Think about how it works and you can see what different shapes mean.
The sonar shoots out a sound wave and receives an echo. The area surveyed has a limited area (cone angle) and at its periphery the distance to the transducer is farther for any given depth than in its center.
So, think about a fish in relation to a circle on the bottom. Also go ahead and assume that the circle isn't moving - just to make it easier to understand - and a fish is not directly on the bottom. If the fish is not moving at all, just hanging there, every pulse of echo shot out will get the same return, so as each dot is painted on the screen the resulting pattern will be a straight line. You can see this by hanging a good sized weight over the side. Downrigger weights work well for this. Let one hang down there far enough that its in the picture and it will show you a straight line.
Next think about a fish that moves through the circle in a straight line and goes right through the middle. When the fish first falls into the range of the machine (is on the edge of the circle) its return echo will be the farthest distance from the transducer it will ever be until it reaches the far side of the circle. So the first dot painted on the screen will be low. Each succeding pulse from the fish finder will find the fish (which is moving) closer to the dot on the screen will be higher up. This will continue until the fish is in the middle of the circle, and then as the fish proceedes on its way it will be moving farther away from the transucer. So you get a shape like this / as the fish moves closer and then one like this \ as it moves farther away (it is actually a mirror immage of that because the fish finder reads right to left but we read left to right). The result of all that is /\ and that is a fish arch. The more symertical it is the greater the chance the fish moved though the cone in a straight line. Also, as you have probably already figured out the higher the arch the closer the fish passed to some point on a line extending directly below the transducer.
So that's a perfect world. In the real world a fish will rarely pass under the boat at a constant speed and in a constant direction - the requisites for a perfect fish arch. However that doesn't mean that perfect fish arches aren't seen. They are but its not because of the movement of the fish, its from the movement of the boat.
You see quite often its not the fish that is moving, or at least not moving very much. Moreoften it is the boat moving over the fish that causes the arch. The mechanics behind it are still the same, but once again, sort of a mirror image. With boat movement, particularly when under power (trolling?) you should find yourself with constant speed and direction much of the time. So you will have met the conditons of a perfect arch not because of what the fish was doing, but because of what the boat was doing.
Also, if the boat were standing still you would have no way of knowing which way the fish was moving when it left the arch. The same pattern will show on the screen if the fish is comming up from behind, met you head on, or passed left to right or right to left under the boat. No way to tell. All you know when you see a fish arch is that the fish is already gone. You don't know if its in front of you, behind you, or to either side of you. That aside, most anglers subconsciously think of a fish finders as a TV screen and will cast right behind the boat ... because that is the way the fish finder sort of showed it going. Now you know better
You also know this, the steeper the arch the shorter time the fish was inside the cone and the longer the fish was in there the flatter the arch will be (remember, if he just stayed in one spot you'd see a straight line).
So what about lop-sided arches? Well, it just means that the fish was moving slow and then got to moving fast. That just might happens if the fish were spooked by a boat passing over its head - a slope at the beginning of the arch that was much more gentel than the steep drop at the tail of the arch would indicate a fish that has headed for safer ground, and probably done it quickly.
Anyway once you can picture what is going on to make an arch in the first place it makes it pretty easy to figure out what caused the ones you see on the screen, if you ever see them that is. My fishing partner and I laugh about fish arches all the time. We see fish, and more often we see bait, but we troll in deep water at long range scales and, as you might immagine, are almost never going to see arches.
Thom
Sonar arches
