In a pickle: the effects of fish preservation on otolith morphology

Abstract

Nestled snugly in a cavity underneath the brains of fish are otoliths – tiny calcium carbonate structures that encode immense amounts of biological information. By examining otoliths, life histories of individual fish can be retold and translated into tangible insights useful to humans. For instance, by examining the microstructure of otoliths, age and growth rates of specific fish can be determined and such knowledge then feeds into quantitative fish stock assessments. However, these insights are only maximised if otoliths are whole and complete. This prerequisite for otoliths to be whole and complete may not hold true if post-capture fish preservation methods alter the integrity of the otolith structure. Three of these post-capture fish preservation methods, namely freezing, submerging in hyper-saline seawater slurry, and immersing in 70% ethanol solution, were investigated for its effect on otolith morphology of farmed juvenile Lates calcarifer and Lutjanus malabaricus. Otolith morphology was measured via five morphological features of focus – otolith weight, area, length, width, and perimeter. Based on a two-step analysis (i.e., Principal Component Analysis and Paired Samples T-test), differing impacts of treatment across species were uncovered. First, ethanol treatment did not significantly alter otolith morphology for both species. Second, seawater slurry treatment led to a significant decrease in overall otolith morphology for only L. calcarifer. Third, freezing treatment led to a significant increase in overall otolith morphology for L. calcarifer, but a significant decrease in overall otolith morphology for L. malabaricus. Overall, this study contributes to the sparse research that has been conducted on uncovering the effects of post- capture fish preservation on otolith morphology, providing baseline results and a methodological foundation for future studies to build upon.