t this
time of year most salmon anglers and river enthusiasts are putting away
their rods and canoes for the winter and dreaming about next summer.
However, the Atlantic salmon are still present in our rivers. Having
just completed spawning following an exhausting trip that, in some
cases, began off west Greenland, these marvelous animals are now
preparing to overwinter beneath the ice that covers many of New
Brunswick’s rivers for four to five months. How do they do it? How do
they survive?
These were the questions that stimulated our winter research of
salmon behaviour and their river environment. Surprisingly, despite the
large amount of work done on salmon biology, our understanding of their
behaviour in winter is embarrassingly poor. And yet, for much of its
range in Canada, winter is the predominant season and generally
considered to be critical to survival. Therefore, in order to properly
conserve this valuable resource, it was felt that we needed to address
basic biological questions such as; what are the salmon doing during
winter, and what are their habitat needs?
Working with a team of biologists and hydrologists from the
Department of Fisheries & Oceans (Moncton) and the New Brunswick
Department of the Environment (Fredericton), we spent parts of four
winters monitoring the movements of forty-two post-spawned Atlantic
salmon, or kelts (the more appropriate term for this life-stage). The
research was carried out on the three main branches of the Miramichi
River (the Northwest, Southwest, and Little Southwest Miramichi), and
tracking covered an area from below head-of-tide and as far as 50km
upriver.
The first step was to characterize the winter environment in our
rivers. Before we could identify what habitats salmon preferred in
winter, it was necessary to determine how comparable that river
environment was relative to what we typically measure and see in summer.
Ice surveys carried out in six large main river pools indicated that the
ice regime was extremely variable between years and between pools. In
our rivers, especially in early winter before a complete surface ice
cover forms, another type of ice - frazil ice - forms on cold clear
nights, on the bottom of the riverbed, usually in fast-flowing shallow
reaches of the river. This sticky, slush-like ice often floats downriver
during the day and accumulates as a hanging dam beneath a surface ice
cover. Such surface ice more typically forms in pools. The result is
that many main river pools in the Miramichi accumulate large masses of
frazil ice beneath their surface ice cover.
The problem, from a fish’ perspective, is that this ice mass that
quickly takes up much of the volume of a pool is also relatively
impermeable to water flow. A simple equation of water flow indicates
that its value is a function of the area through which it passes times
the velocity. In the frazil-filled pool, the reduction in available area
for the water to pass through must be compensated by an increase in
velocity. The result is that water flow beneath the hanging dam
(remember, ice floats) and along pool bottom is extremely rapid. In some
of our surveys, 80% of the pool volume was filled with frazil, even in a
pool with a maximum water depth of 9.5 metres! In some cases, large deep
pools like the one at Big Hole on the Northwest (max. depth = 9.5m)
provided less available space than smaller pools with a maximum depth of
only three metres.
Compare this condition with the summer when many salmon accumulate in
these deep pools, easily holding position in the near static water flow.
In winter, one of our divers who managed to "burrow" through
the frazil mass was unable to withstand the torrent of water raging
along the pool bottom. Salmon, like many fishes in winter, are
attempting to conserve energy. As cold-blooded animals, their metabolism
is dictated by water temperature and they feed very little. In fact,
adult salmon do not feed at all upon entering freshwater to spawn.
Until now, all their energy reserves have been used to reach the
spawning grounds and reproduce. In order to survive the winter, they
must conserve the little reserves they have left. Therefore, pool
bottoms beneath hanging dams are probably one of the worst places to
overwinter. This underlines the importance of carrying out research in
all seasons when assessing habitat suitability. Extrapolating from
ice-free periods (e.g., summer) would lead to erroneous descriptions of
winter habitat.
So how do the salmon react? Where do they overwinter? Salmon kelts,
both grilse and large multi-sea-winter (MSW) salmon, were captured in
the late autumn prior to freeze-up, fitted with radio-tags and released
back into the pools where they were captured. The entire procedure took
less than 15 minutes per fish. Each salmon had a unique radio frequency,
and tracking began immediately after release and continued on a weekly
basis for each of three winters. Tracking was accomplished with a
hand-held antenna from land, roadways near the river, canoe, or
snowmobile.
The results confirmed our predictions regarding the unsuitability of
frazil-laden pools. Salmon kelts avoided overwintering in these pools.
Instead, the majority of kelts spent much of their winters below the
head of tide. The situation was the same for salmon farthest from the
head-of-tide (50 km) as for those tagged much farther downriver (less
than 5km from tide head). More interesting was the finding that movement
to these locations occurred before a complete ice cover had formed on
the river. Maximum distances were recorded at this time with relatively
little movement for the remainder of the winter. For example, one salmon
kelt moved a net distance downriver of seventy-four kilometres by the
spring (May), with greater than 80% of this movement happening before
December. It seemed that movement to wintering areas of the river was
being made before freeze-up and ice accumulation, perhaps because
movement thereafter would be limited by ice. During mid- and
late-winter, kelts moved little - whether in the river proper or below
the head of tide. Average movements during these periods was generally
less than three kilometres, compared with ten times that distance in
early winter.
Not all kelts moved to below the head of tide. Some kelts remained in
the river. These fish seemed to prefer locations near tributary mouths,
in the lee of islands, or in "bogans" - backwaters with
negligible flow, but also with negligible frazil accumulation. These
bogans are rarely used by adult salmon in summer but may indicate a
seasonal habitat value previously not realized.
So what have we learned so far? Winter habitat suitability cannot
necessarily be presumed based on habitat preferences at other times of
year, or by the maximum depth of a pool. Salmon kelts can travel long
distances, after spawning, to wintering areas, and many such areas are
below the head of tide. Possibly, the tidal action precludes frazil ice
accumulation in these habitats. No one knows. Indeed, there is still
much to be learned about these animals and the other fishes that inhabit
our rivers, and how they survive the winter. Such information is
critical to conservation.
