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ABSTRACT ObjectiveThe purpose of this study was to understand the possible provenance of a unique population of Atlantic Cod Gadus morhua that is found in the low-salinity Åland Sea region of the northern Baltic Sea. This population consists of large, healthy individuals, in contrast to the Atlantic Cod in the rest of the Baltic Sea. MethodsWe used laser ablation inductively coupled plasma mass spectrometry to measure levels of boron (as B:Ca) in the otoliths of Atlantic Cod in regions throughout the Baltic Sea. We examined both lifetime chronologies and concentrations in the core region that corresponds to birth and early life. ResultsWe found that B:Ca concentrations were 31 to 348 times higher in the otoliths of cod that occupy the Åland Sea, including in the core region. These concentrations were much higher than expected given that boron is linearly, positively proportional to salinity, which is higher in the southern Baltic Sea, and other populations displayed very low concentrations by comparison. ConclusionsBased on the otolith B:Ca as a unique marker, we suggest that the cod that are sampled in the Åland Sea may be a separate population from those that inhabit the rest of the Baltic Sea. This would not prevent it from mixing with other populations but could point to a separate spawning area. The source of the elevated boron is currently unknown, but the widespread occurrence in cod otoliths from the Åland Sea indicates an extensive nonpoint source. 

Typical analyses of otolith microchemistry use calcium, a major constituent, as an internal standard, setting its value as a constant and ignoring any potential variations. In fact, patterns do occur in otolith Ca deposition, as can be observed either by repeating the analysis, by creating two-dimensional maps of Ca, or both. Here we present evidence of Ca variations in fish otoliths from analyses using synchrotron-based scanning X-ray fluorescence microscopy, electron microprobe analysis, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 2-D maps of otoliths created with LA-ICP-MS indicate that Ca is elevated where especially Zn and P are low, and vice versa, suggesting that spatial variations in protein deposition may affect concentrations of Ca. We encourage others to examine Ca concentrations in their biomineralized samples to check for variations, using LA-ICP-MS and other methods. 

Abstract 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) and early twentieth century to the present, in order to understand how recent, accelerating climate change has affected this key species. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record of otolith indicators provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia due to climate warming will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers. 

Abstract: The intensified expansion of the Baltic Sea’s hypoxic zone has been proposed as one reason for the current poor status of cod ( Gadus morhua) in the Baltic Sea, with repercussions throughout the food web and on ecosystem services. We examined the links between increased hypoxic areas and the decline in maximum length of Baltic cod, a demographic proxy for services generation. We analysed the effect of different predictors on maximum length of Baltic cod during 1978–2014 using a generalized additive model. The extent of minimally suitable areas for cod (oxygen concentration ≥ 1 ml l−1) is the most important predictor of decreased cod maximum length. We also show, with simulations, the potential for Baltic cod to increase its maximum length if hypoxic areal extent is reduced to levels comparable to the beginning of the 1990s. We discuss our findings in relation to ecosystem services affected by the decrease of cod maximum length. 

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 25 Mg, 31 P, 34 K, 55 Mn, 63 Cu, 64 Zn, 66 Zn, 85 Rb, 88 Sr, 138 Ba, and 208 Pb 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. 

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 increasingly recognized as a modulating and/or controlling factor. In particular, biomineralization—the complex, enzyme-regulated construction of CaCO3 crystals scaffolded by proteins—is believed to play a critical role in governing otolith chemical patterns. This review aims to summarize the knowledge on otolith composition and biophysical drivers of biomineralization, present hypotheses on how biomineralization should affect element incorporation, and test the validity thereof with selected case studies. Tracers of environmental history are assumed to be dominated by elements that substitute for Ca during crystal growth or that occur randomly trapped within the crystal lattice. Strontium (Sr) and barium (Ba) largely comply with the biomineralization-based hypotheses that otolith element patterns reflect environmental concentrations, without additional effects of salinity, but can be influenced by physiological processes, typically exhibiting decreasing incorporation with increasing growth. Conversely, tracers of physiology are assumed to be elements under physiological control and primarily occur protein-bound in the otolith’s organic matrix. Physiological tracers are hypothesized to reflect feeding rate and/or growth, decrease with fish age, and exhibit minimal influence of environmental concentration. The candidate elements phosphorus (P), copper (Cu) and zinc (Zn) confirm these hypotheses. Magnesium (Mg) is believed to be randomly trapped in the crystal structure and hence a candidate for environmental reconstruction, but the response to all examined drivers suggest Mg to be coupled to growth. Manganese (Mn) substitutes for Ca, but is also a co-factor in matrix proteins, and therefore exhibits otolith patterns reflecting both environmental (concentration and salinity) and physiological (ontogeny and growth) histories. A consistent temperature response was not evident across studies for either environmental or physiological tracers, presumably attributable to variable relationships between temperature and fish behavior and physiology (e.g., feeding rate, reproduction). Biomineralization thus has a controlling effect on otolith element concentrations for elements that are linked with somatic growth, but not for elements that substitute for Ca in the crystal lattice. Interpretation of the ecological significance of patterns from field samples therefore needs to consider the impact of the underlying biomineralization processes of the element in question as well as physiological processes regulating the availability of ions for inclusion in the growing crystal lattice. Such understanding will enhance the utility of this technique to address fisheries management questions. 

Abstract 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 responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring. Graphical abstract 

Abstract 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 otolith incorporation was tested for the first time and compared with the traditional method of visually counting growth zones, using otoliths from the Baltic and North seas. Various trace elemental ratios, linked to fish metabolic activity (higher in summer) or external environment (migration to colder, deeper habitats with higher salinity in winter), were tested for age estimation based on assessing their seasonal variations in concentration. Mg:Ca and P:Ca, both proxies for growth and metabolic activity, showed greatest seasonality and therefore have the best potential to be used as chemical clocks. Otolith image readability was significantly lower in the Baltic than in the North Sea. The chemical (novel) method had an overall greater precision and percentage agreement among readers (11.2%, 74.0%) than the visual (traditional) method (23.1%, 51.0%). Visual readers generally selected more highly contrasting zones as annuli whereas the chemical readers identified brighter regions within the first two annuli and darker zones thereafter. Visual estimates produced significantly higher, more variable ages than did the chemical ones. Based on the analyses in our study, we suggest that otolith microchemistry is a promising alternative ageing method for fish populations difficult to age, such as the Eastern Baltic cod.