Great Lakes Lake Erie Thermocline and Hypoxia: When Colder Water Isn't Better
Learn how seasonal hypoxia in Lake Erie's central basin can make cold deep water unsuitable and compress fish and forage above it.
For most Great Lakes salmon and trout anglers, finding colder water is usually useful.
In Lake Erie's central basin, that rule needs an important qualification.
During summer stratification, deep water can lose enough dissolved oxygen that apparently ideal cold water becomes poor habitat.
That creates a problem a normal temperature chart cannot show.
Start with the thermocline
Like the other Great Lakes, Lake Erie can stratify during summer.
Warm surface water forms above colder deep water.
Cold-water species would normally be expected to use the cooler water beneath the thermocline.
But temperature is only one requirement.
Fish also need oxygen.
What is hypoxia?
Hypoxia means dissolved oxygen has fallen to a low level.
In central Lake Erie, seasonal hypoxia develops in deep water beneath the thermocline.
Research examining conditions from 1987 through 2005 found hypoxia occurred annually in the study period, generally from mid-July into October.
The timing overlaps directly with the period when anglers would normally expect summer stratification to create useful cold-water habitat.
Why deep water loses oxygen
Once the lake stratifies, the thermocline restricts mixing between warm surface water and cold bottom water.
Oxygen in the deep layer can then be consumed through biological and chemical processes faster than it is replaced.
The colder water remains cold.
But its oxygen concentration declines.
That produces habitat that looks attractive if you examine temperature alone but may be unusable or less productive for fish.
Rainbow smelt show the problem clearly
Rainbow smelt are important cold-water forage.
Under normal summer conditions, older Lake Erie smelt tend to occupy colder water in the metalimnion and hypolimnion.
Research found older smelt largely absent from the warm epilimnion during strongly stratified July conditions.
But hypoxia can remove part of that deep-water refuge.
A long-term central Lake Erie study found the reduction in habitat quality associated with hypoxia was greatest for adult rainbow smelt and round goby among the species and life stages evaluated, averaging about 35%.
Fish can become squeezed
Think of the water column as a narrowing sandwich.
At the top, water may be too warm for a cold-water fish.
At the bottom, water may be cold enough but contain too little oxygen.
The usable habitat is compressed between the two.
This can concentrate fish into a relatively narrow vertical band.
Research in central Lake Erie has documented fish moving horizontally away from low-oxygen areas or vertically into the metalimnion as hypoxia develops.
For an angler, that can make the thermocline and oxycline together more important than temperature alone.
Why this matters for steelhead
Lake Erie steelhead anglers commonly target deep offshore water in late summer.
NYSDEC notes that trollers often target steelhead in roughly 70 to 90 feet of water beginning in mid-August and recommends working spoons near the thermocline.
But if you are fishing a part of the central basin experiencing low deep-water oxygen, simply dropping lures farther below the thermocline may not improve your odds.
The deeper, colder water may contain less useful habitat than expected.
It matters to predators because it affects forage
Even if the target salmonid can tolerate conditions somewhat differently from a rainbow smelt, forage distribution still changes.
If hypoxia pushes adult smelt upward or horizontally, predators feeding on them can respond.
That means an oxygen event can indirectly change the best salmon or steelhead depth by moving their food.
Temperature alone cannot identify the usable zone
This is the main lesson.
Suppose your temperature probe shows:
65°F near the surface.
55°F around the thermocline.
45°F near bottom.
A simple trout temperature chart might make the 45°F water look excellent.
If dissolved oxygen near bottom is severely depleted, however, that cold water may be much poorer habitat than the temperature implies.
The fish may instead concentrate higher, where temperature and oxygen overlap.
Where hypoxia matters most
This issue is especially associated with the central basin of Lake Erie.
Do not automatically apply the exact same oxygen assumptions to every location or every Great Lake.
The severity, timing and location of low oxygen vary.
That is why local conditions matter.
How to adjust your fishing
During the late-summer period, pay attention to more than the coldest available water.
Look for:
The thermocline.
Concentrated bait.
Predator marks.
Known hypoxia conditions.
A narrow band where fish consistently appear.
If marks disappear below a particular depth despite colder temperature, do not automatically assume you need to fish deeper.
The fish may be showing you the lower edge of usable habitat.
The Lake Erie rule
On most Great Lakes summer trips, anglers learn to search for cold water.
On Lake Erie, especially in the central basin during hypoxia season, the better rule is:
Find water that is both cold enough and oxygenated enough, then find the bait inside it.
Cold water is only useful if the fish can live and feed there.
Sources
- USGS, Seasonal and interannual effects of hypoxia on fish habitat quality in central Lake Erie.
- Lake Erie rainbow smelt vertical-distribution research.
- Central Lake Erie forage and hypoxia research.
- NYSDEC Lake Erie steelhead fishing guidance.