ALBERT

Description: Dates of Cruise: 16.05. – 28.05.2016 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Klaipeda, Lithuania, 21.05. – 23.05.2016

Description: Dates of Cruise: 13.05. – 29.05.2017 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Visby, Sweden, 19.05. – 21.05.2016

Description: The disappearance of larger individuals and the decrease in individual body condition suffered by Atlantic cod Gadus morhua in the eastern Baltic during the past two decades can be expected to affect the stock reproductive output. To investigate this, female G. morhua were collected during the spawning and pre-spawning period in 2015-2016. The current individual potential fecundity (FP) of eastern Baltic G. morhua was estimated and analysed in relation to total length (LT) and indices of nutritional status such as body condition (K) and hepato-somatic index (IH) using generalized linear models. In addition, the current prevalence of atresia and its potential relation to K were investigated. Moreover, a calibration curve to estimate FP from oocyte diameter, based on the autodiametric oocyte counting method, was established for the first time for eastern Baltic G. morhua and can be used for future fecundity studies on this stock. The results showed that FP was mainly positively related to fish length, but K and IH also contributed significantly to the variation in FP. The model predicted that fish with K=1·2 have a FP 51% higher than fish of the same LT with K=0·8. The prevalence of fecundity regulation by atresia was 5·8%, but it was found only in fish in the pre-spawning maturity stage and with low K. Temporal changes in biological features such as the length composition and individual body condition of eastern Baltic G. morhua, should be accounted for when estimating stock reproductive potential.

Description: Wochenbericht der Forschungsreise mit R.V. „Alkor“ AL 493 13.05.2017 – 20.05.2017

Description: 16.05.2015 - 30.05.2015 Physical, chemical, biological research and fishery oceanography This multidisciplinary cruise extended a long-term data series on (eco-)system composition and functioning of the Baltic Sea, with a focus on the deeper basins. The series has been collected in similar form since 1986. A key characteristic of the cruise is the integration of oceanographic and biological information to enhance understanding of environmental and (fish) population fluctuations, and evolutionary processes in this system. The resulting datasets and samples are an essential component of the international EU project BONUS BIO-C3 coordinated by GEOMAR. The spatial focus lay on the Bornholm Basin (the most important spawning area of Baltic cod), but also included the Western Baltic Sea, Arkona and Gotland Basin, Gdansk Deep, and Stolpe Trench. Specific investigations included a detailed hydrographic survey (oxygen, salinity, temperature) of the Bornholm Basin, plankton surveys (zoo- and ichthyplankton, with the goal to determine the composition and the abundance and vertical and horizontal distribution of species, and to take samples for later measurements of nutritional condition), and pelagic fishery hauls for clupeid and gadoid fish. The latter served to determine stock structure, gonadal maturation, stomach contents, and egg production of sprat and cod, and to sample tissue for later genetic analyses and otoliths for age estimatation of individual cod. The abundance and distribution of fish species in the investigated area was also assessed with hydroacoustic methods. Additional cruise components were: (i) cod gonad and liver sampling for the determination of lipid content and histological studies; (ii) vertically resolved phytoplankton and zooplankton sampling for studies of plankton phenology.

Description: R.V. ALKOR Cruise No.: AL 457 Dates of Cruise: 16.05. – 30.05.2014 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Gdynia, Poland, 22.05. – 24.05.2014

Description: Genetic data have great potential for improving fisheries management by identifying the fundamental management units—that is, the biological populations—and their mixing. However, so far, the number of practical cases of marine fisheries management using genetics has been limited. Here, we used Atlantic cod in the Baltic Sea to demonstrate the applicability of genetics to a complex management scenario involving mixing of two genetically divergent populations. Specifically, we addressed several assumptions used in the current assessment of the two populations. Through analysis of 483 single nucleotide polymorphisms (SNPs) distributed across the Atlantic cod genome, we confirmed that a model of mechanical mixing, rather than hybridization and introgression, best explained the pattern of genetic differentiation. Thus, the fishery is best monitored as a mixed-stock fishery. Next, we developed a targeted panel of 39 SNPs with high statistical power for identifying population of origin and analyzed more than 2,000 tissue samples collected between 2011 and 2015 as well as 260 otoliths collected in 2003/2004. These data provided high spatial resolution and allowed us to investigate geographical trends in mixing, to compare patterns for different life stages and to investigate temporal trends in mixing. We found similar geographical trends for the two time points represented by tissue and otolith samples and that a recently implemented geographical management separation of the two populations provided a relatively close match to their distributions. In contrast to the current assumption, we found that patterns of mixing differed between juveniles and adults, a signal likely linked to the different reproductive dynamics of the two populations. Collectively, our data confirm that genetics is an operational tool for complex fisheries management applications. We recommend focussing on developing population assessment models and fisheries management frameworks to capitalize fully on the additional information offered by genetically assisted fisheries monitoring.