Out of Sight, Out of Mind
By James Walker and Jim Hedrick
The study of the physical, chemical and biological characteristics
of freshwater environments is known as limnology. Biological communities
in lakes respond in a predictable way to changes in the environment
including variations in temperature, light and a host of chemicals
in the water.
Let’s explore a few of the critical factors to gain a greater
understanding of what goes on under the surface of our West Virginia
waters. Lake changes occur seasonally and can have a major impact
on fish distribution, feeding, activity and ultimately your fishing
success. Knowledge of the lake environment can help anglers tailor
their fishing gear and techniques to improve fishing success on every
A major factor affecting fish distribution and feeding during the
summer is lake
stratification. In winter and spring, lake water temperature
is uniform from top to bottom and the entire body of water circulates
very slowly. This water circulation replenishes oxygen in the bottom
of the lake and brings nutrients up from the sediments.
Stratification occurs throughout the summer and early fall as warm
water rises to the surface while the cooler water sinks to the bottom
of the lake. This happens because cooler water is denser than warm
water. The water will separate into distinct vertical layers of different
Lakes commonly stratify into three layers know as the epilimnion,
metalimnion and hypolimnion. The epilimnion is the upper layer of
water in which the water
temperature is uniform and in which the
water circulates due to wind and wave action. The hypolimnion is
the bottom layer of water, characterized by cold water that no longer
mixes with the warm, upper layers of water. The metalimnion is the
middle layer of transition between the warm and cool water. The zone
in which there is a rapid drop in temperature is known as the thermocline.
Slight variations in the depth of the thermocline may exist in one
lake due to other factors. If you have ever been swimming in a lake
during the summer and felt much colder water on your feet, you have
hit the thermocline. The graph below of water temperature at various
depths in Stonewall Jackson Reservoir during July 2004 clearly illustrates
In West Virginia , many reservoirs and small impoundments commonly
stratify between May and October. The extent and duration of stratification
depends on the size, shape and depth of the lake. Some reservoirs
may not stratify like natural lakes due to the release of cold water
through the bottom of the dam.
Fish survival is dependent upon an adequate supply of oxygen in
the water. Oxygen enters the water through contact with the air or
it may be released by aquatic plants.
The amount of oxygen present
in water depends primarily on temperature because cooler water holds
more oxygen. Coldwater species such as trout require high levels
of dissolved oxygen. For this reason, we find trout primarily in
headwater streams where the water is cold all year and the churning
action of water as it flows over rock increases the oxygen content
of the water. Coldwater fish may survive in lakes, small impoundments
and farm ponds during winter months when the water is cold. However,
as water temperatures rise with approaching summer, available oxygen
will decrease and species like trout may not survive.
Stratification and dissolved oxygen affects fish distribution and
ultimately fishing success. During hot summer months many fish species
seek cool water in the lower water column. Many anglers target these
cool water areas with deep diving lures and baits.
However, many anglers may not be aware of other important aspects
of stratification. Due to the natural process of decomposition and
decay of organic material including leaves, logs and microscopic
organisms, oxygen can become depleted in the hypolimnion. Although
cool water is capable of holding more oxygen than warm water, the
hypolimnion does not mix with the upper layers and oxygen cannot
be replenished. Despite the cool water that would typically attract
many fish species, the lack of oxygen will prevent fish from occupying
this area. Some small impoundments in West Virginia may only have
oxygen in the top two layers of water. Therefore, fishing deep water
will yield few, if any, fish.
In the fall, water temperature begins to decrease in the epilimnion
and the cooler water sinks. Wind creates waves and adds to the stirring
action. As the water temperature becomes more uniform from surface
to bottom the entire lake will begin to circulate once more. The
return of circulation throughout the entire water column is known
as fall turnover. Following fall turnover and throughout the winter
the water temperature is nearly consistent from the water surface
to the bottom (see graph below).
Fish kills can be associated with fall turnover. If oxygen levels
become low in a large portion of the lake during the summer due to
decay of organic material, fall turnover can result in extremely
low oxygen levels throughout the entire water column causing fish
to die. Fish kills as a result of fall turnover commonly occur in
farm ponds. If farms pond are 10 to 15 feet deep or deeper, they
may have summer stratification. A single weather event like a thunderstorm
may cool the surface water, cause turnover and result in low oxygen.
Future issues of West Virginia Wildlife will examine how
microscopic organisms and nutrients determine the number and size
of fish a lake or pond can support.
Jim Hedrick is a fisheries biologist stationed in Farmington .
James Walker is a fisheries biologist stationed in French Creek.