A HIGH-RESOLUTION STUDY OF OTOLITHS OF PERUVIAN HAKE (MERLUCCIUS GAYI): EXPLORING THE CRYSTALLOGRAPHIC ARRANGEMENT OF ARAGONITE PRISMS TO IDENTIFY TRUE GROWTH INCREMENTS

Biomineral structures, often composed of aragonite crystals, in the inner ear vestibule of demersal fishes (otoliths), are frequently used for monitoring migration patterns and assessing fishery stock. Fishery management depends on this information to ensure the maintenance of healthy, sustainable fish populations. Otoliths readings typically consist of counting couplet bands that form concentric rings about the nucleus, often representing an annual chronology, in order to determine maximum age, age at maturity, and growth rates. In many cases, however, the age determined by counting otolith rings does not match the age determined by other methods (i.e. tagging). This process is further complicated by the frequent presence of “false rings”, which are markings that represent an unknown chronology.

In an assessment of sagittal otolith increment formation in Peruvian hake (Merluccius gayi), counting otolith rings could give an overestimate of age by several years. Conventional counting methods allow readers to count increments down to a resolution of daily rings, but we do not have a clear understanding of the crystallographic relationships of aragonite crystals between the supposed daily and annual increments. Using a combination of high-spatial resolution electron backscatter diffraction (EBSD) with secondary scanning electron microscopy (SEM) and Raman spectroscopy, we attempt to understand the arrangement of the biogenic aragonite blades at the increment boundaries. Continuous growth of individual blades through a boundary would indicate that the development of the increments is not a punctuated event based on time, but related to other factors (i.e. nourishment).

Overall results illustrate the need of combining general otolith data (collection date, sex, and length of hake), with crystallographic and compositional information to compare with established length-at-age models, in order to improve the understanding of otolith increment formation. This understanding is essential for effective management of fish populations, and could have a direct impact on future environmental policy.