Cod is an iconic species for the Baltic Sea, both as a top predator in the ecosystem and as a source of income for fisheries. Reliable estimates of the variation in population size and demography are necessary for sustainable management , and information of the cod's age is a vital piece of information for stock assessment.
"We expect relatively high recapture rates, and when the cod are recaptured and returned, they will provide us with real-life growth rates for different areas and age groups of cod"
Karin Hüssy, Project coordinator
Traditionally, age determination of cod is done by examining the otoliths, or "earstones." The otoliths grow continuously throughout the fish’s life forming annual rings that can be read in much the same way as a tree's rings. In the Baltic Sea however, the readability of the otoliths has deteriorated in recent years.
The interaction of deteriorating hydrographic condition (primarily increased occurrence of hypoxia), prey availability and parasite infection are considered the primary reasons for this phenomenon. These factors result in overall poor growth of the fish which is reflected in the visual appearance of the otolith. As a consequence, The International Council for the Exploration of the Seas (ICES) is not currently able to provide an analytical stock assessment for the management of the Eastern Baltic cod stock. The objective of the tagging project is to solve this problem for a better fisheries management.
Vital information for sustainable management
Tagging is one of the most reliable methods to validate age determination and estimate growth rates in wild fish. Funded by the BalticSea2020 Foundation, DTU Aqua is leading the international project Tagging Baltic COD (TABACOD), which aims at providing reliable growth and mortality estimates through a large-scale tagging program. The project will also develop and validate an alternative age estimation method based on otolith chemical composition. Together with Sweden, Germany and Poland, Denmark will be tagging 18.000 cod in the southern and central Baltic Sea during 2016-2018.
Project manager Karin Hüssy, Senior Research Scientists at DTU Aqua says:
“Cod tagging programs have been performed before in the Baltic Sea, but never at such a big scale or with the ultimate aim to improve stock assessment for management purposes. We expect relatively high recapture rates, and when the cod are recaptured and returned, they will provide us with real-life growth rates for different areas and age groups of cod. Information that is vital for a sustainable management.”
Additional expected results of the tagging-project are improved knowledge on cod migration patterns including the extent of stock mixing between Eastern and Western Baltic cod stocks, and a independent estimation of fishing mortality.
Tagging cod on board DTU's vessel "Havfisken" in June 2016. Together with Sweden, Germany and Poland, Denmark will be tagging 18.000 cod in the southern and central Baltic Sea during 2016-2018.. Photo Line Reeh.
Internal tagging of otoliths
During the project, around 18,000 cod will be caught and marked with traditional external tags and chemical marking of the otolith. Recapturing externally marked fish allows measuring the cod's growth but length measurements are generally affected by high imprecision as they vary depending on whether the fish is alive, dead or frozen. Otolith growth is a good proxy to estimate the fish’s somatic growth. In order to validate otolit growth patterns, the otoliths will be marked with an internal chemical tag concurrently with the external tag. The chemical tag is deposited on the surface of the growing otolith and persists over the entire lifetime of the fish, thereby enabling the researchers to estimate the otolith's growth from release to recapture, and thus the fish's actual growth.
A further 1,000 cod will be marked with data storage tags (DSTs), which register the depth and the water temperature which the cod are exposed to. This makes it possilbe s to determine the fish's movement patterns and behavior in the sea, factors that are known to affect fish growth.
A new age estimation method
A second objective of the TABACOD project is to develop a new age estimation method which can be used to determine age and growth in historic as well as future samples of cod in the eastern Baltic Sea. This method is based on seasonal patterns in the chemical composition the otolith, which will be validated using otoliths and data from the tagging program.
The International Council for the Exploration of the Sea (ICES) is not currently able to provide an analytical stock assessment for the management of the Eastern Baltic cod stock. One of the key issues that prevents understanding the present status of this cod stock is the lack of reliable age information and growth rate estimates, which hampers the use of analytical stock assessment methods. Since 2013, this data has been so uncertain that ICES has moved to recommending that the European Commission set fishing quotas for the eastern cod population according to the principle of caution, which means not increasing the previous year's quota by more than 20 percent so as to avoid overfishing. However, this principle does not ensure that fishing stays within sustainable limits. As a consequence of the missing analytical stock assessment, the Marine Stewardship Council, which certifies sustainable fisheries, suspended its sustainability certification of Baltic Sea cod. Reliable estimations of age and growth rate will allow ICES to provide member states and the European Commission with advice based on definite data, promoting a viable cod stock in the eastern Baltic Sea.