New research claims that it has found little support that herring and mackerel are to blame for the decline in salmon stocks
A paper by Norwegian Marine Scientist Erlend A. Lorentzen concludes that new research has found little support that herring and mackerel are to blame for the decline in salmon stocks.
In recent decades, there have been far fewer salmon in the Atlantic. Research carried out in the North East Atlantic over the last number of years claimed the finding of salmon smolts in the stomachs of predatory species like mackerel, who even eat their own kind.
As recent as January 2020, The Business Post reported that prominent fisheries expert Jens Christian Holst said mackerel are underestimated as a predator despite swimming like “torpedoes” and forming huge shoals similar to pests such as locusts.
He said: “When smolts come into the ocean they are really hunted down. The mackerel is a tremendous feeder. They are incredibly opportunistic. They are a bit like grasshoppers. They spread all over the ocean.
“And from the very moment they enter the sea, the salmon smolts are potential prey for the starving mackerel. You have the biggest concentration of mackerel in the migration path of the smolts.
“A large mackerel of 50cm can eat a smolt of 20cm. In my view the mackerel is the major player in the decline of salmon.”
One of the theories that has attached itself to this is that as mackerel and herring stocks increased, stocks of wild salmon has declined, a theory propagated by Dr Holst as far back as 2018.
In a new article, researchers have combined stomach analyses with statistics without finding support for this theory.
No connection detected
“We found little overlap in the diet for salmon on one side and herring and mackerel on the other. In addition, we found little geographical overlap between herring and salmon, and also no connection between the fluctuations that the various stocks have had,” sums up marine scientist Kjell Rong Utne.
Over nine research expeditions in 2008 and 2009, the researchers analysed the stomach contents of 750 juvenile salmon, 678 mackerel and 204 herring caught between Ireland and Svalbard. They compared the diet between individual prisoners in the same area and period.
The livery varied
All the fish ate some seaweed fleas and krill. But mainly herring and mackerel are aimed at redfish and small zooplankton. The salmon mostly ate fish larvae.
“We were surprised at how different diets the species had, especially fish caught in the same area at the same time. Several times in the same trawl hall,” says Utne.
The mackerel stock is estimated to be up to 1000 times larger than the number of Atlantic salmon in the sea. Therefore, only a small overlap can be thought to make a big difference. And the mackerel stomachs were not completely free of fish larvae.
“In an area with small to medium-sized zooplankton, such as redfish, the mackerel caught other prey. For example, sandeel larvae west of Ireland and Scotland. These are also an important prey for juvenile salmon in the same area, so here the competition for food may be higher than in the Norwegian Sea.
“But this is an area that both mackerel and juvenile salmon migrate quickly through in early summer,” he adds.
How many fish were to be found at the same time and place was also part of the research.
Salmon and mackerel overlap, but not salmon and herring
The researchers used catch data from 170 trawl halls in the Norwegian Sea to create statistics for how likely it was to find salmon where you also found mackerel and herring.
“Salmon and Norwegian spring-spawning herring overlap little geographically. Mackerel and salmon were partly to be found in the same area, but in the summer some of the salmon were somewhat further north than the mackerel,” the researcher said.
The rise for mackerel was not the same as the decline for salmon
Finally, the researchers used statistics from the period 1982-2017 to look for a connection between the number of salmon that came home to the river, and the stock size of herring, mackerel and blue whiting.
Would a recovery in the other pelagic fish species cause a decline in salmon?
“We found no significant correlations when it comes to the decline in salmon access in the five European rivers we checked,” Utne explains.
Will not exclude competition
On the other hand, they found a negative correlation between the spawning stock of NVG herring and the total amount of returning salmon in northern Europe.
“In periods when the herring population increased, fewer salmon returned to the rivers. But it is difficult to establish a causal relationship. It may be due to competition, but also other factors that we have not investigated,” he says.
Do not expect parallel fluctuations in stocks
The researchers conclude that they have not found evidence that can support the theory that smaller salmon are due to competition from herring and mackerel.
“We do not rule out that this may be the case but will not expect major changes in the salmon stocks if the pelagic stocks should fluctuate up or down,” says Utne.
Will look more at whether the picture may have changed
“”Competition can also take place locally in special areas, and the picture can of course also change over time,” he continues.
While the stomach analyses in this study are from 2008 and 2009, the mackerel stock really picked up after 2010. Since then, HI has routinely taken stomach samples of herring, mackerel and salmon on annual research trips.
“We have prepared the salmon stomachs, and for herring and mackerel a large material is available for new analyses. Further research could provide a stronger basis for saying something about diet and competition has varied over time,” says Utne.
Through the SeaSalar project, the researchers succeeded in investigating competition between species. (Source)