Salmo trutta Linnaeus, 1758

Brown trout Salmo trutta Linnaeus, 1758 View in CoL

The brown trout (hereafter trout) is the most widely distributed freshwater fish in Norway (Huitfeldt-Kaas 1918). It is very popular for sports fishing, and was historically extensively used for household fishing. Numerous books have been written about it, both nationally (Qvenild 1994, Nilssen 2017) and internationally ( Elliott 1994, Jonsson and Jonsson 2011, Lobón-Cerviá and Sanz 2018). This large interest has also led to extensive research efforts in Norway during the 40-year time period checked here ( Table 1 View Table 1 ).

A total of 528 publications was retrieved by the search, leading to more than 12 publications per year during the 40-year period. There was a weak increase in the number of publications per year during the period (linear regression, R 2 = 0.152; slope 0.16 ± 0.06, P = 0.012). Over 50% of the retrieved publications could be classified as focusing on ecology, and ecological studies have been published with more or less the same frequency throughout the time period ( Figure 7 View Figure 7 ). Studies on behaviour have, on the other hand, been relatively few, and most of these studies were on migration and movement.

Evolutionary studies were rare in comparison to the ecologically focused studies. Almost all of the studies that could be classified as evolutionary were focused on population genetics of some kind. Over time these studies use different methods, most were analysing variation in allozymes and microsatellites (Skaala and Jørstad 1987, Skaala and Solberg 1997, Sønstebø et al. 2007, Serbezov et al. 2012b, Vøllestad et al. 2012). Most studies were at relatively small geographic scales (within river systems) and focus on classical fixation index estimations (FST), and thus gene flow and level of differentiation on putatively neutral makers. These studies can to some degree be put into a conservation genetics context. Some studies also estimated directly relevant parameters such as the effective population size (Serbezov et al. 2012a, b) or more directly assess the effects of for example habitat fragmentation (Junge et al. 2014) or introgression from stocked fish (Sønstebø et al. 2008). The limited scale of many of these studies, as also was common in northern Europe in general (Vøllestad 2018), contrasts the current trend for the study of Atlantic salmon, and limits the understanding of the genetic structure of the trout at the national scale. This is unfortunate, as it makes it difficult to take part in the ongoing and heated discussion on the phylogenetics of the trout ( Bernatchez 2001, Kottelat and Freyhof 2007, Sanz 2018, Guinand et al. 2021). It is also unfortunate that more modern genetic methods have not been used to any degree.

A limited number of studies have also taken a more quantitative or life history approach. Such studies are clearly useful for understanding how trout may respond to various selective forces ( Haugen et al. 2008, Robertsen et al. 2011). It may also help in understanding to what degree genetic architecture and phenotypic plasticity determine life history decisions (resident vs migratory) (Jonsson 1982, Jonsson et al. 1994). The trout is often highly migratory, and it is important as an anadromous species along the coast. However, very few studies address questions related to variation in level of anadromy. Even the few studies that were classified as behavioural did rarely include the marine phase of life (but see later in relation to management).

Trout are found in all kinds of freshwaters throughout Norway. And over time it has been exposed to numerous pollutants, with variable effects on individuals and populations. In the early part of the investigated period the effect of acid rain and subsequent acidification of surface waters were studies by many. The studies focused on individual level effects (Rosseland and Skogheim 1984, Rosseland et al. 1986, Muniz et al. 1987), population level effects ( Hesthagen 1986, Muniz 1991, Bulger et al. 1993, Barlaup and Åtland 1996), and mitigation measures (Rosseland and Hindar 1988, Traaen et al. 1997, Hindar and Wright 2005). Studies of acidification has become rare during the last years. There are two additional topic that have been investigated by Norwegian researchers – the effect of the Chernobyl accident, and heavy metal pollution. The trout is the freshwater fish that was most studied in order to understand the dynamics of radioactive caesium in the environment (Ugedal et al. 1997, Forseth et al. 1998, Jonsson et al. 1999, Braaten et al. 2019). Such studies were limited in time. However, studies on the importance of mercury in fish and in the environment as such is still ongoing (Skurdal et al. 1985, Amundsen et al. 1997, Olsvik et al. 2001, Thomas et al. 2016). In total 75 of the articles that were retrieved from the search could be classified as focusing on pollution issues.

As a widely distributed and sought-after species, there was a large number of studies with a management perspective. More than 110 such studies were retrieved by the query. In particular, numerous studies have investigated the various effects of hydropower development. Such development, with the building of dams and changing of water flow will impact on connectivity, habitat quality and habitat availability. This is an ongoing issue for study, and will probably be so given the recent incentives to develop more so-called green energy ( Aass et al. 1989, Heggenes and Saltveit 1996, Halleraker et al. 2003, van Leeuwen et al. 2016). Recently there has been some more interest in evaluation of the efficiency of fishways and how to facilitate safe two-way migration past dams and weirs ( Fjeldstad et al. 2012, Fjeldstad et al. 2018, Holter et al. 2020).

Throughout the period there has been strong interest in stocking of trout, and numerous studies have focused on the stocking method ( Fjellheim et al. 1995, Hesthagen et al. 1995, Finstad and Jonsson 2001, Solås et al. 2019). Stocking and translocation have been common for a long time (Huitfeldt-Kaas 1918, Nilssen 2017), but rarely has management goals been formulated and even more rarely have it been tested if the goals have been met (Vøllestad and Hesthagen 2001). Stocking has been used for different reasons, but most often to increase the potential yield in recreational inland fisheries. Such stocking is being discontinued, and studies on the potential population level effects of stopping stocking should be done (Nater et al. 2022). One effect of stocking that should be evaluated further is the effect of introgression of stocked, non-native trout into wild populations (Wollebaek et al. 2010). The genetic effect on native gene pools by the use of non-native stocked fish has been intensively discussed internationally (Ryman 1981, Ryman and Utter 1987, Ryman and Laikre 1991, Araki et al. 2007).

A recent problem for trout is the interaction with the aquaculture industry. The main problem is the interaction between anadromous trout and the sea lice (Vollset et al. 2017, Vollset et al. 2018, Serra-Llinares et al. 2020). Overall, this interaction between sea trout and the Atlantic salmon farming industry has also led to some more studies on the marine phase of the life cycle of the trout. However, were little if any information is published on the harvesting of sea trout at sea. Almost all studies on management-related issues are from fresh water.

Overall, the brown trout has been extensively studied – with a variety of topics being handled. The biology of trout is thus in general well understood. However, the marine (coastal) part of the life cycle is less well understood (Thorstad et al. 2016, L’Abee-Lund and Vøllestad 2018). Fortunately, this seems to be changing ( Davidsen et al. 2014, Jensen et al. 2014, Flaten et al. 2016, Eldøy et al. 2021).