Despite the common view that conditions in winter strongly influence survival and population size of fish, the ecology of salmonids has not been as extensively studied in winter as in other seasons. In this paper, we review the latest studies on salmonid winter survival, habitat use, movement and biotic interactions as they relate to the prevailing physical and habitat conditions in rivers and streams. The majority of research conducted on the winter ecology of salmonids has been carried out in small rivers and streams, where temperatures are above zero and where there is no ice. Investigations in large rivers, regulated and dredged rivers, and under conditions of different ice formations are almost totally lacking, presumably related to sampling difficulties with these systems. The studies-at-hand indicate that a multitude of physical and biological factors affect the survival, behavior, and habitat use of salmonids in winter. The general concept that winter functions as a critical period for the survival of young salmonids is not well supported by the literature. Instead, overwinter survival of juvenile fish appears to be context-dependent, related to specific habitat characteristics and ice regimes of streams. In general, over wintering salmonids prefer sheltered, low velocity microhabitats, are mainly nocturnal, and interact relatively little with conspecifics or interspecifics. Specific descriptions of microhabitat preferences of salmonids are difficult to make due to highly disparate results from the literature. We suggest that future research should be directed towards (1) being able to predict the dynamics of freezing and ice processes at different scales, especially at the local scale, (2) studying fish behavior, habitat use and preference under partial and full ice cover, (3) evaluating the impacts of man-induced environmental modifications (e. g. flow regulation, land-use activities) on the ecology of salmonids in winter, and (4) identifying methods to model and assess winter habitat conditions for salmonids.

In insular Newfoundland, Canada, studies were conducted from 1999 to 2003 on the effects of 'simulated' hydropeaking power generation on juvenile Atlantic salmon (Salmo salar). In 1999, Atlantic salmon parr were released into an experimental reach below a hydroelectric facility and flow was manipulated over a range of discharge (1.0-4.2 m 3 s -1 ) during a series of 'experiments' simulating hydropeaking in both summer and fall. Fish were implanted with radio transmitters, manually tracked, precisely located (±1 m), habitat selection evaluated, and movement response determined. Experiments were continued in 2002 and 2003 to contrast response of salmon between summer and winter, the magnitude of flow changes were greater (0.7-5.2 m 3 s -1 ) and changes were made more rapidly (instantly). As discharge was increased, velocity and depth use by parr increased, and fish adapted behaviourally by increased contact with the substrate. Salmon parr also exhibited two distinct movement patterns in the summer and fall of 1999 studies; high site fidelity or considerable movement during trials. Salmon were more mobile during both static and dynamic flow conditions and throughout the diel cycle in the summer of 2002 experiment, and 2 fish were stranded and died, the only time this happened in the four series of experiments. Within each experiment generally there were no differences between movements at static high and low flows for day and night movements, with one exception, and night time movements were always greatest, again with one exception. During dynamic flow changes, within each experiment, distances moved during down ramping and up ramping were not significantly different except in the summer of 1999. Overall, comparing between experiments for up and down ramping events, distances moved in the summer of 2002 were statistically higher than for all other experiments. Not surprisingly, the home ranges of fish in the summer of 2002 were also the greatest while the smallest home ranges were in the winter of 2003. Results suggest hydropeaking regimes may be energetically costly potentially affecting over-winter survival which is related to energy reserves obtained during summer. Collectively these studies provide comprehensive information on the response of juvenile Atlantic salmon parr to hydropeaking, on both diel and seasonal scales, and will assist hydro producers and regulators design and operate hydropeaking regimes to minimize ecological impact.

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In-stream habitat enhancement is a common remedial action in rivers where degradation/lack of suitable fish habitat can be diagnosed. However, post-project monitoring to assess the response of the biota to modification is rare particularly during winter. We conducted in situ monitoring during the winters of [2004][2005][2006] in the regulated Dalåa River, central Norway, in order to determine if winter habitat requirements of Atlantic salmon (Salmo salar L.) parr were realized in an enhanced (substrate and mesohabitat modification) reach. In total, 140 parr were marked with passive integrated transponder (PIT) tags and the fish were followed by carrying out active tracking surveys under variable ice conditions throughout the winter. Highest emigration (44%) occurred before ice formation started. Emigration was reduced after ice formed and was largely offset by parr re-entering the enhanced area. Dispersal into the non-enhanced, small substrate control area was observed only when the study reach was ice covered, and no parr were subsequently encountered in the control section after ice had melted. In the enhanced area, declining water temperature and surface ice conditions did not affect the spatial distribution of the resident salmon parr at the studied scale. Areas with 'solid' anchor ice precluded access for salmon parr whilst areas with 'patchy' anchor were used throughout the winter. Our results indicate that surface ice creates conditions that allow salmon parr to use stream habitats that otherwise provide only a limited amount of in-stream cover. Ice processes should be taken into consideration when habitat enhancement projects are carried out and subsequently assessed for effectiveness.

Abstract:In northern steep streams anchor ice is commonly observed during winter, and plays a key role when considering in-stream conditions. The understanding, however, of the nature of anchor ice formation is less understood, in particular, under natural conditions. In the following, observations of anchor ice formation in three stream environments with different physical characteristics are presented. Results demonstrate that anchor ice not only form in riffle areas, but also in shallow and slow running stream sections. No linkage between spatial distribution of anchor ice and calculated dimensionless numbers (Froude and Reynolds number) was found. Furthermore, analyses on growth and density showed that anchor ice may be distinguished by two main types. (1) Type I: Lower density forming on top of substrata. (2) Type II: Higher density forming between the substrata filling interstitial spaces. Distribution of anchor ice Types I and II suggests a relation between intensity of turbulence expressed by the Reynolds number, growth pattern and density. As anchor ice has both physical and biological implications on in-stream environments, findings from the present study may be of particular interest to cold region freshwater stream management.

During winter, different types of ice formation are commonly observed in northern boreal stream systems. Although largely overlooked today, river ice has profound effects on in-stream hydraulics and therefore ice processes should be considered in freshwater stream management and assessment. In particular, limited knowledge exists about the impacts of dynamic ice formation on stream environments. Results presented from the changes of in-stream heterogeneity in three steep stream environments caused by dynamic ice formation demonstrate that the formation of anchor ice and anchor ice dams may induce significant backwater effects by increasing wetted areas (maximum 43%) and water depths (maximum 241%) and reducing water velocities (maximum 70%); independent of minimal changes in discharge. Consequently, stream environments are transformed from fast-flow to slow-flow areas, even on a short temporal scale (<12 h). Furthermore, the anchor ice build-up initiated static (surface) ice formation due to reduced local water velocities upstream ice dams. Thus, dynamic ice formation plays a key role in the balanced ice regime in steep stream environments and contributes largely to stable static ice cover in these environments. Observations from the present study suggest that the current paradigm emphasizing the role of discharge as the main controller of in-stream heterogeneity may call for a modification in steep streams that experience seasonal ice formation. This is particularly important if future hydraulic-/habitat models and assessment tools are to be implemented in freshwater management to realistically characterize steep stream environments in cold climate regions on a seasonal scale.

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