https://phys.org/news/2018-07-shark-vertebral-band-pairs-growth.html

In the past, band pairs have most often been assumed to be annual. Some studies using chemical marking, such as Oxytetracycline (OTC), in combination with tag-recapture, and bomb radiocarbon dating have shown that this is often only true for a portion of the life of some species. Many of these studies have shown age underestimation in older sharks. This new study explored the relationship of band pair deposition to growth rather than time, explaining this discrepancy.

Researchers report that band pair deposition is closely linked to growth, specifically to the animal's girth or body width. Band pair counts are related to vertebral size, rather than to time or age, and counts vary among vertebrae along the spinal column. As growth rate decreases with age, band pair deposition decreases, leading to age underestimation based on band pair counts.