Structure: Sticks Stones and Thermoclines
Written by James Athey - Team Experience Outdoors   
Monday, 04 June 2007 10:37

The so called intellectuals in the literary world have lost a lot of respect in my eyes today. Not a single dictionary that I resourced had a reference to fishing in it’s definition of the word “structure”. Looking to the sanity of the fishing world, I checked some fishing dictionaries and got mostly references to changes in the bottom contour. A few expanded their definitions to include vegetation in the form of  weeds and tree stumps and the really forward thinkers went farther yet to also include manmade features like bridges and sunken vessels. I’d like to attempt to expand on the definition of structure as it applies to fishing even further to include the water itself. No I haven’t bumped my head again, well not yet today anyways.

Structure: (n) Any physical feature that affects the movement and behavior of fish.


This definition allows us to discuss physical features that aren’t permanent objects, or for that matter aren’t even objects at all. There are physical features in the water itself that, without a doubt, affect the movement and behavior of fish. Temperature breaks and color lines are physical features that  are often overlooked when structure is discussed.

Image Changes in the color and clarity of the water can be caused by such things as wave action kicking up sediment from the bottom, rain run off, and especially streams or rivers flowing into other streams or rivers or lakes. The result is a color line commonly referred to as a mud-line. Even though the mud line isn’t a barrier to physically direct fish movement, it does just that. To understand how, you have to think in terms of the food chain which is what every fish I have ever caught was doing at the time of their demise. The colored water is not only rich in nutrition for the forage fish, it also provides a perfect ambush along the break. Any fish swimming along in the stained or muddy water is limited in visibility and might not see the hungry predators waiting to snap them up when they cross over into the clear water. Temperature can also play a factor in the off colored waters fish attraction as it is often warmer water and so the best fishing might be on the muddy side of the break, a little experimentation will help determine which side of a color line will be most productive for the species you are targeting. Often crossing from one side to the other will trigger strikes. In stained or muddy water, bigger is often better when it comes to bait selection. Rattles or other noise makers are helpful in a fish successfully making contact with your lure. I opt for gold or copper finishes over silver or chrome. Glows are good and chartreuse is go to color in muddy water. Slow down a little in your presentation. This will give the fish more time to lock in on your offering. Several species of fish will congregate around the mouth of their natal river long before the urge to spawn sends them upstream. They will spend a lot of time feeding in the “mud” so don’t disregard it as a place to start your search for those species at any time of the season.

Image Temperature affects water density and the cold, denser water will usually settle to the bottom as a lake stratifies into different layers of temperature. These layers often have defined breaks between them where the temperature changes significantly.  These temperature breaks form the horizontal pieces of structure referred to as a thermocline. Once again you have to think in terms of the food chain to understand how thermoclines affect fish. At the bottom of the chain are zooplankton which are tossed about by the movement of the water. Along with dead insects from the surface, the zooplankton will settle through the warmer upper layers of water until they reach the thermocline. The cold water below the thermocline is denser and the tiny offerings drifting down from above will jam up and collect at this level of the water column. The forage fish come to feed on them and the predators are never far behind. On larger lakes and the oceans temperature breaks will manifest vertically as well and can be easily seen on the surface. These breaks are much like color breaks in that regard. As with the color break and thermocline, a little experimentation will tell you which side of the vertical break will be most productive. There is often current present at temperature breaks so speed and direction of troll can be big factors in your success.

Thermocline Diagram Note: The temperature through the mixed layer and deep water are fairly constant while it changes rapidly through the thermocline.

Finding a good break or color line is only part of the battle. Water structure like color and temperature breaks can be vast so it is important to try to find areas where they coincide with other pieces of structure such as where the thermocline intersects a drop off to deep water. Having found that but no fish, try to locate a third piece of structure such as a rock pile or hump in the area where the thermocline and drop off intersect. There is an old adage about structure which states that if you find good structure but no fish, there is probably better structure nearby.

Fish are cold blooded killers. Well cold blooded predators might be more “pc”. As with all cold blooded creatures, temperature plays a big role in their lives as does maximizing energy intake with minimal energy output. Whether you agree that temperature breaks and color lines should be officially considered structure or not, you can’t deny that they are physical features that affect the movement and behavior of fish and should not be overlooked as you set forth in search of the big ones.

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