You could easily have water quality problems from several sources you mentioned. Low water resulting from no rain and evaporation causes nutrients and natural elements (metals , nitrates , K , P , carbonates etc ) to concentrate in the pond bottom and remaining water. Soot and smoke can cause the pond to acidify and plankton to die (acid rain). Also low water concentrates 5 acres of fish into 3 acres with the resulting high BOD.

I don't think that the pictured FA is a problem. It may well be a symptom of bigger problems.

In addition LMB natural morts are a fact and they can be high if the fish are under stress. Low reproductive rates due to stress along with low forage numbers can increase those morts. Several years of low recruitment due to high stress and high foraging rates can lead to many older LMB to starve or succumb to stressors because of poor condition.