What are the most important differences?

For an inexperienced angler, it's not so easy to tell the difference between salmon and trout. When you see pictures of salmon and trout side by side, the most obvious difference is the spots on their backs. Salmon usually don't have spots below the lateral line, while trout have many more spots both above and below the lateral line.

‍

Generally, salmon are also slimmer and more elongated than trout, which have a more compact body shape.