New fish aging method promises faster results

Determining the ages of fish is an important step for researchers and resource managers in understanding basic biology and informing management decisions. Otoliths—fish ear bones—grow throughout the lifecycle and display rings similar to those of a tree, and have been used for decades to determine age. Furthermore, the otoliths of juveniles may reveal daily growth increments that can be used to calculate hatch dates and provide insights on early life history. However, determining juvenile age with that specificity from otoliths is difficult and time consuming—processing and counting daily growth rings on a single juvenile otolith may take more than an hour.

Alaska Sea Grant-funded researchers are testing a new method for aging otoliths that has the potential to save time and reduce the lag between collection of fish specimens and compiling age information—a boon for informing timely management decisions.

Fourier transform near-infrared spectroscopy, or FT-NIRS, offers the promise of a rapid non-destructive approach to determining age from an otolith. Zach Stamplis, a University of Alaska Fairbanks graduate student working with Professor Franz Meuter on an Alaska Sea Grant-funded project, is developing a FT-NIRS model to estimate daily age in juvenile Pacific cod, while partners also look at the technique for estimating maturity and body condition of walleye pollock.

To build a FT-NIRS calibration model, intact otoliths are first scanned by a spectrometer. Shining near-infrared light on an otolith causes chemical bonds to absorb electromagnetic radiation and vibrate at detectable frequencies. The resulting spectral data reveals an otolith’s chemical composition. These spectra can then be paired with daily ages obtained from the traditional process to build a model that estimates daily age from new spectroscopic scans.

To further test the efficacy of FT-NIRS calibration models, this project will also be looking at juvenile cod collected from field surveys in other parts of the Gulf of Alaska to see what impact environmental or otolith chemistry variability have on the application of the models.

Rapid daily age estimation could be used to determine if changes in hatch date are occurring. A shift in hatch phenology may have consequences for fish recruitment and survival, drastically impacting commercial fisheries. If successful, FT-NIRS could offer researchers a new tool for monitoring juvenile cod and other species, helping gauge the impacts of climate change on important commercial fisheries in Alaska.