Zebra mussel larvae were present in the same area in 2010 and 2011, but not in 2012 or 2013. "Since we have found zebra mussel young in this area before, and because there are established adult populations upstream in the Otter Tail River in Minnesota, finding a few young this year really didn't come as a surprise," said Fred Ryckman, aquatic nuisance species coordinator for the North Dakota Game and Fish Department.