When to be Concerned About Finding Dead Fish on Maine’s Lakes, Ponds, and Rivers this Summer

By Fish Pathologist David Russell

Maine is renowned for its exceptional inland waters with over 6,000 high quality lakes and ponds, superb fish and wildlife habitat, and vast opportunities for year-round outdoor recreational activity.  These natural resources and others are cherished by our permanent and seasonal residents and by those visitors that come to our great State to experience nature. With the onset of summer, as our waters are enjoyed by more people than at any other time of year, reports are frequently received from members of the public of a “fish kill” or an unexpected and sudden appearance of dead fish over a short period of time, mostly on lakes and ponds.  For some people, the sight of such events instantly raises great concern and speculation that the cause may be the result of pollution or irresponsible human activity. However, most of these fish kills are often due to common natural phenomenon, the most common ones being described at the end of this article. When a fish kill involves hundreds to thousands of dead fish of multiple species of mixed sizes and age classes and dead aquatic invertebrates are also observed, the kill may be the result of unnatural phenomenon. For such events, time is of the essence for investigation and therefore prompt notification from the public is greatly appreciated.  Fish kill investigations may involve coordination with other natural resource agencies including the Department of Environmental Protection and the Department of Marine Resources.     

Regardless of the cause, our staff are interested in knowing about any fish kill to assess the need for a formal investigation, as well as management implications.  Generally, a few dead fish to several dozen (possibly hundreds of dead fish on larger waters) are of less concern, except for when it involves endangered and threatened species such as sea-run Atlantic salmon or sturgeon. Since agency staff can’t be everywhere, it is often a call or email from various members of the public, including that from volunteer lake monitors, citizen scientists, and observant outdoor enthusiasts that alert us to a potential problem. Information that is very helpful from the public includes: the name of water body, location on the water where dead fish were observed, time of day when first observed, ballpark number of fish, species composition, size range of fish, and whether fish are fresh and/or in the process of dying, presence of an oily sheen, or unusual water color or chemical odor. Pictures, if a camera is handy, are also appreciated.

Fish kills that may occur in late spring/summer, in part, due to temperature stress:

• Small numbers of dead adult fish of a single species are fairly common to encounter in late spring and early summer. Often it is the result of spawning stress combined with other stressors such as rapid temperature change, early warming, unusually warm temperatures, and sometimes excessive parasite loads.

• Hundreds to thousands of dead juvenile white perch or yellow perch can be observed annually in some waters of the State.  Numbers and frequency of events are exacerbated by temperature fluctuation and by very warm temperatures.  Such kills may be influenced by several combined stressors and commonly involve parasites that are ingested with certain infected prey items (like copepods), stress of external protozoan parasitization, and temperature-related stress. As these species are prolific, such kills only represent a small fraction of the resident population of each water impacted.  Where these kills occur, the remainder of the fishery may even benefit in the years to come as there is less competition for limited food.

• Small but significant numbers of dead sunfish and perch can sometimes be observed when temperatures are very warm or have warmed suddenly and gill function is impaired due to heavy infestation of a fresh water mussel larvae (glochidia) on the gills of the fish.  Certain mussel species have specific fish hosts that are used for completing their life cycle.  Although the larvae may only be present for 10 days up to a month, gill function can be severely impaired during this time.  When such coincides with very warm water, the increased oxygen demand from the fish and lower oxygen content of warm water may put small numbers of glochidia infested fish into respiratory distress and result in some death.  Such kills can involve a mixed size range of fish, but typically only of one or two species.

• Unusually warm temperatures for an extended period may result in fish kills involving cold water species, such as Lake trout, in select waters which are known to have very low dissolved oxygen levels below the thermocline.  When a water becomes too warm, the trout, which are not tolerant of warm water are faced with the dilemma of succumbing to heat stress or the lack of oxygen in the colder deeper parts of the lake below the thermocline. Although a lack of oxygen in the deep parts of the water is unlikely to be from an acute chemical event, it can be linked to increased plankton production in lakes where phosphorus inputs have accumulated with activities in the watershed.  When these heavy blooms die and decompose oxygen is removed from the deepest parts of the lake and is not replenished until the lake turns over and mixes in the fall.   

• Warm temperatures may also stress select populations of fish that are less tolerant of warm water, like brook trout, particularly in shallower ponds.  In these waters the loss of cold-water inputs (groundwater and streams) that provide critical seasonal habitat during the warm summer months can be catastrophic.  Loss of these cold-water inputs may be related to drought conditions and/or activities in the watershed that affect groundwater recharge and discharge.  Such conditions can disrupt the timing and availability of cold water from mountain streams and springs.

• In late summer and early fall, when water temperatures are still warm and the oxygen demand of fish and other aquatic life is still high, shorter daylength and multiple day periods of significant cloud cover can reduce oxygen production from aquatic plants and plankton, particularly when combined with the dieback of aquatic vegetation and resulting increase in biological activity from decomposers.  This type of kill is mostly likely to be observed first thing in the morning as oxygen levels will be lowest overnight.  Although such events can impact large and small fish alike, the kill will typically affect larger fish. Such kills typically occur over several days with new dead being observed each morning.

If you have a fish kill that you would like to report, please look up the Regional Fisheries Biologist for the water in which the fish kill has occurred.  Contact numbers and emails can be found here.