A new report from the Norwegian Institute of Marine Research has found mackerel fry at the North Cape as the stock’s numbers explode. Photo: Norwegian HI
During last year’s ecosystem tour by the Norwegian Institute of Marine Research, their researchers found mackerel fry at the North Cape, “but it is not because the sea has become warmer”.
In a new report publicised by HI, they say, the mackerel’s breeding areas are mainly from the Bay of Biscay to the North Sea. But now it has been documented that mackerel also grows north of the Nordkapp.
“It is not because the sea has become warmer in the north but is connected with the explosive growth in the mackerel population between 2006 and 2014,” says marine researcher Leif Nøttestad.
The population almost tripled in less than ten years
From 2006 to 2014, the mackerel stock in the North-East Atlantic increased from 2.2 to 5.9 million tonnes, according to the researchers’ calculations. Almost triple that.
Warmer seas in the same period were probably a reason for good conditions and population growth, but not the reason for moving north.
“It was their own need for space with an internal fight for food that meant that the mackerel had to expand its grazing area in a big way,” says Nøttestad.
The adult mackerel subjugated the entire Norwegian coast and the Norwegian Sea, as far north as Svalbard.
“In such a climate, the mackerel is a very flexible fish. He thrives in everything between 4 and 20 degrees. But like everyone else, he needs food for survival and growth,” says the researcher.
Started spawning along the Norwegian coast
Mackerel have traditionally been a summer guest on grazing trips in Norway. He has had his spawning grounds further south – west of the British Isles. But from 2016 something happened.
“The mackerel also began to spawn in the Norwegian Sea and along the Norwegian coast up to Stad. At the same time, we began to observe a lot of pir, i.e. juvenile mackerel, further north than before,” says Nøttestad.
When Master’s student, now research technician, Vilde Regine Bjørdal therefore began to analyse the contents of the stomachs of captured pir on a research trip in Nordlege waters. She finely sorted half-digested food from 153 pir stomachs.
The question was: What kind of fish, and can they be competitors for the food of other fish.
May affect the ecosystem
Bjørdal’s work resulted in a master’s thesis and a recently published scientific article.
“The pir ate mostly the same as the adult mackerel, with equally full stomachs. This indicates that the moorhen is able to grow up and graze in the north,” says Bjørdal.
Mackerel eat mostly zooplankton – that is, tiny animals that float with the ocean currents.
The pir’s diet was small larval marsupials, compared to krill in adult mackerel. Other common prey groups were jumping crayfish and wing snails.
“Adult mackerel are not picky and eat what they come across. Based on the prey in the bellies of the pir, it is likely that they will do the same,” says Vilde Regine Bjørdal.
A new player in the ecosystem
The mackerel’s northern expansion broke new records last autumn. Then the researchers found mackerel fry at the North Cape during the annual ecosystem cruise in the Barents Sea.
Young mackerel establishing themselves in new places can in theory mean that there will be more competition for food. Especially for herring and other mackerel.
“We know that each mackerel lost weight in the same period as the population increased, and that there was probably less zooplankton in the sea in the same period. But it is difficult to establish whether one has led to the other,” says Leif Nøttestad.
The neighbouring mackerel was the biggest competitor
There is always greater competition for the available food within one and the same species, than against other species.
“The threat from the neighbouring mackerel has therefore been the biggest driving force for the stock to expand north in the first place.”
The researchers do not want to rule out that juvenile mackerel in new areas can affect the ecosystem, especially in fjords and bordering coastal environments.
“This is something we are working to find out more about,” concludes Nøttestad.