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Photo: Kjartan Mæstad
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Continued cooperation on the survey on Norwegian spring-spawning herring (NSSH)

As last year, the Institute of Marine Research, in cooperation with the fishermen, carries out a survey on Norwegian spring-spawning herring. Tuesday 2 February, the vessels “Libas”, “M. Ytterstad” and “Vendla” started a two-week long survey to map the herring stock during spawning migration.

This year, the industry suggested covering a wider area on the basis of observations of overwintering herring in the ocean west of Norway. Accordingly, Norges Sildesalgslag (the Norwegian herring sales organization) has funded the hire of the vessels “Midøy Viking”, “Nystrøm”, “Selvaag Senior” and “Trønderbas”. These vessels will map the distribution of the western migration route in cooperation with “Pelagisk dugnad” (pelagic voluntary work), but coordinated by the Institute of Marine Research. Hence, the spawning survey can be carried out more effectively by a limited western coverage.

Sildefangst
Photo: Aud Vold

Up or down?

After several good years, there has been a reduction in the quotas on NSSH. Since 2009, the size of the stock has been reduced. It is now below the precautionary approach-limit, when it is time to reduce the quotas to secure the stock. Hence, the last years ICES (The International Council for the Exploration of the Sea) has recommended reduction in the herring quotas.

The basis for ICES’ recommendations has been the international ecosystem cruise, which is carried out in May every year, together with catch data from the fishermen. But the reduced advice on quotas has not matched the fishermen’s experience of how much herring there is in the sea. 

- Best ever?

To get a better basis of data to estimate the stock size, the spawning cruise was carried out last year. For many years, this cruise was a central part of the herring monitoring, but was stopped in 2008, just before the reduction of the herring stock started.

Last year’s resumption of the spawning cruise was in cooperation with the fishermen. The Norwegian herring sales organization covered the costs of hiring three fishing vessels for two weeks in February 2015, set to map the herring’s spawning migration and size of the spawning stock. The Institute of Marine Research supplied scientist and technicians to the ships.

The cruise resulted in a thorough coverage of the herring stock in time and area, probably the best ever, says scientist Aril Slotte at IMR.

Analyzing herring

Cruiseleader Aril Slotte (right) taking samples.

Photo: Kjartan Mæstad
 

However, the fishing industry was quite critical. They argued that there might be herring outside the covered area, caused by overwintering herring further west. This critic has persisted, and the industry, headed by “Pelagisk dugnad”, should be recommended for having found some of this herring and confirmed by commercial fishery that it is NSSH.

Excited by the western migration route

As last year, Arill Slotte is the cruise leader on the spawning survey. He also coordinates the pre-trials with search vessels. In his opinion, this year’s survey is exciting because of the uncertainty of the importance of the western migration route and the unique possibility to estimate this.

Data from the 2015 spawning survey was analyzed and judged together with all other information the Institute of Marine Research and other research institutions have collected on the stock through several decades. The results contributed to an increase in the recommended quotas from ICES from 283 013 tonnes in 2015 to 316 876 tonnes in 2016. 

Method

This year, focus will be on sonar registrations of herring near the surface and the herring’s migration speed. This may give a good basis to correct any effects of migration on the estimate. There will also be carried out more measurements on target strength through the survey. This is part of the work towards a more absolute measure of the stock size, rather than a relative index as used in today’s stock estimates.

Erling Kåre Stenevik

Erling Kåre Stenevik. 

Photo: Kjartan Mæstad

Expectations

We expect good coverage of the herring stock, and hope that the survey this year will be carried through with results that also will satisfy the industry, says Slotte.

The Institute of Marine Research expects in general that data for the benchmark meeting in ICES in Copenhagen in February/March have undergone quality control. In a benchmark testing, all methods and data will be quality secured and undergo a critical control. Until this is carried out, there is no foundation for estimates of the stock based on this survey alone. There is uncertainty linked to stock asssesments, and, therefore, the examination of the data must be on a scientific basis. Information from single data points cannot be given too much weight. 

To learn and correct

In science it is a good rule that you base your hypotheses on a scientific basis before you say anything  with certainty. If new data or results show that our estimate of the spawning stock biomass needs to be corrected, we will do that immediately, says Erling Kåre Stenevik, responsible scientist for the stock of Norwegian spring-spawning herring (NSSH).
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"Ligrunn" (left) and "Inger Hildur" (right) in Ålesund - ready for the herring survey. 

Photo: Aril Slotte