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Three fishermen photographed in Ballstad (1910), each with his own large skrei.
Photo: Norsk folkemuseum / Anders B. Wilse
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From “the welfare of the people” to scientific time series

Article in Fram Forum 2017: Time series are crucial tools for environmental monitoring. The Institute of Marine Research, in cooperation with the University of Bergen, has developed what may well be the world’s longest biological time series – a 160-year record of liver size in northeast arctic cod.

New version. Clarifies inadvertencies around Hjort and Rollefsen’s compiling of data.

Each year the northeast arctic cod swims in from the Barents Sea towards the coast of northern Norway to spawn. The Lofoten Islands are among the best spawning grounds for the cod Norwegians call “skrei”. The name comes from a word that means to stride and indicates that the cod wander over great distances. In Lofoten, the skrei practically come ashore during the spawning season, making them easily accessible for all types of vessels. For this reason, generations of fishermen from all along the coast have been coming to Lofoten to harvest this valuable resource.



Spawning skrei come very close to shore and can therefore be caught from all kinds of vessels, including these traditional boats manned by four or five rowers.

Photo: Norsk folkemuseum / Anders B. WilseCopyright Norsk folkemuseum



In 1859, Ketil Motzfeldt, then Inspector of Fisheries, began recording statistics on the turnover of liver and fish during the Lofoten fishing season. Several fishing regulations had recently disappeared following amendments to the Lofoten Act in 1857, and Motzfeldt started his registration for the “welfare” of the people whose livelihood came from fishing. Later, the statistics proved important for marine scientists, precisely because Motzfeldt registered both fish and liver. This was also the beginning of official fishery statistics in Norway, statistics that have been modernised on several occasions. At present, the Norwegian Fishermen’s Sales Organisation (Råfisklaget) is responsible for ensuring that the skrei catches are registered and systematised. The organisation registers statistics on all white fish landed between Nordmøre and the Russian border.



A haul of skrei delivered in Senjahopen.

Photo: Gunnar Sætra


Hepatosomatic index

Motzfeldt’s registrations were never intended for anything but statistics. Johan Hjort, on the other hand, had more farsighted goals. He was a pioneer of marine research and ultimately became the first director of the Institute of Marine Research (IMR). When he started up his own measurements of the relationship between the amount of liver and the amount of fish, his objectives were scientific.Hjort believed that the size of the fish’s liver was indicative of its condition. Cod species storefat in the liver: a fatty liver tells us that the fish is in good condition whereas a low fat content means poorer condition. Hjort based his work on registrations from 1880 until 1912. During these years, it was gathered significant amounts of data about fish and their livers, which provided an important data point each year: a hepatosomatic index, that is to say, the total weight of liver divided by the total weight of fish. (Nowadays this is expressed as a percentage and is usually used to describe liver size of individual fish rather than group averages.) This index actually tells us something about the condition of the fish. The figures range from as low as 2-3% to as much as 15%. Fish with a high hepatosomatic index are in good shape, whereas a low hepatosomatic index indicates the opposite.

In 1914 Hjort published his seminal work, the groundbreaking treatise “Fluctuations in the great fisheries of northern Europe”. It concluded about twenty years’ work and documented for the first time that fish population sizes do in fact fluctuate naturally. At the time, natural fluctuations, as opposed to fishing, was the most important reason  why catches varied from one period to another.



Karl Erik Karlsen (left) and Kjell Arne Gamst taking samples from skrei delivered to Axel I. Hansen in Senjahopen. General manager Frank Magne Hansen (right) watches with interest.

Photo: Gunnar Sætra


Various types of data were standarised

Johan Hjort’s collection of fish and liver data is an important part of the time series developed in Bergen, showing the hepatosomatic index from 1859 to the present. The scientists have combined Hjort’s time series with official fishery statistics – the statistics Motzfeldt started recording in 1859 and which are still being registered. They have also used data that are not dependent on fisheries, such as samples taken during the IMR’s annual skrei expedition from the middle of March to early April. In addition, the scientists have used data from a time series for 1882 to 1928, compiled by Gunnar Rollefsen at IMR, in which he compared length and weight data for skrei. Samples taken from skrei catches delivered to the fish industry have also been included. This sampling started in 1932 and has continued up to today, apart from a hiatus in the 1970s.

In order to make the nearly 160-year time series, the marine scientists had to coordinate and standardise the data from the various types of statistics, time series, and other methods of data collection. For example, the liver size in Motzfeldt’s registrations and Hjort’s time series was measured in litres of liver per 1 000 fish. Later, official statistics operated with litres of liver in proportion to gutted weight. Since 1991, the relationship is measured as kilos of liver per kilo of gutted fish. To obtain a standardised hepatosomatic index for the whole period, the scientists used mathematical models. In this way, the different data were coordinated and the result a time series that stretches from 1859 to the present. We now call this time series “Hjort’s Hepatosomatic Index”.



Overview of data sources from the annual Lofoten cod fishery and research monitoring programme.

Upper panel: Fisheries Statistics data on the relative amount of liver and fish, including the data from Hjort, presented either as litres of liver per thousand fish or litres of liver per kilo of gutted fish. The more recent Råfisklaget series reports kilos of liver per kilo of gutted fish, and the research survey data present kilos of liver per kilo of whole fish.

Lower panel: Supplementary data sources of information on gutted weight and length.


Temperature and access to food


As mentioned above, the hepatosomatic index for skrei tells us whether the fish is in good shape or not. This may in turn say something about the natural conditions the skrei has lived in from year to year. For example, there are indications that the temperature in the sea affects the skrei’s condition. We can see this if we compare the hepatosomatic index with another long time series, the so-called Kola Section. The Kola Section is a straight line on the map from the coast of Kola northwards into the Barents Sea. Russian marine scientists have measured the temperature and salinity of the sea along this section since 1900. When we compare findings from Hjort’s hepatosomatic index with temperature measurements along the Kola Section, we see that the fluctuations in the hepatosomatic index have mainly followed the fluctuations in temperatures in the Barents Sea. In other words, the skrei’s condition improved when the temperature rose, whereas it worsened in periods with falling temperature. Temperature and hepatosomatic index correlated well until a few years ago, when something changed and the hepatosomatic index went down at the same time as the temperature went up. This shows how difficult it can be to predict the consequences of global warming and highlights how important it is, for research, that long time series are not broken.



Marine biologist Ragni Olssøn shows off a skrei during the expedition of 2012.

Photo: Gunnar Sætra


Hepatosomatic index and political unrest

In 1903, the hepatosomatic index was at an all-time low. History books and other sources relate that few skrei came to the coast that year. The fishermen perceived a “black sea”, with very little fish. At that time, there was also extensive whaling along the coast of northern Norway. In Finnmark, the biggest whaling companies had built land

stations to receive and process whales from the Barents Sea. The fishermen believed it was the whales that frightened the skrei towards shore. Therefore, they also believed that widespread whaling was to blame for the absence of skrei that year. In Mehamn, the animosity against whaling was so strong that the fishermen destroyed the whaling station. It would be an exaggeration to claim a link between Hjort’s hepatosomatic index and political unrest, but the events of 1903 show that the skrei was (and is) an important factor in the financial well-being of northern Norway – and hence also a political factor.


Subsequent analyses indicate that the fishing crisis of 1903 was caused by the collapse of the capelin population. Lack of capelin also led to an invasion of seals along the coast because the Greenland seal in the Barents Sea could not find food in its ordinary feeding grounds. And the skrei were not doing well either, as is clear from the unique 160-year time series known as “Hjort’s Hepatosomatic Index”.


Read more about time series here.