Mix the poppy seeds, sesame seeds, garlic powder, onion powder and salt in a small bowl.
Brush the baguette slices on both sides with olive oil and spread out on a rimmed baking sheet. Sprinkle with the seed mixture. Bake until lightly browned and crisp, 8 to 10 minutes. Let cool to room temperature.
Meanwhile, with a rubber spatula, mix the flaked whitefish, creme fraiche and dill in a small bowl.
Top each crostini with a spoonful of the whitefish salad and garnish with 2 caperberry halves.