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

Area of Science:

• Ichthyology
• Fish Biology
• Fisheries Science

Background:

• Fish otoliths are crucial for age and growth estimation in fisheries management.
• Eastern Baltic cod (Gadus morhua) present challenges in age determination due to unclear otolith growth zones.
• Existing visual aging methods for this stock yield unreliable data.

Purpose of the Study:

• To introduce and evaluate a novel age estimation method for fish otoliths using trace elemental analysis.
• To compare the precision and accuracy of the chemical method against traditional visual aging for Eastern Baltic cod.
• To assess the potential of otolith microchemistry as a reliable aging technique for difficult-to-age fish populations.

Main Methods:

• Otoliths from Baltic and North Sea fish were analyzed for trace elemental incorporation patterns.
• Seasonal variations in trace element ratios (Mg:Ca, P:Ca) were investigated as potential chemical clocks.
• The novel chemical aging method was compared with the traditional visual counting of otolith growth zones.

Main Results:

• Trace elemental ratios Mg:Ca and P:Ca exhibited significant seasonality, indicating strong potential for chemical aging.
• The chemical aging method demonstrated higher precision (11.2%) and reader agreement (74.0%) compared to the visual method (23.1%, 51.0%).
• Visual aging resulted in significantly higher and more variable age estimates than the chemical method.

Conclusions:

• Otolith microchemistry, specifically using Mg:Ca and P:Ca ratios, is a promising alternative for accurately aging fish.
• This chemical approach provides a more reliable alternative to visual aging for challenging fish stocks like the Eastern Baltic cod.
• The findings support the adoption of otolith microchemistry for improved fisheries management and stock assessment.