The ladder at the millpond is open for piscatorial traffic. So what fish might find such a ladder useful? Shad and river herring are some that come to mind. Both are anadromous, for the non-fishy folk out there (like me) that means that they begin life in fresh water then head for the ocean where they mature. Of course they must return to freshwater to spawn. The American eel is another species that might use the ladder. Eels are catadromous -- they begin life in the Sargasso Sea and then find their way inland to freshwater where they mature. Then it's back to the Sargasso to spawn.
Such complicated life-styles!