taxonID	type	description	language	source
039ECE531C45CA30FF5BFCA1FE93FEE3.taxon	discussion	Climate change implications A near relationship between air temperature and water temperature is generally accepted (Kvambekk & Melvold 2010). The pattern with warm and cold summers is reflected in a variety of other lakes, both on Hardangervidda and elsewhere (Qvenild 2022). On Hardangervidda, an increase in air temperature has been observed since the 1980 s (seklima. no). Hence, this trend is also assumed to be reflected in warmer aquatic habitats as seen in the long-term series in Lake Øvre Heimdalsvatn (Sildre. no). This perpetuating trend of warmer lakes is anticipated to continue and will most likely affect the future status of the lakes. The brown trout has proved to be sensitive to the change in climatic conditions on Hardangervidda, with populations ranging from near collapse in the first decades of the 20 th century, to an almost instantaneous change to very rich fisheries in the 1930 s (Qvenild 2022). Climate induced strong year classes dominated the fisheries in the 1930 s. A similar strong year-class appeared in 1997. In the last decade, the warm summers 2014 and 2018 seem to have produced strong year-classes. Such climatic induced variations in recruitment and growth pattern may be stronger with a warmer, wetter and more unpredictable climate. Such a climate may also induce increased productivity in lakes and rivers and with an increased nutrient input from terrestrial habitats. Due to the removal of Eurasian minnow, G. lacustris and L. arcticus are still available food organisms in Lake Skjerja. Even though these two crustaceans can survive water temperatures above 20 ° C for shorter periods of time, it should not exceed 16 and 18 ° C for longer periods, respectively (Lakka 2020; Wilhelm & Schindler 2000). During the period 2017 – 2023, the water temperatures have sporadically exceeded 18 ° C in Lake Skjerja, but only for short periods. In the warm summer 2018, the maximum temperature was 18.5 ° C. Hence, harmful high temperatures do not seem to be any threat for the existence of the two species at this point. A study of the distribution of L. arcticus in the Fennoscandia Mountain range, indicated that an average water temperature (1 July – 15 September) of 14 ° C is close to the upper thermal threshold for the species (Qvenild et al. 2021). In lakes with higher temperatures, L. arcticus seem to suffer, likely because of life cycle mismatches. In some years (2018 and 2021), the mean water temperature approached this limit in Lake Skjerja. However, the lake still seems to have a viable population of L. arcticus. Lake Skjerja is a very shallow lake and thus, it lacks proximity to a colder refugia in the hypolimnion. However, extreme year to year variations may be problematic for a species like L. arcticus, which has proved to be very sensitive to various environmental disturbances (Lakka 2020; Qvenild 2022). A warming of lakes may be tolerated by G. lacustris through its phenotypic plasticity in the reproduction traits (Qvenild et al. 2020). The water temperature in Lake Øvre Heimdalsvatn has also increased in recent years, and the summer average temperature has exceeded 14 ° C in some years. Despite the invasive Eurasian minnow, G. lacustris is still an important food item for brown trout (Brittain et al. 2019). In contrast, L. arcticus seem to suffer, likely due to the combined effect of a high predation pressure from Eurasian minnow reinforced by the warmer water, which likely is beneficial for the minnows. Lepidurus arcticus, both being a prey and a predator, is assumed to have structural effects of the freshwater ecosystem (Jeppesen et al. 2001). Hence, a collapse of the L. arcticus may have severe effects on the food web dynamics in the lake. As indicated by catches of Eurasian minnow with baited traps in Lake Skjerja, a higher water temperature may benefit this species by increased recruitment and individual growth. This can cause increased predation pressure on the two crustacean species, in addition to a stronger competition for these food items with the brown trout. Increased periphytic production and its effect on the grazer dominated ecosystem, may be in favour of Eurasian minnow. Hence, warmer freshwater ecosystems can be an additional challenge in managing sympatric brown trout and Eurasian minnow populations in the future.	en	Qvenild, Tore, Hesthagen, Trygve, Museth, Jon (2024): A long-term study of the impact of the invasive species Eurasian minnow Phoxinus phoxinus on brown trout Salmo trutta production in a high mountain lake, Southern Norway. Fauna norvegica 43: 1-11, DOI: 10.5324/fn.v43i0.5826, URL: https://doi.org/10.5324/fn.v43i0.5826
