Direct and Indirect Climate Change Impacts on Brown Trout in Central Europe: How Thermal Regimes Reinforce Physiological Stress and Support the Emergence of Diseases

Abstract

Water temperature is one of the most important abiotic parameters in rivers having direct and indirect effects on fish. Especially cold-water species like the brown trout (Salmo trutta) are limited by high temperatures. Beside direct physiological stress, higher water temperatures also reinforce the emergence of diseases. In this study we investigate thermal regimes of rivers based on a large-scale dataset covering Austria (∼70,000 km²). The analyses aim to clarify to what extent water temperatures support the emergence of proliferative kidney disease (PKD) and assemble physiological stress for brown trout under current and future climate conditions. Data from 274 gauging stations at 184 rivers were used to calibrate a water temperature model and to investigate critical water temperature thresholds. Therefore, we developed a risk assessment scheme to identify river reaches that already have or will develop critical thermal regimes for brown trout in respect of PKD emergence and thermal physiological stress. The results revealed severe changes in the thermal regimes of the investigated rivers under climate change. Furthermore, the variable characterizing riparian vegetation played a vital role to explain cooling of the water in downstream direction. In respect of PKD, the amount of river reaches having unlikely outbreaks of PKD decreased from 72.6% under current conditions to 37.7% in the far future RCP8.5 scenario. Within small rivers that currently showed optimal thermal regimes over large extents (10,244 km), the habitat suitability will be reduced by combined effects of PKD and physiological stress to 6,554 km. In general, suitable habitats of S. trutta will shift upstream, thus to higher altitudes, and to smaller, alpine rivers in Austria. The warming leads to physiological stress that induces a diminished growth due to the non-positive transition of caloric values to growth as well as cardiac dysfunction in brown trout. These factors will further restrict the distribution of brown trout. However, the results also underline the enormous importance of the alpine region as a future refuge for brown trout in Central Europe. Thus, this study will help to inform managers to timely develop robust conservation strategies.

Current and projected impacts of the parasite Tetracapsuloides bryosalmonae (causative to proliferative kidney disease) on Central European salmonid populations under predicted climate change

“…The future impact of PKD on brown trout, grayling, and salmon populations is greatly dependent on how water temperature, precipitation, and evaporation change in the context of the climate crisis (Borgwardt et al., 2020; Masson‐Delmotte et al., 2018). Water temperatures in central Europe have already increased by 1–2°C (Daufresne & Boët, 2007; Kędra, 2020; Michel et al., 2020; Webb & Nobilis, 2007) and patterns of precipitation have changed in quality (increasingly torrential rain, less snow) and seasonality (decreases in summer, increases in winter) (Masson‐Delmotte et al., 2018).…”

“…Overlapping maps of habitat suitability for salmonids and T. bryosalmonae indicate that this habitat loss is extensive at lower elevations where salmonids already suffer a high probability of exposure to PKD and experience more severe grades of kidney hyperplasia. By contrast, losses of suitable habitat at lower elevations will be slightly compensated by an increase in suitable habitat at higher elevations (Borgwardt et al., 2020). Even if these elevated areas are small and therefore not relevant for fishery utilisation, they are less suitable to bryozoans and T. bryosalmonae , which means they might remain suitable for salmonids and provide a refuge even under significant warming.…”

“…Local causes include structural degradation through obstruction of waterways and the operations of water power stations, and deterioration in water quality due to influxes of wastewater, nutrients, and other contaminants from human activities (Burkhardt‐Holm & Zehnder, 2018). An important global factor implicated in the decrease in salmonid populations is anthropogenic climate change, which will increase water use and water temperatures, and alter precipitation (Bölscher et al., 2013; Borgwardt et al., 2020; Hari et al., 2006; Masson‐Delmotte et al., 2018). First attempts to mitigate local impacts include restoration and rewilding of rivers (Roni, 2019), which together with stocking programmes have restored salmon to the Rhine catchment (Schneider, 2011).…”

“…The grayling ( Thymallus thymallus ), has a broader thermal tolerance than brown trout (Haugen & Vøllestad, 2000; Jungwirth & Winkler, 1984) and spawn in spring on gravel banks downstream of the rhithral zone (Crisp, 1996). Still, in summer, water temperatures might rise above physiological thresholds of these species, causing stress and increasing susceptibility to pathogens (Borgwardt et al., 2020; Gallana et al., 2013; Klemetsen et al., 2003).…”

Broad but restricted detection of malacosporeans in a Neotropical cradle of diversification

“…The five described malacosporean species are variously assigned to the genera Buddenbrockia and Tetracapsuloides (Patra et al ., 2018). Tetracapsuloides bryosalmonae is the causative agent of proliferative kidney disease, an emerging disease exacerbated by warming waters and changing climates (Okamura et al ., 2011; Borgwardt et al ., 2020). Tetracapsuloides bryosalmonae infects a range of wild and farmed salmon and trout (Family Salmonidae) in Europe and North America (Hedrick et al ., 1993; Skovgaard and Buchmann, 2012).…”