The finalised report of the international mackerel cruise 2021 shows a decease in mackerel but an increase in blue whiting biomass. Photo: HI Norway
Marine scientists have now summarised the findings from this summer’s mackerel cruise with the main result revealing less mackerel being registered from last year.
The international mackerel ecosystem cruise in the summer of 2021 in the Northeast Atlantic was conducted with five vessels from four countries in less than five weeks from 30 June to 3 August 2021. The main purpose of the cruise is to provide an annual age-based quantity index with uncertainty estimates for Northeast Atlantic mackerel.
Another important purpose of the cruise has been to establish a new time series of quantity indices for both blue whiting and Norwegian spring-spawning herring.
The cruise survey that was included in the calculations for the mackerel quantity measurement index was 2.2 million km2 in 2021, which is 24 per cent less than the survey in recent years from 2018 to 2020. This is due to less geographical distribution of mackerel in the western areas. Cruise mapping was reduced in western areas such as Greenland waters, the Icelandic basin south of 62 ° 45’N and Reykjanesryggen south of 62 ° 45’N were not mapped in 2021.
Furthermore, 0.29 million square kilometres were mapped in the North Sea in July 2021, but these stations are currently not used in the calculation of the mackerel index, partly due to too short a time series (2018-2021).
The volume index was significantly reduced in 2021 by 58.5 per cent in biomass (tonnes) and 53.4 per cent in number of individuals compared with 2020. Reduced cruise coverage in the western areas did not contribute to the decline since the zero line for mackerel was established in north, west and south of Iceland.
The most numerous year classes based on the cruise were the 2019, 2016, 2017 and 2012 year classes, respectively. Internal consistency between the different year classes was somewhat reduced compared to last year, especially for 5-8 year old mackerel, when the 2021 index was included in the time series (2010, 2012-2021).
Mackerel were most widespread in the central and northern Norwegian Sea and in the North Sea in 2021, which is similar to 2020, with low densities and limited distribution in Icelandic waters.
The mackerel biomass in the North Sea almost doubled in 2021 compared to 2020. The zero limits for mackerel distribution (where it stops being mackerel) were found in most parts of the cruise area, with the exception of the north-western areas in the Norwegian Sea, southern borders in the North Sea and west of The British islands. The mackerel appeared less dense distributed within the cruise area in 2021 compared to the period 2012-2017 and 2019-2020.
The total number registered of Norwegian spring-spawning herring during this year’s cruise was 19.6 billion individuals and the total biomass index was 5.91 million tonnes, which is at the same level as in 2020 (20.3 and 5.93, respectively).
The 2016 year class (5 years old) dominated the stock and contributed 54 percent and 59 percent of the total biomass and total amount, respectively, while the 2013 year class (8 years old) contributed 13 percent and 11 percent of the total biomass and total amount, respectively.
The 2016 year class is expected to be fully recruited to the adult population in 2021, and in addition fully recruited to the cruise area. The cruise is assumed to contain the entire adult part of Norwegian spring-spawning herring during the mackerel ecosystem cruise in 2021. Blue
The total biomass of blue whiting registered during the cruise in 2021 was 2.2 million tonnes, which is an increase compared to 2020 (1.8 million tonnes).
The population estimate for 2021 is 26.2 billion individuals, compared with 16.5 billion of the age groups 1+ in 2020.
The age group 1 year dominated the estimate in 2021 (59% and 69% in biomass and in number, respectively, for age groups 1+). A good signal of recruiting 0-group was also observed during the cruise this year.
Overlap between herring and mackerel
As in previous years, there was an overlap in the spatial and temporary distribution of mackerel and herring. The overlap took place between mackerel and North Sea herring in large parts of the North Sea and in the southwestern most part of the Norwegian Sea.
There was also some overlapping distribution of mackerel and Norwegian spring-spawning herring in western, north-western, and north-eastern parts of the Norwegian Sea.