"what's on my fish" ?

Gill Maggot (Ergasilus),

Ergasilus is a large genus of small crustaceans, well suited to both marine & freshwater habitats and found around the world.

With most Ergasilus species, the adult females become ectoparasites of fish, attaching primarily to the gills, while the adult males are largely planktonic and harmless to fish.

Over time, gill maggots make fish significantly more vulnerable to secondary illnesses and diseases such as columnaris, koi fungus, and infected skin lesions.

As such, it’s exceptionally important to regularly monitor your water quality, & identify and treat gill maggots as soon as possible.

Gill maggots are the adult females of all species within the Ergasilus genus. The males are planktonic, meaning that they float about harmlessly and feed on tiny bits of organic debris in the water. 

Though gill maggots more commonly plague aquaculture fish species like koi, they can also infect wild fish species such as pike, rainbow trout, and bream.

Females attach to the gill filaments (and occasionally fins) via two sharp, claw-like antennae structures with pincers that cause significant tissue damage.

Serrated tooth-like structures surrounding their mouth enable them to feed on epithelial cells, blood & mucus continuously. Each parasite is typically over one millimetre in size and easily visible to the naked eye.

Their body appears like a tiny scorpion, but behind them trail two long sacs of white or greyish eggs, each of which looks quite like a maggot, hence the name “gill maggot.”

Gill maggots find their way into ponds from newly acquired fish that haven’t been properly quarantined before introduction into the pond.

If they were bred in an overstocked aquarium or fish farm, the likelihood of fish having parasites is fairly high due to poor water quality.

Always quarantine new fish for several days before putting them in your pond, and observe them for any signs or symptoms of gill maggots or other parasites.

Additionally, larval gill maggots can be transported fromlive food. If you purchase live food, such as bloodworms, tiny shrimps, etcetera, for your fish, be sure to only purchase them from a reputable and trustworthy source, and check them very thoroughly for any parasites. Use a pond microscope if possible.

For a parasite, the life cycle of the gill maggots is a relatively long one. After mating, female gill maggots immediately attach to the gills of a host fish.

They will develop two long trailing “tails” of egg sacks that closely resemble maggots.

Each clutch can contain over 200 eggs. These eggs typically hatch within a week. These parasites prefer warmer waters, ideally, those 24° C (75° F) and above, though this depends somewhat on the exact species.

The majority of Ergasilus eggs hatch during the height of summer when development is hastened by warm temperatures and eggs can hatch in as little as 3 days and develop into adults. 

After hatching, Ergasilus larvae, known as nauplii since they are copepods, float about freely, simply feeding on tiny bits of floating organic matter. This primarily consists of algae.

They go through three to four developmental stages as nauplii, and then several stages as copepodites, before reaching reproductive maturity. Reproduction typically occurs in the spring, with males dying after mating and females attaching to a fish host to feed and lay eggs.

This process can occur as many as five times per year per female gill maggot, meaning that a single gill maggot can give rise to as many as 1,000 young per year.

Females are also able to overwinter on fish if needed, and typically live for a year or less. Prevention is the best medicine!

First and foremost, quarantine any new fish before adding them to your pond. Closely monitor them for several days, checking for any visual signs of gill maggots.

You should inspect for any other parasites as well, using a microscope if needed to look at water and skin samples.