Salmon farming, climate change, predation by seals and overfishing have all been blamed for a decline in survival of Scottish wild salmon at sea. Only 5% return to spawn in their home rivers.
But now scientist Jens Christian Holst has pointed the finger at mackerel, arguing that they have multiplied and spread so much in Scotland’s waters that they are the greatest danger to wild salmon.
In a report in The Times, Holst, who used to work with the Institute of Marine Research in Bergen, Norway, says that there are at least 57 billion mackerel in the North Atlantic, more than six times the nine billion estimate of the Denmark-based International Council for the Exploration of the Sea (ICES), a global organisation based in Denmark.
Dr Holst believes that “super shoals” of mackerel are eating some young salmon as they migrate from Scottish rivers to their feeding grounds near Canada, Greenland and Norway, and outcompeting the survivors for food.
According to the theory the mackerel have also destroyed zooplankton and small fish in the Norwegian and North seas, affecting the survival of seabirds higher up the foodchain such as the kittiwake and puffin, which are considered vulnerable.
Holst’s theory challenges the accuracy of fish estimates by ICES, whose advice is considered when setting EU catch levels for mackerel. A survey by ICES last year suggested that mackerel stocks in the North Atlantic had reached a record 10.3 million tonnes, but Holst points out that large parts of the North Sea, Norwegian fjords and British waters were not included.
The coincidence of the explosion in timing, density and range of mackerel numbers with the sharp decline in returning salmon provides a possible and feasible explanation
The theory has the support of Tony Andrews, the former head of the Edinburgh-based Atlantic Salmon Trust, which is a long-time critic of salmon farming.
Holst and Andrews want research to be carried out urgently. If the theory is correct, a relaxation of EU quotas that would let fishermen reduce mackerel stocks in the North Atlantic could be necessary.
In a scientific paper given to MSPs, Dr Holst claims there is “strong evidence . . . [that] the northeast Atlantic mackerel stock has grown totally out of proportion due to gross underestimation, leading to overly cautious fishing quotas and underfishing as a consequence”.
Theory ‘a hard sell’
He cites evidence that mackerel have recently been observed at the southern tip and east coast of Greenland, and around Iceland, Jan Mayen Island and Svalbard, for the first time.
Holst told The Times that his theory “has been hard to sell” because it challenges the widely held notion that sea lice and escaped farmed fish are the biggest threats to wild salmon.
In a foreword to Holst’s paper, Andrews writes: “The collapse in numbers of salmon (to about 5 per cent) returning to their native rivers points to a massive mortality at sea. The coincidence of the explosion in timing, density and range of mackerel numbers with the sharp decline in returning salmon provides a possible and feasible explanation. It deserves to be investigated.”
Holst’s paper can be viewed here.