Using otolith microstructure analysis in studies on the ecology of the early life stages of cod, Gadus morhua L.: A review

Otolith microstructure and otolith size analyses have been important research tools in the study of the early life history of cod, Gadus morhua L., since 1980. This review of the literature covers 41 years (1980–2021) of publications in this research category. Most articles were published in the 1990s and 2000s, while in the past ten years, only four articles were published, and most focused on juvenile specimens. As this review indicates, during the larval period, the lapillus was used more often in age analyses than was the sagitta, while in studies on juvenile specimens, the sagitta was used more frequently. Age estimated from otolith microstructures most frequently served as basic data for analyses of somatic growth, otolith growth, and hatch dates, while it was used less commonly for analyses of age validation, age prediction, and juvenile settlement. Since the otoliths of cod are relatively difficult to read, it is worth noting that the topic of the effect of reader experience on the accuracy and precision of age determination was addressed in one of the articles. The purpose of the otolith microstructure analysis of larval and juvenile cod did not change significantly among the years reviewed with regard to these main categories, and only age validation was conducted notably more frequently in the first 20 years of the period analyzed than in the last 20 years, which seems logical. Undoubtedly, otolith size and microstructure analyses have contributed significantly to the knowledge of larval and juvenile cod ecology, and this review could be a good starting point for any researcher interested in using otolith techniques to explore the early life history of cod.