The scale of the invasion itself is staggering. In western North America, more than 95 percent of larger, deeper high-elevation lakes contain non-native trout, according to earlier surveys cited in the review. In the Pyrenees, invasive fish occupy between 35 and 85 percent of lakes. Introductions of game fish such as trout date back to the Middle Ages in the Pyrenees and to the fifteenth century in the Alps, and they continue today, often promoted by state fisheries agencies and fishing organizations. The researchers conclude that introduced invasive fish may be the single most critical threat to mountain lake biodiversity, driving declines in native fish, amphibians, invertebrates, and zooplankton, and reshaping entire food webs.

The ecological consequences extend far beyond simple predation. Introduced fish displace and extinguish native species, alter trophic interactions, disrupt nutrient cycling, degrade habitats, hybridize with local populations, and transmit diseases and pathogens. Because fish accumulate atmospherically deposited toxins in their tissues, non-native trout in previously fishless lakes can even pose a health risk to wildlife and to anglers who eat their catch. Recent studies have documented that introduced fish reduce the occurrence of shrews in alpine lakes and that non-native minnows cause far larger negative effects on littoral macroinvertebrates than trout do. The message from the literature is unambiguous: fishless mountain lakes are not empty ecosystems waiting to be stocked, but unique habitats harboring species with extraordinary adaptations to harsh conditions.

To map the state of knowledge, the team executed a two-stage search of the Web of Science database, first with a naïve set of 84 expert-provided search terms and then with a refined string built from 343 selected keywords extracted from the initial results. They used the R package litsearchr to identify keywords, drew geographic terms from the hierarchically organized GMBA Mountain Inventory covering more than 8,000 mountain ranges, and validated species names against the GBIF backbone taxonomy. After removing duplicates and screening thousands of records, the initial searches returned 1,278 and 1,920 publications respectively, but only 35 papers survived strict eligibility criteria. These described 184 distinct cases, each a unique combination of a single species at a single location, across 96 aquatic ecosystems ranging from 300 to 3,600 meters above sea level, in size from 0.35 to 34,020 hectares, and up to 133 meters deep.

The geographic and taxonomic biases in this literature are striking. Seventy-seven percent of the studies were conducted in North America, primarily in the western United States and the Canadian Rockies, while 17 percent came from Europe, concentrated in the Italian Alps, the Pyrenees, and Austria. Only a single study came from Asia, despite the vast extent of mountain terrain there. Taxonomically, salmonids dominated: the brook trout, Salvelinus fontinalis, appeared in 21 papers and the rainbow trout, Oncorhynchus mykiss, in 12. Cyprinids such as the golden shiner and minnows formed the second most targeted group. All the targeted species were ecological generalists with broad temperature tolerance and life-history strategies, such as high reproductive output or early sexual maturity, that enhance survival and population growth in harsh environments.

When it comes to actual removal techniques, the review found that physical methods dominated the published record, accounting for 57 percent of approaches, followed by chemical methods at 31 percent and biological methods in a small minority. Within the physical category, gill netting and electrofishing were the workhorses, often deployed over months or, in one long-term American program, over 11 years. Chemical treatments relied overwhelmingly on rotenone, a piscicide that kills fish by blocking oxygen uptake in their gills, with antimycin and deoxygenation agents such as organic matter used more rarely. Biological approaches involved introducing predators or genetically modified individuals. Combined approaches appeared in only two papers, pairing techniques such as shoreline trap nets with predator introduction or integrating drainage, multiple net types, electrofishing, and rotenone.

Success rates varied considerably by method. Among the 157 cases where outcomes were reported, full eradication was achieved in 58 percent. Chemical approaches proved the most effective, with 80 percent of cases ending in eradication, and within salmonids, the best-studied group, 27 of 29 chemical treatment cases resulted in complete removal. Physical methods, though used far more often, achieved eradication in 60 percent of cases overall and only 48 of 77 salmonid cases. Yet the authors caution against a simple prescription, because chemical methods carry serious drawbacks. Rotenone and similar piscicides are not species-specific: they kill native fish alongside invaders and are toxic to other organisms, including amphibians. In the European Union, rotenone has been banned since 2008 and antimycin is not approved, while in the United States its use is tightly regulated with criminal penalties for violations. Physical gear, by contrast, requires no special permits and overlaps with the routine toolkit of fisheries managers, which helps explain its popularity despite lower efficacy.

Method selection also followed the goals of each intervention. When the objective was recovering amphibians, invertebrates, or entire ecosystems, managers favored physical approaches almost exclusively. When the goal was restoring native fish populations, chemical methods dominated, presumably because wholesale fish removal was acceptable where restocking with native species was planned. Most of the reviewed studies, 19 of 35, aimed at recovering native ecosystems and their communities, while 12 were methodological assessments of removal efficiency. Government institutions led the vast majority of campaigns, working alone in 17 of the 20 papers that reported stakeholders, and collaborating with researchers and volunteers in the remaining three.

The review also exposes practical and economic realities that shape what is possible in remote terrain. Rotenone treatments in Norway have been estimated to cost between 10,000 and more than 200,000 euros per year depending on the species and site, with most expenses tied to labor. Motorized electrofishing equipment runs roughly 6,000 to 20,000 euros, nets cost from just over 100 to around 1,000 euros per unit, and helicopter flights to inaccessible lakes add substantial expense. Cost information is chronically underreported, making cost-benefit analysis difficult. The authors argue that collaborative approaches among landowners and stakeholders, watershed-scale prioritization, and treating mountain lakes as social-ecological systems can help optimize investments, but they also note that wilderness fishing carries deep cultural value, making public support for eradication both crucial and hard to win.

Perhaps the review’s most important conclusions concern what happens when eradication is impossible or undesirable. Where populations are dense or lakes are large, suppression strategies, reducing invaders below the threshold at which they cause harm, can still deliver conservation gains, an approach the authors call functional eradication. Prevention, early detection through tools such as environmental DNA surveillance, and rapid intervention before invaders become established remain the most affordable and effective strategy by far. To close the knowledge gap they documented, the team compiled a trait database covering the taxonomy, life history, ecology, and invasiveness potential of targeted species, and built the FishME Toolbox, an open-access online platform hosted within the GMBA Mountain Portal that links publications, species, and study sites for both experts and the general public. As fish introductions continue to rise and climate change reshapes mountain waters, the authors hope that learning systematically from past interventions, including failures that often go unpublished, will guide smarter, faster, and more adaptive management of some of the planet’s most vulnerable freshwater ecosystems.

Subject of Research: Removal of introduced fish from mountain lentic ecosystems

Article Title: Introduced Fish in Mountains: State of Published Research on their Removal from Lentic Ecosystems

Article References: Cogălniceanu, D., Stănescu, F., Tănase, T. L., Vlad, S. E., Snethlage, M., Ranipeta, A., & Urbach, D. (2026). Introduced Fish in Mountains: State of Published Research on their Removal from Lentic Ecosystems. Environmental Management, 76(9), Article 304. https://doi.org/10.1007/s00267-026-02616-9

Image Credits: AI Generated

DOI: 10.1007/s00267-026-02616-9

Keywords: invasive fish, mountain lakes, eradication, rotenone, salmonids, gill netting, electrofishing, biodiversity conservation, lentic ecosystems, FishME Toolbox, systematic review, early detection