There is no doubt we have witnessed an increase in the north Atlantic mackerel stock in recent years and that the stock has occupied new areas. Furthermore, salmon stocks have declined and increased mortality at sea is seen as the main driver for the decline.

Against this background, Dr Jens Christian Holst has presented an hypothesis that overgrazing and predation by mackerel are major factors behind dwindling salmon stocks.

The Marine Institute has examined Dr Holst’s article and makes the following comments on his hypothesis focused on the salmon and mackerel perspective.

JCH presented an early version of this hypothesis on overgrazing and predation at the Atlantic salmon Ocean Silver Conference in 2011.  At the time there was an expectation that a set of field experiments / desk top assessments to test the hypothesis would be undertaken, leading to a publication in the scientific literature. However, there does not appear to be a peer reviewed publication that supports the Holst hypothesis.

Published scientific research has shown that predation and a drop in the abundance of both the quantity and quality of plankton may be important factors affecting the survival of salmon at sea.

Other published works have demonstrated a correlation with sea surface temperatures and mortality of salmon at sea.   These published works appear to have been rejected by Dr Holst whose own hypothesis has gone through several iterations, the latest of which proposes that burgeoning mackerel stocks may be a key factor in the decline of salmon survival at sea.

The hypothesis assumes that since mackerel can predate on smolt-sized mackerel, then salmon smolts must be predated on.  There is no hard evidence for this, except in the common area of sea where both fish occur during certain periods of their migrations.

The extent of this overlap and whether there are behavioural differences that segregate migrating smolts from migrating mackerel are not clear. Experimental trawling for salmon smolts in 2008 and 2009 has shown that salmon smolts can be taken in trawl hauls with both very few mackerel and large concentrations of mackerel.

The current approach by salmon scientists has been to work with colleagues in the Atlantic and the Pacific, to examine all sources of ocean mortality that can be identified.  This approach has been outlined in a recent Atlantic Salmon Blue Book which summarises the conclusions and recommendations of an international Workshop held in Edinburgh in November 2017.

This approach places candidate mortality factors within an overall spatial and temporal framework, covering the full lives of salmon at sea. Such an approach could help to quantify the potential of each factor to influence survival (identify the likely suspects).

These factors would be linked dynamically in such a way that the cumulative effects of these factors could explain the variations in survival of different year classes of salmon.

Scientific hypothesis

A key objective here is to establish a testable scientific hypothesis about the factors influencing salmon survival. It is also planned to learn more about how these factors influence marine survival and how best to target research to further refine the current estimates of the scale of mortality at each part of the marine phase of the salmon’s life cycle.

A particular focus would be on identifying where and how mortality factors had changed between earlier periods of higher marine survival (60s and 70s) and the more recent/current low survival.

There are several assertions in the Holst article that we feel are incorrect and undermine his hypothesis.

The article asserts that underestimation of the mackerel stock size led to ‘underfishing’ and caused the stock to expand. No time period is identified as to when this underestimation occurred. If it is therefore a longstanding feature of the mackerel assessment, one might logically assume that the underestimation and thus the expansion of the stock should have occurred since the start of the assessment and TAC setting for the stock in the 1980s. This is not the case.

The narrative further continues that the expanded mackerel stock then ‘overgrazed’ the food resource, leading to competition with and predation on salmon post-smolts. The reason for this, it adds, is that the ‘southern salmon populations’ are more depleted than the ‘northern populations’,  is that the former have to migrate through a greater volume of predators.

Ecosystem modelling

If this was true, then the decline in all Atlantic salmon populations would follow the same timing (just at a different rate) and their decrease in abundance would be coincident with the increase in the mackerel population. This is not the case.

Ecosystem modelling of the wider north Atlantic to investigate predator-prey linkages is only at the very early stages. Even in areas where there is good knowledge of the effects of a predator fish on a prey species (as in the Baltic Sea), the information has not been used to adjust stock assessments, as scientists feel they do not have enough information for even the simple and well-studied Baltic Sea ecosystem.

The narrative concludes by claiming that a ‘solution’ to the situation of low salmon abundance would be a ‘cull’ of the mackerel population, to bring the ecosystem of the north Atlantic back within ‘normal’ range.

Mackerel and salmon have co-existed in the north Atlantic for millennia before fishing ever started, so the idea that fishing is the solution to an ‘ecosystem imbalance’ is difficult to accept because it suggests ‘lack’ of fishing has caused the mackerel stock to expand.

Overall, we feel the article takes some empirical observations at a relatively small temporal and spatial scale and draws conclusions of cause and effect on an ecosystem scale of the north Atlantic. The cause and effect relationships are based on simple correlated occurrences, which is not any form of proof, but simply conjecture. As such the article is the opinion of an individual rather than the result of a study that draws a scientifically rigorous conclusion.

The general feeling among many scientists in relation to the Holst hypothesis is that it needs to be developed and tested through the normal research channels in order to assess the extent to which the suggested impacts from increasing mackerel populations are affecting the overall marine survival of wild Atlantic salmon.

This testing is very important as it provides the scientific community with the opportunity to challenge the hypothesis based on data and analyses rather than conjecture in the public press.

Extractions from Dr Holst’s article:

As a marine fisheries scientist, I have worked closely on the marine ecology of salmon and the factors affecting marine survival of Atlantic salmon since 1991. Based on my ecosystem-based research in the NE Atlantic, I have developed the hypothesis that overgrazing and predation are major factors behind dwindling salmon and sea bird stocks of western Europe.

“As a marine fisheries scientist, I have worked closely on the marine ecology of salmon and the factors affecting marine survival of Atlantic salmon since 1991. Based on my ecosystem-based research in the NE Atlantic, I have developed the hypothesis that overgrazing and predation are major factors behind dwindling salmon and sea bird stocks of western Europe. 

Based on what I consider to be strong empiric evidence, the NE Atlantic mackerel stock has grown totally out of proportion due to gross underestimation, leading to overly cautious fishing quotas and underfishing as a consequence.” 

Holst believes that it is because of this “very large mackerel stock, the food resources of whales, seals, sea birds, salmon, other pelagic fishes and the mackerel itself are now heavily overgrazed. Today, a 7-year-old mackerel weighs half of its weight of 10 years ago — a clear sign of the overgrazing and lack of food.”

Some scientists claim temperature and climate change is the culprit, but in my view there is no empiric basis for such a conclusion.  If we study water temperatures in the main feeding area of ‘southern’ European post-smolts in the Norwegian Sea, they rose from the 1970s to 2007 and have now dropped to close to or below normal, according to the Institute of Marine Research (IMR) in Norway.  

“The temperatures in the Atlantic water along the Norwegian continental shelf have since 2013 been close to or slightly above normal. The temperatures in 2016 were mainly above normal, except the south-eastern Norwegian Sea were the temperatures were lower than normal. Climate change will probably lead to higher temperatures at the peaks and troughs of the coming cycles, but I expect the cycling to continue as documented in sedimentation layers on the seabed since the last ice age of 10,000 years ago. Consequently, there is no correlation with temperatures and the Irish salmon stock collapse but there is very good correlation with the growing mackerel stock and its potential for competition and predation on the Irish post-smolt salmon.

Holst believes this situation will continue to worsen until the heavy  competition and predation by mackerel is reduced:

The reduction [of mackerel stocks] should be done through an internationally agreed and closely monitored thinning fishery on mackerel, where some of the extra catch goes into meal and oil. Not to give the pelagic fishermen higher quotas but to bring the ecosystem of the north-eastern Atlantic back within ‘normal’ ranges where both salmon and seabirds dependent on plankton and small fishes will return to stock levels where they belong.

He concludes with the question:

What is most likely to kill a northward bound 15 cm Corrib postsmolt salmon today —temperatures close to normal or a starving mackerel?

Dr Paul Connolly