{"id":48691,"date":"2025-12-01T14:00:04","date_gmt":"2025-12-01T13:00:04","guid":{"rendered":"https:\/\/tourduvalat.org\/?p=48691"},"modified":"2025-12-11T11:11:41","modified_gmt":"2025-12-11T10:11:41","slug":"when-rivers-warm-up-what-future-for-trout-in-the-soca","status":"publish","type":"post","link":"https:\/\/tourduvalat.org\/en\/publications-en\/when-rivers-warm-up-what-future-for-trout-in-the-soca\/","title":{"rendered":"When rivers warm up: what future for trout in the Soca?"},"content":{"rendered":"

In the cool, clear waters of the upper So\u010da basin, a <\/strong>preserved <\/strong>region of Slovenia, lives the peaceful marble trout Salmo marmoratus<\/em>, a species endemic to the Adriatic basin. This species almost disappeared from the So\u010da due to hybridisation with the common trout Salmo trutta<\/em>. The latter was regularly stocked, i.e. released, until a conservation plan was put in place by the Tour du Valat, the Tolmin fishing association and the WWF in the 1990s.<\/strong><\/p>\n

Separated by impassable physical barriers (waterfalls), only a few genetically pure populations (nine, including two created artificially) have survived to this day, upstream of this basin. But for how many more years?<\/strong><\/p>\n

\"Vall\u00e9e
So\u010da Valley \u00a9 Delphine Nicolas | Tour du Valat<\/span> <\/figcaption><\/figure>\n

More than 25 years of monitoring<\/h3>\n

In order to better understand the impact of climate change on aquatic ecosystems, and more specifically on the different species of trout found in the region \u2014 marble trout (Salmo marmoratus<\/em>) and brown trout (Salmo trutta<\/em>) \u2014 scientists have been monitoring the temperature of several rivers in the upper So\u010da basin in Slovenia for over 25 years.<\/p>\n

Through this long-term, multi-site monitoring, they compiled a detailed overview of the thermal conditions of the three main rivers \u2013 the So\u010da, Ba\u010da and Idrijca \u2013 as well as several smaller upstream watercourses. They then analysed temperature changes over time and examined the possible consequences for the two trout species.<\/p>\n\n\n\n
\n
\"Truite
Marble trout \u00a9 Florian Leborne<\/span> <\/figcaption><\/figure>\n

Marbled trout, a species sensitive to water warming<\/strong><\/h4>\n

The marble trout (Salmo marmoratus<\/em>) is a species endemic to the Adriatic basin, found only in certain rivers in Slovenia and Italy. Like all salmonids, it is stenothermic: it tolerates only slight variations in temperature and needs cold, oxygenated water to survive.
\nAs temperatures rise, its habitat is becoming increasingly hostile: the water is warming up, contains less oxygen, and promotes the proliferation of parasites and diseases, which directly affects the health and survival of its populations.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\"\"
Map of water temperature monitoring sites. The colours of the dots correspond to their catchment area (brown: upper So\u010da basin, pink: Ba\u010da basin, green: Idrijca basin). The Pregrada Podselo dam separates the upper So\u010da basin from the lower part of the So\u010da river.<\/span> <\/figcaption><\/figure>\n

Results confirm the warming of waters<\/h3>\n

Based on the monitoring of 25 sites, analysis of data collected between 1996 and 2022 reveals a clear trend: waters are warming (+0.04\u00b0C\/year)<\/strong>, especially in summer, with an average increase of 0.1\u00b0C per year at 19 sites. Although temperatures currently remain favourable for salmonid reproduction and growth, periods when the water exceeds 15\u00b0C, the critical threshold for brown trout, are becoming increasingly frequent.<\/p>\n

\"Rvi\u00e8re
Zadlascica River \u00a9 Delphine Nicolas | Tour du Valat<\/span> <\/figcaption><\/figure>\n

“The optimal temperature range for brown trout and common trout is between 5\u00b0C and 15\u00b0C.”<\/p><\/blockquote>\n