In Indo-European studies, the salmon problem or salmon argument (also known by the German term Lachsargument) is an outdated argument in favour of placing the Indo-European urheimat in the Baltic region (as opposed to the Eurasian Steppe), based on the cognate etymology of the respective words for salmon in Germanic and Balto-Slavic languages.
The reasoning went as follows: Since the term for Atlantic salmon in the Germanic, Baltic and Slavic languages could be derived from a common Proto-Indo-European root *laḱs, the urheimat of the Indo-Europeans must be located where both these languages and the object it describes can be found: Northern-Central Europe. This argument was first put forward by German philologist Otto Schrader in 1883. The argument was subject to continued scholarly debate throughout the late 19th and early 20th centuries, particularly in German academia.
In 1953, German indologist Paul Thieme submitted that the descendants of laḱs- found in the Caucasus were used to describe the trout (Salmo trutta) rather than the Atlantic salmon (Salmo salar). American philologist George Sherman Lane concurred in a 1970 conference paper: "In my opinion, the name in question probably did refer originally not to the Salmo salar at all, but rather to the Salmo trutta caspius of the northwest Caucasus region." This lent support to the Kurgan hypothesis.