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The Walleye Habitat Pinch

Some years back I learned that fishing for walleye on Lake Glenville was most successful in late summer and early autumn, near to and following the time of the FLG Annual Members Meeting, but did not understand why. After some years of sampling with the FLG datasonde, I have developed some insight to this observation that I think will be of interest to members.

Walleye are night hunters; their eyes have evolved to enable good vision in very low light conditions. Their very name is derived from the appearance of their eyes. Their visual sensitivity makes them uncomfortable in brightly lighted water near the surface, hence they spend their days in deeper, dimly lighted water. Quantitative measures of their light limits are hard to find. For this presentation I will use twice the depth of a Secchi disc measurement, an estimate of the depth of the photic zone commonly used by aquatic biologists. At this depth the light intensity is reduced to about one percent of its value at the surface.

Walleye are described as cool water species. They are stressed by and try to avoid water with temperature higher than about 24°C (75°F). Like all fish, they require adequate oxygen to sustain life. Walleye become stressed and will avoid water with less than about 4 parts per million (ppm) dissolved oxygen (DO), and will perish in water with less than 1 ppm.

Finally, acidity, measured in units called pH, also is critical as it affects all chemical processes in water. Lower pH means more acidic, and pH = 7 is considered neutral. Walleye are seldom found in waters of pH less than 5.5.

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Turning attention from the fish to the lake, all waters of Lake Glenville are a comfortable home to walleye for much of the year. There are however some seasonal variations that limit their access. Figure 1 displays some data from a sampling trip I did in late August of 2011. It can be seen that in the upper 7 meters (23 feet) the water chemistry is suitable for walleye, but the temperature is uncomfortably high, and of course the sunlight during midday is too intense. At depths greater than about 9 meters (30 feet) where the temperature and daytime light intensity are comfortable, the dissolved oxygen concentration and pH have declined to less than 4 ppm and 5.5. these numbers imply a very thin stratum in which walleye can comfortably survive.

We have no winter data to display the annual cycle of water conditions in the lake. The lake may not “turn over” completely, that is, become completely mixed from top to bottom by winter cooling of the surface water. Some thermal stratification may be maintained by colder water from inflowing streams, but there is ample evidence that even the bottom water is refreshed in winter. With the onset of spring the nearsurface water warms rapidly, forming a thermocline, or level at which the temperature changes rapidly with depth. The existence of a thermocline inhibits vertical mixing, effectively isolating the deeper water from the near-surface water and the overlying atmosphere, thus cutting off the resupply of oxygen from, and venting of carbon dioxide to, the atmosphere. Oxidation of organic detritus continues however, consuming oxygen and producing carbon dioxide. The excess carbon dioxide reacts with water molecules to form carbonic acid. Thus in subthermocline waters both dissolved oxygen and pH values decline through the warm months of the year.

Screen Shot 2017 10 10 at 6.57.25 AMThe effects of these processes on the depths of variables critical to walleye health and happiness are illustrated by Figure 2. During 2011 a region of thermal stress appeared about 1 June and deepened to nearly 8 meters (26 feet) by mid August, after which it abruptly terminated. Concurrently, critical levels of both dissolved oxygen and pH rose to depths of only 10 meters (33 feet) by about 1 July, and to as shallow as 8 meters during the next two months. The comfortable habitat zone for walleye was therefore “pinched” into a stratum of water only 3 to 6 feet thick at depth of about 26 feet for about two months. Hence skilled fishermen are able to make good catches by fishing at the depth where walleye are forced to concentrate.

During the following month water chemistry remained stressful and even toxic at levels deeper than 33 feet, but all depths shallower than that had become comfortable, at least during hours of darkness, to walleye. Each year is different in detail, but a similar pattern has played out during each of eight years since FLG acquired the datasonde.

An even more desirable gamefish (in my opinion) in our lake, the rainbow trout, is classed as a cold water fish; it is even more demanding in its aversion to high temperature and low levels of DO, and is therefore pinched into an even thinner stratum. Research in other lakes and ponds has shown that trout will choose to suffer effects of low DO to escape excessive temperatures, for at least brief intervals. I have found no information on whether walleye do the same.

Our currently most sought-after fish, bass, are comfortable, year round. The lake is a little more acidic than optimal for them, but they can cope. They cannot abide the hypoxic deep water, but they are well adapted to the temperature and light conditions in the upper waters.

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