Determining a sharkâs age is challenging because their skeletons are largely cartilaginous, unlike bony fish which have hard, annual-ringing ear stones called otoliths. Sharks, skates, and rays (elasmobranchs) require scientists to analyze other calcified tissues that record growth over time. Accurate age data is foundational for conservation and fisheries management because many shark species grow slowly, mature late, and have long lifespans. Without this data, managers cannot accurately set quotas or determine the sustainability of populations sensitive to overexploitation.
Reading the Rings: Vertebral Centra Analysis
The most common technique for aging a shark involves analyzing the vertebral centra, the spool-shaped segments of the backbone. Although the skeleton is primarily cartilage, the centra are reinforced with mineralized cartilage that calcifies in concentric layers as the animal grows. These layers form visible bands, or annuli, analogous to tree growth rings.
To prepare the centra, scientists extract a section of the vertebral column, often near the pectoral fins where the centra are largest. Soft connective tissue must be meticulously removed, often by soaking the centra in a mild chemical solution. After cleaning, the centra are typically sectioned using a low-speed saw or ground down to a thin, translucent wafer to expose the internal growth bands.
Because the growth bands are not always distinct, chemical staining is frequently used to enhance contrast. A common method involves immersing the prepared section in a 1% silver nitrate solution and exposing it to ultraviolet light. The silver reacts with the mineralized tissue, darkening the bands and making them visible under a microscope.
The resulting pattern reveals alternating opaque and translucent bands, with each pair generally representing one year of growth, though this periodicity must be validated. Opaque bands form during periods of fast growth (warmer months or plentiful food). Translucent bands represent slower growth (colder seasons or reduced feeding). Scientists estimate the shark’s chronological age by counting the total number of band pairs from the center outward.
Alternative Hard Structures for Age Estimation
For species with poorly calcified vertebrae or smaller sharks, alternative hard structures are used.
Dorsal Fin Spines
In the dogfish family (Squalidae), the dorsal fin spines are regularly used. These spines project from the leading edge of the dorsal fins and accumulate mineralized growth layers that can be counted when sectioned.
Caudal Thorns
In skates and rays, researchers sometimes examine the caudal thorns, which are small, hardened dermal denticles found along the tail. Although their reliability varies between species, they record incremental growth. Using fin spines and caudal thorns also allows for non-lethal sampling in some cases, permitting the animal to be tagged and released.
Length-Frequency Analysis
A third, less precise method, primarily used when hard parts are unavailable (such as for very large or rare specimens), is length-frequency analysis. This technique groups individuals by size and tracks the shift in the average size of those groups over time to infer growth rates. However, because individual growth rates vary significantly and the technique does not rely on counting biological growth markers, it is generally considered less accurate for determining individual age.
Validating Age Estimates: Advanced Techniques
Counting growth bands provides only an estimate, so scientists must perform validation studies to confirm that the bands are deposited annually, a concept known as periodicity.
Marginal Increment Analysis (MIA)
MIA tracks the growth of the outermost band edge (the margin) over a calendar year. Scientists sample multiple sharks throughout the year and measure the width of the forming band, looking for a distinct, cyclical pattern. If the margin width consistently increases for six months and then decreases or stabilizes for the remaining six months, it provides strong evidence that one band pair is formed annually.
Bomb Radiocarbon Dating
This specialized technique offers absolute age verification. Atmospheric nuclear weapons testing in the 1950s and 1960s released a pulse of Carbon-14 (\(\text{C}^{14}\)) into the oceans, which was incorporated into the calcified tissues of marine organisms. This \(\text{C}^{14}\) spike provides an undeniable time marker. Scientists extract material from a specific growth band and compare its \(\text{C}^{14}\) signature to a known environmental reference curve, confirming the year the band was formed. This technique is valuable for validating annuli counts in long-lived species, such as the Greenland shark, where traditional counting methods are difficult to apply to the crowded bands of older individuals.
Chemical Markers
Direct validation also involves using chemical markers, such as the antibiotic tetracycline. In a controlled setting, a live shark is injected with tetracycline, which binds to newly mineralizing cartilage and leaves a fluorescent ring on the centra. If the shark is later recaptured, the elapsed time is compared to the number of new growth bands formed outside the fluorescent ring. This process confirms the annual deposition rate for that species, verifying the accuracy of the growth band count.