Many people get ducks as ducklings for pets or for Easter presents without realizing what they're getting involved in. Unfortunately, many of these ducks end up being left at a public park or lake, or escaping to one. People dump ducks off for various reasons. Some did not realize the nature of the ducks and their needs when they got the ducklings and had to find a place for them quickly. Others may have been forced to give up their ducks after their city code enforcement officer told them that poultry was not allowed where they live. Still others may have had too many drakes or had personality problems with their ducks and didn't have the heart to sell them for slaughter or couldn't find them good homes. Whatever the reason, people often leave their ducks out in the cold at their local lakes.
I've heard people say that they've dropped off their ducks at the local lake and their ducks fit right in and did fine. Others say that it's good to drop unwanted ducks off at the lake because they will get fed and be free to be ducks. But, many of those people do not realize the harm that they do.
The biggest is that domestic ducks are very vulnerable to predators. Most common domestic breeds, especially runners and Pekins, can't fly. Most lakes have some kind of predator around, such as hawks, coyotes, foxes, and even domestic cats are dangerous to ducks. Domestic ducks, not only being not able to fly well, are often not aware of how to evade predators. Some of these ducks may have grown up around another family pet like a dog or cat and think they're all the same and try to be friendly to them.
Another issue that these "drop-offs" have is that they can often carry diseases that the wild ducks may be vulnerable of, or vice-versa. Many domestic ducks also have other health issues that their wild cousins might not have to deal with and could spread diseases to the wild flock. Large ducks originally bred for meat will often have leg problems, especially as they get older. Some domestics will not live very long even if they're kept fed and otherwise healthy in a wild setting.
Domestic ducks also cause problems with the population of wild ducks. For example, most wild ducks are great fliers as it's their main means of escaping predators and finding food. However, when they breed with domestic ducks, their offspring often can't fly or fly very well. Soon, there are too many ducks in one area that can't leave on their own and can't be captured and adopted due to them being wild. This can affect the ecosystem as it tries to support a large population of waterfowl all year long and can be detrimental to all wildlife. This may result in more illnesses and even starvation of some species. Not all lakes allow people to feed ducks and dropping off domestic ducks may result in their starvation if there are too many other ducks and wildlife using the ecosystem.
If someone is in the need of finding a home for their pet duck, there might be some alternatives to "dumping" them:
Place an ad on sites like Craigslist and other areas where they sell or deal with waterfowl. There's a site called Backyard Chickens which has a duck forum as well as a selling/swapping forum to find people who may be looking for certain types of ducks. It can be emphasized that they are to go to pet homes only, if necessary.
Ask the local humane society about placing ducks up for adoption, inquire about if they euthanize, and how long they keep them.
Post an ad at the local feed or tractor supply store.
If you have any kind of duck rescue or sanctuary, ask them if they have any advice.
So, don't drop those ducks off at the local lake! Not only is it bad for them and everyone else already there, they would miss their human companion whom they trusted.