"what's on my fish" ?

Fish Lice (Argulus)

Argulidae, the fish lice family, is well adapted to a variety of environments, including both marine and freshwater systems.

Even carefully built and maintained garden ponds that are kept healthy can become infected, though the fish louse is less common than other aquatic parasites.

Contrary to the implication of their name, fish lice are not related to lice, which are insects, but instead are parasitic copepods. They’re more closely related to shrimp and crabs than lice or insects.

If latched onto by a carp louse, fish become vulnerable to a host of secondary issues such as infection and disease that can become lethal if fish lice are not treated immediately, so it’s very important to be aware of what lice are, the possible symptoms, and how best to treat the infection!

Fish lice, also known as carp lice or koi lice in ponds, are parasitic crustaceans that are thought to be quite prehistoric based on their morphology.

However, due to their small size and obvious lack of bones or other sturdy structures, they possess no fossil record so scientists can only speculate at their exact evolutionary record.

They can live in both freshwater and marine environments, making them a potential problem for almost any type of fish!

With over 170 recognized species, fish lice are found all over the world and are all well-suited to many small ecological niches as well as entire, broad

ecosystems. Most members of this family are between 3 and 35 millimetres in size or ranging from a length equivalent to only the width of two pennies up to over an inch, depending upon the species, location, and life stage. Most Argulidae members have a broad, ovular body shape covered by a carapace, somewhat similar in appearance to the common ticks that are found in forests and grasslands.

However, fish lice have quite visible black eyes, antennae that have fused to form a barbed proboscis that also houses easily observable suckers for latching onto fish, and typically four pairs of spiny, leg-like appendages stemming from their thoracic region (the area that includes the thorax and abdomen). With their sucking mouthparts, they feed on the mucus, blood, or both of the fish that they attach to.

The naked eye can typically determine when fish lice are present, as even the smaller species are just large enough to see clearly, though if you aren’t sure a microscope can of course be used to aid in identification.

Where Do Fish Lice Come From?

Fish lice are somewhat common on commercial fish farms and can be transferred to your pond by purchasing from such places.

Some species will also attach to frogs, salamanders, and the like, and can wind up in your pond as these creatures move about throughout their day to day activities. In some cases, they can spread by being accidentally ingested by birds and then released into new areas via their excrement.

They can also be transferred through water, particularly if you fill up your pond using a nearby natural water source rather than filtered water from a hose. As is the case with most parasites, fish lice can soak up pollutants at higher concentrations,

and still survive, much longer than other organisms. Some parasites that have been studied have been found to have around 2,500 times the amount of heavy metals in their system than the fish that they were feeding on. As such, fish lice and some other aquatic parasites are being studied as a means of early metal pollution detection in freshwater ecosystems.

Life Cycle of Fish Lice Unlike many other parasites, fish lice don’t lay their eggs on the host. Instead, after fertilization, the female lays her eggs in the substrate at the bottom of the water or on vegetation and secures them with a mucousy, gelatinous material that she expels. 

About a day after being laid, eggs will turn a light brown colour and within a few days, two dark black spots will become visible on one end of each egg.

Though this depends somewhat on the species, fish louse eggs typically hatch about 12 days after being laid, so long as water temperatures are between approximately 80 and 82° F (27 to 28° C).

Lower temperatures will result in longer hatch times – at 59 °F (15° C), eggs take closer to 60 or more days to hatch, with warmer temperatures resulting in hatching as early as a week.

After hatching, the larvae have only two or three days to find a fish host before dying of starvation. Once attached, they’ll undergo several moults as they grow and develop into adults. Both males and females are parasitic, and they can either remain attached or detach from the fish to mate at around a month of age.

Females will then detach from the fish to lay their eggs, and then reattach to their host as the cycle begins anew.

How Long Do Fish Lice Live?

Adults can live for as long as 60 days, and up to 3 days without a host.

If hatched in the autumn, juvenile fish lice are capable of surviving over the winter within the fish’s protective mucus membrane.

However, fish lice cannot typically survive for long if water temperatures fall outside of the range of 68 to 95° F (20 to 35° C), and will reproduce quite rapidly when temperatures are between 68 and 82 (20 to 28° C).

When is the Best Time to Treat Fish Lice?

The best time to treat fish lice is during their larval stage, if possible, as they must moult multiple times in a short period of time and will be more likely to be impacted by chemical treatments.