How To Fish Stillwaters

November 29th, 2004

Stillwaters, lakes, ponds and reservoirs are the most underutilized fisheries in the North America. Why? Because the average fly fisher doesn't know how to fish them, or where to start. Stay tuned, you too can master stillwaters! ~ LadyFisher


Limnology, Part 2

By Paul C. Marriner

BIOLOGICAL ZONES

Scientists divide lakes into zones related to biological activity. The commonly used terms are littoral, sub-littoral, and pelagic. Littoral and sub-littoral zones are defined as those portions of the lake in which plants grow. One authority suggests that the littoral zone includes that area where plants emerge above the surface or float on it, and the sub-littoral zone is that area where only fully-submerged plants occur. Hereafter, unless separation is necessary for clarity, the two are combined and referred to as the littoral zone. I believe it's impossible to overemphasize the importance of the littoral zone to fly-fishers. In general this is where the useful action is found. Plant life in the littoral zone provides food and shelter to many insects and small fish, the food-forms of larger fish.

Unfortunately, at least from the classification point of view, some stillwaters, or areas of others, do not divide neatly into these zones. These are lakes where the nature of the benthos (bottom) or chemical composition inhibits plant growth even though there is sufficient light. Examples are gravel, marl, or clay bottoms and stillwaters with very low nutrient levels. Like the profundal zone described below, this doesn't mean such areas are devoid of food-forms. So, to avoid pointless verbiage in later chapters, I include these shallow regions in the littoral zone.


Figure 1: Zonation of stillwaters; for illustrative purposes the thermocline is over-thick. Also, the line above the profundal zone is often considered the 1% incident-light level, that is, by the time the light reaches this level it has been reduced to 1% of its intensity at the surface.

The pelagic zone is the open-water area of a lake above depths that are too deep for plant growth. It's further subdivided into photic and aphotic zones. The photic zone extends from the lake surface to a depth where incident light has been reduced to 1% of its surface value, below that is the aphotic zone. Another phrase used to describe the deep region of a lake that is devoid of plant life is the profundal zone. Note, however, that while barren of plants the profundal zone is not devoid of life. Under certain circumstances it may provide the bulk of a trout's food.

ANNUAL TEMPERATURE CYCLE

Most fly-fishers are familiar with the annual temperature cycle of lakes, so I will be relatively brief. Water is most dense at 39 deg F (4 deg C). Ice, being less dense, floats. Where it's cold enough in winter, the ice and the water immediately below it are less dense than the remainder of the lake water and so a temperature inversion occurs, i.e., colder water resting over warmer water. In the spring the ice melts and the surface water gradually reaches 39 deg F (4 deg C). At this time the entire lake is at the same temperature and, given sufficient wind, a thorough mixing at all depths follows. This is called the spring turnover.

During the summer the surface water continues to warm. If the lake is deep enough, and if there is sufficient wind, currents are generated that mix the warm and cold water, but only to a certain depth. This action creates a thermal stratification. The water is warm on top and cold on the bottom. At some depth, which varies considerably with environmental and physical conditions, a narrow transition zone appears; the well-known thermocline (Figure 1). Under some conditions the thermocline may be the only comfortable area for certain species, both predator and prey. The thickness of this transition zone is highly variable, from a few inches to several yards. Nor is it static, moving downward in the summer and upward as fall approaches, and in shallow lakes may disappear intermittently depending on wind conditions. Finally, the thermocline is not even always horizontal. Certain wind-induced currents can cause it to tilt, meaning that it can be closer to the surface at one end of a lake than at the other.

An associated phenomenon is the chemocline - a transition zone between upper regions oxygenated by wind action and oxygen-depleted (anoxic) depths. Chemoclines occur in lakes that for reasons of size/depth ratios or location never turn over. Trout won't live below a chemocline, and so, when on the "bottom," they will always be suspended above it.

Cooling surface temperatures in the autumn provoke the final phase of the cycle. The cool water sinks forcing the warmer water below to the top. The water slowly reaches a uniform temperature everywhere; then the wind can thoroughly mix it again. This is the fall turnover. Clearly the cycle depends on geographic location. For example, some northern lakes that don't freeze have a fall turnover period that extends through to spring.

The temperature cycle is clearly important to fly-fishers. An example is the effect of spring turnover (fall turnover is generally after the end of the fishing season) on nutrient-rich lakes. Mixing may stir-up large quantities of plant detritus from the bottom, turning the stillwater into a virtually unfishable soup - a good time to avoid. ~ PCM

More next time.

Credits: Excerpt from Stillwater Fly Fishing, Tools & Tactics By Paul C. Marriner, published by Gale's End Press. We appreciate use permission.

Previous Lake Fishing Columns

If you would like to comment on this or any other article please feel free to post your views on the FAOL Bulletin Board!


[ HOME ]

[ Search ] [ Contact FAOL ] [ Media Kit ]

FlyAnglersOnline.com © Notice