It
is easy to think a large lake is like a
bigger version of a small one or a pond. It
is also easy to assume that the information
written about large American or European
lakes applies to the large loughs of
Ireland. Both assumptions are incorrect. We
are different here, mainly due to the fact we
have 2 to 3 times the wind speed of
continental countries, and we have more days
on which the wind is blowing than they do.
A large lake in Ireland (or any other
seaboard country with an oceanic climate) is
highly affected by the winds that sweep in
from the Atlantic ocean. The US and EU have
large still lakes with big seasonal
temperature swings, but ours are windswept
and comparatively wider and shallower and as a result there is considerable
water circulation and mixing caused by the
wind. the result of that mixing is that it reduces the
summer and winter temperature swings that
are present in large lakes in countries
with a continental climate.
So our water moves around our lough much
more, and because of this the various areas
of the lakes are less different in
temperature from each other. Our small
sheltered ponds are more like continental
ones, but our loughs need a period of
settled weather to develop continental
characteristics, and it is often only until
the next low pressure weather system
arrives.
What happens
when a wind gets up:
After blowing for an hour a fresh wind will
setup a current on the surface layer of a
lake flowing downwind at about 1/10th to 1/4
the windspeed.
So within an hour say of a 15mph / bft force
4 / 22kph wind, the lake surface becomes
effectively, a vast 5mph river which is flowing
in a downwind direction. It flows downwind
towards the wave lashed lee shore and
carries all the algae-plankton, tiny
organisms that eat them, surface hatching
flies, floating terrestrial insects, and the
small fish that feed on them. That lee shore
's where many of the bigger fish will go to
feed.
Take a look at this:
Just upwind off the lee shore there is a
magic place just before the water current
must dive vertically down and it then flows
back upwind along the lake bottom, making a
wide underwater "river" flowing upwind.
Here is a side view of the lake now:
All the surface organisms that were in the
surface layer find they have been carried
deeper underwater and don't want to be
there, so they swim upwards towards the
light. This takes them back into the upper
layer flowing downwind again, and back under
again. Soon there is a rotating "roller" of
food items built up in a band of water a
certain distance out from the dropoff
parallel to the shoreline of the downwind or
lee shore, the shore with the big waves.
Fish know all about this food accumulation.
What about the other end of the lake? You
can go upwind to the upwind or windward
shore which is sheltered and calm, at least
it is near the shore, especially where trees
block the wind making a calm zone near the
shore. As you move out it becomes a ripple,
and then becomes a wave as the wind picks up
strength the farther you go away down the
lake and away from shelter.
Anyway, the wind driven upwind sunken
current wells up in this zone. Bottom
dwelling insect food items like chironomid
pupae and larvae (bloodworms) get lifted up
to the surface here against their better
judgment, and get eaten by fish that wait
for them. Those that are not eaten dive down
to safety, but are carried upwind, and then
back to the surface by that upwelling
current. The fish will be waiting here. they
will be probably surface feeding, with a
feeding zone in the place where there is a
"flattening of the ripple" caused by the
upwelling flow.
As well as the above food supply, quantities
of terrestrial insects from trees and bushes
will be blown onto the water and get eaten.
There are two places to watch: the edge of
the ripple which is where the terrestrials
land, and the place at the shallows edge
where the upwelling occurs and midge pupae
and bloodworms are brought up.
In a stratified continental lake, (and a
sheltered smaller Irish lake) this zone
might be deoxygenated and easily dismissed.
But even trout have been proven to enter
both warm and deoxygenated water for easy
pickings, in conditions that are lethal to
them if they stay there. But they are
willing to do a snatch and grab and retreat
to the cooled airy ripple, then come back.
Coarse fish species require less dissolved
oxygen and can stay there to feed and cash
in on the bonanza.
In a diamond or square shape lake the
upwelling can be funnelled into a narrow
spot, and what a transient hotspot it is!
Perfect for fishing, only the calm or slight
ripple, and often on a day when you have the
lake to yourself, due to other anglers nor
fishing on account of "bad windy
conditions"!
There
is another feature to notice.
If there is an island, or headland sticking
out into the lake, the downwind surface flow
tries to get around it in exactly the way a
river's flow surges around a rock or piece
of riverbank sticking out into the flow. The
fish lie that is situated just downstream of
such a spot is replicated in bigger version
in the lake when this happens. If this lie
has 2 - 3 fish in the river, the lake could
have 20 - 100 feeding on this feature. Look
for "the edge of the flow", a long line of
white wave froth starting at the tip of the
headland and drifting downwind.
The smaller fish move in on a food source
and commence active feeding.
The predators move in and commence hunting the prey fish in these conditions
because everything they want is there, cover
from the sun provided by the waves,
oxygenated water (if it was in short supply
in summer/winter time), and prey that is
both concentrated and distracted.
So now let's look at that lake again.
Look
at where the food has been moved from,
and where it is now. And we know that where
the food is, the fish will turn up very soon
.... now it's a lot clearer where we should
be fishing, and the large areas where we can
expect less success. We can make an
intelligent guess as to what category
of flies or baits can be expected to be
eaten by the fish in each place.
A combination of wind, water flow, and tiny
food items trying to get back where they
belong creates two huge long rotating
accumulations of food.
I reckon on windy
days surface feeders go 50% downwind to the
lee shore, 30% upwind to the upwelling zone
and 10% to where terrestrials land beside
the upwelling zone, and the rest of the lake
is cleared out with the exception of minor
"holding areas" around reefs, headlands and
islands which exhibit mini versions of the wind
driven attractions mentioned above.
The
bottom current flowing upwind moves bottom species onto
these places, but other species go into the
windy zones to predate on the smaller fish
feeding there.
For bottom dwelling
fish think in terms of the bottom
zone as a flowing area,
and
raised reefs, humps or sunken islands act
like islands in a river, with a "vee" splitting the current on the downwind
side, a "joining" of the flow on the upwind
side, calm in-between, and a speeding of
current where it surges over these
underwater bumps. The food can be upstream,
to the side, or downstream depending on the
season.
In short, when these wind driven food and
fish concentrations occur, it's a time to
fish, if you can.
Wind Driven
Upwelling Lake Currents confirmed when
viewed from Satellite
NASA's
ASTER satellite website has satellite photos
showing upwelling currents driven by wind on
Lake Tahoe ( a large deep lake in the
US mid west) in their excellent article
here:
http://aster.jpl.nasa.gov/content/03_data/05_Application_Examples/hydrology/default.htm
.
Have a look at the thermal image right.
There is a strong south west wind blowing
bringing up a plume of cold water on the
left or west side and streaming it in a
downwind surface current to the right
eastern side. There it splits and spreads
both north and south along the shoreline
carrying nutrients and sediments, and fish
foods.
It shows up in the image as a blue
hammerhead shape visible against the east
shore within a bigger hammerhead of green.
The red coloured warm water has been swirled
into the north and south ends by the power
of the wind. Remember, there is a big
underwater sunken westward flow underneath
and flowing opposite to the visible eastward
surface flow shown in the satellite image.
The next
time you have a cup of tea in your hand,
raise it to your lips and blow across the
surface. Watch the current, see the micro
swirls at the current's edge, and watch the
tea leaves come up from underneath. Your
lake has certain similarities to this due to
the enormous force of hydraulics at work
when the top 6 feet of water starts moving.
Let's
not ignore when there is no wind. The lake
is still then? No it isn't!
The incoming rivers and feeder streams will
create a gentle drift or flow towards the
place on the lake where the outflowing river
leaves it. The shape of the lake matters
here. Once you understand how it goes in a
wind you can imagine very easily how it goes
in a calm.
The key point is that the inflowing streams
and rivers don't "mix in" with the lake
water. Due to temperature differences they
tend to retain their integrity and insert a
spine of current through the still lough
water. The stream can be on top of the lake
or along the lakebed, it depends on the
conditions. The ever present lake drift will
bend the river's path through the lake and
twist it towards a downwind direction, so
the interesting place where river water
meets trout holding depth moves as the
conditions change.
It looks a bit like this: There is one other wildcard. If there is a
groundwater spring under the lakebed, there
will be an inflow of fresh water in that
location, and a gentle flow towards the
outflow channel from there.
When confronted with a strong wind on a lake
I prefer to commence operations at the
downwind end.
If conditions are too turbulent as to be
dangerous, or prevent accurate presentation
of tackle, then it's the right place but
unfishable.
In that case I move to the upwind end for
the raised cooler (previously-was-under-the-thermocline)
water as it upwells outside the dropoff.
There is sometimes the possibility to try
fishing in both zones but only a little
distance apart, where an island lies between
two deeper zones.
Putting it to
work - A Specific Fishing Day:
On one small lake I have fished it was
possible one day to observe anglers at the
downwind shore catching fish through
binoculars. We were located at the upwind
end due to better presentation necessary for
inexperienced anglers (my son) made possible
by the reduced wind. We were catching too
but not as fast as the lee shore fishers.
An interesting phenomenon was also most
noticeable that day:
In between
the taking zones, a shallow retrieve drifted
downwind, but a deep retrieve drifted upwind.
No fish were caught midwater when I tested
that zone. The first 30 minutes showed that
the topwater takes were triple
the deep takes. Also we observed that the
frequency of takes increased as we
moved towards the upwind upwelling zone.
At the edge of the
ripple, terrestrial insects were
deposited by wind. In that place fish took
from underneath in the conventional way. The
fishing here was good, but not hectic.
Only fifteen feet
distance away in the upwelling zone fish
took by streaking fast across right under
the surface, making an exciting vee wake,
and incredibly hard and fast smash takes.
These fish were in a feeding frenzy, in
competition with each other, and their food
was being washed around swirling under the surface.
On one occasion I had a take where a fish
took off from about 12 feet away, hit the
fly lifting the line tight with a ripping
sound as the line pulled free from the water
surface at breaking strain, and overshot by
another 6 feet from sheer momentum before
coming to a halt with the hook in his jaw.
The upwelling zone had shallower faster takes than
the adjacent edge of ripple nearby (where
windborne terrestrial insects were gently touching down).
This seemed odd at first, until we figured out that
the upwelling was so fast that the fish had
all followed food up from deeper, and just
not got time to go back down before spotting
the next food item. So they were within a
foot or two of the top feeding hard as the
food came to them in a swirling flow. The
food items were swirling around just under
the top like rice grains in a boiling pot. If
the fish didn't move fast it got away, or
another fish would have it. By comparison,
in the terrestrial touchdown zone at the
ripple's edge, the fish came up from below
and quietly sipped in the trapped insects.
I didn't get to fish the lee shore that day.
If I had, it would definitely have been very good.
The two anglers there anchored a boat just
off the lee shore and fished into the waves
for 8 fish each. On the upwind shore I had 5
in an hour less, a comparable result given I
didn't know their skill level, and was also
instructing a beginner, who also caught his
first fish and had plenty of action.
The anglers in the places in-between were
not catching all that much.
Oh one last thing, it's a mixed fishery: the
terrestrial landing zone produced all
browns, but the fish we caught in the
upwelling were mainly rainbows ... on that
particular day.
Here is another thought: if I had been
fishing on a deep European or US lake this
day, I might have caught little due to deep
deoxygenated water from below the
thermocline coming to the top in this
location as the wind tilted the thermocline.
In such a lake in such a continental country
the downwind side would have been a lot
better than the upwind side. But in Ireland
both were good places to be. A windier
country and a shallower lake makes a
difference. I will do an article to cover
the deep lakes with persistent thermoclines
later in this series because things are a
lot different on those places.
The fish had separated into three
behavioural crowds depending on which food
source they wanted to use. The wind driven lake
currents provided a 6 - 8mph water flow
carrying
two food sources one along the surface, and the
other along the bottom and coming up to the
surface. Aerial windfalls provided the third
source of food this day. The breeze
accumulated food in three places, if you do
not count the islands as mini places of the
same type.
So we now have a means of locating a
considerable number of the feeding fish in a
large lake measuring many miles long and
wide, before we leave the house.
More later.
Norm
This is the second of
The Reading Large Lake Series, articles on How to Study and
Read Large Irish Loughs & Large Waters
written and published in the FishingTalkIreland.com Forum
In a diamond or square shape lake the
upwelling can be funnelled into a narrow
spot, and what a transient hotspot it is!
Perfect for fishing, only the calm or slight
ripple, and often on a day when you have the
lake to yourself, due to other anglers nor
fishing on account of "bad windy
conditions"!
A combination of wind, water flow, and tiny
food items trying to get back where they
belong creates two huge long rotating
accumulations of food.
There is one other wildcard. If there is a
groundwater spring under the lakebed, there
will be an inflow of fresh water in that
location, and a gentle flow towards the
outflow channel from there.