The rainbow trout, Oncorhynchus mykiss, is one of the most widely introduced fish species on the planet. Native to the rivers of the Pacific coast of North America, it has been translocated to waters on every inhabited continent, and established feral populations are documented across much of Europe and beyond. In the Ammer River itself, the species does not maintain a fully self-sustaining wild population; rainbow trout in this section originate from regular stocking for recreational angling, with individuals typically 30 to 40 centimeters in length released annually from Bavarian aquaculture facilities. The 39-centimeter specimen that carried the snakes therefore represents exactly the size class that anglers encounter in this reach each season.

The capture itself followed standard practice. The fish was taken on fly fishing gear with a dry fly, humanely stunned by a single percussive blow to the head, and dispatched by bleeding in accordance with German fisheries and animal welfare regulations. Dissection was performed shortly after capture using a standard surgical hemostat of 15 centimeters length, and the entire procedure was documented on video. That footage proved scientifically valuable: by comparing the snakes visible in the stomach against the hemostat in the recording, the observers could estimate that each of the three grass snakes measured approximately 20 centimeters in total length, a level of quantification that most historical predation records lack.

Reptiles as prey of salmonids are exceptionally rare in the scientific literature. Keiz reported in 1963 a predation attempt by a rainbow trout on an adult grass snake at a fish farm. Eder and colleagues in 1988 described an unsuccessful attempt by a rainbow trout of about 30 centimeters on a grass snake of around 50 centimeters, and additionally reported a dead lake trout whose digestive tract contained a grass snake measuring 50 to 60 centimeters. Bernini and colleagues in 2006 found remains of a wall lizard, a slow worm, and an unidentified snake of the genus Natrix in rainbow trout from northern Italy. Smith in 1973 noted snake remains in the stomachs of pike and eel, and more recent records include a hatchling Natrix helvetica in France and several juvenile reptiles, including a Natrix species, in northern Italy. Until now, however, no record had documented multiple snakes of the same species inside a single salmonid.

The new observation is remarkable precisely because it shows repetition rather than a single opportunistic mistake. Three juvenile grass snakes in one stomach indicates that the predator did not merely blunder into one unlucky hatchling; it encountered several suitable prey items in quick succession and consumed them all. The authors interpret this as evidence that predation events on small reptiles are not necessarily restricted to isolated cases but may occur repeatedly when the prey is locally abundant. This is consistent with the observations of Bernini and colleagues, who reported reptiles in the diet of salmonids mainly during the summer months, when these animals are especially active near the water.

The size and season of the snakes carry their own biological clues. Grass snakes in Central Europe usually hatch between July and September, so a hatching event in May can be excluded. The juveniles found in the trout were therefore most likely overwintered individuals from the previous year. During spring, such young snakes frequently occur near the shoreline or swim on the water surface, behavior that makes them particularly conspicuous and vulnerable to visually hunting predators such as trout. A rainbow trout holding position in a river section where juvenile grass snakes were crossing or swimming would have had every sensory cue it needed to strike, and the dry fly capture suggests the fish was feeding near the surface at the time.

The feeding ecology of rainbow trout has been examined in numerous studies across its introduced range. Most of these works focus on aquatic macroinvertebrates, fish, and occasionally terrestrial insects as the main prey. Amphibians can play a locally important role in salmonid diets during certain seasons or in specific regions, whereas reptiles have so far only been reported from single events. The new record considerably expands this fragmentary evidence and documents a specific interaction between salmonids and semi-aquatic snakes that has so far received little systematic attention. It also complements earlier single observations of fish acting as predators of snakes and demonstrates that such interactions may occur in wild populations, not only in the confined and artificial conditions of fish farms.

Opportunistic feeding is widely considered a key factor in the global establishment success of rainbow trout. Studies from Patagonia to the Alps have shown that the species can exploit terrestrial food resources when these are available, switching flexibly between aquatic and terrestrial prey according to season and availability. The authors emphasize that such observations do not necessarily imply negative ecological impacts on grass snake populations but rather reflect the pronounced trophic plasticity of this introduced salmonid. Under certain environmental conditions, the species is capable of consuming unusual prey such as small reptiles without this necessarily having ecological relevance for the prey populations.

There is, however, a risk side to this flexibility, and it falls on the predator itself. Previous observations have documented salmonids that ingested comparatively large or unsuitable snakes, and these cases show that the consumption of oversized prey can in rare instances be physiologically stressful or even harmful to the fish. Keiz’s 1963 record and the 1988 account of a 30-centimeter trout attempting a 50-centimeter snake both illustrate the mismatch that can arise when a visually guided predator misjudges prey size. The three 20-centimeter juveniles in the Ammer River trout, by contrast, were small enough to be swallowed without apparent difficulty, representing a more successful execution of the same opportunistic strategy.

The presently reported finding represents one of the few confirmed records of reptiles as prey of salmonids in Central Europe and highlights the value of incidental dietary records in expanding our understanding of trophic plasticity in an introduced fish species. The video documenting the dissection is publicly available on Figshare, allowing independent verification of an observation that might otherwise be dismissed as anecdote. For a species whose adaptive potential has made it a global invader, the ability to add juvenile snakes to an already broad menu is one more illustration of why rainbow trout thrive almost anywhere they are released, and why even a routine angling catch on a Bavarian river can still yield a genuinely novel scientific record.

Subject of Research: Predation of juvenile grass snakes by rainbow trout in a Bavarian stream

Article Title: Three juvenile Natrix natrix ( Reptilia ) as prey of Oncorhynchus mykiss ( Actinopterygii , Salmoniformes , Salmonidae ) in a Bavarian stream

Article References: Vogelmann, C., Wendt, P., & Martens, A. (2026). Three juvenile Natrix natrix (Reptilia) as prey of Oncorhynchus mykiss (Actinopterygii, Salmoniformes, Salmonidae) in a Bavarian stream. Acta Ichthyologica et Piscatoria, 56, 23-25. https://doi.org/10.3897/aiep.56.176058

Image Credits: AI Generated

DOI: 10.3897/aiep.56.176058

Keywords: rainbow trout, grass snake, predation, Bavaria, invasive species, feeding ecology, trophic plasticity, salmonids, Three, juvenile, Natrix, Reptilia