Over 40 percent of extant large freshwater animals (megafauna), including carp, salmonids, crocodilians, turtles, beavers, and hippopotamuses, have been deliberately introduced outside their natural range often for economic gain. While these alien species can provide substantial benefits to certain groups in the introduced regions, they also posed profound and often underestimated risks to native biodiversity and local people. This is shown in a new study published in One Earth, led by researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) and the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences.

Nile perch in Lake Victoria is a prominent example showing the complex ecological and socio-economic impacts posed by alien freshwater megafauna. The introduction was intended to enhance fisheries, but it has caused serious consequences: stocks of native fish species declined, local fishers lost their livelihoods, and chronic malnutrition among children and mothers increased in the surrounding communities.

“Such detrimental effects – especially on vulnerable or marginalised local communities – are often complex and require long-term monitoring to understand their magnitude. Compared to benefits, detrimental impacts posed by alien freshwater megafauna on local people could be underestimated in many regions”, said Fengzhi He, a professor at the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences and also a guest scientist at IGB. He is the last author of the study, in which the team has conducted the first global assessment of socio-economic impacts posed by alien freshwater megafauna.

Based on the Nature’s Contributions to People (NCP) framework and the Socio-Economic Impact Classification for Alien Taxa (SEICAT) framework, the researchers systematically categorized both beneficial and detrimental effects and also quantified the impact magnitudes.

43 percent of megafauna species in 142 countries and regions deliberately introduced

Small species often reach new waters unnoticed – clinging to ship hulls or fishing equipment, or transported by birds. In contrast, freshwater megafauna species, i.e. animals with a maximum reported weight of at least 30 kilograms, have often been deliberately introduced outside their natural ranges because they have high economic or aesthetic values. The team identified 93 alien freshwater megafauna species (43 percent of 216 extant species) that have been introduced outside their natural ranges. These introductions span 142 countries and regions in all continents except for Antarctica. Geographically, the USA has the highest number of introduced freshwater megafauna species (52), followed by China (28), Canada (23), Russia (19), Belgium (18), and Germany (17).

Almost every second assessed megafauna species poses detrimental impacts on local people

The study shows that their introduction can pose negative impacts on local people, in addition to their profound impacts on native biodiversity. Of 59 alien freshwater megafauna species with documented positive contributions to people, 26 also had detrimental ones. This applies in particular to large fish species such as carp, salmonids, and catfishes. Those negative impacts include increased risks to human health, safety risks from aggressive or venomous species, damage to property and infrastructure, or reduced food security due to declines in native species.

Economic benefits for certain groups drive introductions

The study found that introduced freshwater megafauna frequently provide socio-economic benefits for certain groups of people in the introduced regions. The benefits have been mainly documented in aquaculture and fisheries (57 percent), followed by recreational activities such as sport fishing and ecotourism (20 percent) and the provision of materials and companionship (12 percent). Large freshwater animals have also been introduced in the hope of improving the natural environment. For example, silver carp and bighead carp were used to control excessive phytoplankton growth while grass carp was introduced to remove aquatic weeds.

Some freshwater megafauna species were introduced intentionally for pet trade or leather products. “We were very surprised that some crocodilians have been introduced for the pet trade. For example, spectacled caimans are kept as pets in the USA”, said Dr. Xing Chen, former doctoral researcher IGB and the study’s lead author, “In China, they are introduced often for their skin to produce leather products.”

A call for stronger risk assessment

Because introductions of freshwater megafauna are likely to continue due to perceived economic benefits, the authors emphasize the urgent need for rigorous risk assessments, improved monitoring, and transparent communication of both positive and negative consequences
"Balancing economic development with biodiversity conservation and human well-being requires a comprehensive understanding of both the benefits and risks associated with species introductions”, said Prof. Sonja Jähnig, co-senior author of the study and director of IGB.