A blue-green algae advisory is in effect for Prior Lake in Thetis Lake Regional Park after toxic blooms were spotted in the water.

Island Health is asking visitors to avoid swimming in the water and to keep pets on a leash until the advisory is lifted.

Blue-green algae can produce cyanotoxins, which can cause symptoms like headaches and abdominal pain and can be deadly for dogs.

The toxins can still exist in the water even if the blue-green algae blooms aren't visible, Island health said in a statement Thursday.

Prior Lake is located at the north end of Thetis Lake Regional Park, which is around 10 kilometres northwest of Victoria.