Use of Cathodoluminescence on Blueback Herring Otoliths to Better Understand Population Dynamics in the Mohawk River

Blueback herring are diadromous fish, meaning they migrate between freshwater and the ocean, and they are native to the Hudson and its tributaries. They have been able to access the Mohawk River by using the locks in the Erie Canal to bypass the Cohoes Falls. In recent years, there has been an observed drop in population numbers of these fish in the Mohawk and elsewhere on the Atlantic slope. Understanding Blueback herring population numbers in the Mohawk and Hudson is vital to the conservation of this species, which is seeing its habitat constantly being altered by humans. Otoliths, or ear stones, are aragonite (CaCO₃) that exhibit daily deposition and growth in fish, with chemical properties which reflect the geochemistry of the surrounding water. The chemical changes of these otoliths over time have been used to determine fish migration and environmental history. There is a major chemical difference between seawater and freshwater, but in freshwater, some rivers, like the Mohawk, have a distinctive chemistry as well. Otoliths in diadromous fish show a dramatic difference in Sr:Ca because seawater is relatively rich in Sr.  But some watersheds have bedrock geology with a distinctive chemistry that gets recorded in the otolith. Strontium isotope ratios (87/86) in otoliths have been an important method of determining fish residence, but this is time consuming and expensive. Cathodoluminescence (CL) uses electrons to excite a sample in a vacuum on an SEM. This process produces light of different wavelengths, depending on chemistry. Blueback herring otoliths have an aragonite composition, and as such, they should emit light under electron bombardment. Cathodoluminescence has been proposed as a quicker and less expensive way of determining elemental changes in an otolith, with the goal of better understanding herring migration and population dynamics. However, the results of this study suggests a herring otolith from this region does not luminesce enough to produce substantial data relating to migration and population dynamics, as little-to-no cathodoluminescence was seen in this otolith, perhaps due to the suppression of CL by proteins.