The Norwegian Institute of Marine has found that when the temperatures in the seas gets warmer the greater the likelihood of smaller herring

Norwegian fisheries researcher find that warmer seas means smaller herring. Photo: Norges Sildesalgslag/Roar Bjånesøy

The Norwegian Institute of Marine (HI) has found that when the temperatures in the sea gets higher, the herring grow less.

“It shows a unique times series on Norwegian spring-spawning herring,” writes HI.

In two new studies, HI researchers have examined a time series with data on Norwegian spring-spawning herring that goes back 80 years. The goal was to find out what affects how the herring grows.

“We discovered that there was a connection between temperature and growth in adult herring. When the temperature in the water is higher, the herring grows less,” says researcher Fabian Zimmermann.

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Before the 1980s, they also saw the same trend in young herring. But after that, they saw no signs that temperature affected the growth to the same degree as with adult herring.

The scales tell how the fish has grown

By analysing data from the scales of herring, the researchers can find out how old the fish was and how the fish have grown throughout their lives.

In the sscales and ear stones (otoliths) there are rings that can be compared to annual rings in trees, where a ring represents one year. The rings are made because fish do not grow evenly throughout the year, but faster in summer and slower in winter.

If a scale ring in the fish is particularly large, it means that the herring grew a lot that year. And vice versa if the ring is small.

Advantages and disadvantages of large data series

Although long time series with data may seem like a luxury for researchers, Zimmermann points to the challenges when the time series goes very far back in time.  

“We saw, for example, that the connection we saw between temperature and fish growth became less clear the further back in time we went,” says Zimmermann.

“This means that the results depend on which time series we are looking at. We can see different connections if we look at 50 or 20 years back in time.”

He explains that in such long time series, there will always be a complex interplay of different natural fluctuations and factors that make it difficult to capture the one major influence that explains everything.

“Instead, some factors may be more or less important, depending on the condition of the stock and the ecosystem. Therefore, one should be careful about drawing the major conclusions from short time series that are often used in fisheries research.

“There are many things that affect the growth of the fish. Age and maturation play a big role just as they do for all animals, and young herring generally grow better than adult herring. But here we also saw that there are factors such as climate change and warmer temperatures affect the growth of fish,” says Zimmermann.

The growth of the fish is affected by several things

Another study in the same time series shows that the relationship between the size of the scallop and the size of the fish is affected by both the environment and the growth rate, ie whether a herring grows quickly or little. It also varies over time.

The ring size in sscales or otoliths is today often used to calculate the growth of fish over time. It is usually assumed that the relationship is constant over time and within a cohort, i.e., fish born in the same year, but this is a little more complicated, says researcher Florian Berg.

Larger stocks produce less herring

Norwegian spring-spawning herring is one of the largest stocks in the Northeast Atlantic. In the 1960s, the stock collapsed due to overfishing and poor recruitment. From there, it took about 15 years before the population recovered.

After this, the stock has varied. In 2008, it was at a record high level before a clear decline the following decade.

The size of the stock can also affect how the herring grows. Researchers assume that in years when the population is extra-large, herring will have access to less food and grow less. However, in their study, the HI researchers did not see any connection between the population size and growth.

“We saw that the size of the stock has a stronger effect on fish size than on scale size. That is, even if the herring becomes smaller when the population grows, it still retains the same scale size. This may explain why we did not see any connection between stock size and growth, says Berg. 

“These two new articles emphasize that herring growth is a result of both the environment and how much food the herring has access to in the ecosystem through its long migrations, especially when the fish are still small. “

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