Alewife and Rainbow Smelt: How Baitfish Control Great Lakes Salmon Depth

Salmon anglers spend a lot of time talking about preferred temperature.

The salmon are paying attention to something else too.

Food.

A Chinook may be physiologically comfortable in one temperature range and still leave it to attack a school of alewife. Lake trout can move away from ideal cold-water habitat when feeding opportunities justify it.

Understanding alewife and rainbow smelt can therefore explain why salmon sometimes appear in water that seems "wrong" according to a temperature chart.

Start with the food chain

Alewife and rainbow smelt are major forage species for Great Lakes salmon and trout.

NYSDEC identifies both as important forage for Lake Ontario salmonids and notes that they are commonly found in and around the thermocline.

But the two species do not necessarily occupy the same part of the water column.

Age, season, temperature, light and food availability can all change their position.

Alewife are not fixed to the thermocline

Alewife use a broad range of depths during the year.

USGS describes young-of-year alewife as commonly occupying the warmer epilimnion during summer, while older fish often venture into the thermocline and cooler water.

That alone can help explain why salmon of the same species may be caught at very different depths.

One predator may be feeding on mature bait near the temperature break while another is targeting younger bait above it.

Alewife move between day and night

Lake Ontario research gives an even clearer example.

During summer, researchers found alewife schooling near the bottom of the mixed layer during daylight.

At dusk, many schools broke apart and fish moved upward toward the surface.

The researchers concluded that the movement was likely related to vertically migrating zooplankton, which moved into upper water at night.

That means productive predator depth can shift without the thermocline itself moving very far.

If the food moves, salmon have a reason to move too.

Rainbow smelt tend to run colder

Older rainbow smelt generally show a stronger cold-water relationship.

Research in Lake Erie found young-of-year and older smelt separated vertically during strong summer stratification. Young fish occupied the epilimnion while older smelt were concentrated in the metalimnion and hypolimnion. Almost no older smelt were caught in the warm epilimnion during July surveys.

This creates a very different feeding opportunity from warm-water young alewife.

If adult smelt are the main target, predators may have a reason to stay closer to or below the thermocline.

Why Chinook can appear outside their preferred temperature

NYSDEC gives Chinook a preferred starting range of 52 to 58°F, but Chinook are capable of substantial vertical movement.

Combine those two facts with moving bait.

A king does not have to spend its entire day in one comfortable temperature.

It can move through colder or warmer water to feed and return.

That is one reason a bait ball can be more important than a perfect temperature reading.

If you find a dense school of alewife with large predator marks around it, do not ignore the marks because the probe says the water is several degrees outside the number you expected.

Why lake trout are different

Lake trout are generally more strongly tied to cold water and showed far less vertical activity than Chinook in Lake Ontario telemetry research.

But they also have access to a broader menu.

When nearshore water is cold enough, NYSDEC notes that lake trout and brown trout can feed on bottom-oriented round gobies.

That means a lake trout angler has to consider both pelagic forage and bottom forage.

A suspended laker around smelt is behaving differently from one working gobies near bottom.

What a bait ball actually tells you

Not every bait ball has active salmon around it.

But bait provides several useful clues.

First, it tells you there is food.

Second, its depth tells you where at least part of the food web is operating.

Third, the shape of the bait school can sometimes suggest whether predators are pressuring it.

Fourth, repeated bait schools at similar depths can give you a better starting zone than temperature alone.

The most useful situation is where multiple clues overlap:

A suitable temperature range.

A strong thermocline or other boundary.

Repeated bait.

Predator marks.

Then, eventually, bites.

Do not chase every bait ball

Large Great Lakes fish finders can show bait almost continuously in productive areas.

If you change the entire trolling spread every time a bait school appears, you can spend more time moving gear than fishing.

Look for patterns.

Are most bait schools between 55 and 70 feet?

Are predator marks consistently 10 feet below them?

Are your bites coming from the same band?

Once several clues agree, tighten the spread.

The better question

Instead of asking:

What temperature do Chinook like?

Try asking:

Where does suitable Chinook water overlap with the food they are feeding on today?

That small change in thinking explains a lot of Great Lakes salmon behavior.

Temperature defines the available habitat.

Bait often determines which part of that habitat is worth fishing.

Sources

• NYSDEC, Trolling for Lake Ontario Trout and Salmon.
• USGS Great Lakes alewife species information.
• Lake Ontario alewife vertical-distribution research.
• Lake Erie rainbow smelt vertical-distribution research.