I would think that the low Dissolved Oxygen problem is a late-summer/fall event when water temperatures are warmest. Once winter settles in, water temperatures decrease; cooler waters hold more DO. Winter storms also create a greater degree of turnover which allow DO poor water at depth to come to the surface and re-oxygenate.

Ask your property owner friend when he usually sees shiner perch on his beach. I've done alot of beach seining in Puget Sound and typically shiner perch start showing up in the nearshore in May; by June they're often the most prevalent fish in catches and remain so thru September. By about mid-October, however, they again disappear from the nearshore, probably because of decreasing temperatures. It doesn't surprise me that he hasn't seen any perch this time of year. It would surprise me if he had.