Invasive parasite destroying fish species

Researchers have discovered that a parasite carried by an invasive species of minnow is responsible for the dramatic declines and localized extinctions of a different minnow species in Europe during the past 40 years.

This parasite, which scientists have found can almost totally destroy the spawning success of the small sunbleak minnow, Leucaspius delineatus, may pose threats to the diversity and stability of freshwater ecosystems, and is genetically very similar to a parasite that can be deadly to salmon, researchers say.

The findings were published today in the journal Nature by researchers from Oregon State University, the Winfrith Technology Centre in England, Idaho State University and the Weymouth Laboratory in England.

“This solves a mystery that fisheries researchers have been studying for decades, about why sunbleak minnows have been disappearing from Europe, where they were once quite common,” said Michael Kent, a professor of microbiology and director of the Center for Salmon Disease Research at OSU.

“And to find a parasite that can have such a devastating effect on spawning success is a little alarming,” Kent said. “There are still a lot of unanswered questions here, including knowledge of the host range of the parasite. It’s unknown if it would be pathogenic to other groups of fishes, such as salmonids.”

The minnows involved, both the invasive species and those dying out, are rather small, grey, innocuous fish no more than a few inches long that do not by themselves have any major commercial value, Kent said. But they are an integral part of the biodiversity of freshwater streams and lakes across Europe, and form an important part of the food chain for other fish, he said. The European sunbleak is now listed as a threatened species among European freshwater fish.

“This is another aspect of the concerns about invasive species that you always have to watch for,” Kent said. “The new minnows that moved in did not seem to be a big issue by themselves, but the parasite they carried was deadly to the minnows native to these waters.”

The invading species, researchers say, was a fish called Asian gudgeon, and it carried parasites closely related to an odd group of fungus-like organisms called choanoflagellates. The Asian gudgeon was first introduced into Romanian ponds and the Danube River in 1960, and has since spread rapidly throughout Europe.

The fungal-like parasite carried by this species can infect the internal organs of fish, causing organ failure, severe anemia and death in susceptible species. The mortality rate of infected fish is very high, and they can get easily infected by direct transmission of the parasite during a free-swimming stage in its life cycle.

“As high as the direct mortality is from this parasite, it’s nothing compared to the effect on spawning success,” Kent said. “It appears that in sunbleak, infection with the parasite causes near 100 percent failure in successful spawning. Over a few generations that can wipe out a local population.”

Other fish in the cyprinid family, including carp, may also be susceptible to this parasite, researchers say. And the parasite is remarkably similar to the “rosette agent” found occasionally in salmon, including chinook salmon, which can also cause mortality.

This is one of the first times that introduction of an invasive fish species has resulted in the almost-total suppression of spawning success in a native species, researchers said. The host fish, Asian gudgeon, is only a carrier and does not appear to be affected by the parasite. The presence of this parasite in fish may also be a concern in aquaculture trade, they said.