Dear Lady of the Lake,

I live on the lake, I am a fisherman, and I have noticed that in previous years there have been fish kills in the lake, mostly during summer, sometimes after chemical plant treatments, and when the algae and cyanobacteria in the lake get really bad. Are we going to see fish kills again this year? What is going on?

— Need Help “Figuring out Fish Kills” Frank


Dear Frank,

Thank you so much for this question, it’s very important and very timely for this time of season.

Of course we are all crossing our fingers that we don’t see any mass fish die-offs this year, but if we understand the science of temperature and oxygen dynamics in lakes, it’s probably a futile exercise to pray for no fish die-offs this year, unfortunately.

Before we get into the “weeds” on this topic (pun intended), let’s get some definitions out of the way. Although these terms are confused quite a bit, by laymen and professionals alike, there actually is a difference between a “fish-kill” and a “fish die-off.”

Fish-die off vs fish-kill

A “fish die-off” is generally a natural phenomena and occurs in summer when low oxygen and warm temperatures causes large mortality in a localized population of fish. Fish die-offs can also happen in winter months when temperature stress can kill a fish population. We have seen winter die-off happen during extreme winters here in Clear Lake and her tributaries, especially to the fragile, small bait fish populations like the threadfin shad and silversides.

Fish die-offs also happen often in lakes that freeze over, especially in lakes that have been stocked by wildlife agencies with larger populations than they can accommodate. In these frozen lakes, snow and ice accumulate on the surface of the water which eliminates oxygen diffusion with the water surface at the same time as the light penetration reduces which terminates all photosynthetic processes. No photosynthesis means oxygen is not being produced in the amount to meet biological demand and fish, particularly in smaller or shallower lakes, can die-off in large amounts.

Alternatively, a “fish-kill” is an event that leads to small or large fish mortality from a human-caused specific incident such as an oil or sewer spill or a point source dumping of contaminants.

Lastly, a “fish mortality event” can be a good term that is inclusive of both of these occurrences. Just by observation, we might not know what is the cause of a fish-kill or fish die-off, and sometimes an investigation is warranted.

According to the New England Chapter of the North American Lake Management Society, a fish kill worth reporting and in need of an expensive investigation would involve the death of at least 50 to 100 fish, or more.

If you see one carp belly up in the middle of the lake, don’t panic, it’s probably not a big deal, fish do just die, from natural causes.

Additionally, some fish get stressed after being caught by a fisherman. Sometimes after a fish is stored in a warm live-well or bucket for a long time during the summer, they can stress and die, so this can also lead to some of the “belly-ups” sometimes observed around the lake.

Another thing that is investigated is the size and species of the fish involved in a mortality event. If the event only includes similarly sized fish of one species, that could indicate a specific type of die-off. A good example of this is when abnormal or wild weather fluctuations in spring cause spawning stress die-offs for species like sunfish, pumpkinseeds or crappie.

Now, an alarming fish mortality event would include many fish of various sizes and species. That type of event would indicate a potential fish-kill incident and an thorough investigation by California Department of Fisheries and Wildlife, or CDFW, would be extremely warranted.

If something in the aquatic environment is causing that level of broad mortality, then it might impact our nearby drinking water intakes and could be harmful to those recreating nearby.

Likewise, when there is a spill or leak incident, one of the most important monitoring requirements is any observations of fish-kills or other impacts to wildlife.

Reporting fish mortality events

While Lake County Water Resources does receive fish mortality complaints, the mortality reporting, tracking, and investigation of these mortality events belongs to the CDFW.

No one is just out boating around looking for fish mortality events, so it’s really important that if water users or boaters see something, they say something and submit a report to the right agency.

There is an online fish mortality reporting form here at this link, or you can call the CDFW mortality lab at the State Wildlife Health Lab, 916-358-2790.

If you can’t get to the online form, I would recommend you take photos, note the location, date, time, and you can send that info to Water Resources (This email address is being protected from spambots. You need JavaScript enabled to view it.) or call 707-263-2344 and they can pass along that information to CDFW.

Hot water and low oxygen can lead to summer die-offs

I hope you are still with me Frank, because now we are going to dive into the complex, yet exciting lake science that drives fish mortality and why you see these die-offs sometimes.

But first, there are a few basic water chemistry and limnological concepts we need to understand to be prepared to understand the interactions that have to happen to lead to a summer fish die-off event.

— Aquatic organisms, like fish, require oxygen, in what is called Biological Oxygen Demand, or BOD.
— Warm water holds less oxygen than cold water.
— During hot summer and early fall days, the top layer of the lake (called the epilimnion) becomes warmer and warmer, and this layer gets bigger as the season goes on.
— Aquatic organisms, like fish, have oxygen and temperature thresholds. For example, largemouth bass have an optimum temperature range maximum of 80oF and Bluegill can tolerate temps up to 84oF. Some species, like common carp, are more warm-water tolerant, and can be found in water up to 96oF, but this temperature can be lethal even to carp for extended periods of time.
— As the summer season ends, dying algae, cyanobacteria, and aquatic plants breakdown, stop producing oxygen and decompose (which consumes oxygen from the surrounding water).
As the summer season goes on, abundant algae and cyanobacteria populations die and their tissues sink to the bottom layer of the lake (called the hypolimnion). The hypolimnion is normally cold, but not necessarily oxygen-rich as there is no photosynthesis occurring (it’s too dark) and there is always microbial decomposition occurring, which consumes oxygen and creates carbon dioxide.
— As the summer goes on, the bottom hypolimnion becomes more and more deoxygenated and can get thicker in depth. Sometimes the bottom hypolimnion can get hypoxic, meaning very low oxygen < 5mg/L, or milligrams per liter) or anoxic, meaning no available oxygen (<2mg/L).

Putting all these things together, we can see that in summer the lake creates what is called a “thermal squeeze sandwich” (See figure provided by Ficke et al. 2007). With the top layer being heated water that is too warm for most fish, and the bottom being a thick layer of deoxygenated water. The only “livable” area is the middle layer (called the thermocline or metalimnion), which is cool enough for the fish and still retains some oxygen so they can breathe.

The middle metalimnion section becomes thinner and smaller throughout the summer and into fall, and in a lake with a large fish population, like Clear Lake, there sometimes just isn’t enough space and oxygen for all the fish to survive.

The first thing to consider is that in warm weather, fish metabolisms are more active, and they need more oxygen than cooler parts of the year. This means that all the fish in this small, middle, “sandwich” layer are all competing for limited available oxygen. Usually, fish die-offs happen very early in the morning, because all throughout the night, when there is no sunshine, none of the in-lake photosynthesis is occurring.

No photosynthesis means no oxygen is being produced, even in the top layer of the lake where algae is abundant. Remember that algae and cyanobacteria are oxygen producers when there is light and they can photosynthesize. This nighttime decline in oxygen is like “the last straw” and already stressed fish can suffocate and die.

Secondly, if the water is very shallow (like in a bay, cove, or most of Clear Lake), and the air temperature is very warm, the top and bottom layers can basically squeeze out the middle layer, which would mean the entire water column is hot and deoxygenated.

Fish can suffocate from no oxygen and can become stressed in heated water and a massive fish die-off can result. If this event happens in a bay or cove, larger, mature fish usually swim into open, deeper, cooler waters, but small and juvenile fish, who would be prey in open waters, might not swim to safety.

That’s why we usually see first signs of fish die-offs of smaller, juvenile fish. It’s devastating to see these types of events, because that is next year's cohort, or generation of fish. We recently saw this in a small fish die-off by the Oaks ramp, where a bunch of small fish were dead along the edge of the ramp.

Lastly, the bottom hypolimnetic layer of deoxygenated water can sometimes spread into the rest of the water column if there is a large “turnover” event. The “turnover” basically blends all the layers (epilimnion and hypolimnion).

In a shallow lake, a turnover event can be caused by a big wind or a sudden,drastic change in temperature. Some private homeowners in human-made lakes have tried to “add oxygenation through aeration or bubbles” late in the summer season and have accidentally turned over their lake and caused a fish kill.

Aeration should only be installed by a professional and early in the season to act as a preventive to deoxygenation from occurring in the first place.

Very important: Aeration should not be turned off and on, but should be continuously running in the water column.

Plant treatments and fish-kills

You also asked a good question Frank about the chemical aquatic plant treatments and fish-kills. Clear Lake is a huge, shallow lake, with lots of aquatic vegetation.

The county does provide some public access (beaches, boat ramps) chemical aquatic treatments and so does the California Department of Food and Agriculture (CDFA) as part of the hydrilla management program.

Later in the year, I will write another column about hydrilla and plant treatments, but for now, know that only about 1% or less of the lake surface area is subjected to any sort of chemical treatment in any given year. That is a very, very small amount of space, and it’s all spread out into various squares and strips directly adjacent to the shoreline depending on what is needed and where.

Basically when specific aquatic plants are exposed to herbicide, they die, as the herbicides are formulated to do. When the plants die, they decompose and consume the oxygen in the nearby environment. Guess what? The next few days a lot of algae float in and reproduce in that now “free” and recently-vacated plant apartment space. Since algae is photosynthetic, it produces oxygen. So a treated herbicide area is not really depleted of oxygen for that long, especially during the spring when algae is actively growing all over the lake.

Additionally, for all chemical treatments occurring in the state of California under permits issued by the Department of Pesticide Regulation, aquatic herbicide cannot be applied if the water measures lower than 5 mg/L of oxygen (hypoxic conditions). This means that there is no chance for there not to be enough oxygen after a treatment.

Everyone licensed to do applications has to measure their oxygen first and there are times when the applicators cannot safely do their treatments because the oxygen concentration is too low.

Lake County has also instituted a temperature threshold of 85 F, just to be on the safe side for when water conditions are already particularly stressful for fish and other wildlife. Only some applications are selectively approved and only for certain areas and conditions (such as directly adjacent to a natural shoreline, high flow area, or well vegetated plot for example).

Treatments are usually in strips straight out 100 to 200 feet from a boat ramp or just around a dock here and there, there is lots of space around the treatment zone for fish to swim out and away, if they don’t have enough oxygen at any given time.

After talking to local fishing guides, that is generally what they tend to notice after a localized treatment has taken place near their favorite fishing spot.

Sincerely,
Lady of the Lake

Next time you see one dead fish, look around and see if you see a bunch more. Note if they are the same size or the same species. Submit a report to the CDFW mortality lab here, or you can call the CDFW mortality lab at the State Wildlife Health Lab, 916-358-2790.

Recently, an iNaturalist “Clear Lake Fish Kill Monitoring Project” project has been established. By Big Valley EPA. The “Clear Lake Fish Kill Monitoring Project'' is a citizen science project to record fish kills wherever they occur on Clear Lake and nearby tributaries. This project contributes to ongoing data collection and environmental monitoring efforts by the Big Valley Band of Pomo Indians EPA Office in collaboration with local, state, regional, and national agencies.” If you have iNaturalist on your phone or tablet, add this project and record any fish kills or other lake wildlife issues you spot out on the lake. You can also take photos and notes on location, size and species of fish, and submit to the project once you are off the water. https://www.inaturalist.org/projects/clear-lake-fish-kill-monitoring-project.

Angela De Palma-Dow is a limnologist (limnology = study of fresh inland waters) who lives and works in Lake County. Born in Northern California, she has a Master of Science from Michigan State University. She is a Certified Lake Manager from the North American Lake Management Society, or NALMS, and she is the current president/chair of the California chapter of the Society for Freshwater Science. She can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..