Future perspectives for cod in the eastern Baltic Sea – stock development, fisheries and management challenges in a changing ecosystem

The present report describes results of the project ”Future perspectives for cod in the Eastern Baltic Sea–stock development, fisheries and management challenges in a changing ecosystem (FREMTOR)”, funded by European Maritime and Fisheries Fund and the Danish Fisheries Agency. The project provides new knowledge on some key aspects of the status of the stock and associated drivers, as well as synthesizes existing information, altogether providing state-of-the art basis for evaluating future perspectives for this stock. Improved understanding of the factors responsible for the low productivity of the stock is crucially important for being able to assess its recovery potential. Furthermore, realistic targets for stock recovery in the current state of the ecosystem need to take account of changes in biological parameters, including growth and recruitment as well as other indicators of stock health, such as body condition and parasite load.

In this project, time series of various biological parameters of the Eastern Baltic cod (incl. condition, size at maturity, parasite load, length structure) were extended back to the 1940s, i.e. the beginning of intensive exploitation of this stock. These data show that the cod stock is presently in unprecedented poor state. Cod body condition and infection load of C. osculatum were in similar adverse states in the 1940s–1950s, however, the relatively more favorable states of other biological parameters back then indicate that the stock was overall in a better shape than is presently the case. The long–term data reveal concurrent trends in seal abundance and C. osculatum infection, the latter being inversely related to cod body condition, which provides correlation based evidence regarding potential associations between these variables. Spatial contrasts in the present state of cod are apparent in cod from the Åland Sea being in a substantially better condition and larger in size compared to the average specimen in the main distribution area of the stock. This is despite the much higher number of nematodes and intensity of infection of cod in Åland Sea with C. osculatum. This points at the importance of initial condition at infection time, potentially in combination with different dynamics of infection between cod from the two areas.

The prospects of the eastern Baltic cod stock depend largely on its reproductive potential and resulting recruitment, as well as growth. Growth patterns of the Eastern Baltic cod were investigated using advanced techniques based on otolith microchemistry that provided new insights on the effects of temperature and hypoxia. Concerning recruitment, we investigated the effects of parental quality on reproductive success and implications for cod recruitment. The most remarkable result is that the small-sized females were found to have an extraordinary high potential fecundity. Experimental results suggest that the small-sized spawners in poor condition are still capable of producing viable offspring. However, the high investment in reproduction likely occurs at the expense of growth, probably also leading to increased mortality, rendering the Baltic cod stock in a depressed state.

Fisheries management presently only has a limited impact of overall mortality of the cod stock, as natural mortality substantially exceeds the mortality due to fishing. A factor that could be influenced by fisheries management is the availability of fish prey (especially sprat) in the distribution area of cod, as the overlap between these species is rather limited in autumn. We found that overall fishing impact on sprat biomass in the distribution area of cod in autumn is relatively limited, however with occasionally relatively higher catches of the smallest sprat, which may be of essential importance for the small-sized cod.

The present exceptionally poor state of the stock on all metrics suggests that more drivers and mechanisms are having simultaneous adverse effects than has been the case in the past. This complicates Future perspectives for cod in the eastern Baltic sea our understanding of the recovery potential of the stock. The possible benefits to the cod stock resulting from management interventions influencing the ecosystem conditions is presently not possible to quantify. This is due to the complexity of the processes involved, as different parameters of stock health are interacting with one another, and are influenced by several external drivers, affecting recruitment, growth and/or mortality and ultimately stock biomass. Continued regular monitoring of a wider range of stock health indicators is recommended, and future stock assessment and management should consider multiple indicators of stock health, not only biomass, in defining overall stock status and appropriate management measures.