The month of April is in many angling circles believed to be one of the easiest months. I have never found this to be the case. Although the fish tend to be somewhat naive in their feeding habits, this is of little help since they happen to feed very little at this time of year. This is due to the cold temperatures and it is not really until the end of the month (at least) before things really start to heat up (literally and metaphorically). When the fish do happen to feed, it is generally at a depth where we can't actually see them, which doesn't help much either.
I think April has it's easy reputation due to the fact that when the fish are in the mood, they are not particularly choosy about which fly you have on, and also the fact that persistence during this month actually seems to pay off.
Easy or otherwise, the first thing we have to do is to find the fish. In order to do this it helps to understand a little of the properties of lakes and the water in them. Water is densest (heaviest) at 4 degrees Celsius. Ice, as we know, is formed at 0 degrees C. So, what this all means, is that in the winter, when the water temperature drops below 4 dregrees C, this water starts to float on the heavier water beneath, so that when it freezes solid at 0 degrees celsius the heavier water underneath stays at a nice 4 degrees C. This is why lakes rarely freeze all the way through and fish can swim happily underneath the surface ice.
Incidentally, this is one reason why some people argue for the existence of god(s). But I don't buy it.
Anyway, I am getting diverted from the real issue, which, in this case, is that during the early part of the season, the water temperature near the bottom of the lake is around 4 degrees C and the wind which is usually somewhat warmer than the water, is blowing onto the water, giving it some warmth and blowing all that warm water downwind. And this is where the fish are.
The fish are warmer here (remember they are cold-blooded things) and are more likely to be active and possibly feeding to some extent. Especially during the late morning through to early evening. So all we have to do is go to the bank where the wind is blowing towards and fish into the wind.
It helps if the wind has been blowing in the same direction for a good few days.
There are things called hypolimnion, epilimnion and thermocline. The hypolimnion is the upper temperature layer, the epilimnion is the lower temperature layer and the thermocline is the zone of rapid temperature change in between. Too much signficance is drawn over this. The only real function it can serve is that if you know where the thermocline hits the lake bottom, then you might just get some big browns in the summer months.