“A Fish That Changed the World: Cod Fish in the Past 100 Years”

“A Fish That Changed the World: Cod Fish in the Past  100 Years”
Cod fish in deep sea
Atlantic Cod (adult)

Codfish is the first commodity of international trade, and the great “geographical discovery” was also opened by it. At that time, the rich cod fishery was undoubtedly an offshore gold mine. From the beginning of the 16th century, Spanish, British, and French fishing boats embarked on an expedition to Newfoundland to fish for cod. Since the 1860s, Europe, especially the UK, has increasingly caught cod.

At that time, the main production area for European codfish was around Iceland. Many European countries frantically fished for cod in this area. They feared that the codfish resources on which they depend would be exhausted. As a result, a series of “wars” broke out due to the competition for cod resources launched between Iceland and the United Kingdom.

Geographical distribution of Atlantic cod
Geographical distribution of Atlantic codfish

The first “cod war”

Since Iceland’s independence in 1944, cod fishing has become Iceland’s main economic industry. To prevent fishing boats from other countries keep coming for codfish, Iceland unilaterally stipulated an exclusive fishing zone within 12 miles of the coast and sent its own navy to escort and evict foreign fishing boats.

The second “cod war”

More than ten years after the end of the first ‘cod war,’ the Icelanders felt that there was not enough codfish, so they demanded that the scope of the exclusive economic zone be extended to 50 miles. However, the international community has not recognized this plan. The Third “Cod War” In 1975, the Icelanders made another request to extend the no-take zone from 50 miles to 200 miles. (At that time, many countries proposed the “200-mile exclusive economic zone” to protect various resources along their own country’s coast). In the end, Britain was forced to accept Iceland’s request for a 200-mile exclusive economic zone. In the end, Iceland won! This is the famous “cod war” that began in 1958 and ended in 1976, spanning almost two decades.

Finding the solution

Finally, the European countries have defined an exclusive economic zone of 200 nautical miles through the mediation of the European Community. Why are you stroking the codfish alone? The Atlantic Ocean occupies two of the four most extensive fishing grounds globally: the North Sea fishing grounds and the Newfoundland fishing grounds. Not only there is a large number of fish, but there is also a wide variety of fish, and there are many options.

Why are Europeans preferring codfish? For a reason: 1. The protein content of cod is higher than that of regular fish, and the fat content is also lower; 2. Cod is large, usually about three feet in length, and full of round flesh; 3. Cod is straightforward to bite; 4. Cod migrate to lay eggs and are easy to catch; 5. There are very few cod bones, and it is an enjoyable taste. 6. Besides, cod is a religious food. Marinated dried cod is the only meat that can eat during the fast.

1. Introduction

Atlantic cod (Gadus morhua) is a kind of candidate cod from the order Codidae. It usually inhabits marine areas in cold regions such as the Arctic. As an important economic fish, annual catches of Atlantic codfish have been high value. Its high protein source and low oil source can be an excellent food material, top-rated among coastal residents and sportspeople.

As the demand for codfish increases, cod is in low supply for some time. However, what followed was the increase in the intensity of cod fishing and the warming of the climate caused by human activities, which made the living environment of cod worse. So how did cod react to these changes? What changes have taken place in the population?

Analyzing of otoliths

Recently, scientists from the Norwegian Institute of Oceanography Denechaud and others used otoliths from codfish to construct a century-scale growth timeline to explore the relationship between the growth of cod and environmental factors such as climate change, population dynamics, and population fishing. The results were published in the leading ecology journal, “Global Change Biology”.

The study used otolith data from 3,894 cod (Gadus morhua) in the Northeast Arctic Ocean (NEA) to analyze its growth and established a century-scale growth timeline (1924 -2014).

The results showed that:
  1. The growth sequence of 3894 codfish has an evident fluctuating trend;
  2. Codfish growth slows down as the population increases but will accelerate as the capelin population increases (capelin is an important food source for cod). This finding reveals that the main factor affecting the growth of fish remains its food source.
  3. Cod growth is also positively correlated with ocean temperature, but negatively correlated with a global climate index – the Atlantic Ocean’s multi-year oscillations. In other words, when considering the impact of climate change on fish growth, the effects of climate change on fish growth at different spatial scales must be taken into account.
  4. Fishing has a direct and significant impact on the development of cod. Based on the results of the path analysis, the study also found that there are also relationships between different environmental factors. There is a positive correlation between capelin biomass and ocean temperature, but a negative correlation with herring biomass (cod biomass is primarily affected by fishing mortality). In summary, this study directly or indirectly provides a benchmark for understanding how various environmental factors affect codon growth on an interannual scale.
Literature information

Title: A century of fish growth in relation to climate change, population dynamics, and exploitation

Journal: Global Change Biology

Author: Côme Denechaud

Organization: Norwegian Institute of Oceanography

Publication time: 02 August 2020 doi: 10.1111/gcb.15298

Journal of Global Change Biology

Article context

Article topic: The response of fish growth to various environmental pressures

Time scale: 1924-2014

Spatial scale: Northeastern Arctic Ocean

Environmental data: climate index etc.

Biological data: 3894 cod otoliths, fishing resources, etc.

Data analysis: mixed effects model, path analysis, structural equation model

Scientific question: 

How will future environmental changes affect marine ecosystems, especially those in high-latitude regions?

Cutting edge

Over the past century, the impact of climate change on the global marine ecosystem has become increasingly important. It even has severe consequences on physiology, survival, population dynamics, spatial distribution, and population recovery. The population and biodiversity of each individual in the ecosystem are susceptible to several threats. Among them, in different climate change prediction scenarios, the rise in temperature and warming of the ocean is a crucial factor in the study of climate change.

However, for fish populations, due to the lack of long-term biological data, it is quite tricky to predict the future response of marine ecosystems or communities to climate change through fish. Not only that, but many fish populations are also affected by other environmental factors, especially fishing. Although humans avoid catching small individuals by managing the mesh size, this still affects population continuity, leading to changes in fish populations’ life history and the number of fish populations.

These changes will affect the population’s ability to adapt to the environment, which will make the people more affected in the phase of environmental changes. Under various complex ecological stresses, it is particularly important to predict population changes under future climate change scenarios by estimating fish’s response to the historic environment.

Fish Growth

Fish growth is generally considered an essential indicator for studying the response of fish to environmental changes. By analyzing the differences in fish growth over time and space, a large number of studies have shown that it is closely related to temperature, the intensity of predation, fishing, and even changes. In some species (such as population density). If the growth of a fish population changes even slightly, whether it is the development of the individual or the survival rate, the reproduction rate, and the population’s survival rate have a significant impact. Therefore, the construction of the fish growth chronology is an essential biological data reflecting the fish population.

Fish growth data is usually derived directly from body length data of different ages, and then the theoretical maximum body length is estimated by the growth equation. However, in general, the growth process of fish will affect the internal (genes) and external (environmental changes) factors. These factors will affect the growth of fish by affecting the distribution of the energy supply. In general, the energy consumed by fish is used for metabolism, development, and reproduction. That is to say, the growth rate will directly depend on the energy requirements of various bodily mechanisms in an individual’s life.

2. Research methodology

  • 1. Source of otoliths: Institute of Marine Research (Bergen, Norway)
  • 2. Selection of biological and environmental factors and selection of fixed and random effects: Table 1 Description of the structural equation model of the mixed-effects model.
Description of variables used in the mixed effect of equation modelling

3. Study area

Map of the North-East Atlantic area and the Barents Sea eco-region.
Map of the North-East Atlantic area and the Barents Sea eco-region. The arrows represent the central circulation of water masses. The dark gray area represents the main cod spawning grounds of the northeastern Arctic along the Norwegian coast.

4. Cross-sectional view of otolith

Cross section of the Otolith of a cod fish
Cross section of the Otolith

5. Video of measuring otolith diameter Object: Marking Growth Rings in Fish Otoliths


6. Results

  1. Analysis of the mixed-effects model highlighted the influence of different environmental factors on codfish growth. The study of several climate change factors (temperature, North Atlantic fluctuation NAO, and Atlantic multidecadal oscillation) emphasized that the development of NAO and cod was weak. There is a clear correlation between ocean temperature and the multidecadal fluctuation of the Atlantic Ocean. The growth of cod positively correlated with ocean temperature but negatively associated with the Atlantic Ocean’s multidecadal oscillation.
  2. Second, the analysis of environmental factors related to the population (cod and capelin biomass) revealed that two species with a particular predation relationship between cod and capelin also significantly impact cod growth. It slows down as the population increases but accelerates as the capelin population increases. After considering the three environmental factors (climatic factors, population factors, and human fishing), Research discovered impressive results: the fishing fishery has a direct and significant impact on the growth of cod, but the fishing mortality affected the biomass of cod.
  3. This shows that although fishing has a low impact on cod growth, it will affect the entire population biomass in the ecosystem, leading to negative effects such as population decline. The structural equation model results revealed a positive correlation between capelin biomass and ocean temperature, but negatively correlated with herring biomass.
Unfiltered time series of mean increment widths for ages 2, 4, and 8
Unfiltered time series of mean increment widths for ages 2, 4, and 8 (a); the number of distinct age classes represented for all grades for each year (b)
Differences in observed and predicted growth widths for age
Differences in observed and predicted growth widths for age. The average widths of the observed increments (with standard deviation) relate to the age of training (a). Values ​​in blue highlights the total number of increments representing each age group in the timeline. The model predicted the mean width of increments (with 95% CI) as a function of age and sex (b)
Effects predicted with a 95% confidence interval of cod biomass
Effects predicted with a 95% confidence interval of cod biomass (a), capelin biomass (b), fishing mortality (c), temperature (d), and index d Atlantic multidecadal oscillation (e) on cod growth.
Models of structural equations (SEM) of climatic, population, and fishery factors with Northeast Arctic cod growth extracted from BLUP for the year's random effect.
Models of structural equations (SEM) of climatic, population, and fishery factors with Northeast Arctic cod growth extracted from BLUP for the year’s random effect.


As human activities become more frequent, environmental changes such as climate and fishing have impacted marine life, including fish. In fact, the ocean and fish survival in inland lakes and large rivers are not optimistic. In future research, humans need to explore fish’s historical response in different ecosystems to environmental changes and use existing biological data to predict fish’s response to environmental changes in future protection scenarios. Our largest ecosystem-the earth, make its own contribution.

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Reference :

DENECHAUD, C., SMOLIŃSKI, S., GEFFEN, A. J., GODIKSEN, J. A. & CAMPANA, S. E. 2020. A century of fish growth in relation to climate change, population dynamics and exploitation. Global Change Biology, 26, 5661-5678.

Rajitha Dissanayake

Researcher in the field of Marine Mammals and Bio-acoustic

I'm a master’s scholar in marine mammals and bio-acoustic laboratory, Institute of Deep-Sea Science and Engineering (IDSSE), Chinese Academy of Sciences (CAS) and focusing on the sustainable environmental application, conservation, and exploring all aspects of the ecology and behavior of marine mammals.

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