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CLIMATE CHANGE
PUTTING LAKE TROUT AT RISK
By
Steve Lawrence, OMNR Sr. Fish & Wildlife Technician |
- Climate change and
global warming are constantly in the news these days.
Melting polar ice, rising ocean water levels, drought,
increase in severe weather like hurricanes and tornadoes
seem to be everywhere in the news. These effects are not
something we can ignore, as we found out last August
when a rash of tornadoes ripped through the area. We
can’t help but notice horrifying events like that but
the effects of climate change are not only dramatic,
they are also subtle but no less deadly.
- Our
environment has been finely tuned by many thousands of
years of gradual change. When you throw a monkey wrench
into the system like rapid global warming caused by
human excess and abuse, our fish and wildlife suffer.
Creatures adapted to live in extreme environments in
many cases are most susceptible. Polar bears and lake
trout, adapted to live in extremely cold environments
become the virtual “canary in the coal mine.”
- Lake trout are a species of the ice age and their life
history is dependent on pristine, cold water
environments. In this area which is considered the
southern portion of their range, lake trout exist
tenuously even under normal climatic conditions.
Increasing temperatures in the last 25 years are
negatively affecting lake trout habitat and biological
processes.
- Global warming affects lake trout
in two subtle but significant ways. Increased heat units
pumped into our local lake trout lakes can obviously
reduce cold water habitat but warmer water can also
disrupt the basic reproductive cycle of the species by
reducing survival and recruitment of young fish. Lake
trout have adapted to spawning around mid October.
Historically the water temperatures at this time of year
were 10°C (50°F). Today when lake trout spawn in mid
October the water temperature in some lakes is as high
as 14°C (57°F). So why don’t lake trout just spawn later
in the fall? It would seem that the lake trout is hard
wired to spawn at the same time regardless of water
temperature and consequently this could be their
evolutionary downfall.
- Why could a minor
change of 4 degrees be significant? In the perfect lake
trout world, eggs are deposited in mid October, develop
slowly over the course of the winter and hatch in early
spring. Lake trout egg development is determined by the
amount of heat units they receive from the moment they
are laid. Research has shown that it takes 490
accumulated degree-days until the eggs hatch into sac
fry. The sac fry live in a semi-dormant state for
approximately 6 weeks in the substrate as they absorb
the energy stored in their yolk sac. When the yolk sac
is used up the fry swim up from the shoals and they must
actively feed or starve to death.
- Historically, under normal water temperature conditions,
lake trout fry hatch in February or early March and swim
up in late March and early April when the lakes are
starting to break up. Food supplies at this time of year
are abundant. However, increase the water temperature by
just 2 degrees and the effects on survival are drastic.
This slight increase in water temperature during egg
incubation results in the lake trout hatching
significantly earlier. Under these conditions the lake
trout will hatch from late December into January, up to
4 to 6 weeks earlier then normal. The sac fry then
absorb their yolk sac reserves, swim up in February into
a sterile environment and consequently starve to death.
Water temperature observations between the 1980’s and
today indicate that many of our local lake trout
fisheries may not have produced many successful year
classes in the last 25 years.
- Some lakes are
more susceptible to heat retention then others. Lakes
with a small surface area to water volume ratio are most
affected. Our smaller, deeper lakes suffer the most from
these effects. As our warmer climate pumps heat into
these lakes they accumulate more and more heat units and
in effect become huge thermos bottles of heated water.
The thermal inertia that builds in the lakes is very
hard to remove and it can take several consecutive below
average temperature seasons to develop conditions
suitable for successful lake trout recruitment.
- It’s not all bad news for our lake trout
lakes. Some of our lakes are doing just fine, but these
lakes generally have large areas of shallower water that
cool rapidly in the fall. If the wind keeps these lakes
open later in the fall because of their size even more
heat is removed. For many years Fisheries Managers have
accused anglers, who would complain about poor fishing,
of just catching too many lake trout but if the truth
were known we would all have to take some of the blame
for the loss of our valuable lake trout resources
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