Great Lakes Salmon Fishing: How to Find and Fish the Thermocline

For Great Lakes salmon anglers, the thermocline is one of the best places to begin searching for fish and bait during the summer. It can help narrow down a large water column and give you a starting point for setting a trolling spread on Lake Superior, Lake Michigan, Lake Huron, Lake Erie, or Lake Ontario.

But the thermocline is not a fixed depth or a guaranteed strike zone. Its position changes with weather, wind, currents, location, and season. The key is to use it as a reference, then adjust your spread based on the fish, bait, and temperatures you actually find on the water.

What is the thermocline?

During summer, the sun warms the upper portion of a deep lake while the water below remains colder. Between those two layers is the thermocline, a transition zone where water temperature drops rapidly with depth.

Above the thermocline is warmer surface water. Below it is colder deep water. NOAA's Great Lakes Environmental Research Laboratory explains how summer stratification separates warmer surface water from colder deep water and how wind can mix the lakes and change water temperatures.

Because the Great Lakes are constantly moving, the thermocline does not sit at the same depth everywhere. Its depth and shape can change between lakes, between different parts of the same lake, and even during the course of a fishing trip.

That is why a productive thermocline depth in one area of Lake Ontario may be completely different from what you find elsewhere, or from what you find a few days later.

Fish temperature, marks, and bait

The thermocline helps anglers narrow their search because both temperature and food can influence where salmon and baitfish concentrate.

There is no single temperature or depth where salmon will always be found. Temperature should be treated as one piece of information alongside sonar marks, bait, currents, and fishing activity.

Summer stratification can also affect the food web. Great Lakes research has documented deep chlorophyll layers, where phytoplankton become concentrated. One Lake Michigan study found a chlorophyll layer below the seasonal thermocline.

Zooplankton feed on phytoplankton, baitfish feed within this food web, and salmon follow the bait. The New York State Department of Environmental Conservation notes that alewives and rainbow smelt are often found in and around the thermocline in Lake Ontario.

That makes the thermocline a logical area to investigate, but the fish do not have to be directly inside it.

Temperature preferences can provide another clue. NYSDEC lists 52–58°F as a preferred range for Chinook and Coho salmon in Lake Ontario, while also noting that salmonids regularly move outside their preferred temperatures.

Chinook can also be caught in much colder water, including temperatures around 42–43°F. For that reason, a deep mark in cold water should not automatically be assumed to be a lake trout. Steelhead often travel higher in the water column, while lake trout frequently favor colder water closer to bottom.

Use those tendencies to help interpret what you see, but let the actual fish and bait marks determine where you fish.

Find the thermocline before leaving the dock

On the Great Lakes Trolling map, select the Thermocline layer and choose the forecast time for your trip. The map colors represent the forecast depth of the thermocline.

Click a location on the map to inspect the modeled water-column profile for that area. If the profile indicates there is no thermocline, the water column is expected to be mixed from the surface to the bottom at that location.

Because the thermocline layer is calculated from NOAA model forecasts, it is best used for comparing areas and deciding where to begin your search. It is not a direct temperature reading from the water beneath your boat.

You can learn more from the map's data sources and forecast limitations.

Check the water on the boat

Once you are fishing, use your electronics to see what is actually happening below the boat.

On some fish finders, the thermocline can appear as a narrow, faint horizontal band in the water column. NYSDEC's Lake Ontario trolling guide notes that increasing sonar gain or sensitivity can make the thermocline easier to see.

Once you identify the break, look closely for bait and fish marks above, within, and below it. If you have a downrigger temperature probe, use it to confirm the actual temperature at the depth where your gear is running.

Do not assume the thermocline will remain at the same depth throughout the day.

Wind and underwater currents can shift cold and warm water around the lake. Wind-driven upwelling can push colder deep water toward shore, while in other areas the thermocline may slope downward or intersect the bottom near a point, ledge, or drop-off.

As you move, continue checking your sonar and temperature readings. Compare what you are seeing with the forecast, but give more weight to the conditions beneath your boat.

Build your trolling spread around the thermocline

Once you have found the thermocline, use it to build your initial trolling spread.

Instead of putting every lure at the same depth, cover several parts of the water column around the temperature break. Run some presentations above it, some near it, and others below it when conditions and target species make sense.

Then watch your sonar and let the fish tell you where to concentrate.

If you are consistently marking salmon 10 feet above the thermocline, move more of your spread into that zone. If bait is stacked below the break and rods at that depth begin firing, shift additional lines deeper.

The same principle applies when targeting different salmonids. NYSDEC notes that Lake Ontario steelhead are often found higher in the water column, while lake trout commonly favor colder water closer to the bottom.

Those patterns can help you decide where to start, but your sonar marks and bites should determine where you finish.

There is no single salmon trolling depth that works across all five Great Lakes. The thermocline simply gives you a way to reduce the amount of water you need to search.

Find the temperature break, locate the bait and fish around it, spread your lines across the most promising depths, and then tighten your spread around the water that produces.

Sources

• New York State Department of Environmental Conservation: Trolling for Lake Ontario Trout and Salmon
• NOAA Great Lakes Environmental Research Laboratory: Water Temperature FAQ
• Deep chlorophyll maxima across a trophic state gradient: A case study in the Laurentian Great Lakes
• NOAA Great Lakes Environmental Research Laboratory: Dynamics of Lake Michigan Phytoplankton