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.
Highlights
Accurate knowledge of the age composition of a fish population is essential for estimating stock parameters such as growth rate, age at maturity, recruitment, mortality and population biomass-at-age
A modelling comparison of otolith biomineralization in Barents Sea cod versus North Sea cod showed that the formation of the translucent zones in cod otoliths can be attributed to low food intake concurrent with increased water temperature, and that small variations in feeding and temperature corresponded to low contrast growth zones (Fablet et al, 2011)
In the Baltic Sea, environmental and biological drivers that further exacerbate unclear seasonal zonation in cod otoliths appear to be factors that affect fish condition and metabolic activity, such as environmental stress caused by hypoxia (Casini et al, 2016; Chabot & Claireaux, 2008), low prey availability (Casini et al, 2016), parasite infestation (Eero et al, 2015; Mehrdana et al, 2014) and thiamine deficiency (Engelhardt et al, 2020)
Summary
Accurate knowledge of the age composition of a fish population is essential for estimating stock parameters such as growth rate, age at maturity, recruitment, mortality and population biomass-at-age. “losing track of time,” meaning inaccurate age estimation, is problematic in the management of fish stocks (Andrews et al, 2009; Beamish & McFarlane, 1983, 1995; Campana, 2001). Atlantic cod in most of its range has clear, optically contrasting seasonal growth zones in its otoliths. In the case of the EBC, detecting seasonal zonation has become increasingly more complicated over time, such that traditional age estimation is no longer feasible (Hüssy et al, 2016). In the Baltic Sea, environmental and biological drivers that further exacerbate unclear seasonal zonation in cod otoliths appear to be factors that affect fish condition and metabolic activity, such as environmental stress caused by hypoxia (Casini et al, 2016; Chabot & Claireaux, 2008), low prey availability (Casini et al, 2016), parasite infestation (Eero et al, 2015; Mehrdana et al, 2014) and thiamine deficiency (Engelhardt et al, 2020)
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