I think you had a DO sag low enough to kill just the HSB and trout in probably the deepest water or a section of the pond. Ponds typically lose the DO in the deepest water first. Those species not 'trapped' in the low DO or those that ventured out of the stressful water survived. Trout and HSB require higher DO compared to other species in the pond. Vulnerable fish in the stressed area were 'trapped' and killed. I say the deepest water because the diffuser is 3-4 ft deep during winter operation and IMO mixing primarily the upper 4 ft of water of the water column. IMO Shallow water diffusers with gentle, intermittent aeration (windmill sheltered by trees) will result in stratified (chemical &/or temperature) layers. Post kill DO tests in the surface waters (0-4ft) will likely show DO is just fine, esp. true if sunny days have followed the kill. The DO was probably 'okay' in the upper 4 ft during the presumed winterkill.
There is electric near the pond, and there are plans for a Koender EL2 compressor with a membrane type diffuser to be used in conjunction with the windmill.
This system is a low air flow unit. I strongly advise against using this air producer in this application due to: 1. pond receives lots or organic inputs creating a unusually high BOD. High BOD requires the strongest aeration possible. Sewage plants are examples. 2. pond is only 7-8 ft deep and has a low DO reserve. A shallow placed intermitent weak diffuser (windmill) in 0.7 ac increases the chances of stratifiation events. 3. this EL2 unit is a low air volume unit compared to the hp involved. The best choice for this application is a unit that has high airflow per hp such as a rocking piston or rotary vane. A pond this size needs a strong or high volume diffuser and several diffusers. IMO essentially the current aerator is undersized. Ideally I would consider a 3/4 hp RV unit with at least 3 diffusers for this specific application. Considering the history, I would operate at least 1 diffuser in the deep section (20% bottom area) all winter.
