David Morgan

University Honors with Honors in Biology

Majors: Biology

Supervisor: Frederick Scharf, Biology and Marine Biology

An Evaluation of Otolith Aging Precision Using Scanning Electron and Light Microscopy

Otoliths are mineralized structures used for hearing and equilibrium in fishes that can be used for the aging of individuals. The methods of otolith preparation and the microscopic techniques used to estimate age may influence the accuracy and precision of the estimates. This study attempted to evaluate bias and precision between standard light microscopic approaches and scanning electron microscopy (SEM) using backscattered electrons. Otoliths from a sample of juvenile red drum (Sciaenops ocellatus) were prepared for light microscopy and SEM, using one otolith from each fish for each approach. Age bias plots were used to assess bias between the two methods, while both within- and between-reader error were assessed separately for each method. Typically, otoliths are etched with a weak acid for the viewing of topographical features using SEM in secondary electron mode. The higher energy backscattered electron mode was used to view the difference in atomic number between the incremental zones that were counted, eliminating the need for acid etching. Otoliths prepared for SEM only required polishing on one side, thus, preparation time was reduced compared with standard light microscopy. The age estimates from SEM were consistently higher (by > 6 days on average), indicating a considerable bias between the microscopy approaches. Both methods generated acceptable levels of precision within the same reader, however the SEM was found to have much greater precision between readers. Combined with previous work, SEM using backscattered electrons may provide more accurate age estimates, and the estimates are likely less subject to reader interpretation error compared to light microscopy. The technique may prove to be a more reliable and less labor intensive approach than traditional light microscopy for aging of young fishes.