Blog.

07.10.2024

The tale of the marble trout

The tale of the marble trout starts about 1.5 million years ago when the split between the Salmo Trutta and the Salmo Marmoratus happened. Then after another 700.000 years a second split happened between the southern and northern population. They are still both Salmo Marmoratus but different lineages. All this time the species slowly evolved into the species we have nowadays. When people arrived in Europe the northern lineage was found in the Po and Soča basins and the southern lineage in the Neretva, Drin and Aoos Basins. Long everything stayed this way but it all changed when stocking of brown trout started around world war 1. Hybrids started to form and pure marble trout started to decline. Pure marble trout of the southern lineage might be present in the Neretva and Morača but they might also be hybrids. In the drin and Aoos basin they are likely extirpated. The northern population also was thought to have gone extinct until a pure population was found in 1985 in the Zadlascica. Lucky in 1993 the Slovenian researcher Meta Povz alerted Alain Crivelli at the Tour du Valat about the imminent extinction of the Marble Trout in the Soča basin. Between 1993 and 1996 (7) more populations of genetically pure Marble Trout were identified at the head of the basin. They had been naturally isolated by waterfalls that were impassable for the hybrid trout downstream. The stocking of brown trout kept going in Slovenia until it was banned in 1996. And then a conservation plan was launched and the breeding with pure fish from the 8 populations started and 2 new populations where created. From 2000 on 20 populations have been monitored and to this day 17 of these are still monitored annually or bi-annually. And marble trout once again rule the Soča basin. Sadly even though the populations are monitored they are still vulnerable. One of the original pure populations in the Predelica was wiped out in a landslide but luckily the genes of this location are still kept in one of the fish farms with marble trout. And together with de fish from the Zadlascica they are used to stock the waters of the Soča above the lake. For the Idrijca and its tributaries the other pure populations are used to restock the rivers.

The pure populations are also used to start populations in Italy and the species is making its comeback there. To see in what kind of streams the species was found I visited one of them, with someone who was involved in the project. And I have to say it was a lot smaller the I expected. This stream was only between 2-5 meters wide and most of it was only 20cm deep with a few holes between 2 and 10m2 where the dept was about 50-80cm. when I asked further I even got to know that in these streams marble trout can still grow to 40-50cm in length. Sadly when walking along the stream no fish where found. My guide even said that the stream looked dead to him. we found a lot of foam on the water which may indicated pollution, or one of the otters roaming the rivers may have killed the population but it did not look good. However we did not walk the whole stream so maybe further upstream it looks better and we still have some hope the populations is not lost. From the 8 pure populations 2 are probably gone and some populations decreased to only a few fish. I hope monitoring and protecting the remaining populations will be successful as it would be a shame to lose these old pure populations while the rivers are being stocked with their offspring.