question: How do they measure the age of a very old shark for example?
comment: I believe that, specifically for sharks, there are indications of age in their teeth. They shed and grow teeth constantly. They grow a new row of teeth so often, and that new growth leaves some sort of tell in their mouth. I could also be dead wrong.
passages: In 1977, Dr. Dick Beamish and Doris Chilton of the Pacific Biological Station published an article showing that cross sections of the fourth to eighth fin rays from the second dorsal fin provided a method for estimating the age of lingcod. This method has since been validated by a mark-recapture study in which lingcod received an injection of oxytetracycline. Other methods of aging, such as those using scales and otoliths, were found to underestimate ages for older fish.
   Scales are the most widely used aging structure in North America because of their non-lethal ease of collection. Counting the number of annuli (rings) on a scale provides the fish age and the spacing between rings is proportional to the growth of the fish. For some examples and uses of scale aging you can go to "Fish scales tell a story..." from the Delaware Division of Fish & Wildlife. The ease of collection of this aging structure is not without its tradeoffs, as the major bias of scales used as an age estimation structure is their tendency to underestimate the age of older fish.
   BULLET::::- Polat N, Bostanci D, Yilmaz S (2001) Comparable age determination in different bony structures of Pleuronectes flesus luscus Pallas, 1811 inhabiting the Black Sea. Turk J Zool 25:441–446.
  
  BULLET::::- Khan, MA and Khan, S. (2009). Comparison of age estimates from scale, opercular bone, otolith, vertebrae and dorsal fin ray in Labeo rohita (Hamilton), Catla catla (Hamilton) and Channa marulius (Hamilton).Fish Res 100:255–259.
   Ages are determined from fins in much the same manner as for other aging structures: sections of varying thickness are examined under a microscope, and the annuli, or rings, that are formed for each year of growth are counted and used to estimate the age. The cross sections must be made at right angles to the length of the fin ray, and it is therefore important that fins be dried flat, with the cut surface at right angles to the fin rays. In addition, the distance the section is cut from the fin ray base is important, so all fins should be collected with the bases intact.
   Age and growth studies of fish are important for understanding such things as timing and magnitude of spawning, recruitment and habitat use, larval and juvenile duration, and population age structure. Such knowledge is in turn important for designing appropriate fisheries management policies.
  
  Section::::Ecology.:Diet research.
   Section::::Aging structures and techniques.:Calcified or bony structures.
   Biological studies carried out by Kirsty Spalding demonstrated that neuronal cells are essentially static and do not regenerate during life. She also showed that the number of fat cells is set during childhood and adolescence. Considering the amount of C present in DNA she could establish that 10% of fat cells are renewed annually. The radiocarbon bomb pulse has been used to validate otolith annuli (ages scored from otolith sections) across several fish species including the freshwater drum, lake sturgeon, pallid sturgeon, bigmouth buffalo, arctic salmonids, "Pristipomoides filamentosus""," several reef fishes, among numerous other validated freshwater and marine species. The precision for bomb radiocarbon age validation is typically within +/- 2 years because the rise period (1956-1960) is so steep. The bomb pulse has also been used to estimate (not validate) the age of Greenland sharks by measuring the incorporation of C in the eye lens during development. After having determined the age and measured the length of sharks born around the bomb pulse, it was possible to create a mathematical model in which length and age of the sharks were correlated in order to deduce the age of the larger sharks. The study showed that the Greenland shark, with an age of 392 +/- 120 years, is the oldest known vertebrate.
   Shortfin mako sharks, as with most other sharks, are aged by sectioning vertebrae — one of the few bony structures in sharks — and counting growth bands. The age of shortfin mako, and therefore important parameters, such as age at sexual maturity and longevity, were severely underestimated until 2006 (e.g. claims of sexual maturity at 4–6 years, claims of longevity as low as 11 years), because of a poorly supported belief that shortfin mako sharks deposited two growth bands per year in their vertebrae. This belief was overturned by a landmark study which proved that shortfin mako sharks only deposit one band in their vertebrae per year, as well as providing validated ages for numerous specimens. Natanson et al. (2006) aged 258 shortfin mako specimens and recorded:
   The works of Hoffbauer, Reibisch, and Heinke are most often cited as establishing scales, otoliths, and bony structures as viable aging structures. Further, Tereshenko (1913) is credited as the first to use cleithra aging techniques on roach; and Holtzmeyer (1924) with using fin rays to age sturgeon.
  
  Section::::Analysis of ages.
   Age structure analysis can be performed by the above methods which are the most direct, through estimates of length and weight, or a combination of both. Once the data is acquired and individuals are arranged in their respective age classes, one can attempt to attribute trends to the age distribution. For example, in Jaurequizar and Guerrero (2009), the researchers were examining the age structure of a population as a function of a period of four years which experienced varying environmental conditions (two average years to El Niño and La Niña years). The dominant age classes were affected by the environmental conditions.
   Section::::Analyzing age class structure.
   In addition, in most species the accretion of calcium carbonate and gelatinous matrix alternates on a daily cycle. It is therefore also possible to determine fish age in days. This latter information is often obtained under a microscope, and provides significant data to early life history studies.
   Section::::History.
   The general process for scale age analysis preparation is as follows. During collection, it is important to make sure to sample the same area on the same side of each individual. Insert the scale into a scale envelope, then press on acetate slides or it can be washed in distilled water and rubbed between the fingers. Mount the scale on glass slides and dry in moderate heat, 37˚C or 100˚ F. The annuli may be counted using a microscope, microfilm reader, or other such magnification device.
  
  Section::::Aging structures and techniques.:Otoliths.
   Not long after Hoffbauer's and Reibisch's findings were published, aging was used in fishery assessments of the early 1900s. One of the first to focus on the applications of fish aging was the Norwegian fisheries scientist Johan Hjort. Focusing on fish scales, Hjort developed an extensive aging program collecting statistics on birth rate, age-distribution and migration. Hjort's research elicited debate from the biomathematician D’Arcy Wentworth Thompson, who later rescinded his criticisms. His research otherwise received glowing praise and would lead to fundamental changes in the way fish populations were studied and managed.
  
  Section::::Aging structures and techniques.
  
  Section::::Aging structures and techniques.:Scales.
   Section::::Other methods.
  
  Cattle age in a carcase is determined checking the physiological skeletal maturity (ossification) (red) of the tips or “buttons” of the thoracic vertebrae. The size and shape of the rib bones are important considerations as well as the colour and texture of the flesh.
  
  The use of number (year) branding, tattoos or ear tags with numbers or different colours are good methods of identifying the age of cattle, if they are used according to standards.
  
  Section::::External links.
  
  BULLET::::- Age Determination in Beef Cattle
  
  BULLET::::- Determining the Age of Cattle by Their Teeth
  
  BULLET::::- Using Dentition to Age Cattle
   One problem associated with using fin rays to age older fish is the center may be resorbed, resulting in the loss of the first two annuli. It is therefore necessary to determine an average width for the first two annuli by examining the fins from juvenile fish. This measurement can then be used to estimate the position of the third annulus on older fish.
  
  Section::::References.
  
  "This article incorporates material from Fisheries and Oceans Canada. This reproduction was not done in affiliation with or with the endorsement of Fisheries and Oceans Canada."
  
  Section::::External links.
   BULLET::::- A study on the taxonomic status and geological age of large shark remains from the Upper Cretaceous of the Castellavazzo locality (Italy) discovered in the 19th century is published by Conte "et al." (2019).
  
  BULLET::::- 15 partial skeletons of lamniform sharks, including the largest specimen of "Cretoxyrhina mantelli" known to date, are described from the Upper Cretaceous Scaglia Rossa Formation (Italy) by Amalfitano "et al." (2019), who also review the taxonomic history of "C. mantelli".
   Fish ages are often examined along with measurements of length and weight which combined can provide information on stock composition, age at maturity, life span, mortality, and production. Other purposes of performing age structure analysis are growth analysis, population dynamics estimates and resource management. Data from a particular study can delineate individuals into specific age classes. Exploited species often have the older, larger individuals removed from the population because they are the first removed by fishers leaving the younger smaller individuals. This effect may have serious consequences for that population. By performing age analysis studies we can identify these types of effects as well their implications to the status of the population.
   Aristotle (ca. 340 B.C.) may have been the first scientist to speculate on the use of hard parts of fishes to determine age, stating in "Historica Animalium" that “the age of a scaly fish may be told by the size and hardness of its scales.” However, it wasn't until the development of the microscope that more detailed studies were performed on the structure of scales. Antonie van Leeuwenhoek developed improved lenses which he went use in his creation of microscopes. He had a wide range of interests including the structure of fish scales from the European eel ("Anguilla anguilla") and the burbot ("Lota lota"), species which were previously thought not to have scales. He observed that the scales contained “circular lines” and that each scale had the same number of these lines, and correctly inferred that the number of lines correlated to the age of the fish. He also correctly associated the darker areas of scale growth to the season of slowed growth, a characteristic he had previously observed in tree trunks. Leeuwenhoek's work went widely undiscovered by fisheries researchers, and the discovery of fish aging structures is widely credited to Hans Hederström (e.g., Ricker 1975). Hederström examined the vertebrae of pike ("Esox lucius") and concluded that each contained growth rings which could then be used to determine the fish's age. In 1859, Robert Bell reported that one could use these growth rings to reliably determine the age of all fish after examination of sucker ("Catastomus sp.") vertebrae and yellow perch ("Perca flavescens") scales that he raised in a pond for two years showed “two rings or circles.”
   Stable isotope values of otoliths have also been used to determine climate in the past Fish otoliths as old as 172 million years have been used to study the environment in which the fish lived. Robotic micromilling devices have been used to recover very high resolution records of diet, life history and temperatures throughout the life of the fish, including their natal origin
   While the age analysis has been around in some form for over 250 years, there has only more recently been a rapid advance in techniques and uses of this information. There are still efforts required to further validate these aging methods and to determine new techniques. As the population of the world's fish continues to decline due to exploitation age structure analysis data will only become more important as we to try understand multiple effects on population dynamics.
  
  Section::::References.
  
  BULLET::::- Helfman, G.S., Collette, B.B., and Facey, D.E. The Diversity of Fishes. Blackwell Science, 1997.
   Age can be determined by counting growth rings in fish scales, otoliths, cross-sections of fin spines for species with thick spines such as triggerfish, or teeth for a few species. Each method has its merits and drawbacks. Fish scales are easiest to obtain, but may be unreliable if scales have fallen off of the fish and new ones grown in their places. Fin spines may be unreliable for the same reason, and most fish do not have spines of sufficient thickness for clear rings to be visible. Otoliths will have stayed with the fish throughout its life history, but obtaining them requires killing the fish. Also, otoliths often require more preparation before ageing can occur.
   Estimating age at death for adults involves observing morphological features in the skeletal remains, comparing the information with changes recorded for recent populations of known age. With subadults, the development of teeth, length of long bones, and union of epiphyses are used to estimate age. With adults, age is measured through macroscopic and microscopic methods. Macroscopic methods do not involve destruction of the specimen while microscopic methods are more time intensive and require equipment, some destruction, and specialized knowledge. Despite some disadvantages, microscopic methods give more accurate results.
   Due to this error, fishery management models and ecological risk assessment models in use around the world were underestimating both the longevity and the age at sexual maturity in shortfin mako sharks, particularly in females, by two-thirds or more (i.e. 6 years versus 18+ years), and some of these inaccurate models remain in use.
  
  Section::::Life history.:Intelligence.
  