Bring a saucepan of water to the boil over medium heat. Add sweet potato and cook for 8 minutes or until tender.
Drain and set aside to cool.
Steam salmon for 3-4 minutes until almost cooked, then remove from the heat.
Flake fish into a bowl. Add the potato, peas, zest, crumbs, egg, salt, pepper and cayenne and mix well.
Use lightly oiled hands to form into 12 patties, then place on a baking tray lined with parchment paper.
Chill for 30 minutes.
For the herbed yogurt, combine all ingredients in a bowl and chill.
Heat ¼ cup oil in a frypan over medium high heat. In batches, cook the patties for 2-3 minutes a side until lightly golden and warmed through, then drain on paper towel.
Season, then serve with lemon wedges and herbed yogurt.