Laguna Niguel Lake Fish Die Off
Friday, August 29, 2025: OC Parks has received the results of the necropsies performed on three bass fish that died during the incident. Bacteriology, histology, parasitology, heavy metals and viral hemorrhagic septicemia virus (VHSV) were tested. The testing yielded the following results:
Bacteriology: No significant issues detected.
Histology: Results were normal and showed no cause for concern.
Parasitology: No notable parasitic presence identified.
Heavy Metals: Some elevated levels were detected (notably cadmium and arsenic), but not at concentrations high enough to explain the mortality of approximately 500 fish. It is important to note that heavy metals (such as mercury, cadmium, copper, etc.) tend to persist in tissues over time.
Viral hemorrhagic septicemia virus (VHSV): Not detected.
View the full necropsy report.
Following this incident, OC Parks is taking steps to have the necessary equipment and resources in place to be more prepared and respond quicker should anything like this occur in the future. Inspection protocols are being thoroughly reviewed to identify where improvements can be implemented to minimize downtime and prevent future equipment failures. This includes increasing the frequency of inspections for critical components like manifolds and aerators and creating a preventative maintenance schedule. Additionally, remote lake monitoring options are being explored which will give better insights into changing lake conditions, allowing for a more proactive response.
For additional information related to this incident, visit FAQs.
Friday, August 8, 2025: OC Parks has completed a thorough analysis and review of multiple water samples collected throughout Laguna Niguel Lake and Sulphur Creek. No substances of significant level of concern were detected and healthy water qualities were observed. The results further add to the hypothesis that the fish die off incident most likely occurred naturally due to changing weather conditions throughout July.
Water sampling sites in Laguna Niguel Lake and Sulphur Creek. Click to view image.
During the summer months, phytoplankton (microscopic photosynthesizing algae) naturally occur in larger numbers. These organisms may bloom into sizable populations in warm weather when there is prolonged sunlight and little cloud cover. When this happens, phytoplankton will photosynthesize during the day, which can increase concentrations of dissolved oxygen in water, critical for fish survival. After sunset, phytoplankton and other plants in the lake ecosystem consume oxygen, causing dissolved oxygen concentrations in the water to fall.
When there is heavy cloud cover during the day, there may not be enough sunlight to sustain the phytoplankton bloom. This can lead to lower dissolved oxygen levels or a crash in the phytoplankton population. The drastic change can deplete dissolved oxygen and threaten fish and other organisms in the lake.
The graph below shows July 2025 cloud cover data for Laguna Niguel. The data reveals little cloud cover in mid-July. The low cloud cover was immediately followed by high cloud cover for the next several days, ranging from 82% to 100% coverage. Experts believe this change likely reduced photosynthesis and dissolved oxygen production within the lake, causing this low oxygen event. While it is not believed to have had an effect on this incident, six aerators were also offline at the time, which could have impacted the conditions in the lake.
Laguna Niguel Lake cloud coverage July 12 – 22, 2025. Click to view image.
In these low oxygen conditions, fish in the lake likely would have sought areas with more oxygen, such as the incoming cooler water upstream in Sulphur Creek or near the Oxygen Saturation Technology Systems (OSTs) that were installed in the deep-water section of the lake in December 2024. Unfortunately, too many fish gathered in the creek at the location where the die off incident occurred. This could have been caused by a combination of factors including the shallow water depths in that area or insufficient dissolved oxygen for the number of fish that were in such a small area.
To ensure a thorough analysis of all possible factors that may have contributed to the die off incident, OC Parks worked closely with OC Public Works to collect water samples in three locations within Sulphur Creek, which is primarily fed by urban runoff. Samples were analyzed for nutrient content, water quality and several major classes of pesticides and metals. Healthy water quality levels were observed, and the findings concluded a toxic event likely did not cause the fish die off.
Water samples analyzed for ph levels, water temperature, nutrients and solids yielded results within normal range. View the full nutrients report.
Dinetofuran and thiamethoxam, two neonicotinoid pesticides, were detected in Sulphur Creek at levels far below scientific benchmarks from the U.S. Enviornmental Protection Agency (USEPA 2023)1 and published studies (Finnegan et al. 2017)2.
An herbicide, glyphosate, was detected in one of three water samples collected from the creek at a very low concentration, just above the method detection limit. This particular water sample was collected outside the boundaries of Laguna Niguel Regional Park at a concentration far below the lowest toxic concentration. View the full pesticides report.
Concentrations of metals were below levels of concern as compared to thresholds in the California Toxics Rule. View the full metals report.
OC Parks is awaiting the results of the necropsy. Once received, further information will be shared.
1 USEPA. 2023. Dinetofuran DRAFT Biological Evaluation: Effects determinations for federally listed endangered and threatened species and designated critical habitats. Environmental Fate and Effects Division, Office of Pesticide Programs, USEPA, Washington, DC.
2 Finnegan, M.C., L.R. Baxter, J.D Maul, M.L. Hanson, and P.F. Hoekstra. 2017. Comprehensive characterization of the acute and chronic toxicity of the neonicotinoid insecticide thiamethoxam to a suite of aquatic primary producers, invertebrates, and fish. Environ. Toxicol. Chem. 2017 Oct;36(10): 2838-2848.
Friday, August 1, 2025: A Town Hall meeting to discuss the fish die off will take place Saturday, August 9 from 10 a.m. to 12 p.m. at Laguna Niguel Regional Park. More information and register for the Town Hall.
Wednesday, July 30, 2025: OC Parks has received the results of a hydroacoustic, sonar-based fish mapping survey throughout Laguna Niguel Lake. This high-tech method uses sound waves to assess fish population, distribution and habitat use in the lake. During the survey, sonar equipment emits sound waves into the water and records the echoes that return after bouncing off objects such as fish. The survey illustrates where fish are actively present within the lake and shows evidence of a healthy and sustained fish population.
As shown below, the map indicates the density and distribution of fish in Laguna Niguel Lake. The numbers and contour lines represent the approximate water depth, and the brightest color yellow represents the presence of 10 or more fish within the zone.
Mapping shows the highest population density of fish are concentrated in the north end of the lake where the water is deeper and cooler. This is also where two oxygen saturation systems (OST) are located, which saturate the water with dissolved oxygen and are most effective when used in deep water. Please note, hydroacoustic mapping software does not show fish populations in water less than 3 feet deep, therefore fish in the shallower, south end of the lake may not be represented on this map. However, this mapping does provide a comprehensive look at fish abundance and distribution throughout Laguna Niguel Lake.
Hydroacoustic mapping completed Monday, July 28, 2025. Click map to access the document.
Friday, July 25, 2025: As of Thursday, July 24, all cleanup efforts from the recent fish die off within Sulphur Creek near the south end of Laguna Niguel Lake have concluded. Approximately 500 fish were affected, predominantly bass and bluegill, and there have been no reports of additional fish surfacing.
OC Parks continues to work closely with experts in the field to monitor the current conditions of the lake. Water samples from multiple locations and depths have been collected to test water quality, including temperature, dissolved oxygen (DO), pH, and conductivity (salinity). Hydroacoustic fish mapping is also being conducted to assess the remaining fish population in Laguna Niguel Lake.
Location of fish die off within Laguna Niguel Regional Park. Click map to access the document.
Wednesday, July 23, 2025: OC Parks is saddened by the recent fish die off that occurred at Laguna Niguel Lake within Laguna Niguel Regional Park and have been working around the clock to address the situation. We understand the importance these lakes have for the community and want to stress that we are taking this very seriously.
While we cannot definitively say exactly what caused this to happen, as fish die offs can occur naturally from time to time, a combination of factors could have contributed to the situation such as changing weather, increase in water temperature, a lower than average rainfall year, the presence of algae and potential equipment malfunction.
As of now, it appears the fish die off was due to a change in weather causing the water temperature to increase, particularly in the shallow waters at the south end of the lake where most of the fish were seen on the surface. When this happens, there is less oxygen in the water, which is critical to fish survival. Catfish can tolerate the warmer, desert-like conditions present in Laguna Niguel Lake this time of year, but other fish are more affected, seemingly leading to this fish die off. While there is no way to control the water temperature of the lake, further steps are being taken that could possibly help mitigate this from occurring in the future.
OC Parks cares deeply about providing the necessary conditions for fish to survive and thrive in the lakes under our management. All equipment has been thoroughly checked to certify everything is functioning properly. Additionally, we are continuing to monitor water temperature and oxygen levels within the lake to ensure conditions remain stable.
As of Wednesday, July 23, most of the cleanup efforts have concluded, with only a handful of fish surfacing over the last 24 hours. When visiting Laguna Niguel Regional Park, should you ever encounter any wildlife in distress, please contact the park office at (949) 923-2240.
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