Thanks for the suggestion, @Courtney. I have not yet done any maturity studies on stingrays but am considering this aspect as part of my thesis.

Central vertebrae is the most common methods used for age determination in elasmobrachs fishes.

Vertebral centra are certainly the most commonly used structure in the aging of elasmobranch fishes. It is important however that you also validate that what you are seeing are actually annual growth marks, if this has not already been done for the species in question. Cailliet et al. (2006) give an excellent review of the different methods of both aging and validation (Environ Biol Fish (2006) 77:211–228). Good luck.