Age estimation of Alaskan skates, big skate (Beringraja binoculata (formerly Raja bioculata)) and longnose skate (Raja rhina), is currently performed by interpreting the accretion of growth bands from vertebrae with the assumption that each band pair is counted as one year. Age and growth curve estimates are available from the GOA, British Columbia and U.S. West Coast; however, ageing precision across agencies, AFSC and the Pacific Biological Station (PBS), is dissimilar (Fig. 15).
Figure 15. Age bias graph for female big skate (red dots, n = 29) and female longnose skate (blue squares, n = 27) vertebral band counts from unstained thin sections by AFSC and PBS. The diagonal line represents the one-to-one equivalence line.
Figure 16. Longnose skate unstained thin section. On the corpus calcareum, the birth year (0) is indicated with a black dot; inner and outer three growth band pairs are indicated with red dots where sample material is milled.
Figure 17. Birth year and Δ14C (‰) for big skate (red dots, n = 8) and longnose skate (blue dots, n = 3) from the inner three growth bands with error bars associated with 14C measurements and ageing imprecision; compared to the GOA Pacific halibut reference curve (gray dots) with Bayesian credible intervals.
With these inter-agency differences, an age determination study was initiated. The objectives are 1) use bomb radiocarbon dating techniques to validate ages of big and longnose skates, and develop age determination criteria; 2) conduct an inter-agency comparison of new age determination methods using a histological method (hematoxylin stained-thin-sections) versus the standard technique (unstained-thin-sections) to assess bias, precision and develop a standardized protocol of age assignment across their geographical range.
Big and longnose skates were collected off Monterey, California. in 1980-81 and were considered alive during the rapid increase in (14C) oceanic radiocarbon during the atmospheric atomic bomb testing from 1967 to 1969. Each vertebra was thin sectioned (≈1 mm thick), microscope slide mounted and aged. At the AFSC, the Carpenter Microsystems™ CM-2 milling system was used to remove the first three inner band pairs and the last three outer band pairs along the corpus calcarea from each vertebra while acquiring a minimum of 5.0 mg of sample material (Fig. 16). For this preliminary study, inner growth bands from 11 milled samples ̶ big (n = 3) and longnose (n = 8) ̶ were assayed for (14C) at Beta Analytic (a carbon dating laboratory in Miami, FL) using accelerator mass spectrometry.
For big skate, the eight samples assayed appear to be consistent while comparing the Δ14C values to the GOA Pacific halibut otolith reference curve for the years 1965-70. The estimated ages are unlikely to be underestimated, although relatively uninformative since they occur late in the bomb-derived radiocarbon signal (Fig.17). The three samples of longnose skate are consistent with Δ14C values that are reasonably close, albeit 1-2 years older, to the true age based on the reference curve chronology for the years 1961-1963 (Fig. 17).
This collaborative study was conducted with this NPRB project lead principal investigators Jacquelynne King (Fisheries and Oceans Canada, Pacific Biological Station) and Thomas Helser (AFSC’s Age and Growth (A&G) Program) with collaborators David Ebert (Pacific Shark Research Center, Moss Landing Marine Laboratories) and Romney McPhie (Fisheries and Oceans Canada, Pacific Biological Station). Milling and assaying the remaining vertebrae, applying histological methods (staining) and comparing inter-lab ageing results from unstained/stained thin sections to big and longnose (14C) validated ages are ongoing. Ultimately, a standardized protocol for age assignment and criteria will be developed for all agencies to ensue, leading to age determination for big and longnose skates in the North Pacific.
