Saving The Marble Trout

Fishermen and scientists in the River Soča basin in Slovenia have been working hard for 15 years to preserve and rehabilitate populations of the revered, but endangered genetically pure Marble trout in the upper parts of Soča basin. Tolmin, Slovenia. 26/12/2009. (Images taken from 06/2008-12/2009)

The habitat of Marble trout (Salmo marmoratus) is geographically restricted to the basin of Po River in Italy, and the Adriatic basin of former Yugoslavia and Albania. It is famed for being illusive, mysterious, that it can grow to enormous sizes and weigh more than 30kg. In Slovenia, the largest one, found dead, weighed 24kg and was 117cm long. But the largest one caught by sports fishing weighed 22,5kg and was 120cm long. However, the Marble trout in Soča river basin are hybrids.

Since 1906, the Adriatic basin was stocked with unautochton Brown trout that resulted in hybridization and subsequent disappearance of genetically pure native Marble trout. Before the start of the international project of rehabilitation and preservation of Marble trout, only one remaining population of pure Marble trout was known. In the next years, after the project started and provided with crucial information, fishermen and scientists have found eight remaining populations of pure Marble trout that were used to breed new populations. It was determined that hybridization with Brown trout was the cause of Marble trout disappearing, which resulted in banning the stocking of these rivers with Brown trout. To reintroduce the native Marble trout, the genetic rehabilitation was chosen. The project was supposed to study the existing pure populations, create new ones by breeding them in captivity, and to try and raise the level of pure DNA in hybrids until one day foreign DNA would be eliminated. Given the high quality of streams and environment the project is on the road to success.

Many new streams have been populated with pure Marble trout since then, and they are being monitored twice a year. Fishermen and scientists go into the mountainous inaccessible gorges to count them, tag them, take fin and skin samples, weigh and measure them. All of this is used to study their growth (skin samples), determine their parents and origins (fin samples), their migratory skills, resilience, reproductory rate, growth rate, mortality etc… They live in these inaccessible gorges in headwater streams that are isolated from hybridization zones (basically all other rivers) by waterfalls or other obstacles that Brown trout cannot defeat to migrate upstream and mix with Marble trout.

In a captive breeding program, they keep males and females of all these populations, and breed them. When that time of the year comes, they take eggs and semen from them, carefully count them to the last egg and fertilize them in a hatchery. They keep them there until they are developed enough to be stocked into streams. To make the development more natural, they have been stocking streams with fish eggs with eyes for some time now. When the eggs are developed enough for the eyes to be seen, they are the right age to be inserted into streams. Fishermen do it the way a fish would, by digging a spawning hole, inserting the eggs and covering them with gravel. In two months the young come out.

But it hasn’t been all smooth sailing for them. One entire population disappeared in catastrophic floods of 2004, another one nearly died out after the 2007 flash floods. But the latter flood brought another fascinating fact into attention. After 95% of the population died, the number of fish has risen dramatically in a year, which means the population has an astonishing reproduction rate, unlike any fish out there. And as if all the floods were not enough, another one last year threatened to destroy the captive breeding program, which would be disastrous. Fortunately, only minor damage was done, the fish farm was repaired and cleaned, fish counted and sorted back into their tanks.

At least fifteen populations are needed to ensure the preservation of this species. They are getting there. One of the aims of the project is to one day stock rivers with only pure Marble trout and eliminate all foreign DNA. Amazingly, they are slowly getting there.