Syndicated google and/or yahoo ads that are automatically based upon the content of each forum page would be a good idea. Just make sure that the ads aren't too obstructive or painfully annoying for your users. Give them adverstisements for items that they may actually be interested in, this could potentially even increase your users' overall experience.

Another route is to sell basic advertising space to sponsors, per Mike H's suggestion. Sell space on either an impression basis or a monthly payment. You may find that the fishing industry would be more comfortable with a monthly rate rather than a PPC or PPI method -- but that's just a hunch.

Best of luck sir!

EP