Improving knowledge base for management of cod stocks in the Baltic Sea and in the Kattegat

The project ”Improving knowledge base for management of cod stocks in the Baltic Sea and in the Kattegat” focused on the key challenges for assessment and management of the Eastern and Western Baltic cod and cod in the Kattegat. The project was funded by European Maritime and Fisheries Fund and the Danish Fisheries Agency.

The activities in the project involved providing new biological knowledge, advancing methods, and conducting data analyses and modelling. For the eastern Baltic cod, a new method for estimating age and growth based on otolith microchemistry was validated, which can in future replace the traditional age readings that are known to be problematic for the stock.

A new approach for determining cod fecundity was also developed. This improves information of cod reproductive potential as well as alternative spawning stock biomass estimates from egg production methods, also applied in this project. The work of the present project as well as earlier related projects was synthesized to contribute to ICES benchmark in 2019 where analytical stock assessment for the eastern Baltic cod was re-established.

Furthermore, the project demonstrated that future possible recovery of the eastern Baltic cod depends on ecosystem drivers affecting stock productivity, and communicates new challenges for management associated with ecosystem changes and the complexity of factors affecting cod. For the western Baltic cod, the project investigated ways to deal with stock mixing in survey data, and provided a currently optimal solution to this that is used in ICES stock assessment today.

The impact of assumed historical recreational catch values on stock assessment was also investigated. The results demonstrated that it is beneficial for assessment quality to include recreational catch information in the assessment even when good quality data are available only for a few latest years.

Impact of uncertainties in different data inputs (amount and age structure of discards and recreational catch, mean weight and maturity, stock mixing) on stock assessment and advice was also explored, based on the example of western Baltic cod. These analyses increased our understanding of the relative impacts of possible inaccuracies in different data inputs and their annual updates on stock assessment and associated management advice.

For cod in the Kattegat, the project provided new genetic information on mixing of North Sea and Kattegat stocks that improves our understanding of spatio-temporal variations in stock mixing. These data on stock mixing are utilized in developing a stock assessment model within this project, which is a modification of the present SAM model.

The stock assessment model was modified to take into account inflow of juvenile cod from the North Sea and estimate their return migration when the fish become mature. This facilitates evaluating status of different stock components. The project additionally provided new knowledge on the impacts of supporting fisheries management measures such as spawning closures and regulations of fisheries selectivity.

We concluded that if spawning closures are applied, these should cover most of the stock distribution area during most of the spawning season, to avoid possible counterproductive effects. For western Baltic cod, understanding of recruitment processes was enhanced, for example, the recent stronger 2016 year-class was found to be likely related to high abundance of favourable food for larvae.

Spawning closures do not appear to be among the dominant factors affecting year-class strength in Western Baltic cod. The project also investigated fisheries selectivity effects on reduction in abundance of larger Eastern Baltic cod in later decades. The results showed that fisheries likely contributed to this development in some years, however other factors must be more influential as the larger individuals disappeared simultaneously from fisheries and survey catches.