Field Notes / October 2026

How do you know what depth to fish?

PROBE Fishing

Quick answer

There is no fixed depth for any fish species. Fish hold where three things line up: the water temperature they prefer, the light level they hunt best in, and their food. In summer, that is often at or just above the thermocline. In dark or stained water it is shallower; in clear water and bright sun it is deeper. The only reliable way to find it on a given day is to measure the water column and log where the strikes happen.

~20 °C

Water temperature where big pike start moving deeper and cooler.

Source: Farm Forum

2×

Light sensitivity of zander compared with pike of the same size.

Source: Jokela-Määttä et al. via Ribarica

10 °C

How much the Baltic can cool over just a few metres of summer thermocline.

Source: Finnish Meteorological Institute

The problem

Your fish finder shows the bottom and maybe some arcs. It does not show the water temperature 5 metres down, how far light reaches today, or where your lure actually is. So most anglers pick a depth out of habit, and when nothing bites, they change colour instead.

01 / Temperature

What decides how deep fish hold?

Most of the predators anglers chase in northern waters are cool-water species. Pike, perch and walleye/zander belong to that group, which is why they stay active in colder water than bass or bluegill do (Bass Pro Shops). Each species has a comfort band. When the surface warms past it, the bigger fish move deeper to find it again. Big pike, for example, get stressed once the water climbs past the high 60s °F (around 20 °C) and move to deeper, cooler water, while small pike stay shallow in the weeds (Farm Forum).

So the first question is never "how deep?". It is "where is the temperature they want right now?".

With PROBE →

PROBE measures temperature at every depth your lure passes through, so a normal cast doubles as a temperature profile. Instead of reading the surface temperature on the sonar, you see the temperature where the fish actually are.

02 / Thermocline

What is the thermocline, and why does it matter?

In summer most lakes stack into layers. A warm, oxygen-rich top layer sits on a cold, often oxygen-poor bottom layer. Between them is a thin zone where the temperature falls fast: the thermocline (Bass Resource). Bait tends to gather right above it, and many lakes have little oxygen below it in summer.

Professional angler Jacob Wheeler puts it simply: "Locating the thermocline eliminates half of the water on any given lake in the summer" (Wired2Fish). In much of Sweden the thermocline sits less than 10 metres down (Wikipedia). In the Baltic Sea, the summer thermocline is 5–10 metres thick, and the temperature can fall as much as 10 °C over a few metres (Finnish Meteorological Institute).

03 / Light

How does light change fishing depth?

Light sets how high or low a fish holds within its comfortable temperature band. In clear water and bright sun, many predators hold deeper or tighter to cover. In stained or muddy water light fades quickly, and fish and bait move up. The common rule among anglers is "the muddier the water, the shallower you should fish" (Mossy Oak Gamekeeper).

Low-light hunters like zander and walleye use this to their advantage. A Finnish study found that zander have at least twice the light sensitivity of pike of the same size (Ribarica, summarising Jokela-Määttä et al. 2019).

With PROBE →

PROBE's light sensor records how bright it is at each depth. One cast tells you whether the light fades at 2 metres or reaches 8 metres today, and that tells you whether to fish shallow or deep.

04 / Food

Does food matter more than temperature?

Food is the tiebreaker. Predators follow their prey. In autumn, the baitfish that pike chased over deep water in summer move shallow to spawn, and the pike follow (The American Outdoorsman). If temperature and light say "anywhere between 3 and 8 metres", the bait tells you which metre.

05 / Method

How do you find the right depth, step by step?

Measure the water column, not the surface. In summer, surface temperature tells you very little about the water where the fish are.
Find the thermocline, if there is one. Fish above it, or at its top edge.
Check the light. Bright and clear: go deeper or tighter to cover. Dark or stained: go shallower.
Run your lure through the band, and confirm it is really there. Most lures don't run at the depth printed on the box. Line diameter, line out and speed all change it (Bassmaster).
Log where the strikes happen. After a few sessions, the pattern stops being a guess.

Measure the water column on your first cast, not after your fiftieth.

See how PROBE works →

06 / PROBE

How PROBE helps you find the depth

Every step above needs a measurement most anglers don't have: temperature at depth, light at depth, real lure depth and strike depth. PROBE gives you all four from one 7-gram sensor on your line, on every cast, without watching a screen. It stores the data underwater and syncs to the app over Bluetooth the moment it surfaces. Every cast is saved with GPS position and local weather, including the casts where nothing happened, so after a few trips your own water stops being a guess and becomes a map.

Field log / real session

Late September, clear lake, 14 m max depth. Target: pike and perch

Surface temperature	14.2 °C
Thermocline	Weak, between 6 and 8 m: 13.8 °C above, 10.1 °C below
Light	Fades out at about 5 m
Casts / strikes	58 casts, 4 strikes
Strike depths	4.6 m, 5.1 m, 5.3 m, 5.8 m

All four strikes came between 4.5 and 6 m, just above the thermocline and around the depth where the light faded. Casts that ran shallower than 3 m produced nothing.

07 / FAQ

Frequently asked questions

What depth should I fish in summer?

Start at or just above the thermocline, where the water is cooler but still has oxygen. In many lakes that is somewhere between 3 and 12 metres, shallower in murky water and deeper in clear water.

Is surface water temperature useful?

In spring and autumn, when the water column is mixed, it gives a rough idea. In summer, the surface can be many degrees warmer than the water a few metres down, so it says little about where the fish are.

Why do I catch only small pike in summer?

Small pike tolerate warm water and stay in the shallow weeds. Big pike move to deeper, cooler water once the surface gets warm. Fish deeper edges and open water near the thermocline to find them.

Should I change lure colour or depth first?

Depth. A lure in the wrong colour at the right depth still gets seen. A lure in the perfect colour a metre above the fish often doesn't.

How can I measure temperature at depth without stopping fishing?

Use a sensor on your line. PROBE logs temperature, light and depth on every normal cast, so you build a temperature profile while you fish.

From the PROBE team

[PLACEHOLDER: a short first-hand note about a day when depth made the difference.]

Stop guessing the depth. PROBE shows temperature, light and lure depth on every cast.

Get PROBE →

Sources

depth

What is a thermocline, and how do you find it?

An invisible floor in the lake. Above it: warm water, oxygen, bait. Below it, often: not much. Worth knowing where it is.