Have a client here in East Central Ia. that has a privately stocked brown trout stream. Nice trout to 2 1/2 lbs-even reproducing. Two years ago he began finding brownies on the bank with just a bite out of their back. As the days went by he would find them piled in bunches along the stream with the one bite out of them. DNR had intoduced river otter in the area the previous summer. They cleaned out the streams for miles around, including the state stocked trout streams. Would guess you have a similar problem.
