Fish Louse or Gill Louse (Salmonicola sp.)

Volume 2, Issue 2
March 2000
Updated November 2002

By G. Russell Danner MS, DVM
Fish Pathologist

Brook trout Salvelinus fontinalis and Atlantic salmon Salmo salar in Maine with an unsightly white-cream-tan organism attached to their gills, fins or mouths are occasionally caught by anglers. These copepod organisms are Salmonicola edwardsi but they are commonly referred to as a “fish louse” or “gill lice.” They are Crusteaceans related to lobsters, crayfish, sowbugs and amphipods. Salmonicola attach to fish by burrowing an attachment organ called a “bulla” beneath the skin or gill tissue. This bulla anchors the young salmonicola organism in place generally to a part of the fish’s bone or cartilage. It lives its entire life then in that spot on the fish.


Fig. 1. Head, mouth and arms of Salmonicola.

Damage to gill tissues can be extensive, and result from attachment , feeding, and the mechanical presence of the parasite. The parasite feeds on the fish’s body tissue and body fluids. As the parasite grows, the females develop two caudal egg sacs. These can be easily seen with the naked eye. In some species of salmonicola the egg sacs are dark brown to black in others they are cream to tan in color.

Salmonicola have a direct life cycle. The egg sacs on the caudal portion of the female’s body release eggs into the water column. The eggs develop into larva within the egg shell and then hatch ready to infect a new fish. The young parasites die within a day or two of hatching if they do not find a suitable fish host. Many young parasites are eaten by other small fish and aquatic organisms. Many young fish feed for a time on copepods and some such as herring, sardines and smelts, continue to feed upon them throughout life.

Salmonicola edwardsi

Fig. 2. Salmonicola edwardsi infestation on gill of landlocked Atlantic salmon.

Salmonicola generally only infect a specific species of fish. Salmonicola edwardsi will selectively infect brook trout over lake trout or brown trout even in the same tank or pond. Once the young salmonicola attaches to a new fish, it burrows its bulla into the fish’s skin and connective tissue and begins to feed on its flesh and body tissues.

Control Measures

Don’t transport fish from one water to another without an IF& W stocking permit.

Don’t dispose of fish parts into water - it can spread the parasites eggs.

Clean boats and equipment before moving between different waters.


Gill lice infect specific species of fish.

Gill lice are related to lobsters and crayfish.

Gill lice do not infect or harm humans.

Images: Thanks to Colin Widener, Colby College, Waterville, Maine. Scanning Transmission Micrograph of head, mouth and arms of salmonicola. Other images made possible in part by a grant from the Maine Outdoor Heritage Fund.