Advice explainer: Why are precautionary considerations applied to the northwestern and Viking cod substocks?

The Northern Shelf cod assessment has created new and complex advisory and management dilemmas.
Published: 27 September 2023

​ICES released advice on 2024 fishing opportunities for Cod in the North Sea, West of Scotland, eastern English Channel, and Skagerrak on 19 September 2023. This advice includes precautionary considerations for Viking and north-western sub-stocks. 

Previously, advice was given separately for the West of Scotland cod stock and the North Sea cod stock. In 2023, based on biological data and available catch data, these two stocks were merged into one (Northern Shelf cod stock). In turn, this has been divided into three reproductively isolated substocks (northwestern, Viking, and southern cod substocks). These substocks are isolated during spawning season (January–March) but are assumed to mix throughout the rest of the year. Outside the spawning season, with the current available tools, it is impossible to distinguish which of the substocks is caught in each area. 

The southern substock is estimated to be at a low biomass level and requires a stronger reduction in fishing mortality to follow a maximum sustainable yield (MSY) approach. Theoretically, catch advice can be calculated for each substock independently, however in practice, this advice cannot be implemented at substock level as all three substocks are caught together throughout most of the year.

As a consequence, the Advisory Committee considered that it was more appropriate to provide combined advice for all three substocks and to implement the relative fishing mortality reductions required for the southern substock to the Viking and north-western sub-stocks as well. ICES has not provided independent catch advice for the three individual substocks and has instead implemented a type of mixed-fisheries approach.

Is it possible to provide separate advice for each substock?

The Advisory Committee considers that it is unrealistic to have separate fisheries outside the spawning season as cod are caught throughout the year in mixed-demersal fisheries. 

To implement independent catch advice for each substock, real-time genetic data is needed. This would assign catches to substocks in real time and provide better information about the mixing of the substocks throughout the year.

Why have three substocks if the advice is merged?

At present, knowledge of the substocks dynamics is limited and only allows a multi-stock approach in the spawning season. This represents a step forward, as it provides an estimate of the size of each of the three substocks. For the rest of the year, the substocks are combined into a single stock, which results in similar fishing mortality trends over time and supports the application of a mixed-fisheries approach to the stock complex. Because fishing mortality trends are not independent, a 60% reduction in fishing mortality is needed for all three substocks to protect the southern substock, which is the weakest.

What is the trend and status of Northern Shelf Sea cod?

ICES assessment that the stock size of Northern Shelf cod generally increasing in recent years is in line with the observations of the Scottish fishing industry. The northwestern substock is in good condition, above MSY Btrigger and increasing, the Viking stock, while below MSY Btrigger, is also increasing, and the southern stock is below Blim but also increasing. A key issue is that fishing mortality for all three substocks has been declining but remains above the fishing mortality associated with maximum sustainable yield (FMSY) and needs to be reduced.

The new assessment for Northern Shelf cod represents progress in terms of acknowledging the biological complexity of cod sub-populations in the area. It does, however, create new and complex advisory and management dilemmas. ICES has implemented precautionary considerations in its advice which is appropriate and in line with international policy requirements.

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Advice explainer: Why are precautionary considerations applied to the northwestern and Viking cod substocks?

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