Studying Atlantic salmon eggs in Canada’s freezing winter – the forgotten study season

by Michelle Lavery

[Reworked from a post on The Fisheries Blog and an article in Fisheries Magazine]

 For many hydrologic regimes of the world, streams and rivers are ice covered for the majority of the year, yet minimal research is conducted during this period compared with the more “researcher-friendly” open-water period. Without a doubt, scientific progress is hampered by the logistical difficulties and high cost associated with conducting “winter” research. (Prowse, 2001 (part II))

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[Credit: Michelle Lavery] A beautiful, -26°C or -15F° day on the Little Southwest Miramichi River

It seems as though every winter ecology paper contains some variant of this sentiment – we know that winter is important, but we’re not crazy enough to study it. As researchers, we’ve built sampling regimes that ignore an entire season because winter is considered harsh and unforgiving. It’s cold, sharp, and sometimes deadly to us, and so we operate under the assumption that the same goes for the creatures we study.

Alas, it is not so. There’s a lot going on under the snow, and even more going on under the ice. For example, Atlantic salmon eggs incubate in the gravel under river ice in Eastern Canada for six frigid, snowy months at water temperatures barely above freezing. They emerge from the gravel during the spring melt period, when ice jams bulldoze forests and water levels climb metres in minutes. These tiny fish are at the mercy of a dynamic and unpredictable season, yet we barely know anything about it.

As a pampered girl from ‘tropical’ Toronto, I never imagined myself riding a snowmobile and hacking through river ice in the middle of the woods. However, through a serendipitous connection, I found myself doing both – while pursuing a Masters degree supervised by Dr. Richard Cunjak at the Canadian Rivers Institute.

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[Credit: Michelle Lavery] Winter is a dynamic season – here a seeping plume of warm, long-residence groundwater melts a thin trail through thick surface ice on the Little Southwest Miramichi River in northern New Brunswick, Canada.

In Eastern Canada’s Miramichi River system, salmon eggs incubate in the gravel riverbed from late October to early May, during which time they experience highly variable winter conditions. In November, air temperatures can drop dramatically overnight (usually to about -20°C or -4°F), causing water to reach its freezing point quickly and inconsistently. As water crystallizes, it can stick to itself and the bottom of the river, forming anchor ice – a squishy carpet of ice crystals on the riverbed. If this ice forms on top of salmon nests (or “redds”), it can block water flow through the gravel and alter the temperature and oxygen levels surrounding the developing eggs.

Once full ice cover forms and precipitation is locked up in the snowpack, long-residence groundwater may be the major contributor to river discharge. “Long-residence” groundwater refers to water that has spent a considerable amount of time in an aquifer deep underground. Consequently, it is often warmer than the surface water in the winter, and can have significantly lower oxygen concentrations (since it has not been recently aerated). As this groundwater seeps through the river bed, the conditions in salmon redds can change dramatically. Depending on the size of the seep, eggs may develop faster due to warmer water temperatures and require more oxygen to sustain this accelerated rate of development. However, the oxygen-poor groundwater is usually unable to meet their biological demands. Without enough oxygen, these eggs may die or experience “sub-lethal” effects – consequences that impact their survival later in life as free-swimming fish. These may include stunted growth or developmental deformities that impair gas exchange, swimming ability, neurological function, etc.

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[Credit: Michelle Lavery] Studying an ecosystem during the season without snow only gives us half the story… There’s more going on under the ice than meets the eye!

During the spring melt period, silt and clay can be eroded into rivers by meltwater from the river banks. Depending on the grain size, these sediments may clog the egg membrane and prevent oxygen delivery to the embryo, effectively suffocating the fish. Furthermore, as ice breaks up and moves out of rivers, scour along the riverbed may significantly disturb the gravel and damage the embryos buried in the redds.

It is hard to believe, after considering all of the variation inherent in winter and its potential effects on one life stage of one species in one type of habitat, that winter goes largely unnoticed in the scientific literature. It is, certainly, a challenging season to research. I’ve had my fair share of winter mishaps, including digging a snowmobile out of a slush puddle for three hours, miscalculating ice thickness (not ideal!), hypothermic near-misses, and tethering myself to a tree during the spring melt. However, if we can get past our numb fingers and dripping noses, there’s a whole season waiting to be studied. One could argue that winter research is the last true frontier of freshwater ecology – there are so many unknowns to explore, and so many questions left unanswered. It might not be a “researcher-friendly” season, but it’s definitely exciting! Plus, who doesn’t love a good mid-river snowball fight?

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[Credit: Aaron Fraser] When fighting Jack Frost, it helps to have a great pair of neoprene chest waders and some GoreTex mitts. You can take or leave the camouflage…

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