The former reader on the left gave up an exciting and rewarding career in fish aging, but still works for the IPHC doing other exciting stuff.   Check out our  "satellite" reader.

In fish, the otolith serves as a sensory organ, playing a role in hearing and balance. We collect otoliths then use them to age halibut at the IPHC, and that's my main job here. As the fish grows, so does the otolith, and we have used otolith weight and length to estimate the size of halibut. Other properties such as shape have been used to distinguish between stocks of fish in other species. At IPHC, we have tried using otolith shape and a combination of otolith weight, length and age to discriminate sex. (When otolith samples are collected from the commercial halibut catch, the fish have been eviscerated and sex of individual halibut is not known, so we are looking for a way to get sex information from the commercial catch so it can be included in our stock assessment).

Each year, alternating opaque (summer) and translucent (winter) rings are deposited on the otolith. Here is a 3-D illustration of an otolith in cross-section. A year's growth consists of both an opaque and translucent zone. Since halibut are spawned in the winter and have an arbitrary January 1 "birth date", the  translucent or winter zones are counted to determine the age of the fish in years. The winter growth zones are also referred to as annuli (singular: annulus).  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 one of our setline surveys.  He was aged by Cal Blood. 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. This is the former reader who aged her. Here are photos of a couple more old halibut otoliths. The mean age in years of the commercial catch has been 12-13 for the last several years.

Currently there are four of us doing production aging of survey, commercial, and tag recovery halibut otolith samples.  A fifth reader ages sport-caught halibut otoliths from Alaska. The following are some notes/guidelines for aging halibut.

Aging methods used at the IPHC include the following:
Surface ages: These are made with the otolith immersed in water on a piece of black cloth in a container (to minimize glare from the light source and maximize contrast, respectively). We look at the distal surface of the otolith for surface reads; the proximal surface has a deep groove and annuli are obscured.
Break&burn: Our break&burn technique is fairly standard, except that we don't usually sand or polish the broken surface before burning. The otolith surface is scored through the nucleus with a razor blade then snapped in two. After burning one of the halves (keeping one unburned to repeat a surface reading), the sections are held in position by mounting in plasticene then coated with mineral oil or glycerin solution before viewing.
Break&bake: In 1999, one of our group compared the break and burn technique with the "break and bake" method.  Break and bake involves breaking the otolith through the nucleus in the same manner as is done for break and burn, but the sections are baked in a toaster oven at 500 degrees F for 10-15 minutes.  In the study, the two halves of the otolith were given either the bake or burn treatment. Treatment was alternated between anterior and posterior halves.  There was no significant difference in ages produced by the two methods, however the variance was somewhat lower for ages of otoliths that underwent the bake treatment. Break and burn is more time efficient for small numbers of otoliths, whereas baking is more efficient when there are a large number of otoliths to be burned, since many can be baked at once.  We use metal trays for baking that are divided into 50 indented cells, which keep the otoliths from getting mixed up.

Through 2002, Pacific halibut otoliths were all surface aged at IPHC.    The criteria used prior to 2003 included performing break&burn or break&bake age determinations in cases where we were not confident of the surface age, (e.g., thick/steep edge, opaque or cloudy surface, odd growth pattern, high surface age, etc.). In 2002, the criteria were expanded to include performing break&burn/bake readings on all otoliths with surface age greater than or equal to 15 years. Since 2003, all longline (survey and commercial) and sport caught halibut are aged by break&bake technique. Since 2003, 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 we feel they give less accurate ages.  On the year 2000 setline and trawl surveys, both right and left otoliths were collected from a portion of the halibut caught.  The paired ages, both surface and break and bake, were compared.  We had only compared surface ages between right and left side otoliths in the past.  We found no significant difference between ages determined from the two sides of a pair for fish under 6 years of age. For fish older than 6, the age read from the right side of the otolith pair was lower than that of the left side for both surface and break&bake methods.  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.  In crystallized otoliths the form is called vaterite, in normal otoliths it's called aragonite.  The survey otolith pairs collected in 2000 were also part of a crystallization study in which  rates of right and left side crystallization and  rates of left side crystallization by area of capture were compared. Crystallization was found to be slightly more common in females; fully crystallized otoliths were more common than partially crystallized otoliths, and in the paired otolith collection, the rate of crystallization in left otoliths was higher than that of right side otoliths.

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 otolith surfaces.  Agreement is somewhat less for break and burn/bake.  A recent validation study confirmed the annual periodicity of hyaline zones.  Halibut were caught, injected with oxytetracycline (OTC), tagged and released.  OTC leaves a permanent mark on bony structures which is visible under ultraviolet light.  Comparing the number of annuli counted after the mark with the number of years at large gives an idea of the accuracy of the age determination.  Photos of OTC marked halibut otoliths: (here)

Many of the commercial otolith samples are collected in May and June, and often the translucent or winter zone has not yet been deposited, or is still in the process of forming on the otolith edge. There is a problem of deciding whether opaque edge growth on a particular otolith is new (from the current spring or summer) or from the previous summer. As a general rule for fish captured in this time period, IPHC readers have included the opaque edge in the annulus count if the edge growth is greater than half the width of the previous opaque (summer) zone in fish older than 10 years, or almost the same width as the previous opaque zone in fish younger than 10 years. The opaque edge is not counted in younger fish unless it is about the same width as the previous summer's zone because young halibut start their growth season earlier in the year when compared with older fish, and may already have close to half the previous year's width of new growth by late May or early June. Sometimes fish collected in fall or even late summer may have translucent growth on the edge. In this case, we believe the fish has started laying down the coming winter's translucent zone early (rather than including the translucent area on the edge in the age, thereby assuming the fish had no summer growth.

A marginal increment study is underway to measure the amount and type of edge growth on otoliths collected throughout the year to aid in edge interpretation. Otoliths will be thin-sectioned for this study. Thin-sections will also be evaluated as an alternative preparation for age determination.

Here is a picture of a very clear otolith from a 11-year-old halibut (here). Some of them look more like this.
Download IPHC Pacific halibut aging manual (low resolution: 3.15 MB, high resolution: 27.2 MB)

Here are some otolith-related sites on aging:

• CARE (committee of age reading experts), composed of otolith readers from the west coast of Canada and the U.S., which meets biennially in Seattle, WA.
• Alaska Fisheries Science Center (AFSC) Age and Growth Task in Seattle, WA
• AFSC interactive age reading demonstration site
  
• age determination methods for Northwest Atlantic fish and shellfish species
• ADF&G Tag/Otolith Lab in Auke Bay, Alaska
• the Otolith Research Lab in Dartmouth, Nova Scotia, Canada
• Otolith morphology page from SUNY Stony Brook
  
• the European Fish Ageing Network (EFAN)

• Introduction to biochronology
• PIXE (Proton Induced X-ray Emission) analysis of otoliths, University of Manitoba
• temporal signatures