A lot of fish agency staff would like to have that proverbial nickel for every phone call they get about dead fish in the spring. Consultants get this too, and one can pretty much count on a couple of calls a week about dead fish in late May and June, but sometimes right after ice out. There are lots of things that can kill fish, including disease, angling mortality, toxic substances, low oxygen, and high temperatures, but the vast majority of fish-kills boil down to two main influences: low oxygen under the ice and spawning stress in late spring.
First, consider what defines a fish-kill. Two dead fish washing up on shore doesn’t really qualify; most agencies apply some number like 50 to a species or 100 to multiple species. It would be very wasteful to investigate every dead fish that shows up. If 50 fish (or whatever reasonable threshold is applied) of a species can be found at once, that is considered to represent some kind of event. Likewise, if 100 fish of multiple species are found in a short time span, that suggests something more than typical die off. So please don’t get on the phone to your fish agency or consultant when 5 dead sunnies float to shore. By all means, take a lap around the lake and see if this is a widespread phenomenon, in which case a call may be in order, but don’t panic over a few dead fish.
So if we do find enough dead fish to raise an alarm, what might this mean? The first conclusion that many jump to is that there is something undesirable in the water and fish are dying from it. Well, that may be true, but even if so, that doesn’t mean people will die from it if they go in the water, but that is a leap that many people make. If dead fish are plentiful after the ice goes out, it probably means that oxygen got too low and less healthy fish died. This could be one species that lives in the affected area (catfish on the bottom come to mind), or it could be multiple species that coincide in an isolated area (a shallow bay where thick ice met the bottom off shore and prevented them from leaving). These winter fish-kills may not be detected until the ice goes out, but the fish did not likely die after ice-out.
The other, and far more common cause of spring fish-kills is spawning mortality. Many species, most notably perch and sunfish, spawn in shallow areas in the spring. Perch tend to be earlier spawners, and the water is colder and better oxygenated, so they are less likely than sunfish to experience stress while spawning. Sunfish, on the other hand, start spawning in May in much of New England and may continue to do so well into summer. They make nests in shallow water subject to fluctuations in oxygen and temperature within and among days. They don’t eat but they do defend the nests against other fish. The energy balance just doesn’t work out in some cases, and some fish die. If we have extremely variable weather, the stress is often greater.
If a June fish-kill is one species, all of similar size, most commonly 5-6 inch sunfish, you can pretty much count on it being spawning mortality. If the kill involves multiple species or a wide range of sizes, then there might be something worth reporting. It should be noted that the timing of fish-kills in late spring often coincides with a period of herbicide application, leading to another leap to a conclusion that is rarely justified. Over 20 years of fish-kill investigation in Massachusetts, covering hundreds of investigated events, only a handful were linked to herbicides and these were almost always a function of low oxygen created by dying vegetation; toxicity to fish from properly applied herbicides is very, very rare.