There is something terribly wrong within the water, i.e., algae outbreak producing bacteria induced toxins, sewage, fertilizer or other man-made run-off. Fish are more susceptible to the toxins because they cannot leave the area. The birds and other wildlife /waterfowl that notice the problem can escape by flying or leaving the area and looking for a more suitable habitat. In this case, the adult swans knew something was occurring and evacuated with their young before the young stayed too long and ingested or interacted with the toxins.

Hopefully, someone from your environmental department will get out and investigate and offer some remediation to prevent and control further damage. The Regal Swan