Hi,
Whoever you ask will give you a different personal opinion when it comes to most pond matters and although I am in the pond and aquatics trade I also can only give you my personal opinion. Please note that as a newbie to this forum I wish to point out that I am mainly here for the fishing etc, but I cannot leave someone with a problem when I know the solution.
At colder temperatures the motabolism of all cold blooded creatures, such as fish, slows down rather drastically and therefore they naturally become very inactive.
Sterlets are indigenous to very cold climates, such as siberia, and as such can remain comparatively active during these colder months. Most people will tell you various water temperatures that indicate when you should stop feeding, typically around 8 to 10 degrees Celcius for most pond fish but not many people realise that Sterlets are more able to cope.
logically if any fishes motabolism is active enough for it to forage for food then it is also active enough for it to digest the food.
After all if the fish is actively swimming then it is using up energy and this needs replacing (especially if the coming weeks could get cold again).
So if your Sterlets are actively browsing for food then feed them with sinking pellets (lower the protien level the better) but only what they will consumer within five minutes and it is best to keep the food in a localised area so that they do not need to exert more energy than necessary searching for it. As the water temperature can also drop rapidly as soon as it start to get dark it is best only to feed at least four hours before dawn (allows time for the fish to digest the food)
Also as cold water holds more disolved oxygen than warm water, and as sterlet are native to colder climates, it is very important during the summer months to provide them with plenty of aeration (waterfalls, fountains, air pumps etc all help to aerate the water).
Dirty water also reduces the disolved oxygen so maintain your system regularly.
Hope this helps
John