Salmo salar Linnaeus, 1758

Atlantic salmon Salmo salar Linnaeus, 1758 View in CoL .

The Atlantic salmon is the species which has received by far the most attention by fish biologist in Norway. Also, the publication rate (number of published articles per year) has increased significantly (number or articles per year; linear regression, R 2 = 0.396, slope = 0.37 ± 0.07, P <0.001). This interest has also manifested itself globally by the publication of several books ( Aas et al. 2011, Jonsson and Jonsson 2011). The large interest in Atlantic salmon is due to many factors, both economic, societal and political. Without going into detail here, there are large conflicts about how to manage and conserve the wild populations in the face of numerous challenges ( Forseth et al. 2017). A major factor leading to conflicts is the interaction between the Atlantic salmon farming industry and the local wild populations (Liu et al. 2011). This has led to the build-up of large conservation plans with associated programs for collecting relevant data. Further, specific fjords and rivers are set aside where fish farming and other encroachments should not be allowed (Vøllestad et al. 2014, Vøllestad et al. 2018). All in all, this long-term interest and level of conflict has led to the production of a large number of articles on the Atlantic salmon – and as shown earlier, the numbers are increasing.

I tried to classify the articles into different topics – ending with classifying them as either mainly focussing on behaviour, ecology, evolution including genetics, management, or on the effect of pollution ( Figure 6 View Figure 6 ). It was not easy to do this classification, as many publications are at the interface between topics. However, the exercise probably gives an indication of temporal changes in scientific focus.

Throughout the 40 years covered by this summary a large proportion of the published papers could be classified as mainly ecological. Without going into details, it was clear that early in the period focus was on the freshwater part of the life cycle – studying topics such as growth, survival and habitat use of juveniles (Jensen and Johnsen 1985, Berg and Gausen 1988, Heggberget 1988, Heggenes et al. 1991). More recently there has been more focus on the marine phase – mainly by the inclusion of modern methods like telemetry ( Haraldstad et al. 2017, Halttunen et al. 2018), electronic tags of different kinds ( Hedger et al. 2017, Strøm et al. 2017, Strøm et al. 2020), and different types of state-of-the-art modelling (Vøllestad et al. 2009, Otero et al. 2011, Castellani et al. 2018). It is also evident that focus has changed from local-scale studies in individual rivers to more regional and global scales. Local scale studies also tend to become more long-term and focussing on mechanisms. These changes in publication pattern to a large degree follow general trends in many ecological fields, where focus has changed from descriptive small-scale studies to studies focussing on hypothesis testing and synthesizing data over larger temporal and spatial scales.

Behaviour was a topic of interest in particular in the 1980-2000 time period. These studies were focussed on two particular types of behaviours: migration mainly in freshwater (rivers and lakes), and spawning behaviour. The study of downstream migration of smolt has always been interesting, often set in a phenological context (Hansen et al. 1984, Jonsson and Ruud-Hansen 1985, Hvidsten et al. 1995). Also the upstream migratory behaviour of adult fish have been studied ( Heggberget et al. 1993, Lennox et al. 2018). More recently, studies on the swimming behaviour of post-smolts through fjords and older salmon at sea also have been published (Økland et al. 2006, Manel-La et al. 2009). These studies have been facilitated by the development and use of acoustic telemetry, pop-up satellite tags and data storage tags. Several such studies have appeared recently (Strøm et al. 2020, Rikardsen et al. 2021), and more is expected following larger research programs that have been initiated. More classical behavioural studies were uncommon.

Evolutionary questions have been studied throughout the period, but in particular during later years. In this context I include studies on population genetics. During the early period some studies focusing on genetic structure ( Vuorinen and Berg 1989, Skaala et al. 1998) and its relation to aquaculture (Hindar and Balstad 1994, Garant et al. 2003) was published, however usually with limited scopes and limited sets of genetic markers. This was probably due to lack of proper and economically available methods. Recent developments in methods and collaborations have led to many studies on both the general genetic structure of the Atlantic salmon (Vähä et al. 2017, Wennevik et al. 2019), and the level of interaction with aquaculture ( Glover et al. 2013, Zhang et al. 2013, Glover et al. 2018). Many studies in the last category are here classified as focused on management (see below). Numerous studies from the river Tana (Teno) were not retrieved in the search because it did not fit the search criteria, or because they were published after 2020. Also, the publication of the annotated full genome of the Atlantic salmon was not picked up by the search (Lien et al. 2016). However, these publications are important in an international context, as they either produce high-quality results or may help develop major tools for a diverse set of later studies ( Ayllon et al. 2015, Barson et al. 2015, Czorlich et al. 2018). These last studies are trying to understand major evolutionary questions, rather than more local scale questions of applied importance. Overall, also in the published studies that were retrieved by the search such a change from local to global scale questions is evident. Also, studies of more functional character have appeared, such as on the genetic resistance to sea lice (often called salmon lice) Lepeophtheirus salmonis (Krøyer, 1837) infections (Kolstad et al. 2005), the genetic consequences of inbreeding (Roberge et al. 2008), and regulation of growth ( Besnier et al. 2020).

Research focusing on issues related to management has increased in number with time – this is to a large degree related to the increased conflict between wild salmon interests (conservation and recreational fisheries) and the Atlantic salmon farming industry ( Forseth et al. 2013, Forseth et al. 2017). In the earlier part of the time period, management-related research was usually on the effect of hydropower development ( Brooks et al. 1989, Raddum and Fjellheim 1995, Saltveit et al. 2001), managing of the commercial and recreational fishery (Jensen et al. 1999, L’Abée-Lund and Aspås 1999, Thorstad et al. 2007), or the handling of local and regional infestations of the ectoparasite Gyrodactylus salaris Malmberg, 1957 (Johnsen and Jensen 1991, Pettersen et al. 2013, Sandodden et al. 2018). These types of studies have continued to be important throughout the period investigated. However, in the recent decades focus has also been centred on the interaction between wild populations and the salmon farming industry. The two main topics are the effect of escapement of farmed fish into rivers (Skilbrei et al. 2015, Diserud et al. 2019), and the effect of sea lice on wild fish (Torrissen et al. 2013, Kristoffersen et al. 2018). Even if management-oriented studies are published at a reasonably high pace, most of the ongoing research is still published in the grey literature and not picked up in my search.

The final topic that I grouped the papers into was pollution – or rather the effect of pollution. The popularity of such studies seems to have decreased with time, or the pollution problems are becoming less important. In the 1980s acidification was a problem in freshwaters, also in Atlantic salmon rivers (Skogheim et al. 1984, Rosseland et al. 2001, Hesthagen and Larsen 2003, Hesthagen et al. 2011b). The problem has to a large degree been mitigated, either due to reduction in emissions of acidifying compounds or due to local mitigating actions (liming) (Sandøy and Langåker 2001). There was also a small burst in publications on the distribution and effect of radiocaesium following the Chernobyl accident ( Forseth et al. 1998), but the focus was rarely on Atlantic salmon.

Overall, the publication rate on the Atlantic salmon has been high and is still increasing. Still, the wild populations overall are not doing well. This led to listing of the Atlantic salmon as being near threatened (NT) on the 2021-version of the Norwegian red list ( Hesthagen et al. 2021d). On previous versions of the red list the Atlantic salmon was not listed. So, despite extensive efforts by numerous management authorities and organizations, and extensive research activity, the situation is not becoming better. And this is the situation for the species where most basic ecological information is available for any Norwegian freshwater fish, and where most research activity and funding has been directed.