Sustainable management of Kattegat cod; better knowledge of stock components and migration

This report describes the main findings from the EMFF project “Bæredygtig forvaltning af Kattegat-torsk; bedre viden om bestandskomponenter og migration” (J. nr. 33113-B-16-034) which was initiated to improve our understanding of stock mixing and migration in Atlantic cod in the Kattegat in order to improve the data for stock assessment and management.

The stock assessment of the Kattegat cod has recently been challenged due to a large “unallocated mortality”, i.e. a large fraction of fish that disappears from the area, but cannot be explained by mortality due to fishing or natural causes. It has been hypothesized that migration between the Kattegat and the North Sea could explain some of the unallocated mortality.

The major scientific questions we aimed at answering in the project were: 1. Do cod from the North Sea and Kattegat mix within the Kattegat stock assessment and management area? If they do, then 2. During which time of their life do North Sea cod enter the Kattegat? 3. Are there any spatial patterns of mixing and are there indications of different magnitudes of mixing for different age classes? 4.

Do cod migrate between the Kattegat and North Sea? The project had three main components: Genetic analyses to identify population of origin of fish, chemical analyses to identify individual fish migration histories and finally integration of results with stock assessment and the development of future procedures for data collection and stock assessment in Kattegat cod.

We used a unique combination of methodology that allowed us to both identify the population of origin, and hence mixing of populations, with high spatial and temporal resolution, and the migration trajectories of individual fish. Genetic data revealed that North Sea and local Kattegat/transition zone cod indeed co-occur (mix) within the Kattegat, and that there is a gradient in mixing proportion from high proportion of North Sea cod in the northern parts of the Kattegat to lower proportions in the south.

We also found variation between ages and sizes of fish with higher proportions of North Sea fish among the younger/smaller fish. Chemical signatures in the cod otoliths also suggested that North Sea and Kattegat early juvenile stages have different environmental, and thus geographic, origins, and that the North Sea fish enter the Kattegat as larvae or juveniles and leave at approximately age 4, presumably for spawning.

Collectively, the results indicate that North Sea cod enter the Kattegat as early life stages and migrate back to the North Sea when they reach sexual maturity. Analyses of spawning fish confirmed that spawning activity in the southern Kattegat is dominated by fish of local origin. Results from our study have improved our understanding of the complex interaction of different biological populations with the Kattegat stock assessment and management area.

The project has thus contributed directly to improving the basis for sustainable management of cod in the Kattegat, in particular through the identification of future data collection and stock assessment strategies that are able to take migration into account. The project results have been communicated to the scientific community, including the ICES advisory working group, as well as stakeholders from industry and management at several occasions in Denmark and abroad.