Rainbow Trout junior fishing guide

Ergasilus briani is a tiny parasitic crustacean that sticks to a fish’s gills, a bit like a microscopic crab. It irritates the gills and makes it harder for fish to breathe. Heavy infections can stress the fish and slow them down.

Ergasilus sieboldi, also known as the gill maggot, is a tiny parasite that attaches to the gills of freshwater fish. The adult females have long white egg sacs that trail behind them, making them easy to spot. They feed on the fish’s gill tissue, blood and mucus, which makes it harder for the fish to breathe and stay healthy. Heavy infections can cause fish to become tired, lose weight, grow slowly, or even die — especially in warm weather. This parasite can spread quickly through a fishery, so good fishery management and strong biosecurity are really important to keep fish safe.

Argulus mongolianus is a newly discovered fish louse recently found in the UK. It attaches mainly to the gills, mouth, and head of fish, causing irritation, stress, and sometimes deaths. It has already been confirmed in carp, roach, and bream, and may infect all freshwater species. Because it is new and potentially harmful, reporting any suspicious lice or fish behaviour is very important.

Blackspot is a common parasite that forms tiny black cysts on the skin and fins of freshwater fish. It looks alarming but is normally harmless and part of a natural cycle involving snails and birds. Only very heavy infections or small fish are affected. In most cases, blackspot is nothing to worry about.

Ichthyobodo necator (Costia) is a rapidly spreading parasite that attacks a fish’s skin and gills. It causes lethargy, breathing problems, weight loss, and can be deadly—especially for young or stressed fish. It thrives in both cold and warm water, so good management and low-stress conditions are essential to prevent outbreaks.