Fish Finders and Thermoclines

Fish Finders and Thermoclines

By Iain Loveman

I have seen that some fish finders can measure bottom and surface temperature but I want to know if fishfinders can measure temperature levels with the depth?

What we are talking about here are thermoclines but first we need to clarify your understanding of thermoclines.

Thermoclines are layers in a body of water where colder water intersects or mixes with warmer water. These layers separate the cold water from the warm water.

Now, the speed of sound thru water is greatly dependent on water temperature and if the temperature differences are large enough they will allow the return of echoes to the sonar but you have to know what you are looking for and how to find it.

Knowing where the thermocline is an aid in helping you to catch more fish. It is well known that fish will suspend in or around (above or below) the thermocline. Just about any properly set up sounder these days will show you a thermocline and if there is one to be found, up to a depth of 50 to 65 feet.

You could also be referring to current breaks or below surface thermoclines. Current breaks will be detected by a temperature sensor that measure the water temperature at the surface and this really the only reading your going to get from the transducer.

However, thermoclines can show up on a fish finder as a line depending on installation and settings. Now, it’s hard to say which transducer, fish finder combination will show this as there are too many models, settings and parameters to consider.

The transducer is not going to be the only variable in your ability to see a thermocline. If the sounder or head attached to the transducer doesn’t have the sensitivity it will not matter how good your transducer is and the end result will be to see nothing. A lot of anglers have mixed viewpoints on whether or not the unit should be color or grayscale.

In order to find the thermocline you must ensure your unit is in sonar mode. Then increase the sensitivity until you view a weaker sonar reflection from the thermocline. Remember, there has to be a large enough difference and a quick enough change vertically in order for it to display as a thermocline or it will not reflect the sonar signals.

Now, physics dictates that the density of the water changes with it’s temperature therefore the sonar signal will reflect off the change in water density. Remember there still has to be a drastic enough change in water density to change the sonar signal.

Another thing to consider is that to get something you have to give something in return. Now, a lot of debate has been made about, tuning your sounder to find the thermocline.

Do you lose your ability to mark baitfish or fish and are you getting true readings?

The other question is are you really marking the thermocline are you tuning into the density of the suspended material or plankton?

In either case the end result is that you have located an area where fish will tend to locate.

Another option you may want to consider is to attach a speed and temp probe to a downrigger. These units work by attaching a sensor just above a downrigger ball, then lowering them thru the water column at the same time the signal is sent to a receiver or transducer on your vessel.

Depth Sounder Mounting The Transducer

What you are shown is a temperature and speed reading from the depth of the probe and at the surface. Now by lowering and monitoring the probe you can determine the affect of current and temperature at depth. It is my impression that these probes are not designed to be used while cruising. I would say that the majority of serious great lakes salmon fishermen have been using this method for years albeit be in a freshwater situation.

One final point in trying to locate thermoclines is your system mounting. Let’s say you have the top of the line transducer and hooked it up to one of those high end, high priced sounders, it will all go to waste if the location ends up totally destroying the performance.