Fishfinders and Mackerel

Hi All, I hope this doesn't sound like a stoopid question, but it will settle my curiosity. I was talking to a fellow forum member the other night, and he tells me that fish finders can't detect mackerel as they dont have a swim bladder !! I find this very hard to believe.

Is this true ? I have been out on boats where there are loads of markings, which various skippers have said were mackerel, and I've stopped and fished and caught mackerel.

Fellow poster reckons this is the fry which the mackerel are feeding on. Is he talking b*ll*x or what ?

Hi All, I hope this doesn't sound like a stoopid question, but it will settle my curiosity. I was talking to a fellow forum member the other night, and he tells me that fish finders can't detect mackerel as they dont have a swim bladder !! :shock: :?: I find this very hard to believe.

Is this true ? I have been out on boats where there are loads of markings, which various skippers have said were mackerel, and I've stopped and fished and caught mackerel.

Fellow poster reckons this is the fry which the mackerel are feeding on. Is he talking b*ll*x or what ? :?:

Whenever I've seen a concentration of fish on my sounder, it's nearly always Mackerel I catch!
Colin

Same here, the original post is mine by the way, but for some reason my name isn't showing.
I think he's talking b*ll*x. I'm sure not everything showing on a fishfinder has to have a swim bladder - like wrecks !!! Is it not all down to sonar waves etc. bouncing off whatever it hits on the way down/up ?

agree with conor, see below

The sound emitted by the transducer is directed down,
where it comes into contact with various hard objects
(fish and the bottom) and an echo bounces a low-voltage
return signal back up and is then amplified. The return
echo strikes the transducer where the sound energy is
converted back into electrical energy. That returned
electrical energy is then carried to the display unit where
it is interpreted by the Fish Finder's software and exhibited
on the screen.

We've had an Empty Sounder Screen a Brave Few times and Caught Mackeral hand over Fist, We were thinking maybe when you see a ball of Fish up in the water and get a Lot of Marks on the Sounder it could be the Fry that the mackeral are feeding on?

You can see mackerel on most sounders/sonars. Commercial trawlers towing for mackerel depend on it. They don't just charge about randomly with a net in tow.

What you have to remember about sounders/sonars is that they radiate a fairly tight beam away from the transducer - usually pointed down the way ie under the boat. If you look at the specifications for a given unit, you will see a beam width specified, usually about 6 degrees (if I remember from some of mine) from a hypothetical line at right angles to the transducer - effectively a cone with a 12 degree point at the transducer.

Anything outside this 'cone' will not be detected (a bit of a generalisation as there are side lobes and all sorts to factor in, but I'm keeping this simple...sort of). Obviously the deeper the water you are in, the larger the footprint you will 'see'.

However, the energy radiated by the transducer dissipates proportional to the fourth power with distance, so the energy radiated off a target is pretty low. In addition, the same law applies to the power re-radiated from the target back to the transducer.

Certain substances (some fish are denser than others - those with swim bladders are obviously hollow, so less dense; a ray will have a larger reflective surface than a herring etc) do reflect better than others, which is how colour sounders/sonars can differentiate between some different species.

The resolution of a given unit will also depend on its frequency of operation - most run at about 200KHz. The higher the frequency, the better the unit can discriminate between small features below it. However, lower frequencies travel better through the water, so it's a bit of a trade off - hence why you also get dual frequency units.

Beyond a certain distance below the transducer, a solitary fish may be unlikely to mark but a small shoal might. I've seen sounders where the screen is literally black from waterline to sea bed with mackerel, so trust me, they do show up. If they didn't they'd be under serious study to see how they pulled that trick - I'm sure defence contractors in the submarine design business would be fascinated...

Ok, even I'm boring me now...

very interesting reply sand man and well put,,,,,,,

hi all, can anybody tell me what is the best way to fit a transducer. regards dezzer..............

There are two types of transducer, both are fitted differently. There is one which goes through the hull and another type that has a small bracket attached that mounts on the outside of the hull, usually on the transom.

Which type do you have?

If you have both, the through-hull model is the more laborious to fit but will give better results, particularly at speed.

Which type do you have?

I have the type which doesn't detect mackerel because it isn't fitted correctly .