Virginia Institute of Marine Science
MARINE AND COASTAL FISHERIES
We used delta O-18 signatures in otoliths as a natural tag for hatch year to evaluate the scale-based age determination method used for adult American shad Alosa sapidissima in the York River, Virginia. Juveniles of the 2002 year-class exhibited high delta O-18 values in otolith cores that identified adult members of the cohort as they returned to spawn. Recruitment of the 2002 cohort was monitored for three consecutive years, identifying age-4, age-5, and age-6 individuals of the York River stock. The scale-based age determination method was not suitable for aging age-4, age-5, or age-6 American shad in the York River. On average, 50% of the individuals from the 2002 year-class were aged incorrectly using the scale-based method. These results suggest that the standard age determination method used for American shad is not applicable to the York River stock. Scientists and managers should use caution when applying scale-based age estimates to stock assessments for American shad in the York River and throughout their range, as the applicability of the scale-based method likely varies for each stock. This study highlights a promising new direction for otolith geochemistry to provide cohort-specific markers, and it identifies several factors that should be considered when applying the technique in the future.
ALOSA-SAPIDISSIMA; STRONTIUM ISOTOPES; FRESH-WATER; RIVER; FISH; POPULATIONS; CHEMISTRY; VARIABILITY; VALIDATION; DELTA-O-18
The authors thank J. Blusztajn and S. Birdwhistell at the Woods Hole Oceanographic Institution (WHOI) Plasma Mass Spectrometry Facility, D. Ostermann at the WHOI Micropaleo Mass Spectrometry Facility, L. Kerr at Chesapeake Biological Laboratory, and B. Watkins at the Virginia Institute of Marine Science for help with sample analysis. E. Hilton reviewed earlier drafts of this manuscript, and A. Buchheister and P. Lynch helped with statistical analyses. We also thank two anonymous reviewers for their time and constructive comments that helped improve this manuscript. Funding was provided by National Marine Fisheries Service grants to S. Upton (award numbers NA07NMF4050164 and NA08NMF4050610) and grants from the WHOI Academic Programs Office and a WHOI Ocean Life Institute Research grant to B. Walther.
Sally A. Upton , Benjamin D. Walther , Simon R. Thorrold & John E. Olney (2012) Use of a Natural Isotopic Signature in Otoliths to Evaluate Scale-Based Age Determination for American Shad, Marine and Coastal Fisheries, 4:1, 346-357, DOI: 10.1080/19425120.2012.675973