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Lake Topography – An Angler’s Perspective
Lake stratification will be influenced by the depth of the lake. The graph above shows measurements of the temperature of a temperate lake taken at one metre depth intervals. The right hand diagram is an expanded section showing the relationship between the metalimnion and the thermocline.
If you look at the top picture, the levels are nice and flat. The wind is obviously a very minor factor. However this changes with the wind intensity, where the epilimnion is pushed deeper at the opposite bank. If it is blowing from the North, the epilimnion is deepest at the South bank and vice versa. Hopefully the following two diagrams, showing easterly and westerly winds, will make the point easier to visualise.
The epilimnion will be deepest at the bank towards which the wind is blowing. It may be uncomfortable to fish but there are greater depths of warm water while the wind is also pushing airborne food, like insects, to the same point. I have often seen fly fishermen have much better days casting into the wind than their counterparts fishing with it comfortably at their backs.

When the wind stops, people will pop down and fish wherever they like but, sometimes, a little more thought may make a difference to their catches. For myself, immediately after an Easterly or Westerly blow, I try and find a likely looking area near the middle of the North or South banks, the reason for this being the internal seiche – the standing wave at the level of the thermocline - that follows the blow. If you have not heard of an internal seiche, think of it like this. The Easterly wind drives the epilimnion and the metalimnion, containing the thermocline, deeper on the West bank, which means, like a seesaw, that it becomes shallower on the East bank. It then stays like this while the wind is blowing strongly.

When the wind stops, however, the epilimnion does not meekly return to a flat, level plane, far from it. At its very simplest, it may begin to seesaw about a central line – think three dimensionally here - the epilimnion getting deeper at the East bank then the West again, then the East bank and so on. It may last for hours, continually seesawing, or even longer depending on the forces involved. This is an internal seiche.

The central line, from either an Easterly or Westerly wind, can be easily visualised by joining up two points from the middle of the North bank to the middle of the South. This is where there is a good chance that the depth of the epilimnion will vary the least, allowing the fish half a chance to keep the conditions about them as stable as possible under the circumstances. If this coincides with a likely feeding spot then the angler has a good chance of catching.

In reality, the exact form of the internal seiche is probably a lot more complex than this simplistic illustration but it does provide a starting point for selecting a mark from which you can move if the fishing is poor.

In periods of intensely cold weather, the surface of the lake may even freeze, preventing wind energy from mixing the water and redistributing oxygen. At such times an inverse stratification may occur, with the warmest water at the deepest part of the lake - usually around four degrees centigrade - rising to zero degrees at the surface.
Frozen lake showing inverse stratification.
Shallow Lakes and Fisheries

Stratification in shallow lakes and ponds will be the result of settled periods of hot weather and are only likely to last as long as there is little wind to carry out the normal mixing process. They do not tend to last long because storms and strong winds can quickly disrupt their formation and return the lake to normal. This is why they are called polymictic - mixed many times - where a complete mixing of the water happens often throughout the year.

The amount of plant life and the density of the fish population will also impact the amount of dissolved oxygen present in the water. At most fisheries, photosynthesis may be just as important as the actions of the wind since, during sunlight, shallow water plants, algae and phytoplankton put a lot of oxygen into the water. However, during the summer, overstocked venues have oxygen crashes during the night since plants can only photosynthesize during daylight hours. At night, the process reverses and they take in oxygen while simultaneously releasing carbon dioxide. This situation is then made worse by other organisms continuing to use oxygen while breaking down decaying organic matter such as rotting vegetation.

Fisheries, to prevent this causing damage to their stocks, combat these oxygen crashes by installing large, commercial aerators. There are several types of these - splash, air injectors, water circulators and combinations - but they all have one aim; to put oxygen back into the water.
Aside from the temperature, season and wind, the lakes that you fish will all be affected by their shape, depth and the lifeforms present, from the plants to the humblest invertebrates to the species that are either naturally present or which have been stocked. On all of these lakes, the density of food organisms – and therefore the fish predating upon them – will be influenced by how much sunlight reaches the bottom. In the Winter, when water temperature and oxygen levels are uniform, the fish may feed in the deepest parts of the lake, often scouring for creatures such as bloodworm. At such times, deeper parts of the lake may fish well. In the Summer, when the deepest parts are low in oxygen, they may fish very badly.

For Winter fishing, I make more use of contour maps – maps showing not only the shape of the bottom but also giving you an idea as to the speed at which each feature rises or drops off – than I do in the Summer. If you look at a map, you will see some lines gathered closely together. These tell you that the bottom is shelving sharply. In other parts of the map, where the lines are further apart, they give you the idea of a much gentler slope.
An imaginary lake but useful for giving you the idea as to how the bottom changes in both shape and depth. Here the colour coding system has used dark blue for the greatest depth, lightening as the water shallows. Some features are not visible from the surface but may be found by plumbing the depth.
Just by adding a few plants, the whole situation changes.
The same logic applies to water plants. Some provide handy stalks for dragonfly and damselfly nymphs, amongst others, to climb to the surface world, leaving the aquatic world of their nymph forms behind, while others offer handy background camouflage to protect them from the predators that are actively seeking them out. The tench and carp that cruise around the beds of flowering water plants are not, after all, doing so to admire the flowers; they are looking for the creatures that hide in such places, i.e. their next meal.

In the Summer, these areas become noticeably richer in life. They are, however, actively warmed by the sun so that the hunters of such insect protein will find it much more comfortable to explore them in either the early morning or the evening when the water is cooler. For some, such as the tench, this has become a way of life.

Other features will also have an effect. While predators will seek cover amongst the plants so that they can rush out and ambush their quarry, others are quite happy to take advantage of momentary lapses on the part of their prey species. If you watch trout, for example, where they line up facing the head of any little stream that empties into the lake, you will see them hold position and then dart to the left or right as they become aware of insects being swept along by the current. It takes a little energy to hold their position but, in terms of calories, this strategy pays off because of the amount of food that drifts their way.

It is also quite interesting to watch how, when there are multiple trout waiting, it is almost as if a pecking order has been established. In deeper water, the largest trout takes up the prime position nearest the stream while the other trout line up behind it in descending order of size. If the water is shallow, this may not be the case, but it happens often enough to be a noticeable phenomenon.

Manmade features also make a difference. As I have already mentioned, many fisheries have aerators, which not only help to keep the water aerated but also seem to have an effect upon the feeding patterns of the fish, whether they are splash, tulip jets, air injectors, water circulators or even combination aerators. My favourites are the tulip jets, which carp, in particular, seem to like being close to.
Some of my best catches have come from close to tulip aerators. On one occasion, fly fishing for carp, I caught twenty-four doubles. Twenty of these were ghost carp, the only occasion when I caught such a concentration from this particular lake.
Islands can also be great places to target. In the Summer, when the shallows are nice and warm, carp will come into very shallow water, patrolling the very edges of the islands, even if they are only a foot or so deep. In the Winter, especially after a frost, the opposite will apply with the carp going to the deeper slopes although, if the day begins to warm up, they might rise a little bit later on. These are the occasions when localised effects of temperature, especially frost or sunlight in shallow water can make a big difference to the angler’s catches. It just pays to be observant.
A stratified, temperate lake is likely to be a body of water of considerable size and depth.
When visiting a new lake, it is always a good idea to try to understand its topography - the natural and man-made features of the lake - before starting to fish. This will include an understanding of its depth, structure and features, terrestrial cover, i.e. trees and bushes overshadowing the water from the land, time of year and effects of temperature, prevailing winds and manmade features. Some fisheries and lakes will even have contour maps illustrating the depth and underwater features of the lake. This can be very useful though its interpretation will be affected by the temperature and the time of year.

Deep, Temperate Lakes

In a deep lake - and we are talking about something fairly substantial here, perhaps with a depth of twenty five metres - the water becomes fairly uniform in temperature during the early Spring and the late Autumn, even though the shallows may be subject to localised warming when the sun is out. Oxygen levels will be good at all depths because, since the water is isothermal (at the same temperature) there will be enough energy from the wind to mix the lake waters thoroughly, thereby carrying oxygen to every part of the lake and ensuring that it is uniformly distributed throughout.

During the Summer, when the weather warms up, prolonged sunlight will have a stratifying effect upon the lake. As the surface heats up, the top layer of water will begin to be warmer than the layers beneath. Its density changes, becoming lighter, while the cold hypolimnion remains much denser. This has an impact upon the oxygen levels of the lake since oxygen is not transferred easily between water at different temperatures and densities. Before, when the water was all the same temperature and density, there were no problems. In the Summer that is not the case.

As the top layer of water warms, the lake begins to stratify into three distinct areas. At the top is the epilimnion, warm water which is oxygenated by the wind and whatever showers fall, while beneath it is the metalimnion. Think of this as a buffer between the warm waters above and the colder waters below. It is the point where the waters begin to actively cool, which means that the density of the water will be changing too. Within it, at the point of the most rapid temperature change, will be the thermocline. The colder waters below the metalimnion are known as the hypolimnion.

In our lake, with a depth of 25 metres, the epilimnion may be six metres deep, the metalimnion extending from 6 to 10 metres deep and the hypolimnion all the depths below, i.e. 10 to 25 metres deep.

The top layer, the epilimnion, is receiving oxygen from the wind but the normal mixing process does not take place because the layers beneath now have different densities of water. This means that the bottom layers of water are not receiving any new oxygen from wind energy; they only have whatever oxygen was present at the end of Winter and any oxygen produced by plants through photosynthesis. If it is a deep lake, where little light penetrates to the bottom, that will have little impact while decomposing plant matter, which uses oxygen in the process, will have a negative impact upon the amount of oxygen present.

If you think of fish behaviour, in the Winter you may encounter species at any depth. Temperature is fairly uniform, which means that oxygen levels are pretty much the same throughout the lake and the fish can feed wherever they like. In the Summer, oxygen levels are often best at the top while the water is also nice and warm, driving the fish up.

If you think about it, it is no coincidence that carp are seen much nearer the surface in the Summer, when they are very happy to feed upon floating baits, and deeper in the Winter when they can more easily access natural sources of food. They like to bask so as to warm up their metabolism. This aids both weight gain and their capacity to reproduce. With increased bioactivity in the surface layers, there is plenty of food available and this is the major reason why lots of species begin to feed at or near the surface during the Summer.

After stratification, as the weather cools, the top layers of water begin to cool and sink. The bottom layers, which are now warmer, rise, with this turnover of water layers giving the process its name - Lake turnover. When the process occurs twice a year, such lakes are termed dimictic. Other lakes might be monomictic, where lake turnover happens once a year, while some other, shallower lakes might even be polymictic, meaning that lake turnover happens much more frequently, in many cases being so shallow that stratification, when it does occur, tends to last for relatively short periods of time.
Looking at the map, you begin to get ideas of routes that are used by the fish in travelling from one part of the lake to another. You can see, for example, where the island helps to channel the fish into deeper areas – remember they need to evade aerial predators like cormorants etc as well as piscine hunters like perch – and areas which are likely to fish better in the Winter than they do in the Summer. The points might also be useful if the lake holds large numbers of perch or other predatory species. The sides, especially the steeper shelving sides, provide hotspots where predators can corral smaller prey species and gain some high calorific meals with minimal expenditure of energy.

For predators, this is always a factor. If they can get a good meal without spending too much energy swimming it down, then this is an ideal situation. It would, after all, be unwise for a predator to use nearly as many calories capturing its prey as it obtains from the meal. This is why ambush points become so important to species such as perch and why contour maps do not provide all the information that you need to make an effective decision as to where to fish. You also need to look at cover, plant life and areas that receive the greatest amount of sunlight. A tree, for example, might provide shade from which a predator can emerge to target its prey but, in the winter, that very shade stops that particular patch of water from warming up while dead leaves also make it difficult for your tackle to fish properly.

Even the type of tree can make a difference. A native tree is likely to be rich in insect life with some of its inhabitants accidentally, and quite often, dropping into the water below. Many fish species realise this so their life expectancy, at that moment in time, becomes noticeably shorter. Trout, in particular, soon wise up to such free meals, just as they do the hatches of insects that seem to accumulate beneath or around such native trees. By contrast, non-native trees harbour only a tiny fraction of that insect life so their attraction is nowhere near as great. If you ever want to see that in practice then put a white sheet on the ground beneath a native tree and hit some of the lower branches. You will be surprised at how much insect life is held. Repeat the same experiment with a non-native tree and you will see the opposite effect, the tree holding nowhere near the same quantities of insect life. With no free meals to attract them closer, there are far fewer fish to be caught.
Module 1: Fish Biology
Module 2: Invasive Species
Module 3: Fish Diseases
Module 4: Topography - Lakes
The Sea
Module 5: Angling and Mental Health