Seeing dolphins may not seem exceptional to non-New Yorkers. Nevertheless, the only thing I recall in the Hudson River when I arrived in New York some 40 years ago were the “Hudson River whitefish” that I saw that appeared to swim in the currents off the Battery. (The “white fish” were the condoms flushed down city toilets and dumped with raw sewage directly in the river.)
The return of dolphins to the river is indeed a hopeful sign.