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Reviewing stock assessment methods for Northeast Atlantic mackerel

We explore some of the key points from the report published today on the review of stock assessment methods for Northeast Atlantic mackerel.
Published: 4 April 2019

Atlantic Mackerel is a large stock that migrates seasonally from spawning areas west of Ireland to feeding grounds in the Norwegian Sea, around Iceland, and as far west as Greenland. Each year, ICES is requested to provide fisheries catch advice for mackerel in the Northeast Atlantic.

How is the stock advice determined for mackerel?

To provide the requested advice, ICES prepares an assessment of the stock size along with an estimate of the impact that fishing will have on the stock population dynamics.

ICES uses catch data, biological data (e.g. maturity or weight of individual fish), and surveys that indicate trends in the abundance of mackerel over time to prepare a stock assessment. With mackerel in the Northeast Atlantic, the large stock size and highly dynamic nature of its migration makes it difficult and expensive to conduct surveys that include the entire stock. For this reason, stock assessment scientists have employed innovative methods (such as the use of tagging data) to obtain information about the size of the stock.

The resulting stock assessment defines two reference points that are used to manage the fishery:

  • FMSY : the appropriate amount of pressure that the fishery can put on the stock to maximise catch sustainably
  • MSY Btrigger : the lower bound of a stock size, if the stock is to be fished  at FMSY

What is the latest ICES advice on mackerel?

When ICES conducted the mackerel stock assessment in 2018 using the best available data, the findings indicated that the stock size had dropped below MSY Btrigger. Accordingly, the catch advice for 2019 was lower than FMSY to allow the stock size to grow above MSY Btrigger more quickly. The resulting catch advice for 2019 (released in October 2018) was 42% lower than the catch advice given for 2018.

What has ICES published today?

ICES has published the results of a workshop that reviewed the methods used in the 2018 mackerel stock assessment.

In September 2018, mackerel stock assessment scientists found that Radio-frequency Identification (RFID) tagging data was having a stronger influence on the assessment results than when it was first used in 2017. The RFID time-series is relatively short (releases from 2011 onwards) and it was clear that the addition of one new year of data had modified the influence of the different data sources in the assessment. The stock assessment scientists recommended to ICES Advisory Committee (ACOM) that a review of the use of RFID tagging data in the stock assessment be conducted.

A series of expert meetings culminated in March 2019 with a review of the current methods at the Interbenchmark Workshop on the assessment of Northeast Atlantic mackerel. At this workshop, the configuration of the assessment model was changed to improve the use of the various datasets. The outcome of the review is that a new assessment method has been proposed to be used by ICES in the autumn mackerel stock assessment to provide catch advice for the stock in 2020.

Has ICES changed the 2018 advice on Northeast Atlantic mackerel?

No, ICES has not changed the advice.

Will anything change because of this workshop?

The recent workshop was initiated to further improve the knowledge base for advice. ICES acknowledges that the proposed new method for assessing the mackerel stock will likely change the perception of the state of the stock, both the size of the stock and fishing mortality, and the status of the stock relative to the reference points. The change in perception highlights the significant uncertainty, due to the quality and length of the available data series in the stock assessment of Northeast Atlantic mackerel. ICES is in conversation with fisheries managers about the implications of the results from the workshop.

How does a change in methods affect future mackerel assessments?

The newly proposed assessment method is an incremental improvement on the previous approach and reflects further progress towards understanding the dynamics of the mackerel stock. Based on an analysis of the performance of the model, using diagnostic data, the newly proposed method appears to make more appropriate use of RFID tagging data.

However, the mackerel stock assessment uses a number of short time-series and the assessment may remain unstable until longer time-series are available. The assessment also uses data from an egg survey that only provides new information every three years. In 2019, a new egg survey will take place, the results of which could potentially lead to further changes in the perception of the stock. Therefore, the proposed assessment method may also be subject to revision as the scientific process of improvement continually reviews the data, assumptions, and methods.

It is difficult to maintain stability in an assessment when introducing new surveys with short time-series. The current distribution of mackerel is far wider than previously observed, and the implications for the productivity of the stock are still being examined.

There are still important knowledge and data gaps preventing more stable management advice for the mackerel stock. To this effect, ICES will hold a workshop to develop a research roadmap for mackerel in May 2019. Stakeholders and fisheries managers have been invited to discuss management needs for the mackerel stock and how ICES can best inform this.

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‚Äčin March 2019, the Interbenchmark Workshop on the assessment of Northeast Atlantic mackerel reviewed the current methods used to compile the Northeast Atlantic mackerel stock assessment.

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Reviewing stock assessment methods for Northeast Atlantic mackerel

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