Yikes! Glad you were able to save the duck!! Yes, dogs would certainly help. We have three great pyreneese/great pyre crosses to guard our chickens. They have made a huge difference, we used to lose several dozen birds a year, now we hardly lose any. I hope you can catch that pesky fox before you lose anything else.
If you have any outdoor animals, Livestock Guardian Dogs make a huge difference! My male stayed up all last night protecting my bantam ducks. I was woken up so many times last night. I would look out and see him sitting by them and growling and barking into the night. His voice was hoarse this morning but he never left them. And he is still a puppy.
This was him around 5 a.m. protecting my ducks. I have never seen him so angry before.