Great Lakes Salmon and Trout: How Each Species Relates to the Thermocline

The thermocline can narrow down a huge Great Lakes water column, but different salmon and trout species do not use it in the same way.

Some fish regularly travel above and below the temperature break. Others spend much more of the summer in cold water beneath it. Brown trout can relate closely to the nearshore thermocline, while steelhead are often found higher in the water column.

Understanding those differences can help you decide where to place your first trolling lines.

The thermocline is a reference, not a dividing line

During summer stratification, warmer surface water sits above colder deep water. Between those layers is the thermocline, where temperature changes rapidly with depth.

It can be tempting to imagine the thermocline as a horizontal boundary with warm-water fish above it and cold-water fish below it. Great Lakes fish behavior is more complicated.

Temperature affects where fish can comfortably spend time, but feeding opportunities also matter. A salmon may leave its preferred temperature to chase alewife. A lake trout may move into shallower, warmer water to feed. Currents can also move the entire temperature structure while the fish and bait respond.

NYSDEC lists preferred Lake Ontario temperature ranges of 52 to 58°F for Chinook and Coho salmon, 55 to 65°F for steelhead, 42 to 52°F for lake trout, and 54 to 63°F for brown trout. The agency specifically warns that salmonids regularly move outside these ranges.

Chinook salmon

Chinook are highly mobile around the temperature break.

Telemetry research in Lake Ontario found summer Chinook generally occupied warmer and shallower water than lake trout, but they also used a much broader thermal and depth range.

The most striking difference was vertical movement. Researchers estimated Chinook traveled a cumulative median of about 103 metres vertically per hour during summer, compared with about 7 metres per hour for most lake trout in the study.

That does not mean a Chinook swims 103 metres straight up every hour. It represents the total of its repeated movements up and down through the water column.

For anglers, the important lesson is that Chinook can cross the thermocline repeatedly while searching for food.

A king marked well below your preferred-temperature target is not necessarily out of place. Neither is one feeding above it.

Coho salmon

Coho often provide a stronger thermocline and thermal-front relationship.

Lake Ontario research comparing catches inside and outside spring thermal fronts found that Coho catch rates were significantly greater in the spring thermocline than in nonfrontal water. The same study found Coho and steelhead were generally caught shallower than Chinook and lake trout.

During summer, the thermocline is therefore a logical starting point for Coho, especially where the temperature break overlaps concentrations of bait.

That still does not mean Coho will always sit directly inside the thermocline. Think of the break as the center of the search rather than an exact trolling depth.

Steelhead

Steelhead are commonly encountered higher in the water column than Chinook and lake trout, particularly in offshore water.

Lake Ontario catch data showed rainbow trout and Chinook moved progressively deeper and farther offshore as spring turned into summer. Rainbow trout were generally farther offshore during June through August, and they were sometimes caught in cooler water than Chinook despite being shallower vertically.

That is an important distinction.

Steelhead should not simply be described as fish that stay above the thermocline because they prefer warmer water. Their position reflects a combination of temperature, offshore habitat, forage, season, and light.

When steelhead are part of your target, it often makes sense to keep at least part of the spread above the main salmon program.

Lake trout

Lake trout show the strongest cold-water relationship of the major Great Lakes salmonids.

NYSDEC lists 42 to 52°F as a useful preferred range, substantially colder than its ranges for Chinook, Coho, brown trout, and steelhead.

Lake Ontario telemetry also found summer lake trout generally occupied deeper and colder habitat than Chinook and moved much less vertically. However, individual lake trout sometimes made substantial trips into shallower, warmer water.

In practical terms, start searching on the cold side of the thermocline and toward bottom structure, but do not assume every lake trout is pinned to bottom.

Brown trout

Brown trout can show a remarkably strong relationship with the nearshore thermocline.

A Lake Ontario study found that during summer, 88% of the brown trout sampled were in the thermocline or within 3 metres of it. All were within 3.2 kilometres of shore, most were over water 30 metres deep or less, and 70% were more than 3 metres above bottom.

That makes summer brown trout especially interesting.

The familiar shallow spring fishery does not necessarily disappear when nearshore water warms. The fish can shift deeper and begin using the nearshore temperature break instead.

Atlantic salmon

Atlantic salmon should be treated more cautiously because Great Lakes-specific thermocline research is more limited.

NYSDEC lists a preferred starting range of 55 to 65°F for Lake Ontario Atlantic salmon, but that should not be interpreted as a precise fishing boundary.

For practical purposes, search cool water containing forage and treat Atlantic salmon as another mobile predator rather than assigning them a fixed position above or below the thermocline.

Build your spread around overlap

The most useful way to think about the summer water column is not as separate layers assigned to individual species.

Think of overlapping zones.

Steelhead often provide a reason to keep lines high. Coho frequently relate well to the temperature break. Chinook can travel throughout a large vertical range. Lake trout usually shift the search deeper and colder. Brown trout can follow the nearshore thermocline.

Use those tendencies to build the first spread.

Then let sonar, bait, temperature and bites tell you where the fish actually are.

Sources

• NYSDEC, Trolling for Lake Ontario Trout and Salmon.
• Raby et al., Pop-off data storage tags reveal niche partitioning between native and non-native predators in a novel ecosystem.
• Olson et al., Spring Thermal Fronts and Salmonine Sport Catches in Lake Ontario.
• Seasonal movements and habitats of brown trout in southcentral Lake Ontario.