Drain the tuna. Transfer to a small bowl. Mix with mayo. Season with ground black pepper. Taste and season with salt if needed. Cover with plastic wrap and keep in the fridge.
Preheat the oven to 450˚F (230˚C).
Cut the salmon into 8 pieces to about the same size of the toasts.
Spread butter on one side of the toasts. Transfer to the oven and toast until crisp on the outside and soft inside, about 4 minutes.
Place the salmon on top of the toasts. Top with about 1 tablespoon of tuna salad on each toast. Sprinkle with chives. Serve hot or warm.