Understanding Fish & Aquatic Thermocline Habitat
Our walleye populations, as well as our general fish populations in Wisconsin, have declined despite fish management efforts to preserve them. FISH HABITAT decline on shorelines is attributed to human development and increased recreational use. Recently, shoreline tree drops and a “Fish Sticks’ program has been developed to reverse this. But more is needed!
The Walleye Restoration Foundation, Inc. is looking at the bigger ecological picture of this decline in the original walleye range of the Northern Forest of Wisconsin. Recent research has indicated that walleyes fail to recruit into the population in the first year of life, stating there is adequate spawning and egg hatching into fry and into small fingerlings, but by July these fingerling population decline significantly.
Prior to this decline walleye post larval fingerlings and other early April May spawning fish (white suckers, yellow perch, northern pike, and muskellunge) post larval fingerlings feed on small aquatic organisms called zooplankton. All these young-of-the-year-old larval fingerlings follow the zooplankton vertically as light (night & day), wind, and changing temperatures in late May make the zooplankton/ phytoplankton also move vertically. This is the first plankton bloom of the year when nutrients from the bottom are circulated into the water column as the lake temperature rises and these plankton populations increase. This occurs in the deeper areas of the lake.
In late June, the lake temperatures increase further, bringing even more nutrients off the bottom causing an even stronger plankton bloom. It is this bloom that feeds the post larval warm water fish post larval fingerlings (large & smallmouth bass, black crappies, bluegills, pumpkinseed, rock bass) emerging from late May and early June shallow water spawning beds. Lake temperatures from top to bottom begin to stratify into layers.
By late June, most lakes in the North stratify creating a thermocline, which is a layer of water where there is a sudden drop in temperature and oxygen levels. Higher temperatures and oxygen levels are found above this thermocline due to wind mixing and sun radiation. Below this thermocline lower temperatures are found. As summer proceeds, low dissolved oxygen levels develop below the thermocline because of bottom sediment oxygen demand and lack of wind mixing. This thermocline is present from late June to September.
Fish research indicates that young-of-the year walleye fingerlings in northern lakes switch from zooplankton to young-of-the-year yellow perch and the lack of yellow perch is suspected to be the cause of the decline in walleye. In general, spawning habitats for adult yellow perch are in shallow sloping areas (often sandy) where their egg ribbons are deposited and incubate until they hatch into fry before becoming small post larval fingerlings seeking zooplankton in deeper waters. This is where woody debris or remains of last year’s vegetation holds these ribbons off the bottom. Removal of woody debris or winter drawdowns freezing out roots of shallow vegetation would be a detriment to yellow perch incubating egg ribbons as they would collapse and fall to the bottom. This limits yellow perch hatching success.
If one is to consider the survival of all species and sizes of fish in the clear to tannic lakes of the Canadian Shield deep water habitat is critical. Drop off areas near the summer thermocline are places to find many species of fish but any habitat cover of woody debris at this depth is also used by all species and sizes of fish year-round. This is where divers should build fish and aquatic habitat. Attached are diagrams of proven habitat utilized by all fish and confirmed by divers.