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Advection (directional movement) and diffusion (dispersed movement) were applied for the first time to describe movement patterns in Atlantic cod in the North Sea and Baltic Sea between 1955 and 2020. The advection-diffusion approach provided more detailed estimates of movement that corresponded to previously observed patterns using different analytical techniques. Spatial patterns were evident with greater movement distances in cod from the North Sea and eastern Baltic Sea compared to the western Baltic and Kattegat-Skagerrak. Furthermore, comparative case studies on different ecotypes in the western and eastern Baltic suggested that inshore cod were more resident compared to offshore cod. This preliminary study highlights the usefulness of the advection-diffusion method to describe movements in fish populations, and can be further expanded by incorporating information on environment and mortality and providing information to spatially explicit population models.

Knowledge of the movement patterns and area utilisation of commercially important fish stocks is critical to management. The Eastern Baltic cod Gadus morhua, one of the most commercially and ecologically important stocks in the Baltic Sea, is currently one of the most severely impacted fish stocks in Europe. During the last 2 decades, this stock has experienced drastic decreases in population size, distributional range, individual growth and body condition, all of which may have affected the movements between different areas of the Baltic Sea. In this study, we investigated the seasonal movement patterns of Eastern Baltic cod by re-analysing historical tagging data collected by the countries surrounding the Baltic Sea (1955-1988) and compared historical patterns with contemporary data from a recent international tagging experiment (2016-2019). Our re-analyses of historical data showed the presence of different movement behaviours, i.e. resident or seasonally migratory, with larger distances moved by cod released in the northern and central Baltic areas compared to cod released in the southern Baltic areas. Furthermore, trends from the recent tagging experiment indicate a persistent resident strategy in the southern Baltic area. These findings present additional information on general movement patterns and area utilisation of Eastern Baltic cod that could inform future management actions and aid stock recovery.

The main objective of the Working Group on Biological Parameters (WGBIOP) is to review the status, issues, developments, and quality assurance of biological parameters used in assessment and management. WGBIOP (1) plans workshops, exchanges, and validation studies on a range of biological varia-bles to review the quality of information supplied for stock assessment and improve quality as-surance and training; (2) investigates data availability and develops documentation and methods to improve communication between data collectors and end-users; (3) delivers new and im-proved functionality for the SmartDots platform. Four otolith exchanges and two workshops were completed in 2020–2021 using SmartDots— eight further exchanges are ongoing. Proposed future exchanges and workshops were reviewed and approved. The development of the SmartDots platform proceeded with the inclusion of the maturity, eggs, atresia, fecundity, and larval identification modules into the software version. A live SmartDots tutorial for event coordinators was conducted. Work to further develop quality assurance guidelines—and review national applications of these—progressed. Age and maturity validation studies were reviewed and a new method for prioritizing future validation work was proposed. Progress with the Stock Identification Database (SID) was reviewed, and the potential for creating a WGBIOP library collection and active involvement of WGBIOP in updating FishBase.org data were evaluated. The importance of identifying and documenting links be-tween all relevant databases and document repositories was identified, and a task to address this was initiated. Work on improving the feedback loop between data collectors and stock assessors on the usage and quality of biological parameters in stock assessment continued. Moving forward, WGBIOP aims to continue collaboration with WGALES and WGSMART on the development of the SmartDots platform, encouraging cross-group sharing of skills and ex-perience to optimize results. WGBIOP aims to improve accessibility to its outputs through up-dates to SID and FishBase.org, and the potential creation of a WGBIOP library collection. WGBIOP hopes to improve two-way communication between data collectors and end-users around the quality and utility of biological parameters used in assessment. WGBIOP also aims to amalgamate all validation activities into one coherent workstream., ICES

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 assess-ments 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 bench-marked in the coming years. Issue lists were scrutinized, problems identified, and information provided to stock coordinators via regular channels and through the Stock Identification Data-base (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 iden-tified 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.

Identified were suitable dosages of tetracycline hydrochloride (TET) in three single treatments and three combined treatments of TET with 2 mg/kg strontium chloride (STR) in wild western Baltic cod (Gadus morhua), in terms of (a) obtainable mark qualities (visibility of fluorescent bands), (b) growth assessment, and (c) induced mortality rates. Isotonic NaCl solution was injected in a control group (25 cod per treatment). The results provide the basis for imperative age validation studies of Baltic Sea cod. Cod originating from pound nets near Fehmarn Island were kept in swimming net cages at the harbor of Warnemunde for 1.5 months. Mean initial total length was 28(+/- 3) cm (salinity: 13, water temperature: 13 to 8 degrees C). Overall average growth of surviving cod was 0.8 mm/day. In single TET treatments, lowest mortality rates and best mark quality were observed for TET concentrations of 100 compared to 50 and 25 mg/kg wet mass. Mortality rates of the 100 mg/kg treatment group were remarkably lower than in the control group emphasizing the antibiotic effect of TET. By contrast, the double treatment in the TET-STR groups resulted in a binding interaction between both markers in the fish body causing either the antibiotic potency being inhibited or TET and STR forming a non-beneficial chelate (increased mortality), and decreased incorporation of TET in the otolith (reduced visibility of TET bands). Consequently, TET (short-term marker) and STR (long-term marker) should not be injected together. Our results demonstrate that the binding interactions between these substances known from homoiotherm animals also apply for poikilotherms such as fish.

The eastern Baltic (EB) cod (Gadus morhua) stock was depleted and overexploited for decades until the mid-2000s, when fishing mortality rapidly declined and biomass started to increase, as shown by stock assessments. These positive developments were partly assigned to effective management measures, and the EB cod was considered one of the most successful stock recoveries in recent times. In contrast to this optimistic view, the analytical stock assessment failed in 2014, leaving the present stock status unclear. Deteriorated quality of some basic input data for stock assessment in combination with changes in environmental and ecological conditions has led to an unusual situation for cod in the Baltic Sea, which poses new challenges for stock assessment and management advice.Anumber of adverse developments such as low nutritional condition and disappearance of larger individuals indicate that the stock is in distress. In this study, we (i) summarize the knowledge of recent changes in cod biology and ecosystem conditions, (ii) describe the subsequent challenges for stock assessment, and (iii) highlight the key questions where answers are urgently needed to understand the present stock status and provide scientifically solid support for cod management in the Baltic Sea

Sustainable management of fish stocks requires knowledge of stock structure and connectivity between spawning and feeding habitats. Cod in the Sound in the western Baltic Sea are an example of a stock component with complex connectivity patterns. Currently cod in this area are managed as part of the western Baltic cod stock, while several studies suggest potential connectivity with the neighbouring stock in Kattegat. Here we assess the degree of ecological connectivity of cod in the Sound with the neighbouring areas using historical tagging data from 1957 to 1987, and contemporary growth data from trawl surveys collected between 2007 and 2021. Furthermore, data from cod tagged outside of the Sound between 1960 and 2018 and recaptured in the Sound was used to examine immigration from outside areas. The ecological connectivity between the Sound and Kattegat appeared to be considerable, primarily during the spawning season. Furthermore, cod tagged in the northern Sound were most likely to be recaptured in Kattegat while cod tagged in the southern part of the Sound were mainly recaptured in the Sound. Only 40 out of 16,789 tagged cod released outside of the Sound were eventually recaptured inside the Sound. Overall, these results highlight the need for further examination into the current stock structure of cod in the western Baltic and adjacent areas.

Accurate age data are essential for reliable fish stock assessment. Yet many stocks suffer from inconsistencies in age interpretation. A new approach to obtain age makes use of the chemical composition of otoliths. This study validates the periodicity of recurrent patterns in 25Mg, 31P, 34K, 55Mn, 63Cu, 64Zn, 66Zn, 85Rb, 88Sr, 138Ba, and 208Pb in Baltic cod (Gadus morhua) otoliths from tag–recapture and known-age samples. Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter – early spring when water temperatures are coldest in tagged cod . The timing of minima differs between stocks, occurring around February in western Baltic cod and 1 month later in eastern Baltic cod; seasonal maxima are also stock-specific, occurring in August and October, respectively. The amplitude in P is larger in faster-growing western compared with eastern Baltic cod. Seasonal patterns with minima in winter – late spring were also evident in Mg and Mn, but less consistent over time and fish size than P. Chronological patterns in P, and to a lesser extent Mg and Mn, may have the potential to supplement traditional age estimation or to guide the visual identification of translucent and opaque otolith patterns used in traditional age estimation.

Long time series of reliable individual growth estimates are crucial for understanding the status of a fish stock and deciding upon appropriate management. Tagging data provide valuable information about fish growth, and are especially useful when age-based growth estimates and stock assessments are compromised by age-determination uncertainties. However, in the literature there is a lack of studies assessing possible changes in growth over time using tagging data. Here, data from tagging experiments performed in the Baltic Sea between 1971 and 2019 were added to those previously analysed for 1955–1970 to build the most extensive tagging dataset available for Eastern Baltic cod (Gadus morhua, Gadidae), a threatened stock with severe age-determination problems. Two length-based methods, the GROTAG model (based on the von Bertalanffy growth function) and a Generalized Additive Model, were used to assess for the first time the potential long-term changes in cod growth using age-independent data. Both methods showed strong changes in growth with an increase until the end of the 1980s (8.6–10.6cm/year for a 40cm cod depending on the model) followed by a sharp decline. This study also revealed that the current growth of cod is the lowest observed in the past 7 decades (4.3–5.1cm/year for a 40cm cod depending on the model), indicating very low productivity. This study provides the first example of the use of tagging data to estimate multidecadal changes in growth rates in wild fish. This methodology can also be applied to other species, especially in those cases where severe age-determination problems exist.

Understanding the growth of commercially exploited fish is crucial in fisheries biology and correct estimations of growth and its change over time are paramount for the evaluation of stock status development. Mark-recapture experiments represent a reliable method to estimate growth when age determination based on otolith reading is uncertain, as is the case of the Eastern Baltic cod stock. In this study, historical data (1955–1970) from tagging experiments on Eastern Baltic cod performed by Sweden, Poland, Denmark, Latvia and Germany were digitised and collated for the first time in a unique dataset to estimate historical von Bertalanffy growth function (VBGF) parameters based on fish length increments using GROTAG model. The estimated VBGF parameters were L∞ = 98.22 cm and k = 0.14 for the period 1955–1964 (n = 1151), L∞ =123.61 cm and k = 0.09 for 1965–1970 (n = 2612), and L∞ = 125.60 cm and k = 0.09 for the aggregated period (n = 3763). A seasonal growth signal was detected for all the periods, with a peak after the spawning season in early autumn. These estimates are the most thorough historical growth baseline now available for the Eastern Baltic cod and can be compared to ongoing and future tagging experiments contributing to the development of stock assessment models for this stock.

The use of growth estimation methods that depend on unreliable age data has previously hindered the quantification of perceived differences in growth rates between the two cod stocks inhabiting the Baltic Sea. Data from cod tagged in different regions of the Baltic Sea during 2007–2019 were combined, and general linear models were fit to investigate inter-regional (defined as area of release) and inter-stock (assigned to a subset of recaptures using genetic and otolith shape analyses) differences in individual growth. An average-sized cod (364 mm) caught in the western Baltic Sea and assigned to the western Baltic cod stock grew at more than double the rate (145 mm year−1) on average than a cod of the same size caught in the eastern Baltic Sea and assigned to the eastern Baltic cod stock (58 mm year−1), highlighting the current poor conditions for the growth of cod in the eastern Baltic Sea. The regional differences in growth rate were more than twice as large (63 mm year−1) as the stock differences (24 mm year−1). Although the relative importance of environmental and genetic factors cannot be fully resolved through this study, these results suggest that environmental experience may contribute to growth differences between Baltic cod stocks.

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measure (total length, LT = 161–890 mm and weighed (mass, M = 45–6900 g) both before freezing and after defrosting. The cod were found to decrease significantly in both LT and M following death and frozen storage. There was an average (± SD) change in LT of –2.91% (± 0.05%) following freezing, independent of starting LT. Total M changed by –2.65% (± 0.14%), independent of starting mass. Shrinkage of LT and M did not differ significantly between 1 and 4 months frozen storage, though LT shrinkage was significantly greater after 1 or 4 months in the freezer compared with after 5 days. There was significant variation in LT and M shrinkage between regions of capture. A significant negative relationship between condition of cod and LT or M change was also observed. Equations to back‐calculate fresh LT and M from thawed LT, M and standard length (LS), gutted LT, gutted LTg and gutted mass (Mg) are provided.

Tagging-induced mortality experiments are an important component of mark-recapture studies, as they can be used to assess the appropriateness of the tagging methodology, and to improve the reliability of estimates of recapture rates used for calculations of mortality rates and population size. Here, short-term tagging mortality of Baltic cod was estimated through containment experiments in the southern Baltic Sea. Experimental cod were selected from trawl catches, and approximately half were tagged externally with T-bar tags and received an intraperitoneal injection of tetracycline-hydrochloride. The rest of the experimental cod formed the control group, and received neither tag nor injection. The tagged and control cod were mixed evenly within submersible cages, and held for 5−8 days. The experiments were conducted in different regions and during different months by different tagging teams. Overall mortality rate was 16 % (n = 324), with the mortality rate of the tagged group 19 %, and the mortality rate of the control group 13 %. A general linear mixed model was fit to assess the effect of tagging, month, experiment duration, fish length and tagging site (i.e. the combined effect of region and tagging team) on mortality. Tagging had no effect on mortality, indicating that mortality can be attributed mainly to the capture and handling procedure. There was a significantly negative relationship between fish length (range: 20−55 cm) and mortality. Mortality did not differ between the months tested, but there was a significant effect of tagging site on mortality. Tagging-related mortality should be accounted for in analyses of data from mark-recapture studies of Baltic cod, and some variability in mortality between tagging sites can be expected.