Age determination in fish

Sagittal otolith from a 74-year-old (84-YO?) freshwater drum, Aplodinotus grunniens

Fish otolith on a sectioning saw

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.”[4] However, it wasn't until the development of the microscope that more detailed studies were performed on the structure of scales.[5] 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.[5] 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.[5] 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.”

In 1898, more than 200 years after Leewenhoek's original insights of scale age structure, this subject was given a thorough review by C. Hoffbauer.[5] Hoffbauer studied commercially grown carp scale growth patterns throughout the year. He noted that during the season of growth, the concentric rings were easily discernible and widely spaced; however, as growth slowed and ceased during the winter months the rings were very compact then resumed normal spacing as the growth season began again. His work convinced other researchers that these aging techniques could be used on marine species. Shortly after Hoffbauer's findings were published, structures other than scales were examined for utility of aging fish. Johannes Reibisch, working for the Commission of Scientific Investigation of German Seas at Kiel, attempted to use Hoffbauer's techniques to age plaice (Plueronectes platessa) but found it difficult to accurately discern annuli. He decided to study a different structure and in 1899 he published the first procedures using otoliths as an aging structure.[5] A fellow scientist also with the German Commission at Kiel, Friedriche Heincke, also frustrated with difficult scale annuli, further studied other structures to age fish. He discovered annuli in the vertebrae, opercula, and pectoral girdle and published his findings in Heicke 1905.

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.

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.[6] 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.[5]

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.

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.

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.