Introduced freshwater megafauna: Benefits and risks of large animals in rivers and lakes

Rivers and lakes are home to some of the largest animals in inland waters—from giant catfish, sturgeons, salmon, and carp to turtles and crocodiles. However, many of these species have been deliberately introduced by humans into new regions. A new global study in the journal One Earth systematically examines for the first time the consequences of such introductions for biodiversity, ecosystems, and human societies.

The results show that the impacts of introduced freshwater species can be extremely complex. While some species bring economic advantages or create new opportunities for use, they can simultaneously cause ecological damage and undermine the livelihoods of local communities.

How do alien species enter rivers and lakes?

Freshwater ecosystems are considered particularly vulnerable to the introduction of non-native species because they are closely linked to human activities. Many species are deliberately released—for aquaculture, commercial fisheries, recreational fishing, or the pet trade. The common carp (Cyprinus carpio), for example, is now farmed in over 100 countries, while the African sharptooth catfish (Clarias gariepinus) has been introduced to at least 30 countries to establish aquaculture and fisheries.

Sometimes, however, species also reach new regions unintentionally. Ships can take up organisms from one body of water via ballast water and later release them in another. Hydrological connections between river systems or canals as well as increasing shipping traffic also facilitate the spread.

Which species belong to the freshwater megafauna?

Freshwater megafauna refers to animals that reach a body mass of at least 30 kilograms and live wholly or partly in fresh or brackish water. This includes not only large fish species such as sturgeons, salmon, catfish, or carp, but also hippopotamuses, giant salamanders, crocodiles, beavers, and large turtles. Many of these species have now been documented outside their natural range.

Among the most widely distributed non-native freshwater fish worldwide are:
– so-called megafish such as the brown trout (Salmo trutta), the African sharptooth catfish, and the grass carp (Ctenopharyngodon idella)
– large turtles such as the alligator snapping turtle (Macrochelys temminckii) or the common snapping turtle (Chelydra serpentina)

Scale of introductions: 93 species worldwide

The study provides a global overview of the introduction of large freshwater animals and their impacts on humans. In total, the researchers identified 93 freshwater megafauna species that have been introduced outside their natural range. This corresponds to 43% of the 216 globally known species in this group. These introductions affect 142 countries and regions.

Particularly many such species were introduced in North America, Europe, and East Asia. The highest numbers were recorded in the USA with 52 species, followed by China (28), Canada (23), Russia (19), Belgium (18), and Germany (17). Comparatively few introductions were documented in Africa and Oceania.

For 59 of the 93 species, concrete data on their impacts on humans are available. In total, the researchers were able to identify 575 documented effects. All of these species were associated with at least one positive contribution to humans, while 26 species simultaneously also caused negative impacts.

The study thus clearly shows: Introduced freshwater megafauna frequently brings both benefits and problems.

Benefits: food, economy, and recreation

Many introductions of large freshwater animals were carried out for economic reasons. Most commonly, humans benefit through food production and aquaculture. Large fish species such as carp, catfish, salmon, or sturgeons are farmed worldwide and represent an important source of protein.

In some regions, such introductions have even given rise to entire industries. An example is Chile, where introduced salmon—including Atlantic salmon (Salmo salar) and Chinook salmon (Oncorhynchus tshawytscha)—reach an export value of more than 5 billion US dollars per year and secure tens of thousands of jobs.

Recreational activities also play a role. Large fish are attractive target species for recreational fishing. The wels catfish (Silurus glanis), for example, has given rise to a thriving angling tourism industry in Spain’s Ebro basin. Some species are also used in the pet trade. Freshwater stingrays such as the ocellate river stingray (Potamotrygon motoro) are bred for the international aquarium market.

The study also reveals clear regional differences. In many countries of the Global South, the benefit as a food source dominates, while in the Global North, recreational activities such as fishing are more often in the foreground. In Asia, around 80% of the positive effects were related to food production, while this proportion is significantly lower in North America. In total, the researchers documented 429 positive contributions to humans, which were frequently associated with economic or cultural benefits.

Disadvantages: ecological damage and social consequences

As diverse as the benefits can be, the negative consequences can be equally severe. The study identified 146 documented cases of negative impacts affecting 25 species.

Some of these problems arise indirectly through changes to ecosystems. When introduced species reduce native fish populations or alter habitats, this can affect people who depend on these species as a food source or basis for income.

In other cases, direct risks emerge: Large animals, such as freshwater stingrays, can injure people, alter fish populations, or impair economic activities. Some invasive species also cause property damage or affect recreational uses of water bodies.

The majority of negative effects were documented in North America, followed by Europe and Asia. However, many of these impacts are difficult to measure and may be underestimated, as around two-thirds of the documented cases are based on indirect evidence, such as potential health risks from pollutants or parasites in introduced fish species.

Top freshwater megafauna species

The following selection shows some of the most well-known introduced freshwater megafauna species and their impacts:

Perception depends on societal values

Whether an introduced species is perceived as beneficial or problematic depends strongly on societal and cultural contexts. While in many countries of the Global South the contribution to nutrition is the main focus, in wealthier regions recreational activities such as sport fishing or angling tourism play a more prominent role.

This means that the same species can be evaluated very differently in different regions—as an economic resource or as an ecological threat.

Why large species can be particularly problematic

Large freshwater animals can have particularly far-reaching consequences in newly colonized waters. Freshwater megafauna species often occupy high positions in the food web, regulate other species, or alter habitats through their behavior. When such species are introduced to new regions, they can change entire ecological relationships. This affects not only individual species but often entire food webs and ecosystems.

Additionally, many of these species were not spread accidentally but were deliberately introduced for their economic value: as food fish in aquaculture, for recreational fishing, as leather suppliers, for the pet trade, or as tourist attractions. This combination of economic interest and large ecological impact makes them so risky. The benefits often accrue only to certain groups, while the negative consequences can be much more widely distributed.

When introduced megafauna species reduce fish populations, reshape habitats, or shift food chains, this often has direct impacts on people who depend on functioning freshwater ecosystems. Affected are not only biodiversity and conservation but also food security, income, safety, health, and traditional ways of life.

The study also shows this: Introductions of large freshwater species can create economic opportunities but at the same time significantly burden local communities—especially when the ecological damage only becomes visible with a delay.

Major knowledge gaps

Despite the global analysis, the study also reveals how incomplete our knowledge of the socioeconomic consequences of introduced freshwater species still is. Reliable data on how significantly economic activities, ways of life, or social structures are actually changed by introduced species are often lacking.

For many megafauna species, long-term data on the development of their populations as well as detailed studies of their ecological impacts are missing. Especially in tropical regions, data coverage is often sparse, even though many introductions have taken place there.

A global experiment with an uncertain outcome

The study’s findings make clear that introductions of large freshwater species should be evaluated much more carefully in the future. Many of these species were originally spread because of their economic benefits—for aquaculture, fisheries, tourism, or the pet trade.

Indeed, such introductions can create new sources of income. At the same time, numerous examples show that they can trigger profound ecological changes that in the long term also undermine people’s livelihoods. Once a large freshwater species has established itself in a new habitat, it is often difficult or impossible to remove.

The authors of the study therefore advocate for investigating and communicating the potential consequences of such introductions more thoroughly in the future, so that risks to biodiversity and local communities can be better assessed.

At the same time, such introductions also raise fundamental ethical questions. Large wild animals are frequently brought to new regions solely for economic reasons and used as a resource—for aquaculture, leather production, fisheries, or tourism. This can not only permanently alter ecosystems but also jeopardize the livelihoods of local communities.

From a conservation ethics perspective, one could therefore also argue that such introductions should be avoided as a matter of principle. Many of the risks now being discussed only arise because animals are brought to new regions for economic interests or entertainment.

Ultimately, the introduction of large freshwater species resembles a global experiment whose consequences often only become visible decades later. Examples such as the Nile perch in Lake Victoria or the globally distributed common carp show how profoundly individual species can transform entire ecosystems.

The economic benefits of such species are real—but equally real are the risks to biodiversity, ecosystems, and the people who depend on them.

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Source

• Chen, X., Evans, T. G., Jeschke, J. M., et al. (2026). Global assessment of alien freshwater megafauna reveals complex socio-economic impacts. One Earth, 101623. https://doi.org/10.1016/j.oneear.2026.101623