The thermocline can be seen most easily on down-scan images in the wide sonar beam of 455kHz (top right) while High CHIRP with a smaller beam (lower right) captures a better image of baitfish and fish. You can see the thermocline in the side-scan image (left) but it is more difficult to discern and interpret. (Image Capture by Brad Wiegmann)
Seeing Thermoclines, Why does it Matter?
by Brad Wiegmann
You will probably see a thermocline in your favorite reservoir while out fishing during the next few months. In the warmer months a thermocline is typically found in areas with deeper water throughout reservoirs where there is little current or water exchange. A thermocline is the transition layer between the warmer water at the surface and the cooler water below.
The thermocline creates an invisible barrier that prevents dissolved oxygen from mixing into the lower, cooler layer of water. Fish often seek out a compromise – deeper, cooler water that still holds an adequate supply of oxygen. Knowing where that oxygen barrier is can help fishermen know how deep fish may be.
The best way to determine if there is a thermocline and/or how deep it is, is by simply turning up the sensitivity on most of today’s modern sonar units. Echoes from an actively pinging sonar unit will hit the cooler, denser water and reflect, resulting in the cloud-like image on your screen revealing the thermocline.
In 2D traditional sonar or CHIRP sonar a thermocline typically looks like a long, reflection of small dots closely grouped below where temperature change is located; in addition to dotting of specks sometimes thick above or mixed in with fish.
Thermoclines are easiest to see with down-scanning sonar from 455kHz, 800kHz to mega 1.2MHz frequencies. The thermocline will appear as a thick dense cloud. This dense cloud appears below the thermocline and will increase in brightness as the water temperature changes. Unlike 2D Traditional sonar or CHIRP sonar down scanning sonar will continue to show clear structure or cover like brush piles even in the thermocline.
Anglers can change beam angles to see more or less areas below the boat. A wide beam 2D traditional or CHIRP sonar image will be more clutter as it reflects a thermocline at great distances with a bigger cone area. This makes side scanning sonar or down scanning sonar a better choice for viewing game fish and baitfish when there is a thermocline.
Changing the contrast settings higher will increase the reflection of a thermocline making it brighter and easier to see especially in deep water. Anglers fishing in reservoirs in gin clear water like Ozark highland reservoir will find thermocline setting up at deeper depths compare to shallow water reservoirs. By lowering the contrast an angler can decrease the clutter, but also make seeing game fish or baitfish harder.
By far the hardest and the easiest to interpret when it comes to seeing a thermocline is live-imaging sonar (LIV). Anglers that have hours and hours of time watching LIS will recognize it quickly. A thermocline will typically appear as a reflection or cloud below a gap where baitfish, game fish or rough fish are being seen. Due to the fact LIS shows everything and more like ghost trees or seams within the beams some of the images will show false returns. Expert LIS anglers will not have trouble making sense of extra clutter appearing on the sonar imaging, however, the biggest advantage is seeing where the thermocline is from more than 100 feet away.
(From Springdale, Arkansas, Brad Wiegmann is a professional photographer, videographer and outdoor writer published in dozens of print and electronic media outlets all across the nation. Every month Wiegmann will be providing CatfishNOW readers with the latest and greatest information on the ever-changing world of fishing electronics. He is also a well-known guide, podcaster and social media influencer. Learn more about Wiegmann and his lengthy experience and expertise at BradWiegmann.com.)
