Karin Hüssy

Technical University of Denmark | DTU · National Institute of Aquatic Resources

Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. Using Baltic cod otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth, we tracked changes from baseline conditions in the late Neolithic (4500 BP)...

Humans have become one of the greatest evolutionary forces, and their perturbations are expected to elicit strong evolutionary responses. Accordingly, during (size) selective overharvesting of wild populations, marked phenotypic changes have been documented, while the evolutionary basis is often unresolved. Time-series collections combined with gen...

Chronological records of elemental concentrations in fish otoliths are a widely used tool to infer the environmental conditions experienced by individual fish. To interpret elemental signals within the otolith, it is important to understand how both external and internal factors impact ion uptake, transport and incorporation. In this study, we have...

Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. We employed Baltic cod as “canaries in the coal mine,” using otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth to track changes from baseline co...

Although otoliths are widely used as archives to infer life‐history traits and habitat use in fishes, their biomineralization process remains poorly understood. This lack of knowledge is problematic as it can lead to misinterpretation of the different types of signals (e.g., optical or chemical) that provide basic data for research in fish ecology,...

Have you ever lost your phone and used its GPS function to locate it? To learn about fish, scientists are interested in where fish go and what they experience but GPS technology does not work underwater. Scientists therefore developed small electronic data loggers that can be attached to fish, to record conditions in a fish’s environment. When the...

Graphical abstract for our recent publication on the use of otoliths in fisheries science.

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle respons...

Fish eye lenses are a protein‐based chronological recorder of microchemical constituents that are a potentially useful tool for interpretations of environmental, ecological and life‐history experienced by fish. Here, we present the first study with data on the chemical composition of eye lenses from Baltic cod examined using laser ablation inductiv...

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 analy...

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 ha...

Capelin (Mallotus villosus) is a marine fish species that spawns along the shorelines of Greenlandic fjords during late spring/early summer, but its migration patterns from hatching to spawning are largely unknown. This prohibits optimal fisheries advice and management of the stock. In this study, we examine spatial population structure through the...

In the Baltic Sea, salinity and its large variability, both horizontal and vertical, are key physical factors in determining the overall stratification conditions. In addition to that, salinity and its changes also have large effects on various ecosystem processes. Several factors determine the observed two-layer vertical structure of salinity. Due...

Native to the Ponto-Caspian region, the benthic round goby (Neogobius melanostomus) has invaded several European inland waterbodies as well as the North American Great Lakes and the Baltic Sea. The species is capable of reaching very high densities in the invaded ecosystems, with not only evidence for significant food-web effects on the native biot...

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.

Temperature-depth data storage tags (DSTs) continuously store individual measurements of water depth and water temperature of the habitat used by free-ranging fish. To analyse the data, the DSTs need to be returned. Usually, they are returned by fishers whereas returns of fish tagged with DSTs that died due to natural causes are rare. During an int...

Seasonal variation in the incorporation of trace elements into the calcified structures of fish can produce intra annual variation in the microchemistry of those structures. Interpretation of these seasonal signals can provide information about fish age. This approach offers great promise for objectively estimating age and corroborating other metho...

This study investigates stock mixing of genetically distinct Atlantic cod (Gadus morhua) stocks in the Kattegat, an area geographically located between the North Sea and the Baltic Sea, by combining genetic population identification with habitat assignments from hatch to capture from otolith microchemistry. Cod captured in Kattegat were genetically...

In the Baltic Sea, salinity and its large variability, both horizontal and vertical, are key physical factors in determining the overall stratification conditions. In addition to that, salinity and its changes also have large effects on various ecosystem processes. Several factors determine the observed two-layer vertical structure of salinity. Due...

Understanding migration patterns and habitat use is of great importance for management and conservation of marine living resources. The chemical composition of otoliths is influenced by the surrounding environment; therefore, they are indispensable data archives. To extract migration patterns and historical habitat use of individual fish, we analys...

The small pelagic fish capelin (Mallotus villosus) is widely distributed in the Arctic, where it plays a central role in the marine food web as prey for numerous fish, birds, and mammals. Sustainable fisheries management advice for capelin that spawn in Greenland is non-existent due in part to a lack of biological information on population structur...

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 es...

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. H...

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 B...

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 investigat...

Fish otoliths' chronometric properties make them useful for age and growth rate estimation in fisheries management. For the Eastern Baltic Sea cod stock (Gadus morhua), unclear seasonal growth zones in otoliths have resulted in unreliable age and growth information. Here, a new age estimation method based on seasonal patterns in trace elemental oto...

Otolith chemistry has gained increasing attention as a tool for analyzing various aspects of fish biology, such as stock dynamics, migration patterns, hypoxia and pollution exposure, and connectivity between habitats. While these studies often assume otolith elemental concentrations reflect environmental conditions, physiological processes are incr...

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...

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...

An aggregated sample of 925 Atlantic cod Gadus morhua collected by four countries in different regions of the Baltic Sea during different seasons were measured (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...

This study applied the Systems Approach Framework (SAF) to address the issue of declining Atlantic cod fishery in coastal areas. Interviews of 58 fishers from 26 harbours and meetings with national fisheries organisations and managers revealed the perception of an offshore movement of coastal cod. Numerical modelling based on fishing survey data di...

Worrying trends such as deteriorating condition, decreasing growth rates and diminished distribution range have been observed in the eastern Baltic cod stock in recent years. However, the mechanisms underlying these changes are still poorly understood. In an effort to better understand the observed changes in this stock, since 2016 >23,000 eastern...

The eastern Baltic cod stock is in distress, but the underlying mechanisms are still poorly understood. To improve the understanding of their behaviour and ecology, 1260 cod were internally tagged with data storage tags (DSTs) during 2016 to 2019, within the international project TABACOD. The cod were caught, tagged and released in the Baltic Sea o...

This chapter was written during the Workshop on Age Validation of Gadoids (WKAVGS 2013) which was held 6–10 May 2013 at Imedea in Esporles, Mallorca. The terms of reference for the workshop were to: 1. Review information on age estimations, otolith exchanges, workshops, and validation works done so far on the following species: European hake, cod,...

Genetic data have great potential for improving fisheries management by identifying the fundamental management units – i.e. 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 appli...

The ICES Working Group on Biological Parameters (WGBIOP) general aim is to review the status of current issues and developments associated with biological parameters, supporting the Data Collection Framework and end user (stock assessment) requirements.

This report documents the fishery, assessment, and management of lumpfish (Cyclopterus lumpus) across its distribution range. Targeting lumpfish for their roe on a large scale began in the 1950s in Iceland and Norway and then in Canada in the 1970s and Greenland in the 1990s. When the fishery began, there were few regulations, but limits on vessel...

Recent environmental changes have influenced the ecology and biology of eastern Baltic cod. Declining somatic condition, maturation at smaller size and restricted size distribution of the population suggest that growth rates have decreased between the early 2000s and the 2010s. Extensive age estimation problems have until now precluded testing of t...

Fish otoliths, also called ear stones or statoliths, are calcified structures functioning as movement and equilibrium indicators in the inner ear of fish (Fig. 1). From hatching to death these structures grow incrementally, with new material accreted daily (Pannella 1971) in successive layers of protein (1–8%, Degens et al. 1969) and calcium carbon...

The incorporation of a number of readily measured trace elements into otoliths is considered to be under some sort of physiological control, but rarely are explicit mechanisms proposed. Studies of the incorporation of the trace element magnesium reveal that in some taxa there exists strong seasonal patterning, taking on the characteristics of a “ch...

In the Baltic, the first observation of the round goby (Neogobius melanostomus, Pallas 1814) was made in 1990. Within the past decade the species became invasive and spread rapidly throughout the Baltic Sea. Studies about the fishes potential impacts on resident species promote the need for an increasing knowledge of their basic stock structures su...

Identification of essential fish habitats (EFH), such as spawning habitats, is important for nature conservation, sustainable fisheries management and marine spatial planning. Two sympatric flounder (Platichthys flesus) ecotypes are present in the Baltic Sea, pelagic and demersal spawning flounder, both displaying ecological and physiological adapt...

In this study the drift of eastern Baltic cod larvae and juveniles spawned within the historical eastern Baltic cod spawning grounds was investigated by detailed drift model simulations for the years 1971 to 2010, to examine the spatio-temporal dynamics of environmental suitability in the nursery areas of juvenile cod settlement. The results of the...

A multi-disciplinary study was conducted to clarify stock identity and connectivity patterns in the populations of European plaice (Pleuronectes platessa) in the Skagerrak-Kattegat transition area between the Eastern North Sea and the Baltic Sea. Five independent biological studies were carried out in parallel. Genetic markers suggested the existen...

The Eastern Baltic cod abundance started rapidly to increase in the mid-2000s as evidenced by analytical stock assessments, due to increased recruitment and declining fishing mortality. Since 2014, the analytical stock assessment is not available, leaving the present stock status unclear and casting doubts about the magnitude of the recent increase...

The objective of this study was to resolve key mechanisms driving individual growth patterns of Atlantic cod (Gadus morhua). Growth dynamics were analysed by linking growth patterns with stomach content composition and environmental temperature. Samples were collected in August/September of the years 2009, 2010 and 2011 in the north-eastern part of...

Over the recent decades, the International Council for the Exploration of the Sea (ICES) has set guidelines for best practise quality control of age estimation procedures. The applicability of these guidelines is assessed by reviewing the ageing issues of eastern Baltic cod (EBC) as a case study. Since the implementation of an age-based assessment...

Connectivity of pelagic, early life stages via transport by ocean currents may affect survival chances of offspring, recruitment success, and mixing of stocks across management units. Based on drift model studies, transport patterns of particles representing exogenously feeding cod larvae in the transition area between North Sea and Baltic were inv...

Abstract In the Western Baltic Sea two genetically distinct cod stocks “Eastern Baltic cod” and “Western Baltic cod” occur with considerable mixing of stocks. In this study we evaluated the applicability of otolith shape analysis for classification of individuals caught in the mixed stock cod fishery, using SNP (single nucleotide polymorphism) base...

In the Baltic Sea, two genetically distinct cod populations occur, the eastern and the western Baltic cod. Since 2006, cod abundance has increased substantially in the Arkona Basin (SD 24), the potential mixing area between the two stocks management areas, presumably due to spill-over from the eastern stock. In this study, the spatio-temporal dynam...

Round goby Neogobius melanostomus is currently one of the most wide-ranging invasive fish species in Europe and North America. The present study demonstrates how the distribution of round goby has expanded from 2008 to 2013 at a rate of about 30 km yr−1 along the Danish coastline in the western Baltic Sea. Further analyses showed that fish from an...

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 succ...

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a speci...

The objectives of this study were to evaluate whether temperature changes in the Northeast Atlantic influence the growth and recruitment dynamics of boarfish, Capros aper. Two geographically separate areas were examined, ‘north’ at the northern distribution range west of Ireland and ‘south’ on the main fishing grounds south of Ireland. No significa...

Accurate age estimation is important for stock assessment and management. The importance of reliable ageing is emphasized by the impending analytical assessment of whiting (Merlangius merlangus) in the Baltic Sea. Whiting is a top predator in the western Baltic Sea, where it is fished commercially although less extensively compared to the North Sea...

Information from data storage tags (DSTs) is conventionally used to infer movement patterns or reveal characteristics (e.g. temperature or salinity) of the environment surrounding tagged fish. Here we link data derived from DSTs with the reproductive state of tagged fish. Individual vertical activity of adult male and female Atlantic cod Gadus morh...

Hüssy, K., Hinrichsen, H.-H., and Huwer, B. 2012. Hydrographic influence on the spawning habitat suitability of western Baltic cod (Gadus morhua) – ICES Journal of Marine Science, 69: 1736–1743. Recruitment variability of marine fish is influenced by the reproductive potential of the stock (i.e. stock characteristics and abundance) and the survival...

Hüssy, K., Coad, J. O., Farrell, E. D., Clausen, L. W., and Clarke, M. W. 2012. Sexual dimorphism in size, age, maturation, and growth characteristics of boarfish (Capros aper) in the Northeast Atlantic – ICES Journal of Marine Science, 69: 1729–1735. Boarfish (Capros aper) have, in recent years, become of increasing commercial importance due to th...