Dear Lady of the Lake,

Every summer I come up to Clear Lake to go fishing. This year, I noticed a bunch of dead fish in the northern part of the lake. Do you know what is causing this fish kill and if this is a “new normal” for Clear Lake?

Thank you,
— Catfishin’ Carl

Figure: A small bass fish kill, off Lakeport in August, resulted from catch and release fishing during extreme warm weather. These types of fish kills don’t impact large numbers of fish, but generally larger members of the population are commonly included.

Dear Carl,

Thank you for this important and relevant question. I have received several calls, emails and face-to-face questions about the fish die-offs that occurred this year in Clear Lake. This is a great topic to discuss in today’s column.

Unfortunately, fish die-offs are normal for lakes that have healthy and abundant fish populations, like Clear Lake.

Healthy lakes that have abundant fish populations are rich in fish food, including small insects (called zooplankton) and green algae (called phytoplankton), which is the base of the food web.

Clear Lake, as we all are familiar, is very rich with phytoplankton and has a very large food web base. That is why the fishing here is so plentiful.

In these types of “fish” lakes, fish are pretty much growing and dying all the time, and the majority of the dead fish bodies sink and decompose on the bottom of the lake and never float back to the surface.

Sometimes gasses get trapped or are produced from the decomposition process and gasses cause the fish carcass to float. Then due to currents, winds, and waves, we will sometimes witness the sad and stinky event of a fish die-off.

Sometimes fish die-offs are completely natural, and sometimes they are triggered by an external, or man-made event. There are technically two different names for these types of events.

Fish mortality events: Fish kills vs. fish die-offs

Although these terms are confused quite a bit, by laymen and professionals alike, there actually is a dictionary-difference between a “fish-kill” and a “fish die-off.”

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. Most fish-kills that are naturally occurring are actually die-offs, but get mistaken for fish-kills.

As opposed to fish-kills, a “fish die-off” is generally a naturally occuring phenomena and usually 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.

Here in Lake County, we have seen winter die-offs 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.

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.

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 a thorough investigation by a state or public health agency would be extremely warranted.

If something in the aquatic environment is causing that level of broad mortality, then there is a high probability that other uses and users could be impacted, such as drinking water treatment processes or exposure through recreation. Likewise, when there is a contaminant spill or leak incident, one of the most important monitoring requirements is any observations of fish-kills or other impacts to wildlife.

Investigating fish mortality events

In general, a fish mortality event that is in need of reporting and investigating would include the death of at least 50-100 fish, or more. Basically, if there is one or two fish, of different species or sizes, belly up in the middle of the lake, there is no need to panic and report a fish mortality event.

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

It’s important that fishers, during warm summer months, take extreme care of fish that they catch and don’t release right away. The California Department of Fish and Wildlife (CDFW) imposes fishing tournament restrictions in the height of summer, to prevent game fish from becoming too stressed or over-fished when the temperatures are extremely warm.

These restrictions include reduced hours between blast-off and weigh-ins, requirements to report mortalities, and general reductions in the number and species type of permits issued during any one week or weekend.

It’s important for visiting and residential fishermen to practice the methods necessary to protect game fish during warm weather. A great resource for useful tools and strategies are online fishing blogs and magazines. According to Bass West Magazine and BassResource.com, the key to reducing fish mortality is responsible anglers.

Obviously, not all fish kills are the result of fishing. Usually fishing related kills are minimal, and don’t impact more than 5-20 fish. Of course angler-related fish-kills are limited in size and species of fish, and pretty easy to identify. If the fish kill consists of several large, prize game fish, all of one species, and localized to one area, then it's highly likely that its probably an angler or fishing tournament related.

Angler and tournament fish kills are rare because CDFW permit requirements are structured to prevent fish mortality and tournaments are usually accompanied by CDFW oversight, such as a on-site regional biologist, game warden, or local fishing club members that provide observance or support to prevent fish mortality.

When the fish mortality event is not related to fishing, it’s key to investigate the size and species of fish impacted. 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.

Additionally, if a fish is carrying an additional burden of a sickness, parasite, or malnutrition, then any type of external stress could lead to dead fish. If a sickness, virus, or parasite is impacting a population of one species of fish, then a fish kill can result if there is any additional environmental stressor, either natural or man-made.

Interaction of disease and weather can lead to fish mortality events

Earlier this year in June (2023) there was a mass carp die-off throughout the northern part of the Upper Arm of Clear Lake. Shoreline property owners along North Lakeport, Nice, and Lucerne found their shorelines littered with large, stinky, slimy, rotting carp bodies (cue the gross!).

After notification to the CDFW Mortality Lab state personnel collect dead fish specimens to identify possible causes of the event. Of the five carp samples collected and evaluated, five of the five samples were positive for koi herpesvirus (KHV) also known as cyprinid herpesvirus 3 (CyHv3).

Cyprinis herpesvirus effects only koi and common carp fish species. KHV is highly contagious and affects younger carp more than older members of the population, but death is almost certain once contracted. Most importantly is that warm temperatures can increase mortalities in a population suffering from KHV.

In Clear Lake, we have a very healthy and robust carp and goldfish population. In general, carp are considered an invasive species; they can grow rapidly, reproduce several times throughout the year, consume native plants and animals, and out compete desirable native fish species.

When populations of fish increase, and many fish are living closely within the same space, disease can spread easily. For carp, mating is an active and boisterous event, so the chance of spreading a highly contagious disease, like KHV, is high.

The large carp die off earlier this year resulted from too many members within a population, having just closely interacted due to mating season, and a sharp increase in water temperatures as the season moved from spring to summer, causing stress to fish that were already compromised from the KHV.

In this instance the combination of environmental conditions and disease caused a fish die-off mortality event, but that was only able to be ascertained through laboratory analysis and study of the collected fish tissues.

Sometimes mortality events happen, where the cause cannot be as easily identified, but could result from natural causes.

Impacts to fishing

In general, unless there is a wide spread disease or climatic event (i.e.drought), fishing in Clear Lake will remain strong, with normal ebbs and flows, that are mostly season related, as populations go through normal booms and busts. To answer your question Carl, the “normal” for Clear Lake is some years there are fish die offs or fish kills, but many years there are not.

However, Clear Lake is consistently a strong contender for being rated one of the best bass fishing lakes in the west. In fact, for 2023, Clear Lake was rated third for the top 10 Best Bass Lakes of 2023 by Bassmaster. 

Fish in Clear Lake are generally very healthy, and rarely do large scale diseases break out and impact whole populations. This is due to the fact that Clear Lake is huge and is highly productive, so it can provide the food and resources needed for fish to grow and be healthy.

For more information on specific diseases impacting fish, such as viruses other than KVH, fungus, and parasites, I will refer you to the comprehensive “Field Manual for Investigation of Fish Kills” edited by Meyer and Barclay in 1990 and produced by the US Fish and Wildlife Service. This manual can be found for free download at the USGS publications warehouse online here.

Natural causes of fish kill

One of the most natural causes for fish kill is oxygen depletion. When the demand for oxygen by fish, and all the other organisms living in the water, exceeds the oxygen that is available, or being held by the water, then there can be fish kills.

Some fish have been able to swim to other areas where more oxygen is present, like deeper, cooler waters in the lake. If we remember our basic chemistry, warmer water holds less oxygen while cooler water holds more oxygen. Likewise, fish metabolism requires less oxygen in cooler waters than in warmer waters. Some fish are more suited for life in cooler waters, and some are more tolerant in warmer waters.

For example, bass are cooler water fish, and require more oxygen, and tend to swim down to the deep parts of the lake in the height of the summer, while catfish and carp are more tolerant of warmer water, with less oxygen, and can be found in the shallows all throughout the year.

When we talk about low oxygen, a threshold that can become fatal for fish is 0 or 1 parts per million (ppm, or milligrams per liter, mg/L). Anoxic conditions are when the water has absolutely no oxygen (0 ppm) and hypoxic is when there is very, very low oxygen conditions (2ppm or less).

Some size and species of fish become extremely stressed when water around them becomes hypoxic, and the majority of fish cannot survive in even short time periods of anoxic conditions.

Large declines in oxygen can occur after a period of rapid growth, such as a warm period where large amounts of aquatic plants and algae grow. During this growth the plants are actually creating oxygen, since they photosynthesize. They take in carbon dioxide and transpire oxygen.

When it’s nighttime, and the sun is absent, the plants will switch from producing oxygen to consuming oxygen, as they respire they take in oxygen and release carbon dioxide. This is called cellular respiration, and green living things need to respire to balance carbon within individual cells and maintain growth of tissues.

This period of respiration can last into the morning for a few hours, until the sun and temperature reach an optimal temperature for the plant to fully switch to photosynthesize. During this time, the oxygen concentrations are extremely low in the water surrounding thick and heavy aquatic plant beds, and any fish living within these areas might be having a hard time breathing.

If too many fish, and other aquatic organisms are present, the demand for oxygen might exceed the supply, and a fish die-off can occur.

This die-off event can literally take place overnight. Lake shoreline property owners might wake up one morning to see fish bodies, of all sizes and species, bobbing around their dock or washed up on their shoreline.

Sometimes smaller fish can survive this type of fish-die off since their oxygen requirements are lower than larger fish, and some species of fish are successful in swimming out of the area to a place with more oxygen.

However, like in Clear Lake, some shallow areas of the Upper Arm, are very large, shallow, and the expanse of thick aquatic plant beds comprises hundreds of acres.

Unfortunately, during this time in the season when plants and algae are heavily growing, the temperature is simultaneously increasing. Fish can become stressed under warmer water temperatures, and can sometimes become trapped between layers of low oxygen in the bottom parts of the lake and the warm layers encroaching from the surface.

I wrote in detail about this phenomena in my column from August 2021 “Figuring out Fish Kills”  but this can be described as a thermal squeeze sandwich and can lead to non-discriminate fish kills.

Reporting fish mortality events

We all need to do our part to report fish mortality events. Both the state and the County rely on citizen reports to initiate response, monitoring and investigations.

Next time you see one dead fish, look around and see if you see any 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.

A local reporting tool has been created, accessed through iNaturalist smart phone application and called the https://” <https://www.inaturalist.org/projects/clear-lake-fish-kill-monitoring-project> Clear Lake Fish Kill Monitoring Project”. This was created by the 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.

Sincerely

Lady of the Lake

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..