How Underwater Currents Affect Great Lakes Salmon Fishing

Surface wind and waves are easy to see.

The water moving beneath the boat is not.

That makes underwater currents one of the easier Great Lakes fishing variables to overlook. They can move warm and cold water, change the shape of the thermocline, reposition baitfish and create very different conditions at your lure than you see at the surface.

The important point is not that salmon automatically seek the strongest current.

It is that currents continually rearrange the habitat salmon use.

Great Lakes water is always moving

The Great Lakes are not giant tubs of stationary water.

Wind, differences in water density, inflows and outflows, shoreline shape and basin-scale circulation all contribute to water movement.

NOAA uses hydrodynamic models to forecast Great Lakes current speed and direction together with water temperature, water levels and other conditions.

That combination matters to anglers because current and temperature structure are closely linked.

Current can move the thermocline

During summer stratification, the warm upper layer and cold lower layer can move differently.

Wind-driven circulation can push the thermocline upward in one area and downward in another.

Near shore, this can result in upwelling or downwelling. Offshore, it can create slopes and undulations in the temperature structure that would be invisible if you looked only at surface temperature.

The practical consequence is simple:

The thermocline is a three-dimensional moving surface, not a fixed horizontal line across the lake.

A 60-foot thermocline at one location does not guarantee a 60-foot thermocline five miles away.

Surface speed is not lure speed

Currents also matter because your lure is moving through water, not across a GPS screen.

A boat traveling at a steady speed over ground can have a different speed through the water depending on current direction and strength.

The difference can become even more complicated when deeper water is moving differently than the surface.

For a trolling angler, that can affect lure action and blowback on downriggers.

This is why down-speed information can sometimes tell you something the GPS cannot.

Current can reposition bait

The more important biological effect is often indirect.

Salmon are predators.

Where their forage goes matters enormously.

Alewife distribution in Lake Ontario has been shown to depend strongly on temperature and zooplankton distribution. During summer research, alewife schools occupied the bottom of the mixed layer during the day and many fish moved upward toward the surface around dusk as their prey moved.

If moving water changes the position of temperature boundaries or plankton-rich water, baitfish can respond.

Salmon can then respond to the baitfish.

That is why the best fishing depth does not always match a textbook preferred-temperature number.

Watch for changes, not just absolute current speed

For anglers, the most valuable current information is often a change.

Examples include:

A current boundary where two water masses meet.

A sudden temperature change while trolling at the same depth.

Bait concentrated along one side of a temperature transition.

A sharp change in downrigger blowback.

A productive trolling direction that becomes noticeably different when you turn around.

None of those automatically proves you have found a current seam containing salmon.

Together, however, they can tell you the water is not uniform.

That is usually worth investigating.

Current direction can affect the trolling spread

Suppose you troll north and every lure has good action.

You turn south and the downrigger cables suddenly angle differently, divers pull differently and your speed probe shows a substantial change even though GPS speed is almost identical.

You have learned something about the water.

Rather than forcing the same boat speed in both directions, adjust your speed through the water to maintain the lure action you want.

If one trolling direction consistently produces strikes, current may be one reason.

It may not be because the salmon "prefer trolling into the current." More likely, your presentation, bait position or both are different on that direction.

Currents and structure

Points, drop-offs, islands and shoreline bends can interact with moving water.

The exact patterns vary enormously among the Great Lakes, so simple rules should be avoided.

Instead, use structure as a place to investigate whether moving water is producing a useful combination of temperature, bait and predator marks.

A point with no bait is still just a point.

A point where a moving temperature boundary intersects concentrated bait is much more interesting.

Forecast first, verify on the boat

NOAA's Great Lakes forecast systems provide modeled currents and water temperatures, and these can be useful for deciding where major changes may occur.

But a current forecast should not replace what you see on the boat.

Use it to form a hypothesis.

Then test that hypothesis with sonar, probe temperature, lure speed and fish activity.

If the forecast suggests a current boundary in an area, go look for the associated bait or temperature change.

If your electronics show something different, fish the conditions you actually have.

The useful way to think about current

Do not ask only:

Where is the strongest current?

Ask:

What has the current done to the water I want to fish?

Has it pushed the thermocline deeper?

Raised cold water?

Moved bait?

Created a temperature boundary?

Changed lure speed at depth?

Those questions connect physical water movement to what actually matters to a salmon angler.

Sources

• NOAA GLERL Great Lakes current and temperature data.
• NOAA Next-Generation Great Lakes Coastal Forecasting System.
• Lake Ontario alewife vertical-distribution research.