Density of water and impacts on lures running at depth..a little help please!

Ok all you rocket scientists and hydro dweebs.  I have a question for ya. 

A little background.  So a few of us were discussing what we saw on underwater cameras.  I made a comment that everything seemed to be in slow motion, like the gear was running through oil and not water.  Somebody else made a similar comment about how the tell tale "thump thump" of a rod tip when trolling a big flasher seems to slow down at depth.  Both of us agreed that something seemed different than what we observed close to the surface.

SO....

Does the density of the water change at depth (say anywhere from 20ft and deeper) enough to make a noticeable difference in a lures action as opposed to running it just a few feet down (like when you are checking the action of the bait)?

Could it just be something else? Or is it just speed over water vs speed over ground difference and we only notice when its different?

Does the density of the water change at depth (say anywhere from 20ft and deeper) enough to make a noticeable difference in a lures action as opposed to running it just a few feet down (like when you are checking the action of the bait)?

I haven't taken the time to research this at great length, but I've always understood liquids to be effectively incompressible.  The way liquid molecules are arranged does not allow for any space between them that can be compressed, hence, no ability for further compression.

Now that I think about it, the only change you may notice is a difference in salinity, which could cause some change in density, but even this would probably be negligible when you're considering the way a spinner blade behaves at depth.

Does the density of the water change at depth (say anywhere from 20ft and deeper) enough to make a noticeable difference in a lures action as opposed to running it just a few feet down (like when you are checking the action of the bait)?

I haven't taken the time to research this at great length, but I've always understood liquids to be effectively incompressible.  The way liquid molecules are arranged does not allow for any space between them that can be compressed, hence, no ability for further compression.

Now that I think about it, the only change you may notice is a difference in salinity, which could cause some change in density, but even this would probably be negligible when you're considering the way a spinner blade behaves at depth.

I think you are right.  The density of water only changes from 1.025g/cm^3 to 1.028  I wouldn't think that's enough. 

Any other ideas?  Maybe just an observational bias...we only notice when its different and can't be xplained. 

Does the density of the water change at depth (say anywhere from 20ft and deeper) enough to make a noticeable difference in a lures action as opposed to running it just a few feet down (like when you are checking the action of the bait)?

I haven't taken the time to research this at great length, but I've always understood liquids to be effectively incompressible.  The way liquid molecules are arranged does not allow for any space between them that can be compressed, hence, no ability for further compression.

Now that I think about it, the only change you may notice is a difference in salinity, which could cause some change in density, but even this would probably be negligible when you're considering the way a spinner blade behaves at depth.

I think you are right.  The density of water only changes from 1.025g/cm^3 to 1.028  I wouldn't think that's enough. 

Any other ideas?  Maybe just an observational bias...we only notice when its different and can't be xplained.

It's a good question, but I would probably just chalk it up to differences in the movements of surface water vs. water at depth.

Rawkfish is right.  Water does not compress which is why it is such a useful tool.  As for deeper trolling making the thumping seem slower, it could be that if you are using a heavier weight to get it deeper, the heavier weight is acting as a damper.

Or, as Rawkfish said, deeper currents may be affecting it differently.

As you go deeper, you have more line out.  That additional line helps dampen the vibration from the lure or flasher.  And it's not just the axial force along the line, but as you troll the line has a slight bend in it.  It's not a perfectly straight line to your lure, so additional force first straightens out the line a bit before it is transferred to your rod.  Of course mono will cause more dampening than braid.

I think that the more line you have out, the more the jerking of a lure or dodger on your line will be damped by the friction of the water on your line. 

A large Flatfish or Kwikfish lure has a very strong wobble action.  When you long line troll one of these lures, when you start feeding out line the tip of your rod will pulse strongly, particularly when you are using braided line or other low-stretch line.  As you feed out more and more line, the pulsing reduces and eventually disappears.  The pulsing is damped not by the depth of the water the lure is running through, because the lure may not dive very deep, but by the friction of the water on the line.

The damping happens more quickly when trolling with a large sinker, because the line between the tip of your rod and the sinker descends into the water at a fairly steep angle, so the line is being pulled at an angle through the water, which damps the action of the lure on your line.  It happens even more quickly when downrigger trolling, because your fishing line and downrigger cable are both descending pretty steeply through the water to your downrigger weight.

However, I've never been able to compare the action of a lure or dodger in shallow water and in deep water by using an underwater camera.  It would be pretty easy to determine if the action of a lure gets slower as the lure descends by raising and lowering a lure with a downrigger while filming the action.  It would be interesting if the action clearly gets slower as the lure gets deeper.  If it does, I don't know why it does.