Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 5 ingredients (through egg whites) in a large bowl, stirring until blended. Divide mixture into 48 level tablespoon-sized pancakes, squeezing each to remove excess moisture.
Heat 1 tablespoon oil in a nonstick skillet over medium-high heat. Add 16 pancakes; cook 3 minutes on each side or until browned. Arrange on two baking sheets coated with cooking spray. Repeat procedure twice with remaining oil and pancakes. Bake at 400° for 5 minutes or until cooked through.
Combine mayonnaise, apple, yogurt, and horseradish in a small bowl; place 1/2 teaspoon mayonnaise mixture on each pancake. Top each with a trout piece; sprinkle with chives.