Great Lakes Thermal Fronts: Why Temperature Boundaries Can Concentrate Salmon

The thermocline is not the only temperature boundary Great Lakes anglers should understand.

Sometimes the more important feature is a thermal front.

A thermal front separates different water masses horizontally. Instead of a rapid temperature change as you lower a probe through the water column, you encounter the change as you travel across the lake.

These boundaries can concentrate fishing activity in a surprisingly small area.

Thermocline versus thermal front

A summer thermocline is primarily a vertical temperature transition.

Warm water is above.

Cold water is below.

A thermal front is generally encountered as you move horizontally from one water mass into another.

In spring, for example, shallow nearshore water can warm much faster than the offshore lake.

The boundary between those water masses can form a front.

The Great Lakes develop several kinds of temperature fronts, and their location changes seasonally. Satellite research has documented extensive spring frontal activity throughout the lakes.

Why fronts matter to salmon

A temperature boundary can bring together several things predators need.

Suitable temperature may exist on one side.

Forage may collect along or near the boundary.

Water masses with different plankton communities or clarity can meet.

Predators can then exploit the resulting transition zone.

That does not mean every visible temperature line contains fish.

But it gives you a feature worth searching.

Lake Ontario research found more strikes in fronts

A classic Lake Ontario trolling study compared fishing in frontal and nonfrontal water.

Only about 20% of trolling time occurred in thermal fronts.

Yet 35% of strikes and 37% of landed fish occurred there.

Overall salmonine catch per unit effort was higher in frontal water than in nonfrontal water.

That is a meaningful difference.

It suggests anglers can benefit from identifying the boundary rather than simply trolling across large areas of uniform water.

Coho showed a particularly useful response

The same study found Coho catch per unit effort was higher in the spring thermocline than in nonfrontal water.

It also found Chinook and lake trout were generally caught deeper than Coho and steelhead.

This makes fronts particularly useful for building a mixed salmon spread.

The upper portion may produce steelhead or Coho while deeper water produces Chinook or lake trout.

Spring is especially important

Temperature fronts are often easier to understand in spring.

Nearshore water warms first.

Offshore water remains colder.

Where those water masses meet, a relatively narrow temperature transition can develop.

As the season progresses, this boundary generally moves and changes.

By summer, the lake's vertical thermocline becomes a larger part of the fishing picture.

How to recognize a front

Surface temperature can be one clue.

If your temperature changes quickly while trolling across otherwise similar water, investigate.

Other clues can include:

A visible water-colour change.

A line of floating material.

A change in wave texture.

Bird activity.

Repeated bait.

A sudden cluster of fish marks.

None of those guarantees a temperature front.

But several occurring together are worth attention.

Do not troll straight through and leave

If you cross a boundary and immediately begin marking bait or take a strike, turn.

Cross it again.

Determine which side of the temperature transition holds the activity.

The most productive path may be along the boundary rather than directly across it.

That keeps your spread in the useful water longer.

Fronts are not stationary

Wind and circulation move water masses.

A front you found yesterday may shift.

That is one reason exact waypoints can lose value quickly.

The waypoint tells you where the feature was.

It does not guarantee the same feature remains there.

Current temperature information can therefore be more useful than simply returning to old coordinates.

The larger lesson

Great Lakes anglers often think vertically:

How deep is the thermocline?

How deep are the salmon?

Thermal fronts add a horizontal dimension.

Sometimes the important question is not only:

How deep should I fish?

It is:

Which water mass should I fish in?

Find the right water first.

Then determine the productive depth inside it.

Sources

• Olson et al., Spring Thermal Fronts and Salmonine Sport Catches in Lake Ontario.
• Research on Great Lakes surface temperature fronts.