Annuli and Age Determination
Scales, bones, fin rays and otoliths have all been used to determine
the age of fish, since these and other bony parts of fish often form
yearly rings (annuli) like those of a tree. However, otoliths
generally provide the most accurate ages, due largely to their
continued growth throughout the life of the fish and their acellular
nature (which implies that they are not subject to resorption). These
features give otoliths a significant advantage over scales and other
structures, particularly in old fish. Therefore, otoliths have become
the preferred structures for age determinations.
Methods for ageing sharks
or skates and
rays are analogous, but use vertebrae.
Despite the long-term global use of otolith annuli as age indicators,
the factors influencing their formation is not clear cut. Each annulus
is comprised of an opaque and translucent zone, which in many
species correspond to fast and slow growth respectively. The
opposite pattern is apparent in other species. In general though, the
opaque zone seems to be formed during periods of increasing water
temperatures, while the translucent zone is formed during periods of
reduced growth, or in association with spawning.
The preferred method for otolith preparation and interpretation
differs among species and age readers. In general however,
microscopic examination of whole otoliths (immersed in a clear
fluid) is acceptable for thin otoliths, while sectioning or the "crack
and burn method" is
required for other otoliths. Since otolith length may cease to grow
in old fish, even while they continue to thicken, some form of cross
section is required of old otoliths. The best papers on otolith
preparation and annulus interpretation in temperate and tropical fish
otoliths are presented in Chilton and Beamish (1982) and Fowler
(1995) respectively. Detailed methods on preparing
thin sections are also presented elsewhere in this site.
|In keeping with ages derived from otolith
microstructure, annular age determinations are an acquired skill,
with a significant subjective component.
|For this reason, age validation is an
important component of any ageing study, not only to confirm that
the annular rings are visible in the otolith, but that they are being interpreted correctly.
|As noted in
Quality Control in Ageing consistency between age readers or with other bony structures is not
a measure of accuracy.
| An additional source of error is the correct
interpretation of the innermost (first) annulus, for which independent validation is
|The Ageing Unit at the Bedford Institute of
Oceanography is responsible for ageing between 5,000-10,000
otoliths annually in support of stock assessments for cod, haddock
and silver hake (see Production
|Cod and haddock otoliths are
mass-processed: batches of 100 otolith pairs are carefully aligned
and embedded in a black polyester resin. Transverse cross sections
of the resin block are subsequently cut with a high-speed,
water-cooled saw, yielding 100 otolith pair cross sections per
plate. After coating with a transparent coating (Krylon
Crystal Clear), the plates are dried and aged. All ageing is carried
out under an image analysis system. Quality control through the
ageing process is rigorous (see Quality Control