Each and every spring fish kills often happen, especially in northern latitudes, as ice thaws and open water appears. Fish populations in northern climates have evolved to manage with these seasonal changes each year. It is not always a pleasant site to see a shoreline littered with dead or dying fish. However, don’t be alarmed in many instances.
See this great story from New York state on this topic in this link: http://poststar.com/blogs/adirondack_outdoors/blog-dec-says-fish-kills-are-nothing-to-be-concerned/article_109ac6fc-433c-11e8-bb35-0f5113fe781b.html