Professor Harald Yndestad writes (here): ‘The Kola-section data analysis revealed, for the first time, that Arctic climate variations are controlled by the earth’s rotation and the moon’. He refers to ‘the riddle of a 6-year cycle in the cod population’ and comments: ‘My estimates reveal we are at a turning point and moving into a colder period and more ice extent.’ The 6-year cycle reminded me of a Talkshop post featuring a paper by astronomer Willy de Rop, in which he wrote, with illustrations: ‘We will now consider how often such a situation of maximum tides will occur. The perigee moves 0.164 358 002 0 a day relative to the node, corresponding to 360° in a period p 2 190.340565 days. If at the same time this moment of maximum influence coincides with the moment at which the Earth is in the perihelion of its orbit, the tides will reach an absolute maximum. So, when the perigee of the Moon’s orbit coincides with the ascending node, then this situation repeats after 2190.340565 days. This period p corresponds to 5.996 667 350 anomalistic years, thus nearly an entire number of anomalistic years.’ In other words, almost six years. It’s the beat period of the (lunar) anomalistic and tropical months, also of the full moon cycle and draconic year. Whether this plays a part in the cod discussion is an open question.
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In nature, nothing acts alone, writes Prof. Harald Yndestad. Therefore, something outside the cod stock, causes recruitment in periods of 6 years.
The source may be a 6-year temperature cycle in the Barents Sea. If the temperature in the Barents Sea has a period of 6 years, it must also have a source outside the Barents Sea. This means that there must be a first cause. A cause of causes, for temperature variations in the Barents Sea.
So, what is periodic in nature? Could the source be the tides or in the earth’s rotation? I contacted an astrophysicist.
He was not familiar with a 6-year cycle. But says that there is a period of 18.6 years in the earth’s rotation . At the same moment, I imagined a possible explanation. The 6-year period can be a 3rd harmonic period. If the temperature in the Barents Sea has a period of 18.6 years, the biomass will be able to adapt to a temperature variation of 18.6 years, by recruiting in periods of 18.6/3 = 6.2 years.
The sea temperature at the Kola-section
I e-mailed PINRO institute in Murmansk. The question was whether they have temperature records from the Barents Sea. To my surprise, I immediately got back a temperature data series monitored in the Kola-section.
Russian scientists has been measuring the temperature at the Kola-section, every month, for 100 years. Except during the Russian Revolution and during World War II. Now, this was the world’s longest oceanographic data series. Thanks to their work, I could get an answer to the riddle of a 6-year cycle in the cod population.
The problem was that, until now, no one had found anything but random variations, in this temperature data series.
. . .
If it turns out that the temperature in the Barents Sea is driven by predictable tides, it may form a basis for better predictability of future sea temperatures, and better sustainable ecosystem management.
Full article here.
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Image: Barents Sea [credit: Norman Einstein @ Wikipedia]
Tallbloke's Talkshop 
Are tree rings a reliable proxy for temperature? Probably not, they depend on too many variables. Is a cod population a reliable proxy for temperature? For tides? Can black magic explain it? Can science explain it?
Recent paper:
The Effect of the 18.6-Year Lunar Nodal Cycle on Steric Sea Level Changes – 17 April 2024
Plain Language Summary
The orbital position of the moon and the gravity pull it exerts on the earth varies with a period of 18.6 years. This cycle is called the lunar nodal cycle and it results in small variations of yearly averaged sea level (∼1–2 cm). Understanding this variability is important because it allows, for example, to quickly detect an acceleration in local sea-level rise due to global warming. Here we show that the lunar nodal cycle also has an influence on the temperature and salinity in the surface 400m of the ocean. As a result, the ocean density changes and amplifies sea level variations along the western European coast. We make the hypothesis that since the lunar nodal cycle also influences the amplitude of the semidiurnal tides, and since those tides are known to be responsible for a large part of ocean mixing, a change in ocean mixing could be the cause of the ocean density variability that we observe. [bold added]
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL106563
“Can science explain it? No of course not, but an additional input of funding will allow the researchers to get a little bit closer and discover ever more correlations.