It's how you fish minnows and roaches with a cane pole. "Dawgling" refers to the way you drop and hang your line. When you fish like that on rivers, you never know exactly what you're gonna pull up, but it's all good eatin. The "coon" refers to it being a grab bag scavenge.

Could the caption be accusing the young lad of dawdling? I've always heard it used referring to lagging behind distractedly, but one dictionary says, "to waste time; idle; trifle; loiter."

No idea what "Coon dawgling" is unless it's a black man cooling it on a bench.

The sign is for a bait shop whether the photographer knew it or not. Shiners are small, shinny fish used as bait.