Great Lakes Upwelling and Downwelling: What They Mean for Salmon Fishing

A Great Lakes angler can find perfect temperature and bait one day, return to the same water after a wind event, and discover that everything has changed.

The fish may be 30 feet deeper. Cold water may suddenly appear close to shore. A productive waypoint may show warm water all the way down through the depth where you caught salmon the day before.

Upwelling and downwelling are two major reasons this happens.

What is upwelling?

During summer, the Great Lakes normally develop a warm upper layer and a colder deep layer separated by the thermocline.

When persistent wind moves warm surface water away from a shoreline, colder deep water rises to replace it. This is called upwelling.

NOAA describes Great Lakes coastal upwelling as a process in which persistent winds move warm water away from the coast and cold bottom water moves upward toward the surface.

For anglers, this can dramatically change the fishing conditions without changing the bottom depth.

Water that was warm yesterday may suddenly be cold.

What is downwelling?

Downwelling produces the opposite effect.

Wind and circulation can push warm surface water toward a shoreline. That warm water accumulates and forces the colder layer and thermocline deeper.

A temperature that had been available at 50 feet might now require considerably more depth.

This is one reason fixed fishing depths are unreliable on the Great Lakes. You are not simply fishing over a bottom contour. You are fishing within a moving three-dimensional body of water.

Why salmon anglers should care

Salmon and trout do not respond to depth alone.

They respond to the temperature, forage and other habitat conditions present at that depth.

If an upwelling event raises the thermocline 20 feet, the productive fish zone may also become shallower.

If downwelling pushes cold water deeper, salmon and bait may follow it down.

The important word is "may." Predator location also depends on forage, light, season and species.

The temperature structure gives you the habitat. Your sonar tells you whether the fish are actually using it.

Upwelling can create surprising shallow-water opportunities

Upwelling can be particularly important close to shore.

Imagine a shoreline where 40 feet of water has become too warm for cold-water salmonids. A strong upwelling event can replace much of that warm nearshore water with considerably colder water.

Suddenly, suitable habitat can extend much closer to shore.

This can affect Chinook, lake trout and brown trout differently, but all three can respond to the changed temperature structure.

Brown trout are particularly worth watching because research in Lake Ontario found a strong summer association between brown trout and the nearshore thermocline.

Downwelling can remove productive water from shallow structure

The reverse can happen during downwelling.

Warm water can deepen along the shoreline and push the temperature break below the depth of otherwise productive structure.

You may still be trolling over the same point, ledge or contour, but the water occupying that structure has changed.

That distinction matters.

Structure does not disappear, but its suitability for cold-water fish can.

Do not memorize one wind-direction rule

A common mistake is turning upwelling into a universal rule such as:

"A west wind brings cold water."

That might work for a particular stretch of shoreline under particular conditions, but it is not a Great Lakes-wide rule.

The effect of wind depends on shoreline orientation, lake shape, wind direction, duration and the existing water structure.

A wind that moves surface water offshore on one side of a lake can push water toward shore somewhere else.

Instead of memorizing compass directions, ask a better question:

Did the wind move the warm surface layer toward this shoreline or away from it?

Then confirm the result with temperature information.

The effects can continue after the wind changes

The water does not necessarily stop moving when the surface becomes calm.

Wind can displace the thermocline and set up internal oscillations within a stratified lake. Research has documented fish responding to vertical thermocline movements associated with these internal waves on timescales of hours to days.

That helps explain why your probe temperature can continue changing after the obvious weather event is over.

How to fish after a major wind event

Do not begin by assuming yesterday's program still applies.

Start by finding the temperature structure again.

Compare the forecast thermocline with what your sonar and temperature probe show. Watch how quickly temperature changes as you travel alongshore and offshore.

Then find bait.

If cold water has moved dramatically shallower, search the new edge rather than immediately returning to your old waypoint. If warm water has been pushed down, be prepared to search deeper or move away from the affected shoreline.

The goal is not to predict every fish movement from the wind.

The goal is to recognize when the water itself has moved.

Once you understand that, a fishing spot becomes more than a latitude and longitude.

It becomes a particular combination of depth, temperature, forage and moving water.

Sources

• NOAA Great Lakes Environmental Research Laboratory, Water Temperature FAQ.
• Research on thermocline oscillations and fish distribution.
• Lake Ontario brown trout habitat research.