Thermoclines, wind and oxygen: why banker swims suddenly die
Course pack: Depth-temperature and wind log. Coach Willy, our virtual AI coach, can explain each step, set you a practice task and check your understanding.
Who it's for
You're confident on the bank and want the finer details. Aimed at carp, bream, tench, pike.
Where and when
Done on the bank. Best in: all.
What you'll work through
- 1Thermoclines: how deep lakes split into layers
- 2Wind: where it pushes oxygen and food
- 3Weed's day and night oxygen clock
- 4Turnover, seasons and low summer rivers
Step 1: Thermoclines: how deep lakes split into layers
Alright mate, let's get stuck into thermoclines, wind and oxygen: why banker swims suddenly die. I'll walk you through what to look for, what to try and how to tell whether it helped. We'll start with thermoclines: how deep lakes split into layers, then work through a proper practice job at each step. Take it at your own pace and keep a few notes so you've got a useful plan for next time.
By the end of this step
Have a go at: thermoclines: how deep lakes split into layers.
Willy’s take
Alright, this is the course that explains why a 'banker' swim can suddenly go dead. Deeper lakes, pits and reservoirs don't stay the same from top to bottom. In summer the sun warms the top layer, and because warm water is lighter it floats on the colder, heavier water below.
Given enough depth and a spell of settled weather, the lake splits into layers.
The top layer is the epilimnion: warm, mixed by the wind and full of light. The bottom layer is the hypolimnion: cold, dark and cut off from the air. In between is the thermocline, a band where the temperature drops quickly over a short depth.
On many UK pits that sets up somewhere around a few metres down in high summer, but it varies with depth, shelter and weather, so measure rather than guess.
Here's why it matters. The bottom layer can't get fresh oxygen from the surface. In rich, weedy or silty waters, all the rotting stuff on the bottom uses oxygen up through the summer, so the deepest water can become poor in oxygen. Fish then avoid it, even though it's cool.
That's when a deep hole that fished brilliantly in spring goes completely quiet.
Shallow lakes, usually under about four or five metres, often get mixed right down by the wind and may never layer properly. So this matters most on deeper pits and reservoirs. The rule: in summer on deep water, don't assume deep means safe.
Fish often sit above or around the thermocline, where it's cooler than the surface but still has oxygen.
On the bank
Worked example
Imagine this: it's August on an eight-metre-deep gravel pit. In spring your deep marks fished well. Now baits on the bottom in the deepest water sit untouched for two days, while fish show and bubble over the three-metre bars.
Your turn: Thermoclines: how deep lakes split into layers
- 1
Find the deepest and shallowest parts of your water from a map, plumbing or the fishery
- 2
On a warm, settled day take a water temperature at the surface and as deep as you can safely drop a thermometer
That's the free sample — this is where it stops
The full step keeps going with the rest of the practice job, common mistakes and fixes, a check-your-understanding quiz and your Depth-temperature and wind log. Then 3 more steps, each with its own task, and Coach Willy on hand to talk it through.
Start this course free ↓
You've read the free sample
Start this course free
Free members choose one available course per calendar month. Joining doesn't use your choice — you pick when you're ready.
Join free to start this course