Combine the mayonnaise and pesto in a small bowl; set aside.
Cook the bacon in a large non-stick skillet over medium heat until crisp, 6 to 8 minutes. Transfer to a plate with a slotted spoon; reserve the skillet.
Season the fish with ¼ teaspoon each salt and pepper. Cook in the reserved drippings until opaque throughout, 1 to 2 minutes per side.
Divide the pesto mayonnaise, bacon, bass, and arugula among the bread.