Lake Trout and the Thermocline: Finding Summer Lakers

Lake trout are the cold-water anchor of a Great Lakes trolling spread.

When summer surface temperatures climb and the lakes stratify, lake trout generally shift toward colder habitat beneath or around the thermocline.

That makes temperature especially useful for locating them.

But "lake trout equal bottom" is still too simple.

Start on the cold side

NYSDEC lists a preferred Lake Ontario lake trout range of 42 to 52°F, with 48°F as a useful optimum starting point.

Those values are considerably colder than the agency's starting ranges for Chinook, Coho, steelhead and brown trout.

If you are targeting lake trout in summer, the cold side of the thermocline is therefore a logical first place to look.

Lake trout and Chinook use the lake differently

Lake Ontario telemetry provides a useful comparison.

During summer, lake trout generally occupied colder and deeper water than Chinook.

Most also moved far less vertically.

Researchers estimated a median cumulative vertical movement of about 7 metres per hour for most lake trout compared with roughly 103 metres per hour for Chinook.

That supports the familiar observation that lakers can be more tightly associated with a particular depth or bottom zone.

But there was important individual variation.

Some lake trout made repeated trips into warmer, shallower water.

So even this cold-water specialist is not locked to one layer.

Why lake trout are often near bottom

Cold deep water frequently overlaps the lake bottom.

Lake trout also have feeding opportunities there.

NYSDEC notes that round gobies have become prey for lake trout and brown trout when nearshore water is cool enough for those trout to access bottom habitat.

This creates a strong reason for lake trout to associate with structure.

Bottom can provide both suitable temperature and food.

Suspended lake trout are different

Lake trout also feed on pelagic prey such as alewife and rainbow smelt.

When that forage is suspended, lakers do not have to remain on bottom.

A mark 20 feet above bottom in cold water can absolutely be worth fishing.

This is why simply dragging every lake trout presentation directly along the bottom can miss part of the population.

Look for the thermocline-bottom intersection

One particularly useful summer zone is where the thermocline approaches or intersects bottom structure.

There, a relatively small depth change can create a large temperature change.

Move slightly shallower and the bottom may be too warm.

Move deeper and cold lake trout habitat appears.

Wind-driven thermocline movement can shift this intersection substantially, which means a productive contour can change after weather.

Research in stratified lakes has documented rapid temperature variability where a moving thermocline intersects bottom.

Upwelling can move lakers shallower

A strong upwelling event can lift cold deep water toward shore.

For lake trout anglers, that can open shallower water that was previously too warm.

Downwelling can do the opposite and push suitable cold habitat deeper.

This is one reason the same piece of structure can fish very differently from one day to another.

How to start a summer lake trout search

Begin with three questions:

Where is the thermocline?

Where does cold water meet useful structure?

Where is the forage?

If you find all three together, you have a strong starting area.

Use sonar to determine whether the marks are tight to bottom or suspended.

Then place the spread accordingly.

Do not assume every cold deep mark is a lake trout

Species overlap matters.

Chinook can move through very cold water. Lake trout can move shallower and warmer.

Your sonar does not identify species simply because a fish appears at a particular temperature.

Use depth and temperature to develop probabilities, not certainties.

Bottom fishing versus suspended fishing

If the marks are glued to bottom, keep the program tight.

If repeated marks appear 10 to 30 feet above bottom, move gear up to them.

If bait is suspended farther above the structure, consider testing that zone too.

The thermocline tells you where cold habitat begins.

The graph tells you how the lake trout are using it.

Sources

• NYSDEC Lake Ontario trout and salmon temperature guidance.
• Raby et al., Chinook and lake trout telemetry research.
• Research on wind-driven thermocline movement near bottom.