Freshwater trolling 101: How deep do I need to go?

Your blog author with a small lake trout caught in Sebago Lake in late-April trolling 10 ft deep in 25 ft of water

Your blog author with a small lake trout caught in Sebago Lake in late-April trolling 10 ft deep in 25 ft of water

Trolling refers to a fishing technique where a lure or live/dead bait is positioned in the water column behind a boat and pulled forward by the movement of that boat. It is a highly-efficient approach to target certain fish species and cover a lot of water in ponds and lakes during the open-water period. One common question I get from novice trollers pertains to how deep they should place their lure or bait. The answer hinges on two key variables, namely: (a) the time of the year, and (b) the target species.

The time of the year affects water temperature in predictable ways. The basic principle is simple: water of different temperature has different buoyancy. Water expands (i.e., becomes less dense) when it warms up, causing it to stay on the surface, whereas colder water is more dense and sinks.


In early spring following ice-out, the temperature of the water column in a lake is uniformly cold at around 40°F from top to bottom. The water is therefore equally dense everywhere and can freely mix. As spring progresses, and the air temperature rises, the surface water warms up and becomes more buoyant than the deeper water which remains colder and denser.


This process starts to separate the water column into two distinct layers by mid to late spring. That divide becomes intense in the summer with the formation of a thermocline, which represents a sharp temperature boundary, typically located anywhere between 25 and 40 ft deep and which physically separates the warm and buoyant surface water (60°F or more) from the cold and dense water below (50°F or less).


This whole process reverses in the fall when colder air temperatures start cooling down the warm surface water to the point where it is as cold and dense as the bottom water, thereby allowing both layers to mix again in late fall in an event called “turnover”. The take-home message is simple: lakes have distinct water temperature profiles during spring, summer and fall, which profoundly affects where certain fish species will be found.


Freshwater sportfish species in New England fall in two broad categories, namely cold-water species and warm-water species. The former consist of the salmonids (i.e., lake trout, landlocked Atlantic salmon, brook trout, splake [a sterile hybrid obtained by breeding a lake trout with brook trout), brown trout, and rainbow trout), whereas the latter include largemouth bass, smallmouth bass, white perch, pickerel, and several others.


Each group has different temperature preferences which affects where they can be found in spring, summer, and fall and hence how deep to troll for them. The salmonids, and particularly lake trout and landlocked salmon, need cold (<55°F) water in order to thrive. In the spring, when the water temperature is still comfortably cool, all salmonid species will be found throughout the entire water column, including right up to the surface. This pattern is particularly true from late April (I live in southern Maine) into early May when the rainbow smelt emerge from the depths of lakes and swim in large schools into shallow shoreline areas to migrate up their spawning streams to reproduce. The cold water at that time allows lake trout and landlocked salmon to venture right up into those shallow areas to gorge on these smelts.


As spring progresses and the smelt migrate back out of their spawning streams to return to deeper water (typically in early May, depending on when ice-out occurs), the lake trout will start avoiding the warming surface waters. However, the landlocked Atlantic salmon and the trout species will continue feeding right up to the surface into early June until the mayfly hatches cease. Then, the salmon mostly disappear into or below the thermocline to avoid the warm waters above during the hot summer months.


The seasonal movement patterns for the warm-water species are different from that of the salmonids. While they obviously cannot avoid the ice-cold waters of early spring, the warm-water species will reliably stay in the top 20-25 ft of the water column as spring progresses and the thermocline gets established. In the summer, they will stay above the thermocline, with a preference for cooler deeper waters during the day and warmer shallower waters in the evening and early mornings.


My son Joel with a much nicer lake trout caught in Sebago Lake on July 4th trolling 50 ft deep in 143 ft of water.

My son Joel with a much nicer lake trout caught in Sebago Lake on July 4th trolling 50 ft deep in 143 ft of water. The take-home message? Adjust your trolling depth depending on the season.

How does all of that information translate into trolling depths? The depth ranges presented below provide rough estimates by season and species for the state of Maine based on my personal experiences. Keep in mind that the best trolling depth on a particular day or lake will be greatly influenced by a combination of factors, such as the time of the day, the clarity of the water, the amount of cloud cover or sunshine, the choppiness of the surface water, and the time within a season, among others. The key is to stay flexible and attuned to the prevailing conditions and adjust accordingly.


Spring is defined here as the period between ice-out (typically, around mid-April in southern Maine) and early-to-mid June.

Summer is defined as the period between mid-to-late June and late-September

Fall is defined as the period between early-October and mid-December.

Landlocked Atlantic salmon: (a) spring: 3 ft – 15 ft deep; (b) summer: below the thermocline (> 35 ft deep); (c) fall: 15 ft – 35 ft deep.


Lake trout: (a) spring: 5 ft – 35 ft down (towards the deeper end of this range as the season progresses); (b) summer: well below the thermocline (> 45 ft deep); (c) fall: > 25 ft deep.


Other trout species: (a) spring: 5 ft – 25 ft deep (towards the deeper end of this range as the season progresses, particularly for splake); (b) summer: 20 ft – 30 ft deep (stay right above the thermocline; the trout may also swim closer to the surface to feed in the evenings or early mornings); (c) fall: 5 – 15 ft deep.


Warm-water species: (a) spring: surface – 10 ft deep; (b) summer: 10 – 25 ft deep (stay above the thermocline, and shallower in the evenings and early mornings); fall: 5 – 20 ft deep.


One last note of caution: make sure to understand the lake you plan to troll if you’re going after salmonids. If you know that your target lake has landlocked Atlantic salmon and/or lake trout, then you can be certain that it will also develop a thermocline in the summer since those fish species cannot otherwise survive. But here’s the twist: not all lakes containing trout species develop a thermocline. Unlike the landlocked salmon and lake trout which require large expanses of cold, oxygenated waters, the other trout species can survive in smaller ponds and lakes by seeking shelter around groundwater springs that emerge from the bottom. Catching them trolling in the summer becomes a matter of pure luck and probably not worth the effort. It is only in the spring and fall that these trout can be found swimming and feeding throughout the water column.


Was the information in this blog useful? I invite you to share your thoughts and opinions. Also, feel free to discuss your fishing experiences at this location.


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2 thoughts on “Freshwater trolling 101: How deep do I need to go?

  1. I asked this question on another blog, I will ask you also.
    Are the characteristics of the Atlantic Landlocked salmon in Northern California the same as the salmon in Maine?

    • Hi Jeff, here’s my opinion… I’m assuming that a largemouth bass in CA will behave substantially the same as a largemouth bass on the east coast. I’m therefore assuming that the same principle would hold for the landlocked Atlantic salmon. Please let me know what you find out at your end of the country. Stan

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