This can't be right?

[DTro]

I read elsewhere that someone had sent a question to Lowrance specifically asking about the arch sizes on the sonar units.

The tech replied saying that the sonar works by bouncing a signal of the swim bladder of a fish and that the size of the fish has nothing to do with the size of the arch on the screen. So fish with big swim bladders will show up as bigger fish.

Generally, the bigger the fish the bigger the bladder, but there are some fish, such as a salmon, that have a small swim bladders.

Is this right? I always assumed the bigger the arch the bigger the fish.

[prov1900]

As far as I am guessing, with my experience, is that the larger arch, being the stronger signal, typically shows the bigger fish, provided 100 % of the fish is in the cone angle, and I think that is how I at least interpreted the Lowrance manual. Now on color finders, you can make out the hollow portion of the fish, assuming that is the bladder. I am not the expert, but again, it is all assumption/guesstimation, when you are using your fish finder. It all boils down to the percentage of the fish in the cone, right??? Anyone else have any insight?

[waskawood]

In my opinion the arch has nothing to do with either the size of the fish or the size of the bladder. The arch is measuring distance. If the fish goes directly through the center of the cone area of the transducer the arch will be largest. If the fish is off to one side of the cone area the arch is smaller. A larger fiah may show up as a stronger signal, thus your gray line or color line may be brighter. Doc Samson has lots of info on the subject of locators and gps. It is hard to explain without drawing a picture so I hope I have explained this clearly.

[Pat K]

About 10 years ago at the Midwest Sports Show a Garmin rep told me pretty much the same thing. The body of the fish is close in density to the water around it and would take an extremely strong signal to show up. The air in the swim bladder is what shows up on sonar.

[mcmsek]

If it is the air bladder then why does a lure, downrigger, etc. show up on the graph?

[Ralph Wiggum]

A lure or downrigger has a density way different than water and a fish doesn't, that's why.

If I remember correctly, I went through Lowrances online sonar tutorial last winter and it did mention that the air bladder is generally what sonar detects.

[Surface Tension]

The air bladder reverb is somewhat true. Consider the air bladder a drum, the bigger the drum the louder the noise.

The louder the noise the stronger the return signal which gets marked on your screen as darker. So weak return = light, strong return = dark

Arch length is determined by the speed you drove over a target.

The upper and lower reading of an arch are caused from the cone angle. The outside of the angle will show up as deeper,(distance from point A to B is greater) when you get directly over the target it reads the actual depth(shortest distance from A to which is the upper reading of the arch. Its in this position you have the strongest signal over the target and it will read darker then the outside of the angle.

Someone pointed out that you can see your jig or weight and theres no air bladder there. Thats true, that object is reflecting the ducers ping back. In fact you can send down a minnow head down the hole and still see it fall to the bottom so right, you don't need a bladder to see an object but all fish have bladders, the bigger the bladder like a drum, the louder and we know that louder signal reads as a darker signal.