Overwintering Atlantic salmon: How do they even make it through?


Working on the river in the winter can be a little chilly, sometimes dangerous, and ice formation limits the range of work. This area was one of the only open sections of water in this stream.

As I sit here and type these words, the temperature outside is a couple degrees below 0 (Fahrenheit) and the wind is blowing a gale.  As cloud cover abates and the wind continues to blow, the chances of warmer temperatures tonight are about the same as the reading on my outdoor thermometer (less than zero…).  I add a couple logs to the fire that has been burning in the woodstove since Thanksgiving, freshen up my cup of tea and warm last night’s remaining chowder to get me through what we all hope will be the coldest night of the winter.  On cold nights like these, I often wonder how our faunal friends are making it through.  Specifically, given my significant bias, how the many thousand young salmon we recently stocked and their fellow community members are making it. 

The winter stream environment is a very dynamic setting – frazil ice formations can change the stream channel completely, or turn a pool into a fast moving river (Cunjak 2013); Ice anchors to substrate, moving it when it drifts away; cold temperatures and changes in day light intensity and duration change the behavior and eating habits of fish (Watz 2013).  One obvious problem when it comes to monitoring overwintering habits of fish species is that significant portions of the watershed become covered in ice.  Access to the river may also be challenging due to snow accumulation.  On top of all of this, it is just plain uncomfortable doing in-stream field work in the winter, and can even become dangerous.  Despite all of this, it is important to understand fish behavior through the winter and how their habitat, and their use of it, changes so we can make more educated decisions when it comes to management and restoration.    

In the article “How Atlantic salmon over winter in the river: an ice strategy” by Rick Cunjak, Biologists observes post-spawn adult Atlantic salmon (kelts) moving out of the river after spawning in the fall to the head of tide, a distance, in some cases, of nearly 45 miles.  While the majority of the kelts observed in this study moved out of the river to head of tide, some kelts were observed overwintering in the river; however, this environment is very dynamic and unpredictable, making it difficult for kelts to find adequate pools to overwinter in.  In some watersheds it is very likely that kelts are using lakes to overwinter – in fact people catch the kelt through the ice while ice fishing.  An interesting finding in this research was the observation that 80% of the movement to overwintering areas was made directly after spawning, before ice-up. 


Lower mainstem of the East Machias River after a cold night which no doubt caused a lot of frazil ice to form.

For the little guys, the juvenile salmon that stay in the river, their options as to where they go in the winter may be a little more limited.  Overwinter survival for these juveniles can greatly depend on ice formation, or lack thereof.  As a moving stream or river begins to ice over, it seems to be an ever changing environment.  Until the majority of the stream surface ices over creating cover ice, frazil ice, ice forming on particles in the water column creating a kind of slushy, can accumulate and cause a stream to dam and completely divert (check out this awesome video on frazil ice formation in Yosemite National Park!).  This can have immediate effects on fish species by displacing them and/or altering the physical habitat of the stream.  In the absence of cover ice, anchor ice may also form.  Anchor ice forms on the substrate (rocks and other debris) on the stream bottom, filling all of the interstitial space that is created by the substrate.  Many fish species call these interstitial spaces home, using them to avoid predators and stay out of the main flow of the stream channel, and may suffer greatly if these spaces are not available.  The impact that these two ice formations have likely depends on the weather.  If there is a mild winter where cover ice breaks up often or never completely establishes, frazil ice and anchor ice may form more frequently and survival of juveniles residing in the stream may decline.  However, if the winter is cold and cover ice forms (and remains through the winter), it can stabilize the stream environment and survival will likely increase (Watz, 2013).   


Anchor ice forming among the substrate on the river bottom.

Fish behavior changes as the temperatures drop in the winter as well.  Salmon are ectothermic, they rely on their surrounding environment to provide them with warmth or cool them down.  As temperatures drop, so does their metabolic rate.  They move very little to conserve their energy and the less energy they use in predator avoidance the better.  Juvenile salmon have been observed feeding mainly at night, and though their feeding success at night is low, the energy they save not having to avoid predators makes up for this.  The presence of cover ice may decrease predation on juvenile salmon which may correlate to an increase in over winter survival (Finstad et.al.2013, Watz 2013).   

Our rivers and streams are a dynamic, ever changing environment.  They move over time, by design – constantly scouring new bends, creating new side channels, forming pools for any number of species from fish to amphibians to macroinvertebrates to thrive.  They are almost a living thing, the arteries of our ecosystems, moving life from the headwater streams to the ocean, and as everything locks up in the winter, the rivers and streams stay alive.  As I sit in my cozy living room, keeping warm with my wood heat and tea, I can at least take some comfort in knowing that as long as the winter stays cold, as long as the cover ice forms as it should and remains through the coldest parts of the year, our fish friends actually stand a better chance of surviving – it is a lack of ice, more frequent thaws, and a decreasing snow pack that we need to be wary of!


     Cunjak, R. How Atlantic salmon overwinter in rivers: an ice strategy.(2013)  Miramichi River Association http://www.miramichisalmon.ca/how-atlantic-salmon-overwinter-in-rivers/

     Finstad, A.G.;Fiske, P; Hedger, R.D.; Naesje, T.F.; Ugedal, O; Thorstad, E.B. (2013),  Ice-dependent winter survival of juvenile Atlantic salmon.  Ecol Evol. 2013 March; 3(3): 523–535

     Watz, Johan; Licentiate Thesis, Karlstad University Studies, 2013

Winter behaviour of stream salmonids: effects of temperature, light, and ice cover