Pathogen-carrying invasive fish from China threatens US waterways

One little-known legacy of communist countries during the Cold War is invasive species. The accidental introduction in 1960 of the topmouth gudgeon from China into countries bordering the Black Sea, including Ukraine, Moldova and Romania, is a striking example.

Small in size but large in terms of the ecological and economic disaster that it causes, the topmouth gudgeon has invaded numerous waterways throughout the world. It carries a parasite with features from both an animal and a fungus which has probably been present in China for millions of years but is deadly for most other fish species.

As my colleagues and I from France’s Institute for Research and Development and our partners showed in Emerging Microbes & Infections-Nature, this farmed freshwater fish spreads a formidable fungal disease that is a cousin of the infamous chytrid fungus, which has decimated frog and toad populations around the world in the last few decades.

Our research also shows that the fish’s invasion of Europe happened very rapidly – in less than 30 years. And it’s only a matter of time before the fish comes to the US, where it would threaten a wide range of important species for conservation, sport fishing and fish farming.

The risk to the US

The arrival of the topmouth gudgeon in the US would be similar to the introduction of round gobies in the Great Lakes or Asian carp (bighead, black, grass and silver) in the Mississippi River. The most likely route into the US would be accidental, through contaminated live fish stocks either wholesale (into fish farms) or through retail fishmongers.

Lake in south east Turkey which has been invaded by the topmouth gudgeon.Author provided

There are several hotspots around the world with a suitable climate that could represent a potential future point of entrance. In North America, the key states at risk are Mississippi, Louisiana, Arkansas, North and South Carolina, Georgia, Alabama, Tennessee and the North of Florida.

Effective management of invasive fishes begins with blocking introductions in the first place, as this gives the best chance of minimizing adverse effects and their associated costs. Prevention also follows the precautionary principle of taking steps to avoid potential risks.

One might expect managers to focus on preventing the introduction of invasive species; however, managers frequently wait until invasive species have been introduced and only then act to limit the consequences.

There is a perception that post-introduction management is a safer choice than prevention because even with strict and expensive prevention protocols in place, introductions still occur. But the spread of the topmouth gudgeon in Europe and Asia shows that preventing its introduction is the best course of action.

Farmed salmon and bass under threat

Our study shows that in only three years, the introduction of the topmouth gudgeon in a catchment area in south east Turkey provoked a rapid decline in local fish populations, almost to extinction (the number of individuals fell by 80 to 90%).

The parasite it carries is from a bygone age and exists at the borderline between animal and fungus. Known as Sphaerothecum destruens or rosette agent because of the shape of the pathology in the affected areas, this recently discovered agent appeared when animals and fungus separated several million years ago.

In addition to the serious ecological problem that it causes, the topmouth gudgeon has a potential economic impact that is even more worrying. It has been shown, for example, that 90% of salmon die when infected with the parasite it carries. And its recent discovery in Turkish bass farms does not augur well for the future.

In the lakes studied, topmouth gudgeon now represent 95% of fish caught.Author provided

Contrary to what had previously been thought, our research found that estuaries can act as contact zones with freshwater species. That means that contamination of strictly marine species such as bass is possible. Furthermore, feeding of these farmed bass with other infected fish including topmouth gudgeon from neighboring rivers has contributed to a potential secondary infection.

The parasite that lives on the fish itself has entered US waterways in the past.

It was first identified in 1984 among Chinook salmon held in seawater net pens in the state of Washington, then in Atlantic salmon in a private facility in California, and then in Chinook salmon held at the Bodega Marine Laboratory at the University of California Davis.

How this happened is still unknown as the topmouth gudgeon is not yet present in the US. But the most likely route is through the mixing of Asian and US salmonids – the family of fish that includes salmon, trout and others – in the North Pacific feeding zone. Once the outbreak in California emerged in the early 1990s it led to extremely high salmon mortalities.

These events have until now been rare due to limited contact with the pathogen. But with the introduction of the topmouth gudgeon as a healthy carrier of this fungal-like pathogen, mathematical models predict recurring disease outbreaks.

React quickly

The research team responsible for this study has sounded the alarm: the risk of a global spread of this infection is real. The US represents a highly suitable terrain for future invasions and it is becoming urgent that animal health and environmental protection agencies act to contain the rapid spread of this pathogen to the rest of the world.

Diagnostic tools are in place, scientific evidence is robust and mathematical models have been established in order to predict the evolution of this epidemic in fish communities.

To address this effectively, a partnership between researchers and stakeholders needs to be rapidly put in place. A map of the damage needs to be established and the topmouth gudgeon invasion needs to be contained using aquatic resource management tools such as early warning systems via targeted monitoring of potential entry routes. The World Organization for Animal Health’s (OIE) list of pathogens should be updated to include the rosette agent the topmouth gudgeon carries.

Since the discovery in the UK in 2005 of this “partnership in crime” between the topmouth gudgeon and the pathogen, the British government has put into effect eradication programs. Although this is a step in the right direction, it still isn’t enough to limit the decline and the diversity of European fish and the risk of spread to the US.

With over a hundred freshwater fish species already threatened in the US, have we learned anything from the catastrophic impact on US sport fisheries arising from whirling disease?

What was originally considered a mild pathogen introduced from Europe via infected trout shipments in the 1950s, one outbreak of whirling disease led to severe fish mortalities (up to 90%) and costs for some states over $300 million.

Will this type of scenario be repeated or prevented with the introduction of the topmouth gudgeon and its rosette agent?

Rodolphe Gozlan has received funding L'Agence Nationale de Recherche et de l'Onema but it was not directly linked to this research.