I have an euryhaline mermaid species meaning they can cope with both fresh and saltwater. This species are born in freshwater lakes at high altitudes and will swim along the river into the sea or ocean. Recently due to human activities rivers are dammed or dried up cutting off the route connecting their place of birth and the vast ocean. The mermaid's numbers are declining rapidly and little effort (from wildlife conservation groups) is being done to conserve this marvelous species. Now the mermaids must compete with us for freshwater, how do the mermaids find their breeding ground before the humans drive them to extinction?
Follow the salmon
Salmon have much the same breeding requirements and as a valuable "cash crop" humans have a tendency to build fish ladders and other features to allow them to bypass dams in rivers. They're also highly sensitive to pollution so the rivers they're breeding in will be clean.