Thymallus thymallus (Linnaeus, 1758)

Grayling Thymallus thymallus (Linnaeus, 1758) View in CoL

The grayling is distributed in the south-eastern and the far northern part of Norway, and is usually considered to be a purely riverliving species (Huitfeldt-Kaas 1918). However, it is also found in numerous lakes, including the largest Norwegian lake – Mjøsa (Huitfeldt-Kaas 1917). Phylogeographic studies have shown that two different evolutionary lineages have immigrated into Norway following deglaciation (Koskinen et al. 2000), however this has not been followed up by more detailed studies.

A total of 37 articles were retrieved by the literature search, most of which (almost all) were based on studies from the Glomma riversystem in the south-eastern part of Norway. Glomma is the largest river in Norway, splitting into two main upstream branches; Glomma and Gudbrandsdalslågen. In the upper part of Gudbrandsdalslågen the introduced grayling has been intensively studied during over 30 years. Most of these studies are initiated from the University of Oslo (Vøllestad and Primmer 2019). These studies originally focussed on classical ecological topics ( Haugen and Rygg 1996b, Haugen 2000a), but later focussed on evolutionary processes including population genetics ( Barson et al. 2006, Junge et al. 2011) and adaptation to differential temperatures ( Haugen 2000 a, b, Kavanagh et al. 2010, Mäkinen et al. 2016). Lately it has led to genomic studies of different types (Papakostas et al. 2010, Papakostas et al. 2014, Mäkinen et al. 2018). For example, the annotated genome of grayling was published recently (Varadharajan et al. 2018, Sävilammi et al. 2019), facilitating even more detailed evolutionary studies. Most of these studies are based on grayling in the upper reaches and high-altitude systems. Further downstream in the two rivers and the lake Mjøsa detailed studies with applied focus on the population structure, dispersal, and the effect of lack of connectivity has been performed (Linløkken 1993, Kristiansen and Døving 1996, Heggenes et al. 2006, van Leeuwen et al. 2016, van Leeuwen et al. 2018, Holter et al. 2020). All these studies have increased our understanding of grayling life history. However, there is still only limited knowledge about feeding ( Haugen and Rygg 1996 a, Amundsen et al. 2010) and parasitism (Mo et al. 1998, Ieshko et al. 2001), etc. The grayling is host to the monogenean parasite Gyrodactylus thymalli Zitnan, 1960 , and this parasite is of great interest as it is very similar to the pathogenic G. salaris ( Fromm et al. 2014, Mieszkowska et al. 2018). Some work has been done on the biology of G. thymalli (Pettersen et al. 2015, Pettersen et al. 2021), but clearly more work should be done.

The biology of the grayling is relatively well known, but the information is geographically biased towards one particular river system. And there are no studies focusing on population dynamics or on how grayling populations might vary over time.