Fish arch Interpretations

[Seabass81]

Alright . I have had my Humminbird 788ci all summer . Although I use it all the time its rare I am actually looking for fish arches . I am more focused on depth and gps .

While camping with my daughter we where on Leslie Lake which has lake trout . I decided to go see if I could find any on the graph . While cruising at 17 mph I went over deep humps that held many nice arches. I marked these humps so I could hover over top and try and jig a couple .

Now when I was off the motor I never saw an arch on the screen. There where sometimes a couple of persistent lines running in a horizontal direction . Would these be fish or should they still show as arches when not moving .

In a couple of days I will be trying to find some deep smallies and figure this could help alot .


John

[bl8ant]

Well I can tell you that that is completely normal. Also, I have the 788 on the bow of my boat as well.

When moving, fish will appear as arches or hooks—when stationary and under the boat they will appear as a solid line.

[moonshine]

I have found that as well on my Hummingbird. But I have discovered, that if I were to slow down the chart speed I do end up with some arches. The lines are due to stationary fish, not fish that are feeding. If i am trolling all fish show up as arches unless the trout are following my cannonball (which does happen occasionally).

[Hookup]

I believe that this is normal. and I think I can explain why.

There are one of four scenarios when we simplify the sonar and fish echos.

1) the fish is moving and the boat is not
- fish appear as arc's

2) the boat is moving and the fish is not
- fish appear as arc's

3) both the boat and fish are moving
- fish appear as arc's

4) neither the fish or boat is moving
- fish appear as line's

To get something to appear as a line, it must stay "stationary" under the sonar.

The easiest way to visualize this is both the boat and "fish" are not moving at all.

So the sonar, which is constantly pushing old-data to the left, is getting a single, constant return from the stationary fish, say at exactly 23 feet, and thus a line is created, at 23feet on the sonar.

In the scenario when the boat is moving and the fish is not, the fish, which is suspended at 23 feet and not moving, is first picked up from the sonar at the out-side edge of the cone. This "distance" to the fish will be greater than 23 feet, say 24 or 25 feet.

As the boat get's closer to the fish, the distance shrinks, until such a point as the fish is directly under the sonar, where the distance is "correct" at 23 feet.... then the boat moves past the fish, and the distance increases again from 23 feet out to 24/25 feet at the trailing edge of the sonar cone.

This movement of the sonar over the stationary object creates the arc'.

Hope that helps.
Tim

[moonshine]

In the scenario when the boat is moving and the fish is not, the fish, which is suspended at 23 feet and not moving, is first picked up from the sonar at the out-side edge of the cone. This "distance" to the fish will be greater than 23 feet, say 24 or 25 feet.

As the boat get's closer to the fish, the distance shrinks, until such a point as the fish is directly under the sonar, where the distance is "correct" at 23 feet.... then the boat moves past the fish, and the distance increases again from 23 feet out to 24/25 feet at the trailing edge of the sonar cone.

This movement of the sonar over the stationary object creates the arc'.

Hope that helps.
Tim

while this theory does have some merit, it is actually not true. what causes the arch is the taper on the ends of the fish. ie tail and head. The tail being smaller than the head, is the part of the arch that is the lower end. if you take notice of the arches, one end is always lower than the other.

[RJ]

I am far from an electronics fan or expert but I'm sure that last comment was incorrect Moonshine....you can't tell on a sonar which is the head of a fish and which is the tail......or even species for that matter....and if you claim you can.....prove it...Sorry I don't buy that.

Sonar is relatively simple how it works....and the way arches have been explained to me is this....

As you are passing over an object it will enter the cone...and begin an arch....as you drive over it more of the object is in the cone so the signal gets stronger then as you pass over the end of the object it gets weaker and the arch fades......simple as that.

A response from Sheldon Hatch or Paul Shibata would clear this up as they live and breathe this stuff....but it may take 20 minutes to read their responses....

Flasher Units I can fill you up with info on....graphs....not so much...

RJ

[Hookup]

while this theory does have some merit, it is actually not true. what causes the arch is the taper on the ends of the fish. ie tail and head. The tail being smaller than the head, is the part of the arch that is the lower end. if you take notice of the arches, one end is always lower than the other.

Now that's funny... damn! hahaha.

[Hookup]

Further... check this diagram, it illustrates what I was saying above.



See how the distance changes as the fish moves, which translates to depth, the only measurement that a sonar can display.

[horsehunter]

Moonbeam are the straight lines eels?

[Out4trout]

while this theory does have some merit, it is actually not true. what causes the arch is the taper on the ends of the fish. ie tail and head. The tail being smaller than the head, is the part of the arch that is the lower end. if you take notice of the arches, one end is always lower than the other.

Moonshine - I respectfully disagree.
It is a distance function per Tim's description.

[Billy Bones]

Further... check this diagram, it illustrates what I was saying above.



See how the distance changes as the fish moves, which translates to depth, the only measurement that a sonar can display.

Right on Hookup. Has nothing to do with speed has everything to do with distance. The deeper the water the wider the cone from the
boat. Distance is farther on the outside part of the cone and shorter when directly over the object, that's why fish show as an arc. Next time
you're drifting for Lakers, when you first see the line of a fish, it will be lower than when you are directly over it.

while this theory does have some merit, it is actually not true. what causes the arch is the taper on the ends of the fish. ie tail and head. The tail being smaller than the head, is the part of the arch that is the lower end. if you take notice of the arches, one end is always lower than the other.

This is not true, the difference between each side of the arc has to do with the angle of the transducer mounted on the boat. Could be pointing a little forward or a little backward.

[moonshine]

i suppose now that i think about it, i am wrong. my apologies gents. its the taper in the arches that reflect the density of the fish. i guess that would be why the arch is thickest in the middle.

[Billy Bones]

Here's a picture of my old Sonar, an X96 Lowrance. The arcs are almost even because I worked on making the transducer as straight as
possible on the boat. You can even see the downrigger ball at 162 feet.

Billy Bones

[moonshine]

my unit has much higher definition and is also colour. it shows the middle of the arch thicker and of a different colour as well.

[MLR]

If you are sitting still and see arches, then you are looking at debris or very small fish.

I highly recommend spending an hour or so reading just about everything on Doc Samson's website in regards to sonar. He uses Lowrance, but sonar is sonar.

http://www.hightechfishing.com/sonar.html