Cod were larger, older and more numerous at the time of the settlement

A groundbreaking new study by UI scientists has shed new light on the impact of 1100 years of fishing on the cod population around Iceland. The study reveals that cod in the 10th, 11th and 12th centuries were on average 25% larger and up to three times older than cod living today. However, cod during the settlement of Iceland grew at a much slower rate than now, since the size of the population had a significant impact on access to food. Increased fishing in the 14th century had an immediate effect on the size of the cod stocks – there is significant evidence suggesting that European fishing in Icelandic waters was much more extensive than previously thought. These findings were recently published in the prestigious journal Science Advances.

The team behind the study is multidisciplinary, but the principal investigator is Steven Campana, professor of marine biology at UI. The other researchers in Iceland include Guðbjörg Ásta Ólafsdóttir, biologist and director of the UI Research Centre in Bolungarvík, Ragnar Edvardsson, archaeologist at the same centre, and Árni Daníel Júlíusson, historian at the Vigdís Finnbogadóttir Institute and the Stefansson Arctic Institute, and Einar Hjörleifsson from the Marine and Freshwater Research Institute

Fishing began earlier than previously thought 

The team’s findings provide a unique insight into the natural cod stocks before fishing began and also how increased fishing had an immediate impact in the 14th century. Slower growth, for example, is evidence of a density-dependent impact – that is, because the population was so large in this period, competition for food was more intense.

“It was rather surprising to see that changes to the cod stocks began to occur as early as the 14th and 15th centuries, at which point fishing had increased to meet the demand for stockfish on the European markets. We can see that cod mortality rose,” explains Guðbjörg Ásta Ólafsdóttir, biologist at UI.

“We can calculate mortality from the age distribution of the population, since in a natural stock without fishing pressures, there are far more older individuals. This is an extremely important finding, both for understanding the scope of historical fishing but also because knowing the natural mortality of cod is vital for assessing the population today. Until now, it has been impossible to verify cod mortality before fishing began.”

Otoliths are like memory chips recording a cod’s life

This population ecology study, covering a period from the settlement of Iceland to the modern day, used otoliths (‘earstones’) from cod heads excavated at historical fishing stations. The findings reveal that cod around Iceland in the 10th century were much larger and older than they are today, which reflects limited fishing pressures during the first hundred years after the settlement as well as the immediate impact of fishing in the 14th and 15th centuries.

Guðbjörg Ásta explains that Steven Campana, professor of marine biology, joined the team with extensive experience of using otoliths in ichthyological research and it was this approach that really increased the scientific value of the project for modern ichthyology.

A large number of otoliths were discovered through archaeological excavation of old fishing stations. These stations were established at the time of the settlement and in some cases remained in use well into the 20th century.  Information gleaned from the otoliths has revolutionised what we know about cod population ecology in Icelandic waters going back as far as the settlement. 

Steven explains: “Otoliths are really like a kind of memory chip which we can read to get a record of the cod’s life. Cod and all other bony fish have otoliths, which are calcium carbonate structures in the saccule of the inner ear. Otoliths are used a lot in modern ichthyological research and are key to evaluating fish stocks.”

Guðbjörg Ásta agrees, adding that otoliths grow at the same rate as the fish themselves: fast in the summer and slowly in the winter.  Otoliths therefore form noticeable annual growth rings, which can be used to determine the age of the fish and its growth rate each year. 

Study originated in archaeological research on fishing stations in the West Fjords 

The story behind this study is long and fascinating. It all began with the doctoral research of the archaeologist Ragnar Edvardsson, looking at ancient Icelandic fishing stations, e.g. in Skálavík and Breiðuvík. Ragnar’s research sparked interest not only in historical cod fishing but also the hugely valuable organic materials discovered at these old fishing stations. 

“It was clear from my research that the development of Icelandic society was closely linked to changes in the cod population, at least from the later half of the 12th century,” says Ragnar. 

“So we needed to research the population ecology of the cod from the start of the settlement in order to evaluate how changes to the cod stocks affected Icelandic society. We also needed to try to get an idea of the extent of foreign fishing in Icelandic waters through history, since its impact on Icelandic society and the cod population has long been underestimated.”

Several studies have been conducted over the past decade looking at materials from the fishing stations and the sites have been thoroughly excavated in order to add to the collection of ancient cod otoliths, which have been the scientists’ main source of information. According to Guðbjörg Ásta, a major turning point for the project came when Steven Campana took on analysis of the otoliths. 

The role of historical research

A large part of the study is historical and this side of the research was led by Árni Daníel Júlíusson, a historian at the Vigdís Finnbogadóttir Institute and the Stefansson Arctic Institute.

“I managed the work of student historians as they created a database of written Icelandic sources about fishing, dating from up to around 1600. I also collected data from all kinds of studies that have looked at cod fishing around Iceland. I then analysed fish consumption in Iceland, possible exports of cod caught and processed by Icelanders, and from that I evaluated the extent of Icelandic cod fishing.”

Árni Daníel explains that it was important to estimate the population of Iceland and that as part of the study, he produced a new population estimate going up to 1700. 

“I also compiled various older estimates about European cod fishing in Icelandic waters, for example the work of Professor Poul Holm and his team in Dublin, as well as the historians Gunnar Karlsson, Jón Jónsson and others. Our research shows that we need to rethink all estimates about foreign fishing around Iceland, primarily after 1400. This will require a lot of new historical research, because we will need to go over all the sources about foreign fishing around Iceland between 1400 and 1900 and re-evaluate them, the impact on Icelandic society, and so forth. This study is also going to reveal a lot more, because there are all kinds of sources that are still being evaluated. Among the outputs will be two doctoral theses,” says Árni Daníel. 

Steven Campana believes that it will be fascinating to use the findings of this study to look back in time and see what cod fishing was like in Iceland at the time of the settlement. “There was so much cod at that time that if there had been a quota system, the quota would have been three times what it is today and fishing would have been much easier.” 

Main findings

The main findings from the study are as follows:

• Changes to size and age distribution: Cod in the 10th to 12 centuries were on average 25% larger and up to three times older than cod today.
• Impact of fishing: We can see the impact of fishing as soon as the export fishery started, with changes to cod stocks occurring immediately in the 14th century when fishing increased to meet demand for stockfish on the European markets.
• Growth rate: Cod in the 10th to 12th centuries grew 22% slower than cod today, probably due to the density of the population, which was much larger before fishing began. 
• Mortality: Since the study goes all the way back to a time when fishing pressures were insignificant, the scientists were able to evaluate the natural mortality of cod around Iceland. This is important, since natural mortality is used to evaluate current cod stocks. The research also showed that cod mortality started to rise immediately in the 15th century when fishing increased.
• Fishing pressures: The scientists were able to evaluate historical fishing pressures based on changes in fishing mortality rate. This indicates that historical fishing in Icelandic waters by other European nations was more extensive than previously thought.
• Environmental impact: Despite fluctuations in the climate, this study emphasises that fishing pressures rather than environmental changes have been the driving force behind changes to the cod stocks over the long term.

Roles within the research team

Steven Campana (UI) was the principal investigator and analysed the age and growth rates of individual fish. He also produced all statistical analysis and conclusions from this data.

The project originated in and uses materials from archaeological research by a group of archaeologists who are named as co-authors, in particular Ragnar Edvardsson (HÍ) but also Grace Cesario (PhD student at HÍ) and Lilja Pálsdóttir (Hólar University College).

Árni Daníel Júlíusson (Vigdís Finnbogadóttir Institute and Stefansson Arctic Institute) contributed analysis of the Icelandic population and likely domestic consumption of cod, which strengthened conclusions about the role of other European nations in fishing and the markets.

Einar Hjörleifsson at the Marine Research Institute contributed data on current cod stocks and related analyses.

The study was largely funded by an NSF grant led by George Hambrecht (University of Maryland) and Nicole Misarti (University of Alaska). Under their supervision, two PhD students worked on chemical analysis and other analysis.