Seal Slough algae bloom history
California's Water Boards hold 3 bloom reports for Seal Slough going back to 2021. Reports came in every year since then.
State staff recommended an advisory 2 times: 2 Caution. Caution is the lowest level and means people and pets should stay away from scum and discoloured water.
Blooms have been recorded every year for the last 3 years in the data.
- 2021 1 report
- 2022 1 report
- 2023 1 report
When do algae blooms happen at Seal Slough?
At Seal Slough, reports are most often logged in May and June.
About 100% of reports fall between May and September, when warm, calm, sunny weather lets blue-green algae multiply.
Blooms can appear, move with the wind and fade within hours or days, so the time of year is only a guide.
Recent Seal Slough algae reports
Recent reports came from 3 different locations around Seal Slough, including Seal Slough Marina Lagoon Parkside Aquatic Park, near Parkside Aquatic Park and South end. A bloom in one cove or beach doesn't mean the whole lake is affected, and conditions can differ a short distance away.
| Date | Location | Result |
|---|---|---|
| 8 May 2023 | Seal Slough Marina Lagoon Parkside Aquatic Park | Bloom reported |
| 11 May 2022 | near Parkside Aquatic Park | Caution |
| 28 June 2021 | South end | Caution |
What the California results mean
California recommends one of three advisory levels when a bloom is checked, matched to the signs posted at the shore.
- Caution
- Toxins may be present. Stay away from scum and keep pets and livestock from drinking the water.
- Warning
- Toxins are at higher levels. No swimming, and keep children and pets away from the water.
- Danger
- High toxin levels. Stay out of the water and off the shore until further notice.
- Bloom reported
- A bloom was reported, often by a member of the public, without a formal advisory level.
Is Seal Slough safe for dogs?
Dogs are at the greatest risk from toxic algae. They swallow water while swimming, eat scum on the shore and lick algae off their fur afterwards, and a small amount can kill a dog within hours. With blooms recorded in 3 different years here, take particular care at Seal Slough in May and June.
- Keep your dog on a lead near water that looks green, scummy, foamy or discoloured, and away from algae washed up on the shore.
- If your dog gets in, rinse it straight away with clean water and don't let it lick its coat.
- Call a vet immediately if your dog vomits, drools, has diarrhoea, staggers, shakes or seems weak after being near the water.
Read the full guide to toxic algae and dogs: symptoms, what to do in the first minutes and how vets treat it.
Health effects of toxic algae
Blue-green algae (cyanobacteria) can produce toxins that affect the liver, nervous system and skin. Swallowing water during a bloom can cause stomach pain, vomiting and diarrhoea. Skin contact can cause rashes, and breathing spray from the water can irritate the eyes, nose and throat. Children are at higher risk because they are smaller and more likely to swallow water.
How to spot a bloom
Toxic blooms can look like pea soup, spilled paint or green streaks on the surface, and they can also be blue, brown, white or red. Scum often collects along the shore downwind. If you can't see your feet in knee-deep water, or you aren't sure, stay out.
Don't drink untreated lake water, and don't boil it to make it safe: boiling doesn't destroy algal toxins. If you feel unwell after contact with the water, contact a doctor or Poison Control on 1-800-222-1222.
Nearby lakes with algae reports
- Redwood Creek14 km away, 2 reports, last in 2023
- Arastradero Lake22 km away, 2 reports, last in 2025
- San Francisco Bay23 km away, 12 reports, last in 2026
- Shoreline Lake23 km away, 3 reports, last in 2026
- Lake Chabot24 km away, 14 reports, last in 2024
Questions about Seal Slough
Is there algae in Seal Slough right now?
There are no bloom reports for Seal Slough in the last 30 days of California's data. The most recent was a bloom report on 8 May 2023 at Seal Slough Marina Lagoon Parkside Aquatic Park. California's data is checked every day, so for today's conditions check the California HABs Portal map and the signs at the shore.
When was the last algae bloom at Seal Slough?
The most recent entry in California's data is from 8 May 2023 at Seal Slough Marina Lagoon Parkside Aquatic Park: a bloom report.
Has Seal Slough had toxic algae before?
Yes. Blooms are recorded every year from 2021 to 2023, most often in May and June.
Can dogs swim in Seal Slough?
Keep dogs out of the water whenever a bloom is reported or the water looks green, scummy or discoloured. Dogs are the animals most often killed by algal toxins because they swallow water and lick their fur. If your dog swims in water that might have algae, rinse it with clean water and call a vet if it vomits, drools, staggers or seems weak.
Can you eat fish caught in Seal Slough during a bloom?
If you keep fish caught during a bloom, health agencies advise removing the guts and liver and rinsing the fillets in clean water before cooking, because toxins collect most in the organs. Some agencies advise not eating fish from an active bloom at all, so follow local guidance.
About this data
This page uses official records from the California State Water Resources Control Board, which updated daily. Reports are grouped by waterbody. The site was last rebuilt on 24 October 2026.
States only test some lakes, some of the time, so a lake without a recent report hasn't necessarily been checked. This page is independent, isn't affiliated with the State of California, and isn't health advice. View the source data.