Argulus mongolianus fish health guidance for clubs & fisheries

What is Argulus mongolianus?

Argulus mongolianus is a newly discovered type of fish louse — a small crustacean parasite that attaches to fish. It was recently found in the UK for the first time, and only very few studies exist on it worldwide.

Unlike normal Argulus (which usually attach to the skin), this species prefers the gills, mouth, and head of the fish, which can make infections more serious.

Why is it a concern?

This parasite:

• Feeds on the fish’s blood and skin
• Can cause irritation, stress, and tissue damage
• May lead to loss of appetite, weakness, and in some cases, fish deaths
• Spreads easily if fish or water are moved between lakes
• Can infect many different freshwater species, not just carp

Because it is new in the UK, experts don’t yet know how harmful or widespread it might become.

What does Argulus mongolianus look like?

Fish lice are small, flat, disc-like creatures that move on the fish’s body.
 However, you cannot identify this species by eye — only lab specialists can confirm it.

Signs to look for:

• Fish lice around the gills, mouth or head
• Fish rubbing against surfaces (flashing)
• Lethargy or poor condition
• Visible distress

Female parasites may appear larger and filled with eggs.

What is being done?

The Environment Agency has:

• Placed movement controls on infected fisheries
• Started monitoring its spread across UK waters
• Begun working with experts in the UK and Asia to understand the parasite
• Encouraged anglers and fishery managers to report anything suspicious

If you find Argulus on a fish, it can often be gently removed and sent for lab identification.