The older ducks are being territorial.

Pen the young ones in the area, in a secure pen so no predators (weasels, etc) can get to them. Tie the door closed in addition to latching it, make sure nothing can squeeze through the wire, or dig under. Some predators can fit in a 1"x2" opening. This will allow introductions to take place, they can get used to each other w/o the little ones getting picked on, and after a week or 2 the older ducks should come to realize that the new ducks are now a part of the flock (though issues can arise if you have too many males).

If you can't secure a 24/7 pen, daytime penning would suffice but will probably take a little longer.
Good luck. Be sure to provide food, water, and shelter from sun & wind.

Brendasue

I have only one use for farm animals that choose to be a problem instead of useful; sounds like dinner "buff"ette to me.
with a nice sauce of tangerine (or orange) juice and dates cooked down into a nice syrup and drizzled over the duck with some rice and maybe some tender asparagus spears.

Ducks are similar to chickens in that they will just have to establish a pecking order. Just keep them all penned in together and let them work it out. You should only be concerned if anybody stops eating or you can see skin from feathers being pecked at. Brendasue is right, the older ducks are being territorial. And pekins are so sweet by nature they probably allow the bullying to some degree. If anybody gets seriously injured or stops eating then seperate them for sure! Otherwise, I'd say they are probably just being birds.

Thanks for all your responses.

The ducks have worked it out among themselves. After a couple weeks of fighting they seem to be one happy family of ducks. I suspect it's mainly due to pekins out-growing the buff and the crested. Pekins are very plump now.

Now I ran into another serious issue. It seems that my pond is infested with turtles who, it seems to me, bite ducks, and now ducks are afraid even entering the pond.

When I chase ducks into the pond, they swim for while, but then start sort of jumping in the air, and rushing out of the pond.

Any ideas how to deal with the turtles?

Thanks

Look up "turtle trap"

The big ones can take off an entire duck foot.