We had this same occurrence at one of our facilities which we consult. There is no 100% way of protecting the swans, even a floating island except penning the birds at night in a pen that is completely enclosed, top to bottom so that predators cannot enter on top by climbing or dig underneath to get into the pen. Even if you provide a floating island, there is no guarantee that the swans will use it at night.

If the birds come out of the water just for a second or stay in shallow water, the predator will get to them. Therefore, our best suggestion is to bring them indoors inside a safe pen at night and let them out around 8AM. Bring them in around 5PM (as the time changes, you want to make sure that the sun is high morning and evening as predators hunt throughout the night from dusk to dawn).

Predators are smart and will watch for easy prey and any usual patterns. Therefore, it is important that you rotate the times you let the birds out, 1 hour early or later, etc. Also, during winter months, some predators may even hunt during the daytime, so this is another added concern. The Regal Swan