What is an otolith?

halibut otolith photo

Otoliths, also called ear-bones, are structures made mostly of calcium carbonate that are found in the head of most fish. Otoliths act as sound receptors and also play a role in balance and orientation. As the fish grows, so does the otolith, by deposition of concentric layers of material. Seasonal changes in the fish's growth rate are reflected in the otolith. A year's growth consists of a wider summer zone (reflecting faster growth) and a narrower winter zone (reflecting slower growth). Because halibut spawn in winter, the winter zones are counted to determine the age of the fish in years. These annual growth rings, or "annuli", are very similar in appearance to the growth rings of trees.

Ages are used for estimating growth and mortality rates as well as population age structure. Age data are incorporated into the IPHC's annual stock assessment. In the past, the IPHC also used otolith weight and length to estimate the size of halibut, though this later proved to be inaccurate.

Each year, alternating opaque (summer) and translucent (winter) rings are deposited on the otolith. The oldest age recorded for Pacific halibut is 55 years for a 118 cm male (~36 lbs, net) captured in 1992 in the Bering Sea on IPHC's setline survey. The oldest recorded age for a female is also 55 years. This female was 161 cm long (about 100 lbs, net) and was captured in the Bering Sea in June 2000, also on an IPHC survey. The mean age in years of the commercial catch has been 12-13 for the last several years.

Currently there are four staff members doing production aging of survey, commercial, and tag recovery halibut otolith samples. Sport-caught halibut otoliths from Alaska are also aged. Approximately 30,000 otoliths are read per year.

3D otolith drawing

Notes/guidelines for aging halibut

Aging methods used at the IPHC include the following:

  • Surface Ages: Otolith are read while on a piece of black cloth, immersed in water (to minimize glare from the light source and maximize contrast). The distal surface of the otolith is observed during surface reads; the proximal surface has a deep groove and annuli are obscured.
  • Break & Bake: Break and bake involves scoring the otolith surface through the nucleus with a razor blade, then snapping the otolith in two. We bake one of the halves and keep the other half unbaked so we can still do a surface reading. Baking the sections enhances the contrast between summer and winter zones (winter zones turn dark brown). Previously we heated the otolith sections one at a time over an alcohol flame (Break & burn); however, baking allows us to heat many sections at a time, saving time. We use metal trays for baking; they are divided into 50 indented cells, which keep the otoliths from getting mixed up. The baked sections are then mounted in plasticene and coated with mineral oil or glycerin solution before viewing under a dissecting microscope.

Through 2001, Pacific halibut otoliths were all surface aged at IPHC. The criteria used between 1992 and 2001 included performing break & burn or break & bake age determinations in cases where readers were not confident of the surface age, (e.g., thick/steep edge, opaque or cloudy surface, odd growth pattern, high surface age, etc.). The break & burn/bake method of age determination was validated by a bomb radiocarbon study and since 2002, all longline (survey and commercial) and sport-caught halibut are aged by break & bake technique. Since 2002, only otoliths from the trawl survey collections are surface aged. If the surface age is 5 or greater, the otolith is broken and baked. Trawl-caught otoliths that are obviously older than five are not surface-aged first.

We only collect and read the left or blind side sagittal otolith at IPHC. The right and left otoliths are not mirror images as they are in some species, and right otoliths are harder to read and give less accurate ages. We also do not age crystallized otoliths. Reasons for crystallization are unclear, but crystallization occurs in other fish species as well and one or both of the pair can be affected. The inorganic portion of crystallized otoliths is made up of calcium carbonate, just as in "normal" otoliths, but the crystalline structure is different and growth patterns are difficult to interpret. Total between-reader percent agreement of between 55 and 80% or agreement within one year for 80-95% of the readings is usual for halibut otoliths.

Otoliths contain other useful information besides age. They can be used to identify fish species in stomach contents of other fish or mammals, and have been used as biological "recorders" of environmental changes using growth patterns or trace elements within the structure of the otolith.