Summer Brown Trout and the Nearshore Thermocline

Great Lakes brown trout are famous for shallow spring fishing.

Cold shoreline water, creek mouths, stained water and warming bays can all concentrate early-season browns within easy reach of small boats.

Then summer arrives.

The shallow water warms and many anglers stop looking for brown trout.

The fish have not necessarily disappeared.

They may simply have moved to the nearshore thermocline.

Brown trout can have a strong summer thermocline relationship

Research in southcentral Lake Ontario found a surprisingly strong connection.

During summer, 88% of sampled brown trout were caught either inside the thermocline region or within 3 metres of it.

Seventy-eight percent were in water between 8 and 18°C, and all sampled browns were within 3.2 kilometres of shore. Most were over water 30 metres deep or less.

That provides a much more useful picture than simply saying "browns go deeper in summer."

They were still near shore, but they were relating vertically to the temperature break.

They were not all on bottom

Another useful finding is that 70% of those summer brown trout were more than 3 metres off bottom.

That matters to trolling strategy.

If you automatically put every summer brown trout lure close to bottom, you can fish underneath suspended browns.

Search the thermocline itself.

Why the thermocline makes sense

Brown trout require relatively cool water, but they also feed heavily.

The thermocline gives them access to a transition zone between warmer nearshore water and colder deep water.

It can also overlap with pelagic forage.

NYSDEC lists 54 to 63°F as a useful preferred range for Lake Ontario brown trout and notes that alewife and rainbow smelt commonly occur around the thermocline.

That creates a useful overlap between temperature and food.

Follow the temperature, not last month's depth

A spring brown trout spot may still be useful in summer, but the productive vertical zone can change dramatically.

Instead of thinking:

"Brown trout were here in 20 feet in April."

Think:

"Where is suitable brown trout water along this shoreline today?"

That may put you over 40, 60 or more feet of bottom while the fish themselves remain suspended.

Upwelling can completely change the fishery

Nearshore browns are especially exposed to changes in temperature structure.

Upwelling can bring cold deep water toward shore and raise the thermocline.

Downwelling can push warm water toward shore and force the break deeper.

A location that is marginal for brown trout one day can become excellent after the temperature structure changes.

NOAA identifies persistent wind as an important driver of Great Lakes coastal upwelling.

Brown trout and bottom forage

Do not ignore bottom completely.

Round gobies provide an important food source for brown trout when nearshore temperature allows the fish to reach bottom habitat.

That creates two potential summer patterns.

One is a suspended thermocline fish feeding pelagically.

The other is a brown using cold enough bottom water where gobies are available.

Your sonar and the temperature profile can help distinguish between them.

How to troll summer browns

First find the nearshore thermocline.

Then look for places where it overlaps useful structure, bait or both.

Do not place the entire spread at bottom.

Run presentations around the temperature break and cover some water above and below it.

If your sonar repeatedly shows suspended marks, put lures through them.

If bottom-oriented rods produce consistently, tighten part of the spread closer to structure.

Where to search

Good starting areas can include:

Nearshore drop-offs.

Points.

River or creek areas with suitable temperature.

Edges where the thermocline approaches bottom.

Areas containing concentrations of alewife, smelt or gobies.

The productive feature is rarely structure alone.

It is structure combined with the right water.

The summer brown trout lesson

Brown trout do not necessarily leave the nearshore fishery when the surface gets warm.

They can remain surprisingly close to shore while moving vertically into thermally suitable water.

Find the nearshore thermocline before assuming the brown trout are gone.

Sources

• Seasonal movements and habitats of brown trout in southcentral Lake Ontario.
• NYSDEC Lake Ontario trout and salmon guidance.
• NOAA Great Lakes upwelling information.