NATURAL MORTALITY HIGHER THAN EXPECTED FOR MACKEREL

Natural mortality higher than expected for mackerel – small changes for nvg herring. Researchers have revised the methods for calculating the populations.

Author: Øystein Rygg Haanæs.,  Norwegian Institute of Marine Research (NIMR) 

The International Council for the Exploration of the Sea (ICES), has just revised the methods for estimating the stock of north-east Atlantic mackerel and Norwegian spring-spawning herring (NVG herring).

Here you can find the new benchmark report from ICES. 

According to the revised methods, the spawning stock of mackerel has been larger than in previous stock assessments – approximately 60 percent higher.

“The major changes for mackerel primarily concern the historical stock level. The current stock situation has changed less, and the negative development of the spawning stock in recent years is still very clear,” says Are Salthaug, marine scientist and stock manager for mackerel.

Strong recruitment of mackerel in the period 2007-2016

“The revised approach shows even more clearly that mackerel recruitment was strong in the period 2007-2016, especially in 2012-2013, and that it has fallen to a lower level in recent years. When recruitment becomes lower without sufficient reduction in catch, the spawning stock falls sharply,” says Salthaug.

The researchers believe that the revision provides a higher quality stock estimate.

“The revision of the population when new years of data are added, the so-called retrospective pattern, is significantly better in the new population calculation,” says Salthaug.

Revised regularly

In a method review, marine scientists thoroughly review both the data included in the stock assessments and the assessment methods themselves.

Such reviews are carried out every few years for most stocks on which ICES provides advice. The researchers then take into account available, updated scientific knowledge in all relevant areas.

“The previous method review was in 2019 for mackerel and in 2016 for nvg herring. After a review, the methods and data sources agreed upon in the annual stock assessments are used until the next review,” says Salthaug.

Changed assumptions about mortality

The change in methodology that has the greatest impact on the assessment of the mackerel stock is that the assumed natural mortality is changed. This is all mortality in the stock that is not due to registered fishing, which is very difficult to calculate.

“Previously, natural mortality for mackerel was assumed to be 14 percent per year for all ages. This was based on historical data from tagging and recapture,” says Salthaug.

Now, a higher natural mortality is assumed, which also varies with age. It is highest for the youngest individuals and lower for older mackerel.

“The change is based on recently published knowledge (see reference) and is considered to be more realistic. This is related to both the mackerel’s high maximum lifespan of approximately 23 years, and to the fact that small and younger individuals are more susceptible to being eaten by others,” explains Salthaug.

The level of fishing mortality – mortality due to registered fishing – is adjusted downward by 26 percent on average for mackerel.

Here you will find more detailed information about the revision of the methods and data basis for mackerel stock assessment.

Small changes for nvg herring

The revision of the methods did not have a significant impact on the stock assessment of Norwegian spring-spawning herring.

“The changes led to minor adjustments to the trend in the spawning stock and fishing mortality, but not to the stock level itself,” says Erling Kåre Stenevik, marine scientist and stock manager for NVG herring.

Here you will find more detailed information about the revision of the methods and data basis for stock assessment of nvg herring.

No new quota recommendations for 2025

Although the methods have been revised, ICES will not issue new quota advice for mackerel and nvg herring for the current year.

“Although a revision may provide a changed picture of how the stock and mortality have developed over time, it is not common for ICES to update quota advice for the current year,” says Salthaug. The focus of a methodology revision is on the historical changes in the stock, not on the current situation.

The next quota advice for mackerel and nvg herring will apply to 2026, and will come on September 30 this year.

References

ICES. 2025. Benchmark workshop on Mackerel and Norwegian spring-spawning herring (WKBMACNSSH). ICES Scientific Reports. 7:64. 509pp. https://doi.org/10.17895/ices.pub.29279615

Lorenzen, K. 2022. Size- and age-dependent natural mortality in fish populations: Biology, models, implications, and a generalised length-inverse mortality paradigm. Fisheries Research 255: 2-13

Image: Catch of mackerel south of Jan Mayen. Illustrative photo: Leif Nøttestad / Norwegian Institute of Marine Research (NIMR

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