Decline of European grayling (Thymallus thymallus L.) populations in Belgian rivers: What are the main environmental factors involved?

This study investigated population trends of the European grayling (Thymallus thymallus L.) across Belgian rivers in relation to environmental changes. Freshwater biodiversity has been declining dramatically and facing many threats those past decades. Among freshwater species, the European grayling is present throughout Europe, but its populations have declined across their distribution area. It is an exigent fish species, having specific physiological and ecological needs that are in the frontline of many consequences of human activities, with climate change bringing additional pressures. This study aimed to assess the decline of graylings and to determine the most impactful environmental factors. We analysed data from fishing events from 2000 to 2022 and environmental data of the Walloon region of Belgium. When comparing the first two decades of the 2000 s, the fishing data highlighted a decline of 42.8 % of grayling population abundance. In parallel, water flow tended to decrease with a higher occurrence of very low-flow, and maximum water temperature tended to increase with higher occurrence of days with water temperature > 18 ◦C. Indeed, there was an average of 18.6 additional days at very low water flow and an average of 9.6 additional days at water temperature > 18 ◦C. These parameters showed an
impact on graylings abundance giving the significant correlation found between them. This study quantifies the rate of grayling’s decline in Belgium and the change of water flow and temperature over time, and it highlights the environmental variables that have shown an influence on European grayling abundance.