A new assessment method reveals a larger and healthier mackerel stock

Gepubliceerd op
29 juli 2014

The method and data used by scientists at ICES to estimate the size of the Northeast Atlantic mackerel stock has been re-evaluated last spring. This so-called benchmark assessment was a 6 month effort by scientist from the EU and from the northern mackerel fishing countries. The previous method dated from 2007 and had a number of weaknesses, related to both the scarcity of the data used, and the calculation methods. This resulted in increasing problems through the years, until eventually the method was abandoned last year.

The new method is based on a more modern calculation software and incorporates more sources of information. Both improved the reliability of the mackerel estimation. In addition to the mackerel egg survey, already used in the past, new scientific survey data are used for the abundance of young fish (bottom trawl survey on the western European continental shelves) and for the abundance of old adults (pelagic trawl survey in the Nordic seas). The assessment of mackerel now also incorporates tagging-recapture information from Norway. This proved to be useful to compensate for the bad quality of commercial catches information for the period before 2000.

This new assessment gives us a new perspective on the history of the mackerel stock and on its current state. First, the stock appears to be larger than previously estimated (figure 1). It was already known by the scientists that the previous method gave a too low estimation of the stock. This was also pointed out by the fishing fleet. Thanks to the use of all new data, the new method gives a more realistic estimate. The adult stock is now estimated to have varied between 2 million tonnes in the late 1990s and early 2000s and 5 million tonnes in the recent years. With the previous assessment, the minimum was at around 1.6 million tonnes and the maximum at 3 million tonnes (figure 1). In addition, the new assessment indicates that the fishing pressure has been continuously decreasing since 2003, and that the stock is now being underexploited. The former model indicated an increasing fishing mortality since 2010, and an overexploitation of the stock. This means that despite the strong increase in catches in the recent years, the current exploitation level is not threatening the stock.

ICES recently updated their catch advice for 2014 to reflect this new state of the stock. The scientific advice is now to fish between 927 kt to 1 mt in 2014. However, the managers had already set their quotas for 2014, which sum up to around 1.35 mt. The coastal states (EU, Norway and Faroes Islands) have agreed to catch a little over 1mt, Russia and Greenland have announced at catch limit of 100 kt each, and Iceland set its own quota at nearly 150 kt.

ICES currently gives advice on the basis of a long term management plan, which was adopted by the managers in 2008. With the new perception of the stock, this plan is still consider to be safe for the stock, but no longer results in optimal catches. For this reason, ICES is currently working together with the stakeholders, on a revision of this plan which should be completed in the autumn of 2014.

The management of mackerel also faces new challenges, with the ongoing expansion of the summer distribution. The latest scientific observations indicate that, after extending their migration to Iceland in 2008, mackerel are now migrating as far as eastern Greenland or Svalbard. Similarly, changes in the distribution and migration are also observed when mackerel are spawning in spring. Spawning now starts one month earlier that in the past, and extends further north and further west.

Scientists don’t have firm proofs yet, but they believe that these changes may be caused by a combination of changes in the environment, such as increasing temperatures, and the effect of a very large stock, in which fish have to move further to avoid competing with each other. That mackerel have to compete more with each other for their food is also a possible explanation for the decrease observed in the size of the fish in the recent years compared to the past. More investigation is needed on the ecology of mackerel and new studies are being planned in collaboration with the fishing industries. Getting a better knowledge on what triggers the changes in the mackerel stock, and how permanent or reversible those changes are, will also help the stakeholders making decision on the management of the mackerel stock.

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Figure 1 : The new mackerel assessment (red line, with confidence envelop) estimates a larger stock compared to the old one (black line). Fishing mortality in the recent years is now thought to be lower than previously estimated, and below the new Fmsy value (horizontal line).

Thomas Brunel, IMARES Wageningen UR

Thomas Brunel (37) comes from France, where he studied fisheries science and completed a PhD on fish recruitment and climate change. He has been working at IMARES since 2007. At this moment he is member of the ICES assessment group on widely distributed stocks (WGWIDE), where he is responsible for assessment of the Northeast Atlantic mackerel.