Being an ice fisherman out in the cold, he likely wants to fall asleep from the ill-effects of frigid temps and cold winds. The audible alert will awaken him when Mr. Fish nibbles the bait. The sudden awakening sends an adrenaline rush through his body, thereby increasing heart rate, blood flow and finally body warmth. All he has to do is take his mother-in-law along for company inside the ice fishing hut... that will leave him uncomfortable enough to boil and remain awake.
OK, how about keeping things real simple... attach a small bell to the pole's tip. No batteries to replace, no electronics to succumb to the weather!