Ask SAIL: Fix a Misleading Sonar

We just returned from cruising the Bahamas and were intrigued to see that our new Si-Tex CBS-128 color sounder/sonar unit regularly showed a thin orange blanket of fish hanging above a sandy bottom about 50 feet down. My kids kept trying, but couldn’t catch a thing! This happened regularly throughout our trip.
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H. Grossman of Pensacola, Florida asks:

We just returned from cruising the Bahamas and were intrigued to see that our new Si-Tex CVS-128 color sounder/sonar unit regularly showed a thin orange blanket of fish hanging above a sandy bottom about 50 feet down. My kids kept trying, but couldn’t catch a thing! This happened regularly throughout our trip.

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Gordon West replies:

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Your powerful Si-Tex sonar was showing you things a simple sounder would miss. The sonar can detect abrupt changes in water temperature where warm water overlays much colder water below. Remember diving down to retrieve your sunglasses and how the water suddenly went from warm to icy cold? Same deal here. That stratified temperature variation, known as a thermocline, is sharp enough to register on your high-powered equipment. Use the manual gain control to turn the gain down, and you’ll see this phenomenon disappear, but will still have good bottom readings.

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