Marble trout are stunning-looking trout found naturally in the rivers and streams of Northern Italy, Slovenia, Croatia, Bosnia and Herzegovina, and Montenegro. They likely once existed as far south as Albania. In pristine waterways, they can grow to impressive sizes with fish over 50lb (22kg) not unheard of, but like many salmonidae species, their natural range has been greatly reduced due to development and competition with introduced species.
So, what are Marble trout? Are they a form of brown trout, or are they a unique species of trout?
Their scientific name gives much away; they are most commonly called Salmo marmoratus. This places them in the same genus as brown trout (‘Salmo’), giving them the unique species name of ‘marmoratus’. So, in the simplest of interpretations, they are a distinct species from Salmo trutta, the fish most of us refer to as a brown trout.
Now, in taxonomy, things are never that simple. There is something called the brown trout species complex, a grouping of between 30-60 trout that belong to the genus Salmo. All of these trout are considered to be brown trout, and counted among them are both Salmo trutta and Salmo marmoratus. Put simply, all trout that belong to the genus Salmo are brown trout, but they are different species than Salmo trutta, which is the Atlantic brown trout.
To further complicate matters, many scientific papers looking at trout genetics and diversity seem to refer to the entire brown trout species complex as Salmo trutta. Within the complex, they then split it into 5 main lineages, with Atlantic and Marmoratus being among the key lineages. These authors seem to prefer classifying both the normal brown trout and marble trout as more like sub-species rather than distinct species in their own right. It is all very complicated, and it is still far from being settled.
Just how different are they? Well, genetic analysis has discovered that Salmo trutta atlantic and Marmoratus lineages shared a common ancestor sometime between 500,000 and 2.5 million years ago. Although, a recent article I just read seems to suggest much of the diversification occurred within the last 500,000 years during the glaciation cycles of the Pleistocene. The Atlantic lineage is the oldest lineage, which also explains why it is much more widespread than the likes of marmoratus.
So, to answer the original question: Marble trout are brown trout, but they are a closely related but different species from the more common brown trout we all know and love.
Can marble trout be caught in North America?
Marble trout, Salmo marmoratus, have never been introduced to North America, and as far as I know, they have never been stocked outside their natural range. While brown trout with marble patterns can be occasionally caught in North America and elsewhere, they are not brown trout but just a unique phenotype.
The main stocking of brown trout in North America consists of Salmo trutta of the Atlantic lineage. Most of these were introduced from Germany, with additional stock from England, Scotland, and Norway.
Although there have been some limited and unsuccessful attempts to stock Lake Ohrid trout (Salmo letnica) in the states.
Marble trout, Salmo marmoratus, are visually distinct from S. trutta but belong to the same genus, which means they are considered part of the brown trout species complex. They are generally considered a separate species, although debates exist within the brown trout species complex about their classification. Genetic studies suggest a shared ancestor between marble and Atlantic brown trout, dating back hundreds of thousands to a few million years.