Interlinked patterns in movements and otolith formation of cod (Gadus morhua) in the southern Baltic Sea

Some of the best studied fish species with the longest monitored stock time series worldwide occur in the North Atlantic. Over the decades numerous publications added knowledge to our understanding of ecology and behaviour of these fish species and stocks. However, there are also gaps in knowledge, obvious uncertainties in ecological interactions and potential biases. These are rarely profoundly questioned unless environmental changes or inconsistent stock assessment results challenge our established points of view. In the Baltic Sea, Atlantic cod (Gadus morhua L.) has been intensively surveyed and investigated for decades. Yet, our understanding of this marine species living in a semi-enclosed, brackish environment is still incomplete. For example, despite numerous otolith exchanges to improve the age reading and large scientific efforts to understand the ecology of Baltic cod, the analytical stock assessment of the Eastern Baltic cod (EBC) stock was suspended in 2014; obviously age data were uncertain and the aut- and synecological knowledge was incomplete to explain the changes occurring in this stock in distress. For instance, spreading hypoxic areas in the central Baltic basins forced EBC to contract in the southern Baltic Sea but behaviour and movements of cod in response to the consequences are poorly understood. This dissertation aims at providing a more thorough understanding of cod ecology in the southern Baltic Sea, with a particular focus on patterns in movements of wild, free-ranging cod and validation of ring patterns in otoliths. Tagging performance, data storage tags recording depth and ambient temperature and otoliths of recaptured cod were analysed. Tagged cod had been released in the southern Baltic Sea within the scope of the international mark-recapture project TABACOD (“Tagging Baltic cod”). In chapter 1 (“Short-term tagging mortality of Baltic cod (Gadus morhua)”) the post-release short-term mortality of cod after tagging with T-bar tags and tetracycline was assessed. The mortality associated with catching and on-board handling was estimated to be 16% with no significant influence of the tagging process itself. While no effect of tagging month was observed, mortality rate was decreasing with increasing fish length. The low mortality rate confirmed the appropriateness of the tagging technique and potentially explained the lack of smaller recaptures. In chapter 2 (“Validation approaches of a geolocation framework to reconstruct movements of demersal fish equipped with data storage tags in a stratified environment“) a geolocation framework was adapted to reconstruct the movements of cod in the southern Baltic Sea tagged with temperature-depth data storage tags (DSTs). The adapted geolocation framework was tested with five validation experiments including i) simulated tracks, ii) stationary, nearshore moored DSTs, iii) temperature data from an offshore measuring station, iv) a temperature-depth probe attached to an otter board of a commercial vessel, and v) DSTs mounted on the CTD and otter boards of a survey vessel. The difference between true and modelled positions was on average between 2 and 19 km and generally better when there was a pronounced stratification of the water column and simulated individuals regularly conducted vertical movements. In chapter 3 (“Movement of cod (Gadus morhua L.) in the southern Baltic Sea: evidence from data storage tags”) the adapted and validated geolocation model was applied to the temperature-depth DSTs from 28 recaptured Baltic cod assigned to the EBC or Western Baltic cod (WBC) stock by genetics or otolith shape analysis to reconstruct daily positions. The temperature and depth profiles were supplemented with information on salinity and oxygen estimates from the regional ocean model also used for geolocation. Individual movements could be classified into three behavioural types: 1) coastal, shallow-water WBC, 2) resident EBC, and 3) migratory EBC. Unlike WBC, EBC generally occupied deeper waters, were exposed to higher salinities and regularly spent short period in hypoxic waters. While resident EBC stayed within the Bornholm Basin year-round, migratory EBC moved between spawning grounds in the Bornholm Basin during summer and coastal feeding grounds during autumn and spring. This study highlights the importance of coastal shallow-water feeding grounds, especially in autumn and spring which are underrepresented in the current bottom trawl survey. In addition, the temperature-depth profiles of all EBC revealed daily vertical movements in the water column which were triggered by twilight and partly followed the lunar cycle. Regular defaecation below the thermocline of small pelagics ingested during nightly feeding excursions above the thermocline may exacerbate hypoxia in the stagnant water bodies of the deeper basins. In chapter 4 (“Validation of otolith zone formation and otolith growth of adult wild cod (Gadus morhua L.) in the southern Baltic Sea through mark-recapture and tetracycline marking“) the chemically time-stamped otoliths of 258 T-bar tagged cod assigned to the WBCor EBC stock were analysed. WBC in the southern Baltic Sea confirmed the recent age validation result from the Belt Sea that the translucent zone is formed during summer when cod are stressed by adverse temperatures and reduced feeding. The translucent zone of EBC was also laid down during summer, but under very different conditions, i.e. during spawning coinciding with hypoxic conditions and food limitation. The faster otolith growth of WBC were in line with previous findings showing that EBC exhibit slower somatic length growth and, hence, severely reduced productivity of the EBC stock. Through this dissertation, it was possible to interlink the environmental conditions experienced by cod tagged with data storage tags to the growth and patterns in ring formation in the otoliths of cod tagged with T-bar tags. The evaluation of movement patterns in the present centre of cod distribution thus provided new insights into the behaviour and ecology of cod inhabiting the permanently stratified brackish-water ecosystem of the southern Baltic Sea