Syddakyd since when are any questions stupid. You didnt have to make a comment. No stupid questions. I would go with the fishing line. I know in MN it is a huge fine and jail time for even shooting at heron.
And no one has the right to build a pond, put brightly colored, slow fish in it and then get mad at the heron when it does what comes naturally. Thousands of dollars in koi or not you put them there and the bird is doing what comes natural.