​​​​Since 2023, ICES has assessed Northern Shelf cod as three substocks: northwestern, southern and Viking. The available scientific evidence shows that these are distinct populations of cod that spawn separately and may mix to some extent during the rest of the year.

However, the current lack of knowledge on how these stocks mix across time and space makes it difficult to implement stock‑level advice which maximises catches from substocks in good condition and enables rebuilding of substocks with low biomass. Gaining a better understanding of the spatio‑temporal mixing dynamics would help to implement stock-level advice and also refine the assessment model.

The advice issued today aims to address this gap in knowledge and charts an operational pathway towards area-based management of Northern Shelf cod substocks. 

“By detailing how genetics can be used to identify the three Northern Shelf cod substocks and how these data should be sampled, this advice provides a clear implementation path," says Vanessa Trijoulet, a senior researcher at the Technical University of Denmark (DTU) Aqua and the Danish representative on ICES Advisory Committee.

“With coordinated sampling and adequate funding, the resulting data will improve our advice in the short term while laying the foundation for robust area‑based advice in the future."

This advice is based on the work of the GenDC project and two ICES workshops the Workshop on Scoping Data collection for Northern Shelf cod sub-stocks (WKCODSCOPE) and the Workshop on Sampling Data for Northern Shelf cod sub-stocks (WKCODSAMPLING). The results from GenDC showed that genetics data can be used to distinguish between substocks and this informed the development of a genetics sampling plan.

Plan of action

ICES advises a multiyear approach is required to move towards area-based advice. Genetic sampling could be complemented by other approaches, such as tagging, to improve understanding of the substock migration and mixing.

The timeline begins with pilot studies, to start ideally this year, which should collect genetic samples from current biological sampling programmes on both catches and fisheries-independent surveys.

The outcomes of these pilot studies, together with a simulation study incorporating alternative assessment models and management strategies, should guide the design of a routine, multiannual genetic sampling programme to ensure the continued availability of the data required for effective stock assessment and advice.

The timeline outlined in the advice lays out a pathway that will take a minimum of three years to complete and depends on adequate funding and resources.

Why do we need more data?

Mark–recapture tagging data for North Sea cod collected between 1961 and 2015 were re‑analysed and indicated limited migration outside the substock areas. However, the spatial coverage of the tagging programme was restricted, there is no information on which substocks the tagged fish originated from, and no new data have been collected since 2015. These limitations reduce the usefulness of the dataset for informing current assessment models

Short-term steps

ICES advises that, in the short term (while the pilot studies are ongoing), full genome sequencing could provide both new insights into stock mixing and essential information to refine advice for Northern Shelf cod and revise the proposed timeline.

This approach is likely not cost-effective in the long run, but could inform potential spatio-temporal management measures while the pilot studies are still under way. For example, it could lead to ICES providing more specific advice to avoid fishing in certain areas.​

Read ICES advice regarding genetic work on Northern Shelf cod substocks.