You can always fight it. I am sure this isn't any different than other situations regarding tickets issued by officers where they (the officer) is the only proof. You might win by default if the officer doesn't show and for that amount, it might be worth the chance. Of course you each will have an assigned court date, although I am sure all the tickets written that day will be assigned to the same docket. You could go out and take pictures showing there are no signs, but you say there are trampled no trespassing signs so that might not fly but it might. I am certain that no trespassing must be CLEARLY marked and you would have a valid point in saying it wasn't. If it does turn up that they say there was signs, no Trespassing covers no fishing pretty well. Also, the prosecutor will probably argue that not knowing is no excuse and that argument usually wins but definitely doesn't all the time.