Pike can be found in just about every kind of freshwater environment available. You can catch them in rivers, drains, reservoirs and even your local pond but there’s one kind of water that’s almost synonymous with pike fishing, the great glacial lakes that exist in the more mountainous regions of our islands. Big, deep, ice-cut waters present a wonderful environment for the pike and in many ways, a tremendous challenge for the pike angler. Many fail to get to grips with the big lakes and often return to their local pools disgruntled and crestfallen. Deep waters have a character that is quite different to lowland waterways and in particular, they exhibit a yearly cycle that dominates the lives of the creatures that live within them. Understanding this cycle is a fundamental part of pike fishing in the big lakes and failing to take account of it can so often be the route to failure.

Let us take a look at this cycle. We’ll start our year in May, a time when many might be hanging the rods up, and find out what’s happening to the lake and more importantly, how the pike are affected.

As spring progresses and eventually moves into summer, so the water in deep lakes starts to stratify, or form layers. There are essentially three layers, the surface layer or epilimnion, the thermocline and the deep layer or hypolimnion. The science behind the thermal stratification of lakes is well established and there is a lot of information available on the internet so I won’t go into great detail about that here - what you’re really interested in is how it affects the fishing.
A great deal of attention is paid to thermoclines by the angling world – especially in America – and in some ways this excitement is justified. The other two layers, each in their own way, can be hostile environments to fish like pike and so it is no surprise that the thermocline sometimes becomes the place to fish. I will take each layer in turn and see if we can work out what it is about them that makes the pike love them or loathe them.

Epilimnion
This is the topmost and warmest layer of water; it’s also called the mixed layer and has a fairly uniform temperature. As summer progresses, the epilimnion becomes more stable and extends deeper. The water in it gets warmer and warmer and it is this that makes the pike uncomfortable. Warm water holds little oxygen while a warm fish needs a lot, so a cold water species like the pike is going to struggle to stay in the epilimnion during the heat of the day. You’re most likely to find pike here during the cooler parts of the day but there are circumstances when they will stay in the epilimnion, most notably in amongst weed beds, where there is shade and the oxygen levels may be higher, or on a windward shore where wave action has caused oxygen enrichment.
Ok, you’ve fished the weed beds and caught nothing – where are you going to find your pike then?

Thermocline
To understand the nature of this layer, one only has to look at the word itself. Thermo (as in thermometer and thermostat) refers to temperature while cline (as in incline or decline) refers to a gradient or slope. So a thermocline is an area where there is a temperature gradient and in fact the top of the thermocline will be at the temperature of the eplimnion while the bottom of it will be at the temperature of the lower layer, the hypolimnion.
The thermocline is unlikely to start shallower than fifteen feet deep and may extend as deep as 40 feet in hot weather.
Significantly, while there are areas of cooler water in a thermocline, there is still a reasonably good level of dissolved oxygen, though this will vary considerably from lake to lake.
In theory, pike and bait fish will be found in this area, making the most of comfortable temperatures and oxygen levels but does the theory stand up to scrutiny when catches are looked at? More of that later.

Hypolimnion
This is the deep, cold layer of water that forms because water at around 4 deg. C is at its densest. The biggest problem with the hypolimnion is that it may have no oxygen in it. Dead phytoplankton and vegetation fall into this layer and start to decay, thus using up the available oxygen. There is no mixing with the upper layer as the water is too far away from wave action and the thermocline acts as a barrier between the top and bottom layers.

Once again, there is variation between lakes but in some cases, particularly in nutrient rich (eutrophic) waters, the hypolimnion can become completely devoid of oxygen about a month after stratification starts to occur in the spring. Lakes with very low nutrient content (oligotrophic) on the other hand may well have oxygen in the lower layer right through the year.

Phew, that’s the jargon out of the way! After all that, we still aren’t quite sure where the pike are during the summer months. Well the theory states that the pike won’t be very deep due to the lack of oxygen in the lower layer, but neither will they be very shallow due to the high water temperature, which increases their need for oxygen. They are most likely to be sitting in the thermocline and this presents a problem. The thermocline forms a kind of “false bottom” to the lake and if fish are sitting in it, they could be just about anywhere, suspended over any depth of water. Anglers are used to finding pike close to “structure” such as drop-offs, gullies and plateaux but a thermocline has none of these, it’s just a band of water so the pike are naturally going to be very hard to find.

Now to be honest, I don’t do that much pike fishing in the summer, preferring to spend my time fishing for tench and barbel – but I know a man who does, a certain Mr Kelbrick. I gave Dave a call in the middle of August to find out if his catches coincided with the theory – here’s what he said;
“We’ve been catching on lures mate.” He said “but they’ve been at a depth of thirty feet, fishing over water seventy feet deep.” Dave’s a prolific catcher of pike and his findings bore out the theory perfectly! So the summer pike Dave was catching were sitting suspended on the thermocline and could be well spread out all over the lake over any depth of water greater than thirty feet. Summer can be a tough time to find the pike, especially if you are a bank-only angler.

Turnover
As summer gives way to autumn the upper layer will start to cool and become more comfortable for the pike. It’s at this time that many people start to catch consistently and water between ten and thirty feet deep can be very productive on both lures and baits. October is a favourite fishing month for many pikers and this is because the pike have come back close to the banks where they can be found in around that “structure” we talked about. The deep water is still going to be a hostile place for them so they become fairly well concentrated in the relatively shallow, but cool areas.
As this cooling continues, so the temperature of the upper layer starts to approach that of the lower layer, the thermocline becomes thinner and less significant and eventually a point is reached when the upper and lower layers are the same temperature and the phenomenon known as “turnover” occurs.

Turnover is the point in time when the whole water column becomes mixed. To get there, we really need periods of frost, to cool the upper layers down close to 4 deg C followed by a strong wind to set up the underwater currents needed to mix the layers. When turnover occurs the deoxygenated water that has been trapped for so long in the lower layer is mixed in with the rest of the water and the fishing is likely to suffer as a result, maybe for a couple of weeks.

The post-turnover period is the time that excites me the most in these big lakes. Many American articles will cite turnover as occurring in October but the onset of winter is much faster and more severe in a huge continental landmass like North America than it is in our islands so turnover will be later here. Indeed, the influence of global warming and the very mild winters we have had in recent years are tending to push it back even further, probably into December and I find that it takes until around Christmas for the fish to settle down and start feeding seriously.

Bait fishing seems to work best just now and the fish are often to be found in quite deep water. It’s common to catch on the bottom in 35ft and I’ve had many post-turnover pike in depths of more than 50ft. We’re close to spawning time and the fish need to feed in order to put on weight and develop eggs and so on occasions, a bonanza can be on the cards. It’s at this time that the smaller fish seem to disappear and most of the fish I catch are double figures or better!

The Spring bloom
The turnover of the column of water during the Autumn/Winter period affects much more than water temperature. All those nutrients released from decaying vegetation and phytoplankton that have been locked away in the lower layer are released into the upper layers where they become subjected to the effects of sunlight.
In fact, this has little impact at first since when the turnover takes place, sunlight is in short supply and may be decreasing, it is in the springtime that this phenomenon becomes important. During the spring, day length increases and the angle of the sun becomes steeper, so allowing light to penetrate more deeply. Moreover, the water begins to warm up a little and biological activity becomes more intense. It is at this time that algae in the water start to make use of the supply of nutrients and multiply, resulting in a spring bloom.
The spring bloom comes at an awkward time as far as pike fishing goes. It usually happens shortly after spawning and at a time when the pike, though rather lean, should be feeding hard to get themselves back in condition. I find that the coloured water that the bloom brings about can be detrimental to piking and to lure fishing in particular, though the fish can sometimes still be tempted to take a deadbait, as is often the case in conditions of low visibility.

All is not lost
If a spring bloom affects your fishing, all is not lost; there are a few things that you can do to keep catching. The first one, as already mentioned, is to switch methods. If the lure fishing has slowed up, try bait fishing for a while. The fish may not have moved very far from where you were catching them on lures before the bloom set in so there’s no need to go chasing all over the lake but a bit of faith is needed if you’re going to make such a fundamental change in your approach. Of course it’s easy enough to mix the methods anyway, just fish one rod on a deadbait and fish a lure with the other – but don’t take your eye off that float!

Another thing to bear in mind is that different waters will exhibit a Spring Bloom at different times and to a different extent, so it’s possible to avoid the worst of the bloom altogether. I had a startling example of this early this year when, during early April, the lake I was fishing (let’s call it lake “A”) suffered a bloom and the fishing slowed up. I switched to another lake (lake “B”) where the water remained clear, continued lure fishing and caught a stunning 26lb 11oz fish on a Squirrley Burt.

The two lakes are actually in different countries and, though both are large and very deep, have quite different characters. Lake “A” has extensive areas of shallow water, so warms up relatively quickly, resulting in an early bloom whereas lake “B” is sheer sided with hardly any water of less than 30ft depth. What’s more, lake “A” is nutrient rich due to the influence of farming activity and the presence of several towns and villages along its watercourse. In the event, lake “B”’s spring bloom took place fully a month after the other lake.

This article was first published in Pike and Predators magazine