Effects of body size on sympatric shelter use in over‐wintering juvenile salmonids

The study tests two hypotheses: (1) the degree of shelter dominance in Atlantic salmon Salmo salar and brown trout Salmo trutta increases progressively with increasing size differential between heterospecific fish in a pair and (2) shelter dominance, standardized to size differential, correlates with aggression. The results support the first but not the second hypothesis, suggesting that the fitness consequences of high growth performance during the summer are likely to become evident during winter. At this time of year, when mortality is high among both Atlantic salmon and brown trout, shelter dominance may increase the chances of survival.     

Shelter selection in juvenile Atlantic salmon,or why do salmon seek shelter in winter?

During winter, juvenile Atlantic salmon Salmo salar become nocturnal and seek refuge during the day in the stream bed gravel interstitial spaces. The function of this behaviour is unclear, but two major types of hypothesis have been proposed. One is that the fish are hiding from something (e.g. a predator) and the other is that the fish are seeking shelter from the water current. These hypotheses were tested by examining the selection of juvenile salmon for refuges that offered different degrees of concealment or shelter. The fish clearly preferred refuges that allowed them to hide (i.e. they were dark and opaque) but offered little shelter from the current. Therefore, it can be assumed that the primary function of this nocturnal behaviour during winter is most likely to hide from diurnal predators.     

Winter behaviour of juvenile Atlantic salmon Salmo salar L. in experimental stream channels: effect of substratum size and full ice cover on spatial distribution and activity pattern

Activity and choice of areas offering different cover (substratum or surface ice) for juvenile Atlantic salmon Salmo salar were studied in experimental stream channels during winter. Channels were completely ice covered between December and March. During this period, the ice thickness increased from 50 to 300 mm after which 50% of the ice was experimentally removed and followed by c. 2·5-fold increase in discharge to simulate the effects of spring flood. Large substrata provided preferred habitats but areas with small substratum sizes were also used when full surface ice provided above-stream cover and the stream discharge was relatively low. The fish remained nocturnal throughout the study but the level of day activity significantly increased as the surface ice became thicker. Maximum movement distance during a 24 h period and homing-at-dawn behaviour remained at a constant level throughout the main winter, but significantly changed during the simulated spring flood (mean ± s . e . maximum extent of movements within 24 h increased from 1·1 ± 0·1 to 3·0 ± 0·5 m; homing behaviour decreased from the highest level of 89·3 to 34·6% during spring flood). Overwinter survival was high (92·9%). Relative mass increase during the study ranged from –8·3 to 28·5%, and 84% of the juvenile Atlantic salmon gained mass. The highest rates of mass increase were associated with frequent movements between areas of different substratum size. The results indicate that during winter: (1) Atlantic salmon parr preferred large substratum cover compared with surface ice cover at the fish densities studied here, (2) juvenile Atlantic salmon were predominantly nocturnal but diurnal activity increased as surface ice became thicker and (3) increase in water discharge during spring altered the behaviour of juvenile Atlantic salmon and may have caused additional habitat shifts.     

Seasonal and spatial microhabitat selection and segregation in young Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in a Norwegian river

Seasonal microhabitat selection by sympatric young Atlantic salmon and brown trout was studied by diving. Both species, especially Atlantic salmon, showed seasonal variation with respect to surface and mean water velocities and depth. This variation is partly attributed to varying water flows and water temperatures. In winter the fish sought shelter in the substratum. A spatial variation in habitat use along the river due to different habitat availabilities was observed. Both species occupied habitats within the ranges of the microhabitat variables, rather than selecting narrow optima. It is hypothesized that the genetic basis allows a certain range to the behavioural response. Microhabitat segregation between the two species was pronounced, with brown trout inhabiting the more slow-flowing and partly more shallow stream areas. Atlantic salmon tolerated a wider range of water velocities and depths. Habitat suitability curves were produced from both species. It is suggested that habitat suitability curves that are based on observations of fish occupancy of habitat at median or base flow may not be suitable in habitat simulation models, where available habitat is projected at substantially greater water flows.     

Ice‐dependent winter survival of juvenile Atlantic salmon

Changes in snow and ice conditions are some of the most distinctive impacts of global warming in cold temperate and Arctic regions, altering the environment during a critical period for survival for most animals. Laboratories studies have suggested that reduced ice cover may reduce the survival of stream dwelling fishes in Northern environments. This, however, has not been empirically investigated in natural populations in large rivers. Here, we examine how the winter survival of juvenile Atlantic salmon in a large natural river, the River Alta (Norway, 70°N), is affected by the presence or absence of surface ice. Apparent survival rates for size classes corresponding to parr and presmolts were estimated using capture‐mark‐recapture and Cormack‐Jolly‐Seber models for an ice‐covered and an ice‐free site. Apparent survival (Φ) in the ice‐covered site was greater than in the ice‐free site, but did not depend on size class (0.64 for both parr and presmolt). In contrast, apparent survival in the ice‐free site was lower for larger individuals (0.33) than smaller individuals (0.45). The over‐winter decline in storage energy was greater for the ice‐free site than the ice‐covered site, suggesting that environmental conditions in the ice‐free site caused a strong depletion in energy reserves likely affecting survival. Our findings highlight the importance of surface ice for the winter survival of juvenile fish, thus, underpinning that climate change, by reducing ice cover, may have a negative effect on the survival of fish adapted to ice‐covered habitats during winter.     

Effects of ice cover on the diel behaviour and ventilation rate of juvenile brown trout

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Night,day, sunrise,sunset: do fish under snow and ice recognize the difference?

1. Although boreal lakes are ice‐covered for several months annually, little is known about the behaviour of fish under ice. To consider the reasons for diel vertical migrations (DVM) it is important to compare periods under ice as opposed to under open water. Echosounding provides a tool for non‐intrusive continuous monitoring of fish, even in winter. 2. Changes in the vertical distribution of fish through six 48‐h periods were monitored using a stationary, mounted echosounder that beamed vertically either from the bottom up or from the surface down from February to April, 2003. The up‐beaming and down‐beaming transducers were run alternately for 24 h each over the 48‐h period. Standard echo analysis software was used to detect fish traces and estimate the vertical and temporal distribution of fish abundance. Fish were sampled with a winter seine. 3. Prominent diel vertical migration in response to changing light level was detected throughout the study period (late winter to spring). Fish were highest in the water column at sunset and sunrise. In daylight, most detected fish were well below 10‐m depth. The number of fish detected was greatest during the night when they occurred throughout almost the whole water column, sometimes with a considerable number very close to the ice. Fish were mostly vendace and whitefish. 4. It became evident from data from the up‐beaming transducer that at night fish may occupy the layer closest to the surface. These fish would not have been detected if we had only used the down‐beaming transducer. The overall pattern of DVM in winter was very similar to that in summer. The results support the suggestion that DVM is a genetically fixed behavioural trait responding to the contemporary level of illumination.     

Effect of density on competition between wild and hatchery‐reared Atlantic salmon for shelter in winter

The effect of varying the density of hatchery‐reared Atlantic salmon Salmo salar on the ability of single wild fish to occupy a shelter is assessed. Although there was strong density‐dependence on sheltering overall, the ability of wild Atlantic salmon parr to occupy a shelter was not affected by the presence of hatchery‐reared fish even when outnumbered by four to one. These findings illustrate a competitive asymmetry for shelter in favour of the wild fish at the densities tested.     

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.     

Demographic responses to a mild winter in enclosed vole populations

Mild winter weather causing snow to melt and ice to accumulate on the ground has been proposed to cause the decreased survival of individuals, and less pronounced cyclicity, of small rodent populations in Fennoscandia. However, detailed data linking ice accumulation to decreased winter survival is lacking. We live-trapped and monitored with passive integrated transponders enclosed populations of root voles (Microtus oeconomus) exposed to different amounts of ice accumulation through a mild winter. We studied how social behaviour and survival responded to snow melt and ice accumulation. Voles avoided ground ice by moving their home ranges, thus increasing home range overlap in enclosed populations experiencing more extensive ice cover. Winter survival was not affected by the amount of ice accumulation, and was only slightly reduced during ice formation in early winter. The lowest survival rates were found at the onset of snow melt in early spring. These results suggest that ice accumulation does not cause lower survival during mild winters, probably because plastic social behaviour enables root voles to reduce the negative effects of varying winter weather on survival. The mechanisms for lower survival during mild winters may operate during spring and be related to spring floods or increased susceptibility to predators. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Competition for shelter among over‐wintering signal crayfish and juvenile Atlantic salmon

Three separate effects on refuge use by signal crayfish Pacifastacus leniusculus and Atlantic salmon Salmo salar were examined: (1) the effect on Atlantic salmon of an addition of signal crayfish (doubling the total number of animals), (2) the effect on signal crayfish of an addition of Atlantic salmon and (3) intraspecific compared with interspecific competition, compared by holding total density of animals constant and varying the proportion of signal crayfish and Atlantic salmon in trials. Observations were made during winter, when both species are nocturnal. The proportion of Atlantic salmon sheltering was significantly lower in the presence than in the absence of signal crayfish when the interspecific treatment (Atlantic salmon plus signal crayfish) effected a doubling in density compared to the intraspecific treatment (Atlantic salmon alone). The proportion of signal crayfish sheltering was independent of the presence of Atlantic salmon. When total density was constant, the proportion of Atlantic salmon sheltering was significantly higher in intraspecific (52·8%) than interspecific trials (27·3%). Atlantic salmon out of shelter during the day in winter are believed to be very vulnerable to predators and the capacity for fish to share shelters with one another is known to be very low. Therefore, competition from crayfish for winter shelters may lead to detrimental effects on Atlantic salmon populations.     

Seasonal variation in the thermal performance of juvenile Atlantic salmon (Salmo salar)

1. Experimental data on the maximum growth and food consumption of winter‐acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70°N were compared with predictions from published models of growth and food consumption of summer‐acclimatised fish from the same populations. 2. All winter‐acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1–6 °C). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 °C. 3. Growth and food consumption varied significantly among populations. Winter‐acclimatised fish from a Northern population had a higher mass‐specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer‐acclimatised salmon, where fish from the Northern population had the lowest growth efficiency. 4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.     

Perspectives in winter limnology: closing the annual cycle of freezing lakes

Winter has traditionally been considered as an ecologically insignificant season and, together with technical difficulties, this has led winter limnology to lag behind summer limnology. Recently, rapidly expanding interest in climate warming has increased water research in winter. It has also become clear that neither winter conditions of lakes nor under-ice communities are as static as often supposed. Although interannual differences in water temperature are small, close to the maximum density temperature, they may have profound effect on under-ice hydrodynamics. Thus, stochastic variations in weather, particularly those preceding the time of freezing and ice melting, may have important consequences for hydrodynamics which then affect the distributions and conditions of microorganisms and probably further to higher trophic levels. Even fish distributions can be dictated by under-ice conditions and their activities as well as behavior can sometimes approach those in summer. Life in freshwater ice is one of the least studied aspects of winter limnology and recent studies suggest that a thorough evaluation is needed. Altogether there are strengthening signs that winter should be considered as an integral part in the functioning and dynamics of lakes affecting quantitative and qualitative characteristics of aquatic communities in summer. There are great prospects that more thorough understanding of the prevailing limnological conditions in winter will improve our understanding of lake ecosystems in their entirety, and there is no doubt that such an approach requires multidisciplinary and long- term studies at different spatial scales.     

Water velocity shapes juvenile salmonids

Phenotypic plasticity in morphology is often considered adaptive. Stream-living fish encounter considerable spatial and temporal environmental variation in their native habitats, and the ability to adapt to this variation is of utmost importance. We studied experimentally whether water velocity affects the body shape of juvenile Atlantic salmon (Salmo salar m. sebago Girard) and brown trout (Salmo trutta m. lacustris L.). The fish were reared in slow and fast water flow, and their morphology was studied by measuring a number of morphometric characters. We studied which characters differed between the environments in each species, and found that water velocity caused morphological differentiation in both salmon and brown trout. The differences occurred especially in body height as well as in fin sizes, characters that are very likely to be of functional importance for life in the stream environment. Salmon in fast flow became more robust, whereas brown trout in fast flow became slightly more streamlined. The observed variation in body morphology of salmon and brown trout indicates phenotypic plasticity, but the species differed in their response to environmental variation, which may be due to different energetics and cost reduction strategies. Morphological differentiation caused by water flow occurred very rapidly, within 1-month exposure to the different water flows. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Winter movements and habitat use of riverine brown trout, white sucker and common carp in relation to flooding and ice break-up

Movement of radio-tagged riverine brown trout Salmo trutta , white sucker Catostomus commersoni and common carp Cyprinus carpio in the Grand River, Ontario, was minimal during the winter except during periods of high water discharge or the break-up of river ice. Ice break-up and flooding occur when southern air masses penetrate northwards in winter and as air temperatures increase in spring. Both white suckers and common carp moved the longest distances, generally in a downstream direction, during flooding and ice break-up. Brown trout moved less during these times. All three species had positive relationships between distance moved and water discharge. During periods of high water discharge and ice break-up, white suckers and common carp used backwater habitats more and main channel habitats less than during periods of low water discharge. During high flows, brown trout tended to use runs more and pools less. The break-up of river ice and accompanying flooding resulted in several fish being stranded on the floodplain. Backwater habitats appear to be important areas of refuge for riverine fishes during periods of flooding and ice break-up.     

Changes in Northwest Atlantic Arctic and Subarctic conditions and the growth response of Atlantic salmon

There has been a systematic change in the weight at age of Atlantic salmon (Salmo salar L.) in the Northwest Atlantic that is related to climate variability. This relationship emerged from analyses of broad-scale measures of ocean surface thermal habitat, which show that expansion of the area bounding 4–8°C is associated with greater growth. To further elucidate the effect of the environment on salmon growth, time series of sea surface temperature (SST), sea ice coverage, chlorophyll concentration, net primary production and zooplankton abundance were examined temporally and spatially in relation to changes in the weight of salmon. SST and zooplankton data were extracted from in situ analyses, whereas sea ice and chlorophyll-based measures of productivity were collected with satellite sensors. Salmon growth was found to be unrelated to productivity at the base of the food chain but highly associated with thermal regime during winter and spring. Warming conditions during specific segments of the salmon life cycle have been associated with poor adult recruitment; yet, warming during others is beneficial to salmon growth and is assumed to increase reproductive output of spawning fish. Despite these positive influences, climate change will continue to erode the viability of salmon populations while the negative effects of warming on survivorship outweigh the benefits of any increase in reproductive output related to growth.     

Spatial distribution of limited resources and local density regulation in juvenile Atlantic salmon

1. Spatial heterogeneity of resources may influence competition among individuals and thus have a fundamental role in shaping population dynamics and carrying capacity. In the present study, we identify shelter opportunities as a limiting resource for juvenile Atlantic salmon (Salmo salar L.). Experimental and field studies are combined in order to demonstrate how the spatial distribution of shelters may influence population dynamics on both within and among population scales. 2. In closed experimental streams, fish performance scaled negatively with decreasing shelter availability and increasing densities. In contrast, the fish in open stream channels dispersed according to shelter availability and performance of fish remaining in the streams did not depend on initial density or shelters. 3. The field study confirmed that spatial variation in densities of 1-year-old juveniles was governed both by initial recruit density and shelter availability. Strength of density-dependent population regulation, measured as carrying capacity, increased with decreasing number of shelters. 4. Nine rivers were surveyed for spatial variation in shelter availability and increased shelter heterogeneity tended to decrease maximum observed population size (measured using catch statistics of adult salmon as a proxy). 5. Our studies highlight the importance of small-scale within-population spatial structure in population dynamics and demonstrate that not only the absolute amount of limiting resources but also their spatial arrangement can be an important factor influencing population carrying capacity.     

Habitat use of Atlantic salmon Salmo salar parr in a dynamic winter environment: the influence of anchor‐ice dams

The effect of anchor‐ice dams on the physical habitat and behavioural responses of Atlantic salmon Salmo salar parr in a small, steep stream was investigated. Anchor‐ice dams formed periodically, leading to a dynamic winter environment as the study reach alternated between riffle and walk dominated habitat. Parr demonstrated large individual variation in habitat use, utilizing most of the wetted stream width, and were generally unaffected by diel changes in the mesohabitat composition. Furthermore, parr displayed high site fidelity in areas with low embedded substrata, and demonstrated few large movements between the three mesohabitat classes present: shallow riffle, walk and pool. Findings from this study question the importance of hydraulic variables such as water depth, flow velocity and dynamic ice formation as single habitat features for juvenile stream salmonids during winter and emphasize the importance of access to substratum cover.     

Effects of food availability on temporal activity patterns and growth of Atlantic salmon

1. Patterns of sheltering and activity are of fundamental importance in the ecology of animals and in determining interactions among predators and prey. Balancing decreased mortality risk when sheltering with increased feeding rate when exposed is believed to be a key determinant of diel patterns of sheltering in many animals. 2. Despite lower foraging efficiency at night than during the day, Atlantic salmon Salmo salar parr are nocturnal during winter and at low summer temperatures. Nocturnal activity also occurs at warm water temperatures during summer, but little is known about the functional significance of this behaviour. 3. This study aimed to determine: (1) the preferred activity and shelter pattern of Atlantic salmon parr during warm summer months, and (2) their response to variations in food availability when balancing growth rate (G) and mortality risk (M), as expressed through time out of shelter. We differentiated among four potential responses to reduced food availability: (1) no response; (2) G decreases but M remains constant; (3) G remains constant but M increases; and (4) G decreases and M increases. 4. Time exposed from shelter was inversely related to food availability. Fish subject to high food availability were significantly less active during the day than those with restricted rations. However, food availability had no significant effect on the extent to which fish were active at night. There was no evidence of variation in growth rate with food availability. 5. Salmon were predominantly nocturnal at high ration levels, consistent with their previously reported behaviour during winter. Rather than switching to diurnal behaviour at high temperatures per se, as previously was supposed, it appears that the fish are diurnal only to the extent needed to sustain a growth rate, and this extent depends on food availability. 6. Atlantic salmon parr modulate the amount of time they are active rather than growth when responding to variations in food availability over an order of magnitude.