Oddity When Viewing Sonar

[BassQualifier]

Hey guys. I was wondering if any of you could shed some light on something that has intrigued me many times out on the water. I've noticed a couple times when I'm idling the boat over say a point, or either slowly moving through a creek, that on my depth finder, I will see a sort of thin line moving up in a slanted line all the way up to the boat. Like if the depth is 12 feet, I will see a line form at the bottom, then slowly keep building longer and longer until the line finally reaches the boat. What is this? It almost looks like fishing line, but I know it isn't. Could it be a fish that is swimming straight up from underneath the transducer?

It is a really odd sight and I would LOVE to know what this is. It happens quite frequently, probably once every couple outings. Would love to hear your thoughts.

 
Example 1:  (sometimes going down)
+------------------------+
|             \          |
|               \        |
|                 \      |
|                   \    |
| 12ft //\//\//\/\\\/\\\\|
+------------------------+

 
Example 2:  (sometimes going up)
+------------------------+
|            /           |
|          /             |
|        /               |
|      /                 |
| 12ft //\//\///\/\\/\\/\|
+------------------------+