Arctic char Salvelinus alpinus (Linnaeus, 1758)

A total of 318 articles presenting relevant data on Arctic char (char hereafter) were retrieved. The char is distributed throughout most of the country, with anadromous populations in the northern parts. It is also the only freshwater species that has populations on Svalbard and Bjørnøya (Bear Island) (Gulseth and Nilssen 2001, Bytingsvik et al. 2015). Other species have been recorded from freshwater at Svalbard (Svenning et al. 2015), but it seems that only the char has well-established populations.

The char exhibits extreme variability in life history, behaviour and demography (Klemetsen 2010). This variability has by some been called the “char problem” (Nordeng 1983). Numerous articles describe phenotypic and life history variation in the char, many focusing on the existence of alternative morphs or ecotypes within watercourses. The number of morphs within a system may vary from one (called “normal” char) to the newly discovered four-morph system in Tinnsjøen (Østbye et al. 2020). The system in Tinnsjøen is kind of similar to the well-studied char-system in the Icelandic lake Thingvallavatn (Sandlund et al. 1992). In some river systems in the north, the char can be classified into three different morphs (one anadromous, and two resident morphs) (Nordeng 1983). Some sympatric morphs may be part of the same population, whereas other morphs are more or less reproductively isolated (Hindar and Jonsson 1993, Praebel et al. 2016). This leads to a classic discussion about the importance of genetic differentiation and phenotypic plasticity (Nyman 1972, Hindar et al. 1986, Whiteley et al. 2019). For example, some researchers have classified selected populations from Norwegian lakes as separate species (Kottelat and Freyhof 2007). Understanding the structuring of char seems particularly difficult. But it is also a common topic of discussion regarding other species that have invaded lakes and rivers post-glacially (Bernatchez 2004). It is at times difficult to detect the existence of sympatric populations, and potentially also species, as it may require more sensitive genetic methods than has been in use until recently (Jorde et al. 2018). Clearly, the systematics of the Arctic char is not fully resolved.

On average, eight articles focusing on the char were published per year during the 40-year period. It was a tendency that number of published papers increased through the period (R 2 = 0.115, slope estimate 0.13 ± 0.05 y-1, P = 0.014), but the among-year variation was large. Most of the retrieved articles could be classified as ecological (76.8 %), whereas behaviour was investigated to a very limited degree (1.9 %). The ecological studies had a wide range of contexts, but many were focusing on the position of the char in the ecosystem. Many have studied the diet of the char, usually in lakes (Dervo et al. 1991, Dahl-Hansen et al. 1994, Gregersen et al. 2006, Amundsen et al. 2008). In that context, numerous studies have also investigated the transmission of various parasites (Knudsen and Klemetsen 1994, Knudsen 1995, Amundsen et al. 2003 b, Siwertsson et al. 2016). A particular observation is that most of these studies are from North-Norway.

The char is considered to be a very efficient zooplankton predator, and several studies consider the interaction between char and other fish species. In particular, the relationship between char and trout (Saksgård and Hesthagen 2004, Persson et al. 2007, Hesthagen et al. 2011a, Guenard et al. 2012, Persson et al. 2013) has had particular interest. The char have high growth efficiency relative to the trout, but still seem to be out-competed by the trout - in particular in relatively warm and productive lakes (Finstad et al. 2011). Modelling exercises from Sweden indicates that increasing temperatures and interaction with trout may lead to an extensive population loss (Hein et al. 2012). Similar predictions have been made from other locations in Europe (Kelly et al. 2020). No such studies are available from Norway yet. The char may also be at peril due to the increasing distribution of the pike (Hein et al. 2012).

Overall, the feeding biology of the char is well understood as also seems the case with variation in life history (Nordeng 1983, Vøllestad and L’Abée-Lund 1994). Also the anadromous part of the life cycle of char has been investigated – focus has been both on why some individuals or populations are migratory (Finstad and Hein 2012), and the migration process itself (Berg and Berg 1989, Finstad and Heggberget 1995). Even if the ecology of the char is relatively well understood, the drivers of the extensive phenotypic variation are still in need of understanding. In order to acquire such understanding evolutionary questions has to be asked.

In total only 29 papers with an evolutionary focus were found by the search. Some small-scale population genetic studies were found (Hindar et al. 1986, Westgaard et al. 2004, Wollebaek et al. 2011, Praebel et al. 2016, Østbye et al. 2020). However, the number of studies were very small relative to the large phenotypic variability that can be found. More classic evolutionary studies focussed on various aspects of sexual selection (Skarstein and Folstad 1996, Skarstein et al. 2005, Egeland et al. 2015), and also some more functional genetics studies were found (Eliassen et al. 1998, Lysfjord and Staurnes 1998).

A relatively limited number of papers (n = 15) handled the effects of pollution. As expected, some papers focused on the effect of acidification of surface waters (Andersen et al. 1984, Hesthagen and Sandlund 1995), while some also followed the effects of the Chernobyl accident (Forseth et al. 1998, Jonsson et al. 1999). Otherwise, few if any studies focused on other kinds of pollutants and their effect on individuals or populations. Thus, even if the number of papers published on the char is relatively large (n = 318), the number of studies with a direct management application is limited. There are some studies focussing on the effect of harvesting (or different ways of harvesting) (Langeland 1986, Finstad et al. 2000, Smalås et al. 2020), and there are some studies focusing of the potential for the char to be a temporary host for parasites such as Gyrodactylus salaris (Bakke et al. 1996, Robertsen et al. 2007, Winger et al. 2008b). Recently, there has also been some studies on the effect of sea lice infections of sea-run char (Bjørn et al. 2001, Bjørn and Finstad 2002). However, the number of such studies are small relatively to those published on Atlantic salmon and sea trout. Overall, even if the ecology of char is well-understood, there clearly is a wide range of topics that needs better understanding. The important drivers of phenotypic variability are not understood, as well as the importance of genetic differentiation and level of response to natural selection (potentially leading to local adaptation).