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A Two-Story Fishery

Lac Courte Oreilles – a Rare and Fragile “Two Story Fishery”
by Frank Pratt

A “two-story” fishery is a lake capable of supporting warm-water species like bass, northern pike and muskellunge in its warm, “top story”. It also can support cold-water species like cisco or whitefish in its deeper, colder, well-oxygenated “lower story”.

Lac Courte Oreilles is one of these rare two-story fishery lakes. In all Wisconsin there are only about 200 of these lakes. LCO has smallmouth and largemouth bass, walleye, northern, muskies and other warm water species in the top story.

LCO is particularly unique, really rare because it supports both cisco and lake whitefish in its narrow, colder band or layer of water, in its lower story. There are only five such, inland lakes in the entire state.

Cold-water habitat in lakes is by its very nature fragile and imperiled. As organic matter dies and sinks, its decay uses up oxygen in deeper water. The amount of decay and the rate of oxygen loss depend upon how fertile the lake is. LCO is right on the verge of too much fertility. We cannot put any more nutrients into our lake without risking loss of the lower story fishery. LCO is at a tipping point.

The longer the summer, the greater the oxygen depletion. Imagine a first floor (lower story) where the floor and ceiling squeeze together for three or four months. Then a “normal” September brings surface cooling. Cisco and whitefish squeezed by low oxygen in the first floor now have an open stairway to the second floor (top story) because surface waters are now cool enough to meet their survival needs. If, however, summer hangs on well into September, a full month of squeeze is added and the stairs are blocked. The lower story can become devoid of oxygen but there is no escape since surface waters remain too warm. So, the cold-water fish now face a double barreled shotgun – eutrophication (too many nutrients, not enough oxygen) and climate change. *

This diagram depicts conditions in August , 2011. The first floor (lower story) is just a 3 foot band at 34-37 feet. Cisco can occupy the lower story pretty much throughout the lake, including suspending over the middle of the lake. Cisco migrate into warm surface water (top story) in the evening to feed, and when that happens the big predator fish put on the feed bag. This is one reason LCO consistently produces such big fish, including world-class muskellunge.

The whitefish are under more stress, requiring even colder water than cisco, plus they like living near bottom. They occupy the first floor (lower story) near the floor in the corners, only about 5% of the first floor volume. So, whitefish are the real canary in the coal mine for worsening water quality.

Below the first floor (lower story) is the cold, “basement” which is completely devoid of oxygen.

We should be concerned about our cold-water fish for two reasons: 1. They are food for other fish and the prime reason LCO is such a quality fishery; and 2. They are our best index of lake conditions. If cold-water fish go away it will foretell a series of problems for the lake; loss of water clarity, increased weed growth/nutrient loading, imbalanced fishery, decreased property values and other economic damage.

The best management prescription for protecting any two-story fishery is preventive maintenance – keep nutrient levels at or below their current levels. Protecting the watershed protects the lake, protects the fish.

*Note: Can the cisco and whitefish survive both eutrophication and climate change? Probably not. If we hold the line on nutrients they do have a fighting chance. Climate modeling for cisco puts their chance of survival to 2100 in a lake like LCO at about 65%. That model assumes no significant eutrophication. If our phosphorus levels rise just 3 parts per billion then our children’s children’s children will only read that cisco and whitefish were once present in Lac Courte Oreilles. Of the two factors it looks like nutrients are our biggest, most immediate worry.

Cisco and Whitefish

Cisco are also known as Tulibee or Lake Herring. In LCO they are abundant and grow to about 8” maximum size.Small cisco have a long body shape. Larger cisco are deep bodied and can look a lot like whitefish. Whitefish, however, have an over-hanging snout, cisco don’t. Small populations tend to grow bigger cisco. So, if cisco numbers decline in LCO, we should notice them getting bigger. They are somewhat related to trout and salmon; however, cisco are most closely related to lake whitefish. The two rarely share habitat in inland lakes, which makes LCO very unique in that respect. Cisco do not have teeth like smelt do, and they filter feed on microscopic zooplankton, often migrating upward into warmer water to feed. They prefer water in the 60-67 degree range. Cisco suspend in dense schools in mid-water portions of the lake and are perfect prey for large, warm-water, predator fish, and the main reason LCO gamefish get so big. They spawn around Thanksgiving and can be seined. Good eating – smoked or pickled.

Whitefish are probably the most threatened species in LCO. They are much more sensitive to declining water quality than cisco are. Whitefish prefer colder water than cisco and are very bottom oriented, so they rarely suspend. Suitable whitefish habitat is about 5% that of good cisco habitat. Like cisco, they will filter-feed on plankton, but often feed on insect larvae in, on, or around the sediment. They are much rarer than cisco in LCO, but can get very big – up to 12 pounds. Occasionally they can be caught by hook and line vertical jigging with a spoon or blade bait in 35 – 40 feet of water. Also try casting a spinner in the shallows early May and late October. They spawn usually right around Thanksgiving , and can be seined then. Great eating – smoked, pickled, baked, boiled, or fried. In commercial retail markets con- sidered one of the highest quality and most valuable species, worldwide.

**Environmental Protection Agency (USEPA). 2009. National Lakes Assessment: A Collaborative Survey of the Nation’s Lakes.EPA 841-R-09-001.U.S. Environmental Protection Agency, Office of Water and Office of Research and Development, Washington, D.C.