Northeast Arctic cod, an ecotype of cod, returning to Norway with love

The festivities of Saint Valentine´s day are upon us, and this coincides with the arrival of the Northeast Arctic cod to the shores of mainland Norway for spawning, a fish close to the heart of Norwegians. This fish is also known as Barents Sea cod, or in Old Norse skreið, modern Norwegian skrei. Skrei might be one of the earliest recognised subtypes of cod, but not until the 20th and 21st Century have researchers been able to start pinpointing exactly how it is different from other local cod, with the aid of modern sequencing technology.

Figure 1 Atlantic cod, Gadus morhua. The species is easy to identify by the characteristic barble, "overbite" and shape of lateral line. Other characteristics for identification are for example position and shape of fins, though not shown clearly on photo. (Photo: Cecilia Helmerson) 

Northeast Arctic cod have been nicknamed “the love fish” (Norwegian: Kjærlighetsfisken) [1], or “the Valentine fish” (Norwegian: Valentin-fisken) [2], since around this time of the year it returns from feeding grounds in the Barents Sea to the coast of Norway in order to spawn, thus returning to the coast of Norway with love.

Over the geographical distribution of Atlantic cod, (Gadus morhua) in the Northern Atlantic concepts such as “stocks”, “populations” and “ecotypes” are used to describe the aggregations or clustering of similar behaving, looking and related cod. The stock is the managing unit, which is fished upon and regulated by quotas. However; a stock can consist of several populations, fish that mate with each other on a more regular basis than others. In our case this could theoretically be fish belonging to the same stock that have a tendency to spawn in the same fjord. The ecotype concept differs from the stock concept in that it is biologically defined – there is an ecological and evolutionary definition, and the purpose is not for management. Ideally the stock concept is biologically defined, but it does not have to be, and this has been suggested to be one of the possible contributions to the slow recovery of collapsed Canadian cod stocks [3]. When a quota is set for a stock made of several “biological subunits” – be it populations or ecotypes, it can lead to a general overestimate of how much fish that can be taken and/or the disappearance of one or more “units” [3].

In Norway, cod is close to heart, making up 20% of the total fish landed in the Norwegian fishery 2016, and it is the fish reported as having the highest economical value [4]. In 2017, it also made its way onto the new 200 kroner bill [5]. In the Norwegian language there is also a range of names for cod (See below Curios about other “types” of cod).

Norwegian cod trade has been going on since the Viking and Middle Ages [for a timeline see 6]. This is also verified by the application of new sequence technology performing genetic analysis on archaeological remains of fish found in Germany and comparing with genetic data from modern fish, as reported in an article by researchers at UiO, last year [7]. 

Somewhere in history an observation was made - some cod where present all year round, whereas the huge schools of fish only appeared in spring. These schools of fish where named skreið, in modern Norwegian denoted as skrei which is related a word for movement, skride which loosely translates as to “glide” [8]. And perhaps this can be seen as the first observation that there is something special about the skrei.

Figure 2 Ear stone from Atlantic cod (Photo: Cecilia Helmerson)

Somewhere before or during early 1900s researchers where starting to look into the biological differences between the fish that was staying and the fish that was migrating – questioning whether these fish were separate species, or whether they were subspecies. Some might even have embraced the ecotype concept that was introduced in the 1920s [9]. In the 1930s the use of otoliths, ear stones (bony part in the inner ear) started to be used. Like the rings of trees can be used for determining age, so can the rings of otoliths, and researchers also found that the shape differed between the wandering cod and stationary cod [10]. From the 1930s and forward, methods of extracting proteins and the separation of proteins on gel in an electric field based on their charge and size, started to develop (so called electrophoresis), and in the 1960s differences in banding was found between types of cod when looking at haemoglobin [11]. Further breakthroughs came after the development of the polymerase chain reaction and the use of a cutting enzyme as well as development of sequencing technology. The gene Pantophysin was early on identified as a marker to separate the different populations [e.g.12, 13 and more]. In 2011 the cod genome was published [14], which further has spurred discovery, both for the understanding of the species and between the different ecotypes of cod, and how to separate stocks. Several chromosomal rearrangements have been found between ecotypes of cod and they are thought to be the key for explaining the ecotypic divergence and how migratory cod have arisen from non-migratory cod, which have been described as the original state [e.g.15, 16 and more]. Thus the history of skrei is far from finished, new knowledge will be gained in how this important species and its genome is influenced by natural selection and local adaptaion, research aided by methodological and technological advances.

Curios about other “types” of cod …

Coffee cod - (Norwegian: Kaffetorsk) – cod with a weight above 30 kg, caught in the Lofoten area, that entitles a bounty; a bag with one kilogram coffee, to the catcher by the local newspaper Lofotposten [17].

King cod – (Norwegian: Kongetorsk) – Cod with skeletal deformities, showing a characteristic akin to a bulldog, thought to bring luck [18].

Kelp cod – (Norwegian: Taretorsk) – Redish to brown coloured cod, lives among kelp, refereeing to a coastal cod of certain colouration [19]. 



[1] Faltin T., Synøve Dreyer. (2017) Skreien krever bare én ting for å bli perfekt. Dagbladet. Article accessed on the internet

[2] Hurtigruten. (2018). Skrei. Article accessed on the internet

[3] Sterner, T. (2007). Unobserved diversity, depletion and irreversibility The importance of subpopulations for management of cod stocks. Ecological Economics, 61:566–574 

[4] Statistisk sentralbyrå. (2017). Rekordhøg fangstverdi for fiskaren.

[5] Norges Bank (2018). NY 200-KRONESEDDEL.

 Article accessed on the internet

[6] Norwegian Seafood Council. (2018). The Timeline of Norwegian Fisheries. Article accessed on the internet at

[7] Star, B., Boessenkool, S., Gondek, A. T., Nikulina, E. A., Hufthammer, A. K., Pampoulie, C., Knutsen, H., André, C., Nistelberger H.M., Dierking, J., Petereit, C., Heinrich, D., Jakobsen K.S., Stenseth, N.C., Jentoft, S., Barrett, J. H. (2017). Ancient DNA reveals the Arctic origin of Viking Age cod from Haithabu, Germany. Proceedings of the National Academy of Sciences of the United States of America114(34), 9152–9157.

[8] Det Norske Akademis Ordbok. (2018). Skrei. Article access on the internet at

[9] Thuresson, G. (1922). The Species and the Variety as Ecological units. Hereditas III: 100-113

[10] Rollefsen, G. (1933). The otoliths of the cod. Fiskeridirektoratets Skrifter Havsundersøkelser 4(3) : I - 14 and 3 plates.

[11] Sick K. (1961). Haemoglobin Polymorphism in Fishes. Nature, 192, 894-896

[12] Fevolden, S.E., Pogson, G.H. (1995]. Differences in nuclear DNA RFLPs between the Norwegian coastal and Northeast Arctic population of Atlantic cod. In H.R Skjodal., C. Hopkins., K.E. Erikstad, H.P. Leinaas (eds): Ecology of Fjords and Coastal Waters, Proceedings of the Mare Nor Symposium on the Ecology of Fjords and Coastal Waters, Tromsø, Norway, 5-9 December, 1994. Pp 403- 427. Amsterdam: Elsevier Science

[13] Sarvas, T.H., Fevolden, S.E. (2005) Pantophysin (Pan I) locus divergence between inshore v. offshore and northern v. southern populations of Atlantic cod in the north-east Atlantic. Journal of Fish Biology 67: 444-469

[14] Star B., Nederbragt A.J., Jentoft S., Grimholt U., Malmstrøm M., Gregers T.F., Rounge T.B., Paulsen J., Solbakken M.H., Sharma A., Wetten O.F., Lanzén A., Winer R., Knight J., Vogel J-H., Aken B., Andersen Ø., Lagesen K., Tooming-Klunderud A., Edvardsen R.B., Kirubakaran G.T., Espelund M., Nepal C., Previti C., Karlsen B.O., Moum T., Skage M., Berg P.R., Gjøen T., Kuhl H., Thorsen J., Malde K., Reinhardt R., Du L., Johansen S.D., Searle S., Lien S., Nilsen F., Jonassen I., Omholt S.W., Stenseth N.C., Jakobsen K.S. (2011). The genome sequence of Atlantic cod reveals a unique immune system Nature 477: 207–210 DOI:10.1038/nature10342

[15] Berg P.R., Star B., Pampoulie C., Sodeland M., Barth J. M. I., Knutsen H., Jakobsen K.S., Jentoft S. 2016. Three
chromosomal rearrangements promote genomic divergence between migratory and stationary ecotypes of Atlantic cod Scientific Reports 6, 23246; DOI: 10.1038/srep23246

[16] Berg P.R., Star, B., Pampoulie, C., Bradbury, I.R., Bentzen, P., Hutchings, J.A., Jentoft, S., Jakobsen K.S. (2017). Trans-oceanic genomic divergence of Atlantic cod ecotypes is associated with large inversions. Heredity 119, 418–428

[17] Nikolaisen, K. (2017). – Kaffetorsken har eksistert i mer enn 50 år! Lofotposten 2017-15-03 Article accesses on the internet at

[18] Fjelldal, P. G., Jawad L., Bengtsson, K. E., Otterå H., Thorsen A. (2015). KONGETORSKEN: Skapt sånn eller blitt sånn? Havsforskningsrapporten Fisken og havet 1-2015

[19] Det Norske Akademis ordbok. (2018). taretorsk. Article accessed on the internet at

Tags: ecotype, codfish, variety, migration By Cecilia Helmerson edited after suggestions by Leonie Färber and Sissel Jentoft
Published Feb. 14, 2018 8:42 PM - Last modified Feb. 14, 2018 11:23 PM