$5000 post tax money for the entry fee, $10,000 pre tax money winnings. Unless someone is footing your entry fee, the disparity between the entry fee and post tax winnings is not a whole lot. Definitely would need to treat it as a business and keep track of ALL expenses.
I'd rather go to work, get a paycheck/benefits and fish for fun. When you turn a hobby into a business, all of the fun goes away. Not my dream!!