Black-Necked Swans from South America seem to survive in brackish water because they possess a filter that allows them to separate the salt from the water. However, all other species can die in this environment because of dehydration. We have seen a very limited number of other species of swans (not Black Necks) and they don't survive for too many months or years. Salt is hard on their system and they need freshwater. Is there a freshwater resource in your area that allows the swans to spend time on your lake, but can visit the freshwater? This may be the answer as to why they can survive in the saltwater for a short time or visit. The Regal Swan