Blackspot fish health guidance for clubs & fisheries

What is Blackspot?

Blackspot is a parasite that forms tiny black cysts on the skin, fins, and gills of freshwater fish. The dark colour comes from a pigment the fish produces around the parasite — not from the parasite itself. You can see blackspot with the naked eye.

The parasite is a type of flatworm that has a very complicated life cycle involving snails, fish, and fish-eating birds like herons.

What does Blackspot do to fish?

Not very much!
 The parasite burrows into the fish’s skin and forms a small cyst, but it doesn’t feed on the fish’s tissue and rarely causes harm.

Blackspot:

• Is usually cosmetic only
• Does not normally affect swimming or feeding
• Is only problematic when very large numbers are present
• Can make small fish more noticeable to predators

Most fish with blackspot live completely normal lives.

The Life Cycle (Simple Version for Juniors)

The parasite has three hosts:

1. Birds (like herons)
   • Adult parasite lives in the bird’s gut
   • Eggs are released into the water
2. Snails
   • Eggs infect aquatic snails
   • New larval stages develop inside the snail
3. Fish
   • The larvae leave the snail and burrow into a fish
   • The fish’s skin forms a black cyst around them
   • When a bird eats the fish — the cycle repeats

This is a normal part of nature and happens in lakes all over the world.

Is Blackspot harmful?

Most of the time, no.

• Light infections are harmless
• Heavy infections can affect small or young fish
• Seeing blackspot can actually be a good sign, because it means the habitat supports snails — a key part of a healthy food chain

Blackspot is one of the easiest parasites to spot, but usually nothing to worry about unless fish are covered in large clusters of cysts.