Bawplank,
Sorry to hear about what happened. When your dad caught these individuals did they have fish in their possession and how many? If not you are going to have a hard getting any type of compensation from them. If they do not confess to taking all the fish that appear to be missing from your pond you are going to be out of luck. If the fish are in their freezer and the cop does by chance get a warrant to confiscate them how are you going to prove they are from your pond? I dont mean to burst your bubble but if they get an attorney these are the questions that will be asked.
Trust me I had a guy in PA who caught someone tresspassing and fishing on his pond and the only thing I could prosecute him for was fishing without a license and tresspassing. He had no fish culled at the time and the owner of the pond could not prove this was the individual that had caught some of his 5-8 pound bass. The perpetrator sure aint going to admit to it, if he does good but tell the cop to get a written statement from him or it is nothing but hearsay unless he was placed under arrest.
As far as civil litigation you must have proof that these guys indeed had fish in their possession also.

Dreaming of the pond!