Tagging Baltic Sea cod (TABACOD)

Background and Objective

The age of a fish is an influential variable in fisheries biology. Stock assessments which form the basis of management advice use fish age as a key input parameter when calculating estimates of catch, abundance and recruitment. For the eastern Baltic cod stock, age of individual fish is currently estimated using the traditional method of visual analysis of “annual otolith rings”. The otolith (a bone inside the inner ear of the fish) grows throughout the life of a fish, forming annual rings that can be read in a similar way to tree rings. However, the annual rings of eastern Baltic cod otoliths are notoriously difficult to interpret due to irregular growth patterns and low contrast between the seasonal growth zones, and these difficulties have dramatically increased in recent years. The current method for age-reading of eastern Baltic cod is now so unreliable that in 2014 the traditional age-based stock assessment failed due to the lack of high quality input data.

Not only is reliable age information important for stock assessments, it is also key to understanding the current status and condition of the stock. Since 2013, there has been a noticeable lack of larger cod in the eastern Baltic Sea. This lack of large individuals is likely to be driven by either high mortality of older individuals, slow individual growth or some combination of these two processes. However, without accurate estimates of age it is impossible to reliably disentangle the influences of growth and mortality on the status of the stock. This uncertainty hinders the selection of management measures appropriate for responding to this worrying stock development.

There is currently an urgent need to calculate accurate estimates of growth and age of individual eastern Baltic cod, for present and future management. The objectives of the TABACOD project are to provide the data necessary to calculate reliable estimates of growth rates for this stock, and to develop tools which can be used to objectively and continuously derive growth and age information in the future.

Approach

The task of obtaining accurate growth and age estimates for eastern Baltic cod will be tackled by using two interlinked methods: mark-recapture of individual fish and analysis of the chemical composition of their otoliths.

The mark-recapture method is a widely-used and relatively accurate method for calculating the growth rate of wild fish. This method involves marking fish with easily identifiable tags, and then releasing them back into the wild. When these fish are re-captured estimates of growth rate can be calculated based on the change in length of the fish and the length of time at liberty. Three types of tag will be used to tag eastern Baltic cod in the TABACOD project. (1) External tags will be used to identify individual fish and to provide information on recapture time and position. (2) Data storage tags (DSTs) will be used to collect data on depth and temperature experienced by individual cod, important factors which can influence fish growth. (3) Fish will also be tagged with internal chemical tags which deposit a permanent, visible mark on the otolith that corresponds to the time of tagging. As otoliths grow proportionally to the fish, growth can be back-calculated by comparing the distance between the tag mark and the outer edge of the otolith with the distance between the tag mark and the centre of the otolith. Over the 4 year duration of the TABACOD project, thousands of cod will be tagged in a huge international effort between Germany, Denmark, Sweden and Poland. Combined with historical data on Baltic cod growth, this will ensure a large sample size from which to calculate reliable growth estimates.

Additionally, state-of-the-art micro-chemical analysis of otoliths will also be conducted in an attempt to develop new methods for aging this difficult-to-age stock. As certain trace elements are incorporated into otoliths throughout the life of the fish in response to physiology, they can potentially be used as “chemical calendars”. The marks incorporated into otoliths during tagging will be used to validate hypotheses relating elemental fluctuations in otoliths to seasonal cycles.

Results

see final report (publications below) and publications

Links and Downloads

For more information please visit the TABACOD website

Information from the Thünen Institute (in German):

Projekt TABACOD zur Dorschmarkierung startet

Altersbestimmung und Wachstum - Markierte Dorsche

Information about Baltic 2020