmo trutta and native and introduced ampbipods; tbeir implications for fisb diets

An attempt to improve the food base for brown trout Salmo trutta in Northern Ireland was made in 195859 by deliberately introducing English Gammarus pule.x into several Irish rivers. In addition, another amphipod Crangonyx pseudogracilis, was later accidently introduced into Irish waters. Our study represents the first attempt to examine the trophic interactions between a native fish predator (S. trutta) and an array of these native (Gammarus dueheni celticus) and introduced (G. pulex and Cpseudogracilis) amphipods. Feeding experiments, involving young brown trout predators and amphipod prey, revealed that the fish actively selected C. pseudogracilis relative to two alternative Gammarus prey species. Although the trout encountered the Gammarus species more than C pseudogracilis, they were eaten less than Crangonyx. Difficulties in handling and ingestion of Gammarus by trout may be a key component of the preference for the smaller, more easily handled Crangonyx. The microdistribution of the species was altered by the fish, due to predation being greater in particular microhabitats. Our study showed that the introduction of the herbivorous C. pseudogracilis into Irish freshwaters may represent a useful addition to fish diets, particularly for small and or juvenile fish. The repercussions of the deliberate introduction of G. pule.x are less clear. It may improve feeding for fish. but only if it can coexist with indigenous macromvertebrates and thus ultimately improve the range and quantity of possible food items in predator diets. Alternatively, being highly predatory towards other macroinvertebrates including G. d. celticus and C. pseudogracilis, G. pule.x may be deleterious to the diversity of the resident benthic community and hence reduce the diversity of prey available to fish predators.

Three-dimensional microhabitat use by young pool-dwelling Atlantic salmon and brown trout

This study, conducted in deep pools in three rivers, is the first to show a clear three-dimensional habitat segregation in size groups (equivalent to age groups) of juvenile Atlantic salmon, Salmo salar, and brown trout,S.trutta . Young-of-the-year (YOY) held position near the river bed and the river bank; height above bottom and distance from river bank increased significantly with fish size. Brown trout held position significantly further from the substratum, and were on average closer to the river bank, than salmon. The vertical segregation of young salmonids was most evident among young trout, with YOY being closest to the bottom. This size-dependent segregation is probably a result of different outcomes of the trade-off between the conflicting interests of higher food availability and greater predation risk in the upper part of the water column. We suggest that intercohort predation and competitive interactions were the main reasons why YOY of both species and salmon yearlings held positions close to the river bed. We found no evidence of salmon and trout parr preferring particular water depths, as studies in shallow parts of rivers have suggested, as the correspondence of use and availability of microhabitats at different water depths was high in the pools.

Spatial niche variability for young Atlantic salmon (Salmo salar) and brown trout (S. trutta) in heterogeneous streams

Abstract–Habitat is important in determining stream carrying capacity and population density in young Atlantic salmon and brown trout. We review stream habitat selection studies and relate results to variable and interacting abiotic and biotic factors. The importance of spatial and temporal scales are often overlooked. Different physical variables may influence fish position choice at different spatial scales. Temporally variable water flows and temperatures are pervasive environmental factors in streams that affect behavior and habitat selection. The more frequently measured abiotic variables are water depth, water velocity (or stream gradient), substrate particle size, and cover. Summer daytime, feeding habitats of Atlantic salmon are size structured. Larger parr (&gt;7 cm) have a wider spatial niche than small parr. Selected snout water velocities are consistently low (3–25 cm. s−1). Mean (or surface) water velocities are in the preferred range of 30–50 cm. s−1, and usually in combination with coarse substratum (16–256 mm). However, salmon parr demonstrate flexibility with respect to preferred water velocity, depending on fish size, intra‐ and interspecific competition, and predation risk. Water depth is less important, except in small streams. In large rivers and lakes a variety of water depths are used by salmon parr. Summer daytime, feeding habitat of brown trout is also characterized by a narrow selection of low snout water velocities. Habitat use is size‐structured, which appears to be mainly a result of intraspecific competition. The small trout parr (&lt;7 cm) are abundant in the shallow swift stream areas (&lt;20–30 cm depths, 10–50 cm. s−1water velocities) with cobble substrates. The larger trout have increasingly strong preferences for deep‐slow stream areas, in particular pools. Water depth is considered the most important habitat variable for brown trout. Spatial niche overlap is considerable where the two species are sympatric, although young Atlantic salmon tend to be distributed more in the faster flowing and shallow habitats compared with trout. Habitat use by salmon is restricted through interspecific competition with the more aggressive brown trout (interactive segregation). However, subtle innate differences in behavior at an early stage also indicate selective segregation. Seasonal changes in habitat use related to water temperatures occur in both species. In winter, they have a stronger preference for cover and shelter, and may seek shelter in the streambed and/or deeper water. At low temperatures (higher latitudes), there are also marked shifts in habitat use during day and night as the fish become nocturnal. Passive sheltering in the substrate or aggregating in deep‐slow stream areas is the typical daytime behavior. While active at night, the fish move to more exposed holding positions primarily on but also above the substrate. Diurnal changes in habitat use take place also in summer; brown trout may utilize a wider spatial niche at night with more fish occupying the shallow‐slow stream areas. Brown trout and young Atlantic salmon also exhibit a flexible response to variability in streamflows, wherein habitat selection may change considerably. Important topics in need of further research include: influence of spatial measurement scale, effects of temporal and spatial variability in habitat conditions on habitat selection, effects of interactive competition and trophic interactions (predation risk) on habitat selection, influence of extreme natural events on habitat selection use or suitability (floods, ice formation and jams, droughts), and individual variation in habitat use or behavior.

Effects of Water Chemistry and Habitat on the Density of Young Brown Trout Salmo trutta in Acidic Streams

We examined the relationship between young brown trout ( Salmo trutta) density in lake tributaries, and water chemistry and habitat variables. The study was carried out during the autumn in three acidic, softwater river systems in western and southwestern Norway; Gaular and Vikedal (1987–1993) and Bjerkreim (1988–1993). The streams had mean calcium concentrations of 0.35 mg L-1 (Gaular), 0.52 mg L-1 (Vikedal) and 0.84 mg L-1 (Bjerkreim). The concentration of inorganic Al was generally low, with mean values of 8.40 (Gaular), 22.22 (Vikedal) and 43.36 μg L-1 (Bjerkreim). In multiple regressions that involved different water chemistry variables, brown trout density correlated best with calcium concentration and with a combination of calcium and pH; the Ca2+:H+ ratio. In Vikedal and Gaular, calcium explained 51 and 57%, respectively, of the variability in brown trout densities. Althoug alkalinity exhibited the best correlation with brown trout density in Bjerkreim ( r2=0.33), it was similar to that of the model that included all major ions plus pH. The Ca2+:H+ ratio had a larger effect for variability in brown trout density in Gaular (r2=0.66) than calcium alone. In Vikedal and Bjerkreim, the Ca2+:H+ ratio also correlated with brown trout density, but considerably less than in Gaular. The predictive power of habitat variables was much lower than that of water chemistry; the single most important factors were altitude in Gaular (r2=0.22), mean water temperature in Vikedal (r2=0.11) and depth SD (index of heterogeneity) in Bjerkreim (r2=0.07). Models that included both habitat and water chemistry variables showed that the density of young brown trout was predicted primarily by calcium concentrations in Gaular (r2=0.75) and Vikedal (r2=0.54), as opposed to pH in Bjerkreim (r2=0.25). Habitat had low effect in all three river systems (r2=0.01–0.04). The final model explained 86, 68 and 32%, respectively, of the variability in brown trout density in the three catchments. Thus, water chemistry variables seem to be factors that limit the density of young brown trout in acidic softwater streams.

Seasonal changes in the body composition of young riverine Atlantic salmon and brown trout

The body composition of protein and fat in Atlantic salmon Salmo salar and brown trout Salmo trutta before and after winter was investigated in a temperate, small river, normally ice covered from the middle of November until the end of March. Fat, protein and specific energy declined greatly in winter but were replenished rapidly in spring. Rates of decline were slower for the smallest fish, which also had the lowest specific content of fat, protein, and energy, while the differences in absolute amounts were greatest for the largest fish. The mean specific fat content was reduced by 45–70% during winter, relative to the pre‐winter period (September). Mean daily reductions in specific enegy of the larger size groups of brown trout (3·7 × 10−3 kJ g−1 day−1) were almost half of the corresponding values for the largest Atlantic salmon (6·3 × 10−3 kJ g−1 day−1) during winter. A minor reduction in protein content was found during winter, with mean reductions of 6–10% in comparison to those in September. During spring the fat content was replenished rapidly, particularly for the smallest salmon fry (a threefold increase from April to June). Fat content in the larger salmon and trout increased by about 1·8 times. Based on estimated metabolic rates, digested energy during wintertime may contribute about two‐thirds of the brown trout fry's energy demand. For Atlantic salmon, the corresponding value is about 50%. The winter period put considerable stress on the young salmonids living in lotic environments, in particular for the smallest fry with the lowest energy content before winter and the largest losses during winter. This should make the fry more vulnerable to adverse abiotic and biotic factors.