Unfortunately the full article that they are reporting on is paywalled.

https://www.khm.uio.no/english/about/organisation/archaeolog...

To be fair, Gemini says there are no presumed cases of specifically brown trout populating an isolated water body by means other than human stocking. For other types of fish, maybe (eggs stick to bird legs, or one gets pooped out intact), but for brown trout, no! The eggs only survive in cold water, not inside a bird, are relatively large and non-sticky.

To survive a long schlepp up a mountain in a waterskin, it would presumably have been fertilized eggs that were transported (can be found in river gravel) since they have low oxygen needs. According to Gemini even baby fish might only survive a couple of hours, but eggs (if kept in cold water) could survive days or weeks, and obviously would be more practical to transport a sufficient number to have a decent chance of establishing a breeding colony.

Interesting stuff, but maybe better considered as reconstructive history than science.