Hi
I have a sturgeon which is approx 14 inches long and about 10 years old and has lived with Koi the majority of his life. I have tried so many times to feed the koi first and then feed him, it just, will never work, well it seems that way.
No matter how much you feed the koi, they will still want to eat the rich protein food that sinks for your sturgeon, and the added bonus is that wherever you put the sinking pellets for your sturgeon, he will just swim in the opposite direction! 
And eventually, after he has swum around the sides of your pool, and all along the bottom in various directions, all of the food will be eaten. My sturgeon will swim right over the top of the sunken pellets on occasions, and still not eat anything!
As one person has suggested, using a pipe to put the food in, would not solve the problem with my situation anyhow, as the Koi would still get there first!
Not sure what the best solution is, but would be willing to try any suggestons. 
Tracey