Preferred Temperature vs Actual Fish Temperature: How to Read Great Lakes Temperature Charts

Temperature charts are useful.

They can also give anglers a false sense of precision.

If a chart says Chinook prefer 52 to 58°F, it is easy to conclude that 53°F is good water and 48°F or 62°F is bad water.

Fish do not follow the table that closely.

To use a Great Lakes temperature chart properly, you need to understand what the numbers actually represent.

Preferred temperature is not a wall

NYSDEC provides the following starting ranges for Lake Ontario salmonids:

Chinook salmon: 52 to 58°F.

Coho salmon: 52 to 58°F.

Steelhead: 55 to 65°F.

Lake trout: 42 to 52°F.

Brown trout: 54 to 63°F.

Atlantic salmon: 55 to 65°F.

The agency explicitly says the chart is intended as a starting point and that salmonids venture outside these temperature zones.

That sentence should probably appear beside every fishing temperature chart.

There are several different kinds of "best temperature"

A temperature can describe different things.

Preferred temperature usually refers to a range fish tend to select when other constraints are limited.

Optimum temperature might refer to conditions associated with physiological performance or growth.

Occupied temperature is the temperature where researchers actually observe fish.

Catch temperature is the water temperature associated with fish caught by anglers or sampling gear.

Those numbers do not have to match.

A wild fish has more decisions to make

A salmon does not choose temperature in isolation.

It also needs food.

It responds to light.

It avoids predators when young.

Mature fish eventually respond to spawning cues.

Currents move its habitat.

Forage moves vertically and horizontally.

Dissolved oxygen can restrict otherwise suitable water.

A fish therefore faces trade-offs.

The most comfortable water is not always the most profitable feeding water.

Chinook are a good example

NYSDEC lists 52 to 58°F as a preferred Chinook range.

Yet detailed Lake Ontario telemetry showed Chinook using a broad thermal and depth niche and making extensive vertical movements.

Researchers estimated summer cumulative vertical movement around 103 metres per hour at the median.

That behavior would be impossible if kings refused to cross water outside one narrow preferred-temperature band.

Lake trout are more temperature-oriented, but still flexible

Lake trout show a stronger cold-water association.

Their listed range is much colder, and Lake Ontario telemetry found them generally deeper and colder than Chinook.

But some individual lake trout still made trips into warmer, shallower water.

Even the strongest cold-water specialist in the group shows behavioral flexibility.

Catch temperature can change by season

Long-term Lake Ontario data found Chinook and rainbow trout changed both depth and temperature through the fishing season.

Both moved deeper and farther offshore through early summer before their distributions changed again later in the year.

So asking "What is the best Chinook temperature?" without mentioning the season leaves out important information.

Bait can override the chart temporarily

Alewife move vertically.

Lake Ontario research found many alewife schools shifted upward around dusk, apparently following zooplankton.

A Chinook can follow that feeding opportunity into warmer water.

That does not mean temperature stopped mattering.

It means feeding benefit temporarily outweighed the cost of leaving the most favourable temperature.

Oxygen can override temperature too

Lake Erie provides an especially clear example.

Cold water below the thermocline can look ideal on a temperature chart.

But if dissolved oxygen becomes too low, cold-water fish may not be able to use it effectively.

Research in the central basin found seasonal hypoxia substantially reduced habitat quality, with adult rainbow smelt among the most strongly affected species.

A temperature-only chart cannot show that.

How anglers should use temperature charts

Use the chart to answer:

Where should I begin looking?

Do not use it to answer:

Where must the fish be?

Then add information in this order:

Temperature structure.

Bait.

Fish marks.

Season.

Current conditions.

Actual bites.

The more of those clues agree, the more confidence you can have in the pattern.

A better way to talk about temperature

Instead of saying:

"Chinook should be in 53°F water."

Say:

"53°F is a useful starting temperature for Chinook, then adjust to the bait and fish you find."

Instead of:

"That 42°F mark must be a lake trout."

Say:

"Cold deep water increases the chance of lake trout, but species overlap means the mark cannot be identified by temperature alone."

Those statements are less precise.

They are also more accurate.

Sources

• NYSDEC salmonid temperature guidance.
• Chinook and lake trout telemetry research.
• Chinook and rainbow trout catch-temperature research.
• Lake Erie hypoxia research.