MANKATO, MNThanks to his nitpicking, impatience, and insistence on absolute silence in the boat, avid angler Don Gillespie, 41, forever ruined fishing for his 10-year-old son Douglas Tuesday. "No, no, noyou're casting all wrong," said a visibly seething Gillespie after Douglas' line landed a mere three feet from the stern of the rowboat. "Forget it! Just let me do it, and I'll hand you the rod afterward." Douglas was further put off fishing when his father threw back the only fish the boy caught all day because it was not big enough.