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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.

Das Projekt STELLA (STELlnetz-Lösungs-Alternativen) zielte darauf ab, Lösungen für die Mitigation unerwünschter Beifänge von Seevögeln und Meeressäugern und damit für den Ausgleich der Konflikte zwischen Naturschutzinteressen und den Interessen der Küstenfischerei in der Ostsee zu finden. Das Projekt wurde vom Thünen-Institut für Ostseefischerei (OF) von November 2016 bis Januar 2020 aus Mitteln des Bundesministeriums für Umwelt, Naturschutz und nukleare Sicherheit in Kooperation mit dem Bundesamt für Naturschutz durchgeführt. Von Februar bis Juli 2020 wurden noch ausstehende Arbeiten aus Mitteln des Thünen-Instituts fortgeführt. Die Ziele des Projektes wurden in fünf Arbeitspaketen (AP) bearbeitet: AP1 zielte auf eine verbesserte Erhebung von Aufwands- und Fangdaten aus der Stellnetzfischerei; AP 2 fokussierte auf die technische Verbesserung von Stellnetzen, um unerwünschte Beifänge zu reduzieren; AP3 widmete sich Verbesserungen an zwei Alternativen zum Stellnetz, der Fischfalle und der Hebereuse; das sozialwissenschaftliche AP4 befasste sich mit der fischereilichen Handlungspraxis in Bezug auf die Vermeidung unerwünschter Beifänge. AP5 schließlich bildete aus den Ergebnissen der APs 1 bis 4 die Synthese und leitete Handlungsempfehlungen für die Politik ab. [...] The project STELLA (Gillnet-Solution-Approaches) aimed at finding finding solutions for the mitigation of unwanted by-catches of sea birds and marine mammals, and thus reconcile nature conservation interests and the interests of coastal fisheries in the Baltic Sea. The project was carried out by the Thünen Institute of Baltic Sea Fisheries (OF) from November 2016 to January 2020, funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety in cooperation with the Federal Agency for Nature Conservation. From February to July 2020, pending work was continued with funds of the Thünen Institute. The objectives of the project were executed in five work packages (WP): WP1 aimed to improve data collection from the gillnet fishery; WP 2 focused on technical modifications of gillnets to reduce unwanted bycatches; WP3 addressed improvements to two alternatives to the gillnet, the fish trap and the pontoon trap; the social-science WP4 dealt with the fishing practice in relation to the avoidance of undesirable by-catches. Finally, WP5 synthesised the results of WPs 1 to 4 and derived recommendations to policy makers. [...]

Workshop 2 on age reading of North Sea plaice (Pleuronectes platessa), (WKARP2) was the first age reading workshop focusing specifically on age reading of the North Sea plaice stock (ple.27.420) in the North Sea and Skagerrak. The objectives of the workshop were: to evaluate the level of agreement between age readers for the stock by reviewing results of the 2020 North Sea Skagerrak plaice exchange in consideration of previous calibration and validation work; to standardize laboratory procedures and age reading methods applied; to provide guidelines for reliable age interpretation; to provide age error data to the stock assessment working group; to create an agreed age reference collection of otoliths. Two age reading exercises, one exchange before the workshop (SmartDots ID 281), and one workshop exercise (ID 402) were completed using SmartDots1. Age readers’ annotations of growth structures and ageing results from both exercises were examined using standardized quality analyses based on an R script , presented in this report. Age reading error data has been provided to the ICES WGNSSK2 (Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak) which can be tested in the ple.27.420 stock assessment model. Disagreement between readers is mostly attributable to differences in the identification of the first winter ring as this can vary in width across samples collected from different areas. Results showed that estimated ages in older fish can be unreliable due to a narrowing of the annuli close to the otolith edge. Further work is required to provide guidelines for age readers about which structures should be identified as annuli. Different preparation methods are applied in national laboratories. The group concluded that reading whole and sectioned otoliths viewed under reflected light is optimal; no obvious benefit was identified from sectioning plaice otoliths from fish under the age of 6. Using images of otoliths, the reliability of the age reading results is depending on image quality. To help standardize image format, lighting and calibration a workshop is recommended to establish a set of guidelines for image quality used in age determination.

The main objective of the Working Group on Biological Parameters (WGBIOP) is to review the status, issues, developments, and quality assurance of biological parameters for use in assessments and management that are in line with the requirements of end-users. In this final year of the three-year term, WGBIOP operated under challenging circumstances due to COVID-19 measures. The initial action plan was replaced by a more flexible one, where online plenary and subgroup meetings were spread over the year with intersessional work to finalize the proposed deliverables.WGBIOP continued the review of past exchanges and workshops under the remit of the working group. Since 2019, these calibrations on age, maturity, and larvae identification have been carried out in SmartDots, an online platform for sharing images and facilitating the reading of otoliths, staging of gonads, and identification of early life stages. Developments are underway to include an improved calculation of modal age and error matrices in the SmartDots standard report. WGBIOP investigated ways to incorporate error matrices into assessments and studied the effect of this inclusion together with stock assessors.Requests for new exchanges and workshops were reviewed, with a focus on stocks to be benchmarked in the coming years. Issue lists were scrutinized, problems identified, and information provided to stock coordinators via regular channels and through the Stock Identification Database (SID). Despite close cooperation with stock assessors and continued efforts, it has not been possible to further streamline the WGBIOP workflow with the benchmark process. This will be addressed with the Advisory Committee. The need for validation studies was stressed by the repeated low levels of agreement between readers of some stocks and recurring issues and recommendations to WGBIOP. Lack of resources is the main obstacle. As a first step for measures to prioritize validation studies, WGBIOP identified precision, trueness, and feasibility of validation methods (as well as the urgency for the assessment). WGBIOP continued investigations into new life-history parameters for integrated assessment and advice in cooperation with end-users (Working Group on Integrative, Physical biological and Ecosystem Modelling-WGIPEM and Regional Coordination Groups-RCGs). This included a standardization and quality assurance action plan for stomach sampling. Efforts have also been taken to streamline data and workflows across databases and groups. A step has been taken in the standardization of quality assurance procedures at the regional level. Institute-level overviews of methods and quality assurance protocols used for ageing and maturity are now available. Also, a new method for quality grading was developed, tested, and implemented in SmartDots.