Annual testing of marble trout from zone of hybridization

One of the success stories of fish restoration comes from the west part of Slovenia (the River Soča and its tributaries). Native marble trout was under threat because of hybridization with non-native brown trout that was being stocked in the rivers for decades. In the 90s, the last 8 populations of genetically pure marble trout were identified and used in marble trout restoration program. The native range of marble trout is mostly stocked with offspring of fish from these 8 populations. However, these populations have low genetic variability, therefore individuals from the zone of hybridization are also used in the restoration program so that genetic diversity of marble trout from the zone of hybridization is not lost. The problem is how to select individuals from zone of hybridization that don't have (too much) non native genetic characteristics.

We performed genetic testing of fish from the zone of hybridization for the last 5 years. This year we have received fin clips from 34 individually tagged trout from the River Tolminka, tributary of the River Soča. These samples were checked for non native genetic characteristic on 6 nuclear genetic markers (each genetic marker in 2 copies, one allele inherited from each parent) and on 1 mitochondrial genetic marker (in 1 copy, inherited from the mother only). Out of 34 samples, 14 show signs of hybridization with non native trout. Eggs and sperm will be taken from 20 trout with no detectable brown trout genetic characteristics, fertilised and raised in the hatchery and released back in the zone of hybridization.

Below is a simplified table of genotypes. These genotypes are not publishable, since the initial sampling was biased, only marble trout (by general appearance) were taken from the river, so the actual percentage of non native alleles in Tolminka might differ from our results (6%).

Table of genotypes across 1 mitochondrial and 6 nuclear markers. Samples with non native (=brown trout alleles) are marked red and will not be used in restoration program. f - female, m - male, M - marble trout specific allele, B - brown trout specific allele.