Heterogeneity of otolith chemical composition from two-dimensional mapping: Relationship with biomineralization mechanisms and implications for microchemistry analyses.

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, fisheries management, and species conservation. Otolith calcification relies on a complex system involving a pericrystalline fluid, the endolymph, whose organic and inorganic compositions are spatially heterogeneous for some constituents. This property stems from the particular structure of the calcifying saccular epithelium. In this study, we explored the spatial heterogeneity of elemental incorporation in otoliths for two species of high economic interest, European hake Merluccius merluccius (L. 1758) and European sea bass Dicentrarchus labrax (L. 1758). Two-dimensional mappings of chemical elements were obtained using UV high-repetition-rate femtosecond laser ablation (fs-LA) system coupled to a high-resolution inductively coupled plasma sector field mass spectrometer analyses on transverse sections of sagittae. Results highlighted a clear asymmetry between proximal (sulcus) and distal (antisulcus) concentrations for elements such as magnesium (Mg), phosphorus (P), manganese (Mn), and potassium (K) with concentration gradient directions that varied depending on the element. Strontium (Sr) and barium (Ba) did not show a proximo-distal gradient. These results are discussed in light of current knowledge on the endolymph composition and the mechanisms that lead to its compartmentalization, highlighting the need for further research on otolith biomineralization. Operational implications for studies based on otolith chemical composition are also discussed with emphasis on advice for sampling strategies to avoid analytical biases and the need for in-depth analyses of analytical settings before comparing otolith signatures between species or geographical areas.

Eyes and ears: A comparative approach linking the chemical composition of cod otoliths and eye lenses.

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 inductively coupled plasma mass spectrometry (LA-ICPMS) and compare these spatially resolved data to otoliths from the same fish also analysed by LA-ICPMS, measuring the isotopes 27 Al, 137 Ba, 43 Ca, 52 Cr, 65 Cu, 57 Fe, 39 K, 7 Li, 25 Mg, 55 Mn, 31 P, 208 Pb, 85 Rb, 45 Sc, 29 Si, 88 Sr, 47 Ti, 50 V, 149 Yb, 66 Zn and 90 Zr. Comparison of the variation in element concentrations between eye lenses and otoliths from the same individuals showed minor similarities, suggesting a different governance in the uptake processes. A strong overlap between the concentric growth rings in the eye lenses and the otolith Sr periodicity was observed, where each consecutive minima in the chemical profile with high accuracy correspond to the width of each lens ring. No comparable trends were seen between growth rings and all other elements measured from both lenses and otoliths. The characteristic rings observed in cod eye lenses do not seem to represent seasonal fluctuation nor are they found to be directly linked to age. With this research, we provide a baseline study identifying elements in corresponding eye lenses and otoliths that show potential for unravelling the environmental and biological conditions experienced by fish.

Seeking the true time: Exploring otolith chemistry as an age-determination tool.

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.

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea.

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 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 LS and gutted M are provided.

Genetic analyses reveal complex dynamics within a marine fish management area.

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.