Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

Abstract

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

Bilateral asymmetry of otoliths from Collichthys lucidus of different sizes in Haizhou Bay and Xiangshan Bay

“…The otolith is a calcium carbonate structure integrating auditory and balance function contained within the inner eardrum labyrinth of true bone fish and is divided into the lapillus otolith, sagittal otolith and asteriscus otolith (Harvey et al ., 2000; Wicaszek et al ., 2020). Due to its variability and uniqueness, otolith morphology not only permanently records the life history of fish (Markwitz et al ., 2000; Plonus et al ., 2021) but is also a product of interactions with variable water environments and complex genetic information (Collins et al ., 2013; Elsdon & Gillanders, 2004; Randon et al ., 2020). In the examination of otolith FA and related biological information of fish, the morphological difference between the left and right sagittal otoliths is usually analysed (El‐Mahdy et al ., 2019; Lychakov et al ., 2006).…”