During testing I discovered my game was leaking a small amount of memory each second. Using Instruments I discovered the problem seemed to be connected to PhysicsKit.
In GameViewController I had enabled showPhysics to check the physics bodies of the nodes. This was causing the memory.
skView.showsPhysics = true
It’s not a big problem as this would not be enabled for a shipping game but it might help if you have a similar problem.