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February 2012

NOAA Professional Paper NMFS 13

Age determination manual of the

Alaska Fisheries Science Center

Age and Growth Program

Mary Elizabeth Matta and Daniel K. Kimura (editors)
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NOAA Professional Paper NMFS 13

Age determination manual of the

Alaska Fisheries Science Center

Age and Growth Program

Mary Elizabeth Matta and Daniel K. Kimura1 (editors)

Chapter authors:

Delsa M. Anderl, Irina M. Benson, John D. Brogan, Christopher M. Gburski, 

Betty J. Goetz, Charles E. Hutchinson, Christopher G. Johnston, Craig R. Kastelle, 

Daniel K. Kimura, Mary Elizabeth Matta, Charles E. Piston, and Wes Shockley2

Email address for contact editor: beth.matta@noaa.gov

Resource Ecology and Fisheries Management Division

Alaska Fisheries Science Center

National Marine Fisheries Service

National Oceanic and Atmospheric Administration

7600 Sand Point Way NE

Seattle, WA 98115

1 Retired; email: dankimura@comcast.net

2Deceased.

U.S. Department of Commerce

Seattle, Washington

February 2012
Suggested reference

Matta, Mary Elizabeth, and Daniel K. Kimura (editors). 2012. Age determination 

manual of the Alaska Fisheries Science Center Age and Growth Program. NOAA 

Professional Paper NMFS 13, 97 p.

Online dissemination

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Professional Papers link).

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because of this NMFS publication.
iii

CONTENTS

Chapter 1: Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Chapter 2: Otolith growth pattern interpretation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Otolith morphology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Marginal growth and the international birth date convention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Chapter 3: Collection and preparation of otoliths for age determination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Otolith dissection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Otolith preservation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Methods of preparing otoliths for age determination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Chapter 4: Groundfish age determination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Walleye pollock (Theragra chalcogramma). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Pacific cod (Gadus macrocephalus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Atka mackerel (Pleurogrammus monopterygius). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Sablefish (Anoplopoma fimbria). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Rockfish (Sebastes) species. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

 Pacific ocean perch (Sebastes alutus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

 Northern rockfish (Sebastes polyspinis). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

 Rougheye rockfish species complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Greenland halibut (Reinhardtius hippoglossoides). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Arrowtooth flounder (Atheresthes stomias). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Yellowfin sole (Limanda aspera). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Chapter 5: Precision and bias in age determination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Precision testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Measures of bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Chapter 6: Age validation methods used at the Alaska Fisheries Science Center. . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Direct methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Indirect methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Literature cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Appendix. Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

1

Age data can provide considerable 

insight into fish population dynamics. 

Age determination is particularly 

important for marine fishes because 

they are often difficult to census at 

every life cycle stage. At the National 

Marine Fisheries Service’s Alaska Fisheries Science Center (AFSC), fish 

age data, including catch-at-age data 

collected from commercial fisheries 

and population age compositions estimated from scientific bottom trawl 

surveys, are used to develop age-structured stock assessment models. These 

models evaluate the overall health of 

fish populations and guide fishery 

managers in setting sustainable catch 

limits.

Age determination science relies 

on growth marks deposited on a daily 

or annual basis within the hard structures of marine organisms. Otoliths, 

sometimes referred to as ear bones, 

are of unique value for age determination of teleost fishes because 

across taxa they are the only hard 

structures that continue growing even 

after somatic growth has ceased. In 

cartilaginous fishes such as sharks and 

skates, otoliths are not well-calcified, 

so researchers must rely on other hard 

structures such as vertebrae or spines 

for age estimation.

Although annual marks in otoliths 

and vertebrae are similar to tree 

rings, their appearance is generally 

much fainter and more irregular. 

Another major problem in counting annual marks is that the timing of the deposition of new growth 

zones varies considerably by species, 

age, and exogenous factors such as 

geographic location and climate. 

These issues may increase the difficulty of interpreting annual marks 

in aging structures, thus requiring 

the specialized expertise of biologists trained in age determination.

Abstract—The Age and Growth Program at the Alaska Fisheries Science Center is tasked with providing age data in 

order to improve the basic understanding 

of the ecology and fisheries dynamics of 

Alaskan fish species. The primary focus 

of the Age and Growth Program is to 

estimate ages from otoliths and other 

calcified structures for age-structured 

modeling of commercially exploited 

stocks; however, the program has recently 

expanded its interests to include numerous studies on topics ranging from age 

estimate validation to the growth and lifehistory of non-target species. Because so 

many applications rely upon age data and 

particularly upon assurances as to their accuracy and precision, the Age and Growth 

Program has developed this practical 

guide to document the age determination 

of key groundfish species from Alaskan 

waters. The main objective of this manual 

is to describe techniques specific to the 

age determination of commercially and 

ecologically important species studied by 

the Age and Growth Program. The manual also provides general background information on otolith morphology, dissection, 

and preparation, as well as descriptions of 

methods used to measure precision and 

accuracy of age estimates. This manual is 

intended not only as a reference for age 

readers at the AFSC and other laboratories, but also to give insight into the quality 

of age estimates to scientists who routinely 

use such data.

Chapter 1: Introduction

by Daniel K. Kimura and Mary Elizabeth Matta 

The primary role of the AFSC Age 

and Growth Program is to provide age 

estimates to support stock assessments 

of commercially exploited groundfish 

species in Alaskan waters (Fig. 1). The 

program has aged thousands of fish 

since its inception in the late 1970s. 

However, we also conduct novel scientific research. Our program has performed numerous validation studies 

to evaluate the accuracy of our age estimates. We have also investigated the 

life histories of non-target species and 

developed new aging criteria for species that are difficult to age. Recent 

research has examined relationships 

between environmental variables and 

growth. A summary of the fish species aged and studied by the Age and 

Growth Program is listed in Table 1.

The purpose of this manual is to 

report age determination methodologies used by the Age and Growth 

Program. Without adequate documentation, unwanted drifts in age 

estimation practices could occur. This 

manual provides a means to ensure 

consistency over time and among 

different age readers by acting as a record of the criteria used to determine 

age for many of the key species studied at the AFSC. A subsequent goal 

of this manual is to provide insight 

into our data quality by describing 

the level of accuracy and precision associated with our age estimates. This 

manual represents the culmination 

of decades of work, benefitting from 

the expertise of a diverse group of 

scientists who have worked in or who 

are currently working in the Age and 

Growth Program.

In this extensive document, we begin by discussing fish otolith morphology and edge interpretation 

(Chapter 2) and the dissection, preservation, and preparation of otoliths 

for age determination (Chapter 3). In 
2 Professional Paper NMFS 13

Chapter 4, the most important chapter of this manual, 

we give descriptions of the age determination of all 

major species studied by the Age and Growth Program. 

Each species section within Chapter 4 includes relevant 

biological information, history of age determination 

methods, otolith preparation, growth pattern interpretation, age estimation, and availability of information supporting age estimate accuracy. The remaining 

chapters explain procedures used to maintain the precision of our age estimates (Chapter 5) and validation 

research (Chapter 6).

Age determination is a science that relies heavily on 

technical terminology. Definitions for these terms can 

be found in the glossary at the end of this manual. Images of otoliths have also been included throughout the 

manual to provide examples demonstrating how aging 

criteria are applied for a variety of species. Unless otherwise noted, all otolith images were taken with a digital 

camera mounted on a dissecting microscope, using 

reflected light emitted from a fiber-optic light source. 

Dots or numerals have been used to indicate the position of presumed annual marks in most otolith images.

Supplementary material for this manual, containing 

additional species chapters and more detailed information on age determination, is available from the AFSC 

Age and Growth Program website (Alaska Fisheries Science Center, http://www.afsc.noaa.gov/REFM/Age/

Default.htm). The supplementary material is intended 

to serve as a working document for age readers and 

age data users by describing historic and current age 

estimation practices. Due to the evolving nature of 

fish age determination science, we expect to add new 

chapters to the online document with the advent of new 

technologies.

Figure 1

Major bodies of water where the groundfish species described in this manual are found and harvested.
Chapter 1: Introduction 3

Table 1

Groundfish species currently aged by the Alaska Fisheries Science Center Age and Growth Program. Age type is classified 

as “production” (i.e., routinely aged on an annual or biennial basis to support stock assessment) or “limited” (i.e., aged 

infrequently for stock assessment or research purposes).

Common name Scientific name Age type

Alaska plaice Pleuronectes quadrituberculatus Production

Alaska skate Bathyraja parmifera Limited

Arctic cod Boreogadus saida Limited

Arrowtooth flounder Atheresthes stomias Production

Atka mackerel Pleurogrammus monopterygius Production

Bering flounder Hippoglossoides robustus Limited

Bigmouth sculpin Hemitripterus bolini Limited

Big skate Raja binoculata Limited

Blackspotted rockfish Sebastes melanostictus Production

Capelin Mallotus villosus Limited

Dark rockfish Sebastes ciliatus Production

Dover sole Microstomus pacificus Production

Dusky rockfish Sebastes variabilis Production

Eulachon Thaleichthys pacificus Limited

Flathead sole Hippoglossoides elassodon Production

Giant grenadier Albatrossia pectoralis Limited

Great sculpin Myoxocephalus polyacanthocephalus Limited

Greenland halibut Reinhardtius hippoglossoides Production

Harlequin rockfish Sebastes variegatus Limited

Kamchatka flounder Atheresthes evermanni Limited

Longhead dab Limanda proboscidea Limited

Longnose skate Raja rhina Limited

Northern rock sole Lepidopsetta polyxystra Production

Northern rockfish Sebastes polyspinis Production

Pacific cod Gadus macrocephalus Production

Pacific ocean perch Sebastes alutus Production

Plain sculpin Myoxocephalus jaok Limited

Prowfish Zaprora silenus Limited

Redstripe rockfish Sebastes proriger Limited

Rex sole Glyptocephalus zachirus Production

Rougheye rockfish Sebastes aleutianus Production

Sablefish (black cod) Anoplopoma fimbria Production

Sharpchin rockfish Sebastes zacentrus Limited

Shortraker rockfish Sebastes borealis Production

Shortspine thornyhead Sebastolobus alascanus Limited

Southern rock sole Lepidopsetta bilineata Limited

Walleye pollock Theragra chalcogramma Production

Warty sculpin Myoxocephalus verrucosus Limited

Yellow Irish lord Hemilepidotus jordani Limited

Yellowfin sole Limanda aspera Production

Otolith morphology

Fish otoliths, sometimes referred to as ear bones, are 

calcified structures that play a role in hearing, balance, 

and spatial orientation (Popper et al., 2005). Otoliths 

are composed of calcium carbonate precipitated upon 

a protein matrix (Degens et al., 1969; Campana, 1999). 

All teleost fish have three pairs of otoliths: the asterisci, 

lapilli, and the sagittae. The sagittae are typically the 

largest in size and are the otolith pair most often used 

in age determination applications (Fig. 2).

Otoliths vary widely in shape and size across taxa 

(Fig. 3). In general, otoliths from round-bodied 

teleosts are mirror images of each other, and either 

the left or the right otolith may be selected for age 

determination (Fig. 4). In contrast, flatfish have asymmetrical sagittal otoliths (Fig. 5). Generally, the otolith from the blind side of a flatfish’s body has a centric 

core, whereas the otolith from the eyed side of the 

body has a core located posteriorly. Both otoliths can 

provide valuable information in age determination. 

The eyed-side otolith is generally more useful in estimating age from the surface, while cross-sectioning 

techniques are better applied to the blind-side otolith. 

(See Chapter 3 for a complete description of otolith 

preparation methods.)

Chapter 2: Otolith growth pattern interpretation

by Mary Elizabeth Matta and B. J. Goetz

Otoliths accrete material in concentric layers around 

the core, a process which continues even after a fish 

stops growing in length (Campana and Thorrold, 

2001). The accreted material is composed of alternating layers that differ in density and optical properties. 

These layers are referred to as being either opaque 

or translucent, and their appearance depends on the 

microscope and light source used to view them. When 

viewed against a dark background using reflected light, 

translucent growth zones appear dark and opaque 

growth zones appear light in color (Wright et al., 2002). 

The inverse is true when these growth zones are viewed 

using transmitted light.

In general, a year of otolith growth consists of one 

opaque growth zone and one translucent growth zone 

(Figs. 2 and 6). In otoliths from North Pacific fish, 

the opaque growth zone is typically wider and corresponds to periods of fast growth, and the translucent 

growth zone is typically narrower and corresponds to 

periods of slow growth. Thus, the opaque growth zone 

is sometimes referred to as the “summer zone” and 

the translucent growth zone is sometimes referred 

to as the “winter zone.” However, these nicknames 

can be somewhat misleading, since deposition of the 

translucent growth zone actually occurs in the spring 

or early summer for most of the groundfish studied 

5

Figure 2

Diagram of a typical flatfish blind-side sagittal otolith, cross-sectioned to show concentric annual growth zones and general 

otolith morphology. Illustration courtesy of J. Forsberg (International Pacific Halibut Commission, Seattle, WA).
6 Professional Paper NMFS 13

Figure 3

Composite image of otoliths from some of the groundfish species aged by the 

Alaska Fisheries Science Center Age and Growth Program. From left to right, 

top row: Greenland halibut (Reinhardtius hippoglossoides), Pacific ocean perch 

(Sebastes alutus), walleye pollock (Theragra chalcogramma); bottom row: flathead 

sole (Hippoglossoides elassodon), yellowfin sole (Limanda aspera), Atka mackerel 

(Pleurogrammus monopterygius), sablefish (Anoplopoma fimbria), Dover sole 

(Microstomus pacificus). Viewed with reflected light.

at the AFSC (Kimura et al., 2007). Exact timing of 

growth zone deposition depends on a suite of factors 

such as fish age, temperature, geographic region, and 

species.

Age readers often use the term “annulus” to describe 

an annual growth zone (typically the translucent growth 

zone). However, the phrase “annual mark” may be more 

apt because, as noted by Panfili et al. (2002) and Cailliet 

et al. (2006), the word “annulus” is derived from the 

Latin word “anus,” meaning ring or circle, not from 

“annus,” meaning year (Gove, 1961). The assumption 

of yearly growth zone deposition is critical to age determination, as it provides a timestamp matching otolith 

growth and fish age. Thus, throughout this manual we 

strive to use the term “annual mark” when referring to 

any annually deposited growth zone; the term “annulus” 

may also be used, although less commonly, to refer to 

concentric growth zones that may or may not be deposited on a yearly basis.

“Checks,” irregular translucent growth zones, are 

sometimes present in otoliths (Fig. 6). Checks are not 

annual marks, and often occur as the result of physiological or environmental stresses experienced by the 

fish during life. Checks may correspond with life history events such as settlement, migration, maturation, 

or spawning (Penttila and Dery, 1988). Checks can be 

distinguished from true annual marks by their irregular 

spacing, relatively faint appearance, and lack of continuity throughout the otolith (Fig. 6). “Splitting” is a special 

case of checking, where two or more closely spaced 

translucent growth zones are deposited in a single year. 

The number of checks observed in groundfish otoliths 
Chapter 2: Otolith Growth Pattern Interpretation 7

Figure 4

Sablefish (Anoplopoma fimbria) sagittal otoliths. Otolith pair is 

symmetrical, as is typical in round-bodied fishes. Viewed with 

reflected light.

often varies by species and geographic region and can 

increase the difficulty of growth pattern interpretation. 

Becoming familiar with the general growth pattern of a 

given species through experience helps an age reader 

differentiate between checks and annual marks.

At the AFSC, age readers typically count annual 

translucent growth zones from the core to the edge to 

obtain an age estimate. It is necessary to identify checks 

to avoid including them in the final age estimate. Age 

readers must also interpret any opaque growth observed 

at the otolith margin. Most age readers begin the age 

estimation process by viewing each whole otolith using 

a dissecting microscope with reflected light. Young fish 

with clear otolith growth patterns can often be aged 

solely from the otolith surface. In the case of older fish, 

examination of the otolith surface can act as a guide 

in interpretation of early annual marks in otolith cross 

sections.

Identification of the first annual mark is a critical step 

in age determination. Collections of juvenile fish are 

extremely helpful in this respect, as the act of matching 

length- and age-frequency modes of fast-growing young 

fish may be used to corroborate the first several annual 

marks (CARE, 2006). Checks are frequently observed 

within the first few years of life, often co-occurring with 

ontogenetic changes in diet or habitat; thus, life history 

information can also be useful in guiding age determinations (CARE, 2006). Knowledge of the spawning 

season can also give age readers an idea of the expected 

size of the first annual mark. For example, species that 

spawn earlier in the year may have more time to accrete 

opaque material around the core, resulting in a wider 

first annual mark, than those that spawn later in the 

year.

Morphology helps age readers identify reading axes 

in otolith cross sections (Fig. 6). A reading axis is the 

path from the core to the margin along which an 

age reader counts annual marks. Depending on the 

species and the clarity of the otolith, certain reading 

axes may be clearer and more accurate than others. 

Otoliths are not spherical, and thus new material does 

not accumulate at the same rate among reading axes; 

some grow much faster than others, a fact which may 

influence the type of preparation method used for age 
8 Professional Paper NMFS 13

Figure 5

Northern rock sole (Lepidopsetta polyxystra) sagittal otoliths. Otolith pair 

is asymmetrical, as is typical in flatfishes. Viewed with reflected light.

determination (see Chapter 3). It is best to compare 

age estimates from different axes in the same otolith 

whenever possible to obtain consistent annual mark 

counts. The areas adjacent to the sulcus, a groove on 

the proximal surface, are critical reading axes in many 

species, as the translucent growth zones near the sulcus 

tend to be relatively clear.

Marginal growth and the

international birth date convention

Most fish age determination facilities have agreed by 

international convention to assign a birth date of 1 January to fish regardless of their actual spawning date (Chilton and Beamish, 1982; Penttila and Dery, 1988). The 

international birth date convention allows age readers 

to estimate birth year and assign each fish to the correct 

cohort, which is integral to effective stock assessment.

The birth date convention may affect an age estimate 

when opaque growth is seen on the margin (edge) of 

an aging structure. Age readers must assign the correct 

calendar year to the growth zone observed on the margin. In other words, age readers must decide whether 

opaque growth observed on the margin occurred during the year the fish was collected or during the previous 

year. The collection date is a vital piece of information 

that enables age readers to make this decision. For example, a fish caught in August will often have opaque 

marginal growth that was laid down during the collection year, and therefore the age estimate is equivalent 

to the number of observed translucent growth zones. 

However, a fish caught in January with the same amount 

of opaque marginal growth may not yet have deposited 

the translucent growth zone marking the end of the 

previous year. Therefore, opaque growth on the edge 

of a January-collected otolith is most likely attributable 

to the past growth cycle, and the estimated age is one 

year more than the observed number of translucent 

growth zones because 1 January has passed. The same 

fish would have been assigned an age one year younger 

if it had been collected in December, even if the edge 

characteristics were identical.

For many species, opaque edge growth is observed 

earlier in the calendar year in the otoliths of young 

fish than in those of older fish (Forsberg, 2001). For 

example, a 3-year-old walleye pollock (Theragra chalcogramma) caught in June typically has a greater proportion of opaque growth on the otolith margin than a 

10-year-old walleye pollock caught at the same time. 

In other words, marginal growth is not identical across 

age groups.
Chapter 2: Otolith Growth Pattern Interpretation 9

Figure 6

Otolith from a 6-year-old walleye pollock (Theragra chalcogramma), showing growth zones and terminology used 

to refer to otolith cross sections. Sectioned transversely through the core and baked in an oven for several 

minutes to improve the contrast between opaque and translucent growth zones. Viewed with reflected light.

Otoliths collected from spring to early summer often have the most problematic patterns to interpret 

because the translucent growth zone corresponding 

to the previous calendar year has often not yet been 

deposited (Forsberg, 2001). For example, in a Marchcollected otolith with three translucent growth zones, 

an age reader may observe an opaque growth zone 

at the margin which is almost equal in width to the 

previous opaque growth zone. The otolith would be 

estimated as 3 or 4 years depending on the interpretation of the edge. The age reader must decide which of 

two scenarios is more likely: 1) the opaque growth was 

laid down during the collection year, in which case it 

is determined to be 3 years; or 2) the opaque growth 

was deposited during the previous year and a translucent growth zone has not yet formed. In the second 

scenario, the opaque growth is attributed to the previous year and the age reader assumes that a translucent 

growth zone corresponding to the previous year will 

soon form; thus, the fish is determined to be 4 years. 

Even if a translucent growth zone was deposited late 

in the previous year, it is unlikely that a whole year’s 

opaque growth would appear by March. Therefore, it 

is most likely that a translucent growth zone has not 

yet formed, and opaque edge growth can be attributed 

to the previous year, resulting in an age estimate of 4 

years. If this otolith had been collected in December, 

it would be estimated as 3 years because of the conventional 1 January birth date, even if the growth pattern 

was identical.

For young fish caught early in the year, one factor 

that aids edge interpretation is whether opaque growth 

is visible on more than one reading axis, especially in 

cross section. It is somewhat common to see a large increment of opaque growth on one reading axis but not 

on others. This is probably due to an early growth spurt 

that appears only on the axis of greatest growth for that 

otolith; in such a case, the opaque growth is typically 

attributed to the collection year and not counted in the 

age estimate.

To illustrate the process of edge interpretation, two 

different Atka mackerel (Pleurogrammus monopterygius) 

otoliths collected in January and September 2005 are 

shown in Figure 7. Both otoliths have four fully-formed 

translucent growth zones. However, the specimen collected in September has a fifth translucent growth zone 

starting to form on the otolith margin. This fifth translucent growth zone (and the preceding opaque growth 

zone) most likely formed during 2005, and thus is not 

included in the age estimate. If this otolith had been 

collected in January 2005, the fifth translucent growth 

zone would be counted in the age estimate even though 

it isn’t fully formed because it would have been attributable to the year 2004.
10 Professional Paper NMFS 13

Because age readers often note differences in the 

timing of marginal growth between areas, calendar 

years, and age groups, experience is often a key component in the interpretation of growth patterns. Unfortunately, it is rare to have samples collected throughout a 

single year, so age readers are often expected to make 

Figure 7

Otoliths from two different Atka mackerel (Pleurogrammus monopterygius), both with an age estimate of 4 years, 

demonstrating how collection date affects margin interpretation. (A) Break-and-burn preparation of an otolith collected 

in January 2005, with four fully-formed translucent growth zones. (B) Break-and-burn preparation of an otolith collected 

in September 2005, with four fully-formed translucent growth zones and a fifth translucent growth zone starting to form 

on the margin (indicated by arrow). This zone is not counted because it is not fully formed and most likely was deposited 

during 2005. If this otolith had been collected in January, the fifth translucent growth zone would be counted, resulting 

in an age estimate of 5 years. Viewed with reflected light.

judgments about edge growth with very little information. In such cases, the reader must make a decision 

based on the most logical choice and make an effort 

to apply this criterion with precision, even if there is a 

lack of evidence as to its accuracy.
11

Otolith dissection

Sagittal otoliths are collected for the AFSC during scientific surveys or by domestic fishery observers working on 

commercial fishing vessels. A sharp knife is positioned 

with the blade just posterior to the preopercle to cut 

vertically (transversely) through the fish’s head (Fig. 

8A). As an alternative, the “haircut” method may be 

employed, whereby one holds the knife horizontally and 

slices into the head just above the eyes, cutting along 

the frontal plane toward the lateral line. Once the knife 

breaks through the bony skull, the snout of the fish is 

grasped in one hand and the body in the other, and the 

head is carefully cracked open to expose the brain. The 

sagittae, or sagittal otoliths, are located ventrally and to 

either side of the brain tissue (Fig. 8B). Each otolith is 

carefully removed using forceps.

It is important that otoliths are cleaned before storage 

to prevent degradation. This can be accomplished by 

placing otoliths on a wet sponge and carefully brushing off tissue and blood using a firm brush. Once the 

otoliths are cleaned, they are inserted into labeled vials 

containing preservative.

Otolith preservation

Since the AFSC began collecting fish otoliths in the 

1970s, we have used either glycerol-thymol solution or 

ethanol to preserve and store fish otoliths. Vertebrae 

and otoliths from round teleosts have been preserved 

using at least 50% ethanol. Flatfish otoliths have never 

been stored in ethanol at the AFSC because of concerns 

that it may cause their annual marks to over-clear. Denatured ethanol and formalin are acidic and can cause 

severe damage to otoliths (Butler, 1992; Morales-Nin, 

1992), thus they should not be used as preservatives. 

Recently we switched to glycerol-thymol solution for 

preserving the otoliths of all species because it does 

not pose the safety hazards of ethanol. Ethanol is a 

flammable material, and special care is required for its 

transportation and storage.

Glycerol-thymol solution is made by crushing 5.5 g 

of thymol into a powder, which is then dissolved in 5 

ml of 95% ethanol by stirring or agitating the mixture. 

(The thymol is added as a preservative to prevent mold 

and bacterial growth.) Once the thymol is dissolved, 

the thymol/ethanol solution is added to 0.5 gallons of 

glycerol and shaken well. Lastly, 0.5 gallons of water 

Chapter 3: Collection and preparation of otoliths for age determination

by B. J. Goetz, Charles E. Piston, Charles E. Hutchinson, Christopher G. Johnston, and Mary Elizabeth Matta

is added to the mixture and again shaken well. Care 

should be taken not to add the thymol solution to the 

water before adding the glycerol, as doing so will cause 

the thymol to precipitate.

Methods of preparing otoliths 

for age determination

Examination of the surface of intact otoliths is generally 

the first step in age determination. However, there are 

several other techniques that can be used to visualize 

and further enhance otolith growth patterns. The most 

common method we use to determine fish age at the 

AFSC is the break-and-burn method, which entails halving the otolith transversely through the core and using 

an alcohol burner to burn the cut surface of one of the 

otolith halves (Beamish, 1979; Chilton and Beamish, 

1982; MacLellan, 1997). The break-and-bake technique 

(Barto, 1999; Forsberg, 2001) is a recent variation of the 

break-and-burn method; it is best applied to delicate 

otoliths. Both methods enhance the contrast between 

translucent and opaque growth zones, resulting in clarification of the overall growth pattern. Thin-sectioning, a 

method typically used for species whose growth patterns 

are difficult to interpret, produces uniform reading surfaces. This method is often applied in special research 

studies.

When studying new species, different preparation 

methods should be tested to establish which is most appropriate. At the AFSC, the method chosen is the one 

that most efficiently produces growth patterns that are 

clear enough for age determination. Generally, surface 

examination is the least time-consuming method, followed by the break-and-bake method, the break-andburn method, and finally, thin-sectioning. Validation 

of the age estimates generated by the chosen method 

should be the ultimate goal of any age determination 

laboratory (see Chapter 6). The four otolith preparation methods listed above are described in detail in 

the following sections, and their applications to individual species are further described in Chapter 4 of this 

manual.

Otolith surface examination

Surface examination, while rarely the sole method of age 

determination, provides supplementary information to 

the age reader that can be useful in making age deter-
12 Professional Paper NMFS 13

Figure 8

Otolith dissection from a Pacific ocean perch (Sebastes alutus). (A) 

Proper placement of knife immediately posterior to preopercle 

to cut transversely through head. (B) Right sagittal otolith and 

exposed brain tissue.

mination decisions such as identifying the first annual 

mark. Whole otoliths are submerged in water in a Petri 

dish lined with black felt and viewed with a dissecting 

microscope and reflected light. Surface examination is 

helpful in that it often makes it easier to distinguish between checks and annual marks, as translucent growth 

zones typically appear more coalesced than they do in 

cross section. Surface examination may be adequate for 

determining the age of young otoliths or otoliths with 

relatively clear growth patterns. However, in the case 

of older fish it is usually too difficult to interpret age 

from the otolith surface alone, and further treatment 

is required. This is because as fish grow older, their otoliths become thicker, effectively reducing the visibility 

of growth zones on the surface. Furthermore, annual 

marks become more closely spaced and more difficult 

to differentiate as otolith growth slows with age. Therefore, using only the otolith surface to estimate the ages 

of older fish may result in underestimation of true age.

Otolith break-and-burn method

For most round-bodied teleosts, either otolith may be 

selected for cross-sectioning. However, flatfish otoliths 

are asymmetrical (Chapter 2), and it is usually preferable to section the blind side (centric-cored) otolith. 

Otoliths are cross-sectioned through the core using 

either a scalpel, a low-speed saw, or by snapping them 
Chapter 3: Collection and preparation of otoliths for age determination 13

Figure 9

Walleye pollock (Theragra chalcogramma) otolith being sectioned 

along the transverse plane using a low-speed saw.

in half. The method used depends on the 

species, the size of the otolith, and the degree 

to which the characteristics of the cut surface 

affect age determination. Very small otoliths 

such as those of Atka mackerel (Pleurogrammus 

monopterygius) or sablefish (Anoplopoma fimbria) 

may successfully be snapped in half using one’s 

fingers, whereas most flatfish species are best 

cross-sectioned using a scalpel. Walleye pollock 

(Theragra chalcogramma) and Pacific cod (Gadus macrocephalus) otoliths should be sectioned 

using a low-speed saw, such as an IsometTM 

(Buehler®, Lake Bluff, IL), to minimize topography on the cut surface which can interfere 

with age estimation.

When using a low-speed saw, a reference line 

should first be drawn transversely through the 

core to orient the otolith for sectioning. Some 

readers draw this line on the proximal surface 

whereas other readers prefer to draw it on the 

distal surface of the otolith. A microscope may 

be used to draw the line on smaller otoliths; on larger 

otoliths the line can be drawn with the naked eye. A clay 

support is inserted into a custom-made chuck on the 

saw and lowered onto the blade to press another reference line in the clay. The reference line on the otolith 

is aligned with the reference line in the clay. Using a 

light weight (25–75 g) to provide pressure, the otolith 

is lowered onto the turning saw blade and cut in half 

(Fig. 9). Heavier weights are not recommended because 

they may break the otolith or section it unevenly.

Some readers prefer polishing the cut surface of each 

otolith half before burning. Water is either added directly to fine grit sandpaper or the otolith half is dipped 

in water prior to polishing. Either a back-and-forth or 

a circular polishing pattern works well as long as one is 

gentle enough not to break the edges.

For all species, age readers usually examine the untreated cut surface (“the unburned face”) of each otolith half prior to further processing, obtaining an age 

estimate from it when possible. The otolith half chosen 

for burning varies by species and general otolith clarity. 

For example, the posterior half is preferred over the 

anterior half for burning walleye pollock and Pacific 

cod otoliths. For these species, the anterior half yields 

better surface estimates and is typically preserved when 

possible, especially if the second whole otolith is broken, 

crystallized or otherwise damaged. For other species, 

the otolith half with the clearest growth pattern may be 

selected for burning.

Using forceps, the cut surface of the otolith half is 

passed back and forth through the alcohol flame until 

it is browned (Fig. 10A). With experience, age readers 

learn how long to burn otoliths. Under- or over-burning 

makes the growth pattern very hard to interpret. The 

amount of time required to obtain an optimal burn pattern varies by species and otolith size. Because walleye 

pollock otoliths are very brittle, they cannot be held in 

the flame for long periods of time without shattering. 

For this reason, otoliths should only be burned using 

safety goggles or behind a protective plastic shield to 

prevent eye injuries (Fig. 10B).

When sufficiently burned, the growth patterns are enhanced and more easily interpreted. The burned otolith 

is allowed to cool and is then inserted in a clay support 

and brushed with mineral oil to view the growth patterns. 

Break-and-burn growth patterns are interpreted using 

a dissecting microscope with reflected light. If the cut 

surface is not clean, the burning process can cause 

black deposits to appear. These deposits can interfere 

with the age determination process by occluding annual 

marks. Deposits can be removed by gently polishing the 

broken-and-burned surface on wet fine-grit sandpaper. 

The otolith is much more brittle after burning, so only 

gentle pressure should be used. A circular motion can 

cause breakage, so a simple, back-and-forth motion 

on the sandpaper is the safest way to remove deposits. 

Immersion in diluted hydrochloric acid also removes 

deposits but is more hazardous.

One disadvantage of the break-and-burn method is 

that otolith growth patterns may fade during long-term 

storage. This phenomenon does not affect all otoliths 

and is often unpredictable, but may be prevented by 

encasing otoliths in a clear casting resin (CARE, 2006).

Otolith break-and-bake method

The break-and-bake technique is a variation of the 

break-and-burn method and was first used by the AFSC 
14 Professional Paper NMFS 13

Figure 10

An alcohol flame is used to burn the cut surface of a walleye pollock (Theragra chalcogramma) otolith. (A) Closeup view. (B) Plastic shield used to protect the age reader from burning debris.

Age and Growth Program in 2003. Instead of being 

passed over a flame, the cut otolith is baked for several 

minutes in a conventional toaster oven. This technique 

works particularly well for otoliths that have a tendency 

to crumble when burned over an open flame.

Otoliths are transversely sectioned through the core 

using either a scalpel or low-speed saw, as described 

above. One half from each sectioned otolith is then 

baked in a toaster oven using a metal tray that has 

50 separate cells. Optimal baking times and temperatures vary according to species, but otoliths are 

generally baked between 350°F and 500oF for several 

minutes.

The break-and-bake technique is often more convenient than the traditional break-and-burn method 

because it allows the age reader to prepare multiple 

specimens at the same time. Another advantage of 

oven-baking is that it produces relatively even contrast between opaque and translucent growth zones 

while reducing the possibility of charring or cracking 

otoliths. Barto (1999) found no significant difference 

between the break-and-burn and break-and-bake 

methods for Pacific halibut (Hippoglossus stenolepis) 

age estimates.

Otolith thin-section method

At the AFSC, thin-sectioning is used to determine 

age for skates, some rockfishes, and other species 

for which the break-and-burn method is either not 

possible or does not produce reliable age estimates. 

Thin-sectioning results in an even reading surface and 

enhances both annual and sub-annual growth zones. 

Thin-sectioning can be a valuable method in studies on 

age validation, stock discrimination, and environmental effects on growth.

The AFSC thin-section method is a modified version 

of a similar thin-section technique developed in Australia (Smith et al., 1995). Multiple otoliths or vertebrae 

may be sectioned at once. Polyester resin is prepared 

and poured into blocks within custom-made rubber 

molds to form a thin uniform base layer about 2 to 3 

mm thick. A tongue depressor is used to gently mix the 

resin to minimize the introduction of air bubbles, which 

can cause specimen loss during sectioning. Before 

mounting the otoliths or vertebrae, the resin is allowed 

to partially set to prevent them from sinking to the bottom of the mold.

Using a dissecting probe, the resin base layer is scored 

with parallel lines to create rows for aligning aging 

structures for cutting. Multiple otoliths are mounted 

along each row with the proximal surface facing down 

and the anterior-posterior axis perpendicular to the 

scored lines to result in transverse cuts directly through 

the cores (Fig. 11). For proper alignment of small otoliths, it may be necessary to draw a pencil line on the 

surface transversely through the core (Fig. 11). Vertebral centra are mounted so that their foci are aligned 

along the scored lines, with either the anterior or posterior side facing down to yield longitudinal sections. 

A label with specimen numbers is placed at the top of 

the block.

After mounting, another batch of resin is mixed 

and poured to cover the aging structures completely. 

Ideally, each aging structure should be completely 

encased in resin while keeping block thickness to a 
Chapter 3: Collection and preparation of otoliths for age determination 15

Figure 11

Preparation of rockfish (Sebastes sp.) otoliths for thin-sectioning. (A) Resin is poured into rubber molds to 

create blocks containing multiple otoliths. (B) Otoliths are mounted with their cores aligned to yield transverse 

thin sections.

minimum. Thick resin blocks take longer to cut and 

are more likely to cause overheating and binding of 

the saw blade.

The AFSC Age and Growth Program uses a Buehler 

Isomet® 5000 Linear Precision saw equipped with diamond blades to create thin sections. Because it is difficult to align the cores of all of the otoliths within a row, 

it is usually necessary to make three or four thin sections from each row. Optimum section thickness varies 

by species. Thin sections are affixed to glass slides with 

an adhesive resin after cutting and may be ground and 

polished to attain the desired thickness. Thin sections 

may be viewed using a dissecting microscope fitted with 

either a reflected or transmitted light source. The light 

source selected typically depends on the thickness of 

the specimen and the age reader’s preference.

17

Chapter 4: Groundfish age determination

Over 30 species of groundfish from Alaskan waters are 

aged at the AFSC, 18 of which are aged on a production 

basis for stock assessments. The remaining species are 

aged on an occasional basis for research purposes. The 

following sections describe age determination methods 

and aging criteria for some of the species most commonly studied by the AFSC’s Age and Growth Program.

Walleye pollock (Theragra chalcogramma)

by B. J. Goetz, Irina M. Benson, and Daniel K. Kimura

Biology

Walleye pollock (Theragra chalcogramma) is a semidemersal species found in the waters of the continental shelf and slope of the North Pacific Ocean; its 

range along the North American coast extends from 

southern Oregon to the Gulf of Alaska through the 

Bering Sea to the southern Chukchi Sea (Bakkala et 

al., 1986). Walleye pollock, a key species in the eastern 

Bering Sea ecosystem, is both a dominant predator 

and an important component of the diet of many 

upper trophic level consumers (Aydin and Mueter, 

2007). Alaskan populations of walleye pollock, especially the Bering Sea stock, support one of the largest 

single-species fisheries in the world, representing 

over 40% of global whitefish production (Ianelli et 

al., 2007).

Walleye pollock are relatively fast-growing and shortlived (Ianelli et al., 2007), with a protracted spawning 

season lasting from January through August depending 

on geographic location. Recent studies indicate that 

shelf and deepwater basin walleye pollock in the Bering Sea form independent spawning groups that differ 

in timing and location of spawning, age at maturity, 

and possibly growth rate (Stepanenko and Nikolaev, 

2004). In the Gulf of Alaska, spawning occurs from midFebruary through April (Hughes and Hirschhorn, 1979; 

Dorn et al., 2010).

Larvae grow relatively slowly and metamorphose 

to the juvenile life stage at about 18 mm in length. 

Young-of-year juveniles reach 80 to 100 mm by 6 

months and 120 to 140 mm by the end of their first 

year. Walleye pollock mature sexually at about age-4 

and at a length of about 400 to 450 mm (Bailey et al., 

1999). Maximum reported age differs by region, with 

walleye pollock in the Bering Sea generally growing 

older (maximum age=31 years) than those in the 

Gulf of Alaska. The relative differences in growth 

between the two regions are also reflected by fitting 

length-at-age data from each area with the von Bertalanffy growth function (VBGF), Lt

=L∞(1–e–k(t–t0)), 

where Lt=predicted length at age t, L∞=asymptotic 

length, k=growth coefficient, and t0=theoretical age at 

zero length. Von Bertalanffy growth parameters were 

L∞=673.3 mm, k=0.1937/yr, and t0=–0.205 yr (n=3147) 

and L∞=700.0 mm, k=0.2465/yr, and t0=–0.364 yr 

(n=1177) for walleye pollock collected during trawl 

surveys in the Bering Sea and Gulf of Alaska, respectively, in 2008 (Fig. 12).

Age determination history

Walleye pollock were first aged at the AFSC in 1971 using the surfaces of whole otoliths. The break-and-burn 

method was introduced in the 1980s, followed by the 

break-and-bake method in 2003 (Chapter 3). Today, 

surface examination is still used to age otoliths from 

small fish or otoliths with exceptionally clear growth patterns. The break-and-burn or break-and-bake methods 

are applied to otoliths that have patterns that are more 

difficult to interpret. 

Around 1990, one of the aging criteria used to interpret growth patterns in walleye pollock otoliths was 

slightly modified. Prior to this time, otolith cross sections from older fish were typically aged from the core 

to a notch between the ventral and proximal surfaces. 

Additional growth zones on the proximal surface between the sulcus and notch were not counted (Fig. 13). 

However, after the criterion modification, all marks 

within this area were counted. The criterion change 

was relayed to data end users and only affected fish 

older than about 12 years, a very small percentage of 

the population.

The AFSC has aged large numbers of walleye pollock from the Bering Sea, Gulf of Alaska, and Aleutian 

Islands regions. To date, no obvious differences have 

been noted between walleye pollock otolith growth 

patterns from the Bering Sea and Aleutian Islands 

regions. However, walleye pollock otoliths from the 

Gulf of Alaska often have larger first annual marks than 

those in the Bering Sea and Aleutian Islands.

To date, the AFSC Age and Growth Program has 

aged over 300,000 walleye pollock otoliths. Inter-reader precision is relatively high (coefficient of variation 

[CV]=4.4%; n=55,282). Walleye pollock age estimates 

have been corroborated using radiometric methods 

(Kastelle and Kimura, 2006), length mode analysis 

(Kimura et al., 2006), and edge type studies (Kimura 
18 Professional Paper NMFS 13

Figure 13

Break-and-burn preparation of a walleye pollock (Theragra chalcogramma) otolith 

collected from a 640 mm female on 6 April 2007 in the Aleutian Islands. Age estimate 

is either 16 or 17 years, with a questionable fifteenth annual mark. Before an aging 

criterion change in 1990, age readers did not count any translucent growth zones 

visible from the ventral-proximal notch to the margin along the proximal reading 

axis. (Area not aged prior to criterion change indicated by bracket.) Viewed with 

reflected light.

Figure 12

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for walleye 

pollock (Theragra chalcogramma) collected during trawl surveys in the Bering Sea (BS; 

n=3147) and Gulf of Alaska (GOA; n=1177) in 2008.
Chapter 4: Groundfish age determination 19

Figure 14

Clear walleye pollock (Theragra chalcogramma) otolith collected from a 380 mm 

male in the Bering Sea on 16 October 2001. Age estimate is 3 years. Viewed with 

reflected light.

et al., 2007) at the AFSC; these methods are described 

in further detail in Chapter 6 of this manual.

Current age determination methods

Age determination of walleye pollock is moderately difficult, and the readability of otoliths from this species 

is variable. At the AFSC, walleye pollock otoliths are 

examined using a dissecting microscope and reflected 

light.

Surface examination. Annual marks are typically 

visible on the surfaces of whole otoliths (Fig. 14), and 

young, clear specimens can usually be aged from the 

surface alone. Surface age estimation is often more difficult on older, thicker otoliths. Large walleye pollock 

otoliths are generally aged from the proximal surface, 

and it is sometimes possible to improve visibility of annual marks by tilting the otolith to the side with forceps 

under low magnification. However, for thinner, younger 

otoliths, checks can often be distinguished from annual 

marks by turning the otolith over and examining the 

distal side.

Since the sagittae of walleye pollock are much 

thicker than those of many groundfish species, relying on surface examination alone can be problematic. 

For this reason, most walleye pollock are aged using 

the break-and-burn method. However, the occasional 

sample is comprised of younger or thinner otoliths 

which can easily be aged from the surface. New age 

readers should compare paired surface and break-andburn age estimates from each otolith, but with experience age readers learn how to recognize young, clear 

surface patterns that do not require a break-and-burn 

preparation.

Break-and-burn examination. If a walleye pollock otolith cannot be aged from the surface, further processing 

is required. Age readers typically use either the breakand-bake or the break-and-burn method (Chapter 3) to 

enhance the growth pattern. Both methods have advantages and disadvantages: the break-and-burn method is 

better at darkening translucent growth zones but may 

result in crumbling, while the break-and-bake method 

leaves the otolith intact but generally yields slightly less 

contrast between opaque and translucent growth zones. 

However, the annual growth pattern is nearly identical 

in appearance using either method, and the choice of 

method is left up to the age reader.

It is difficult to break walleye pollock otoliths at the 

desired point manually, so a low-speed saw is used to 

section them in half. The best burn patterns occur when 

the otolith is slowly burned over an alcohol flame. Overburning and under-burning create age determination 

problems, so it is best to experiment with an alcohol 

burner to find the best technique to achieve an optimal 

burn. Over-burning can result in erosion of the dorsal 

and ventral tips and a chalky white appearance, whereas 

under-burning will fail to produce enough contrast 

between opaque and translucent growth zones, making 

them more difficult to count. Walleye pollock otoliths 

are brittle and can shatter when exposed to heat, so 

they should be burned carefully. A protective shield or 

goggles should be used to prevent eye injuries.

Reading axes. Although there is no single preferred 

reading axis on walleye pollock otolith cross sections, 

the areas on either side of the sulcus are often clearest. 

It is useful to compare patterns on multiple reading 

axes to see if translucent marks are continuous around 

the otolith (Fig. 15). The anti-sulcus is usually the least 
20 Professional Paper NMFS 13

Figure 15

Sectioned walleye pollock (Theragra chalcogramma) otolith showing general age reading axes. Collected from 

a 550 mm female in the Gulf of Alaska on 11 July 2006. Age estimate is 16 years. Viewed with reflected light.

Figure 16

Otolith break-and-burn pattern from a 510 mm female walleye pollock (Theragra chalcogramma) 

collected on 19 February 2008 from the Bering Sea. Using other reading axes, one might 

assign an age estimate of 6 years, but this specimen is clearly 7 years on the anti-sulcus. Viewed 

with reflected light.

useful reading axis, although in rare cases it is the clearest 

(Fig. 16).

Marginal growth deposition is not uniform among 

all cross-section reading axes. For example, some 

walleye pollock otoliths tend to grow most quickly on 

the dorsal tip, whereas others, especially those from 

older fish, accrete new material most quickly near the 

ventral tip.

Identification of the first annual mark. The first translucent growth zone, or annual mark, is typically visible 

on the surface of whole walleye pollock otoliths. Bering 

Sea walleye pollock stocks spawn over a broad time 

period, and thus the size of the first annual mark may 

depend on the hatch date of the fish. We expect the first 

annual mark of a late-spawned (summer or early fall) 

fish to be smaller in size than that of a fish spawned early 

in the spring. Age readers should be aware of variation 

in size of the first annual mark (Fig. 17), since the time 

and location of walleye pollock spawning in the Bering 

Sea varies widely.

Information on the first annual mark’s microstructure is a valuable aid in its identification. The central 

area of the first annual zone (Fig. 18) appears darker 
Chapter 4: Groundfish age determination 21

Figure 17

Variation in first annual mark size in Bering Sea walleye pollock (Theragra chalcogramma) 

otoliths. First annual mark of the 1-year-old (upper) otolith is larger than the first 

annual mark of the 4-year-old (lower) otolith. Upper otolith collected from a 150 mm 

male on 21 June 2003. Lower otolith collected from a 410 mm female on 3 July 2003. 

Viewed with reflected light.

1 Nishimura, A. 1998. Personal commun. at workshop, as noted in: 

Report from the second workshop on ageing methodology of walleye pollock (Theragra chalcogramma), Alaska Fisheries Science Center, Seattle, WA, 17–20 March 1998, [Available from http://www.

afsc.noaa.gov/REFM/Age/age_pollock_workshops.htm, accessed 

March 2011.]

2 Nishimura, A. Unpubl. manuscr. False ring observed in the otolith 

of age 1 walleye pollock collected in the Bering Sea. Kumaishi 

Branch, Hokkaido Fish Hatchery, 043-04 Kumaishi, Hokkaido, 

Japan.

because daily increments are arranged densely around 

the otolith core (Nishimura and Yamada, 1988). While 

the core of some otoliths is very dark, in others it is 

barely visible. During the first year of growth, daily 

increments gradually widen in the first annual opaque 

growth zone, which usually forms during spring and 

summer (Nishimura and Yamada, 1988). As growth 

slows down in winter, daily increment widths diminish until increments become obscure within the first 

translucent growth zone (Nishimura and Yamada, 

1988).

One of the problems encountered when interpreting the first annual mark on otoliths of walleye pollock is the occasional formation of checks in the first 

year (Fig. 19). One or two translucent growth zones 

sometimes precede the first annual mark on otoliths 

of young walleye pollock (LaLanne, 1979). These 

zones are seldom apparent after the third and fourth 

year of growth because otoliths thicken with age 

(LaLanne, 1979). An otolith growth check is formed 

during the postlarval period at body lengths of 8–11 

mm, independently of age and growth patterns and 

corresponding to initiation of calcification in the 

endoskeletal system (Nishimura, 1993). Nishimura1

recognized three different growth patterns in age-1 

otoliths: 1) otoliths with a single translucent growth 

zone, 2) otoliths with two translucent growth zones, 

and 3) otoliths with three translucent growth zones. 

In each of these three growth patterns, only the outermost translucent growth zone, ranging between 4.5 

and 5.1 mm in diameter, was identified as the first 

annual mark because, unlike the other translucent 

growth zones, its daily growth increments were compressed (Nishimura2

).

When the first annual mark is not clear on the otolith 

surface, corresponding break-and-burn or break-andbake preparations may increase the prominence of the 

first annual mark and make checks less visible (Fig. 20). 

However, age readers should be aware that variation in 
22 Professional Paper NMFS 13

Figure 18

(A) Otolith surface and (B) break-and-burn pattern from a 1-year-old, 

150 mm male walleye pollock (Theragra chalcogramma) collected from the 

Bering Sea on 21 June 2003. First annual mark consists of an opaque (lightcolored) growth zone around the core and a translucent (dark-colored) 

growth zone. Opaque growth around the otolith edge is not included in the 

age estimate because it likely occurred during the collection year. Viewed 

with reflected light.

size and spacing of annual marks may make it difficult 

to interpret the growth pattern and determine the first 

annual mark.

Age determination problems. A common growth pattern in Bering Sea walleye pollock is a nebulous second 

annual mark. A strong, clear first annual mark is commonly followed by one or two diffuse, darkened areas 

that could be interpreted as either two discrete annual 

marks or a single broad translucent growth zone. Sectioning the otolith and viewing the unburned face or 

burning the otolith may improve clarity. If the mark is 

well-defined, discrete, and continuous, it is considered 

to be an annual mark.

Crystallization is a common problem in walleye pollock otoliths (Fig. 21). In most cases, only one otolith 

is crystallized. Crystallized otoliths may exhibit dense 

nodules on the proximal surface or they may be diffusely crystallized over a large area, sometimes over the 

entire otolith. Annual marks are typically obscured in 

crystallized areas, so otoliths with extensive crystallization usually cannot be aged.
Chapter 4: Groundfish age determination 23

Figure 19

One-year-old walleye pollock (Theragra chalcogramma) (A) otolith surface and 

(B) break-and burn patterns. Checks are evident within the first year. Collected 

from a 190 mm male in the Bering Sea on 3 July 2003. Opaque growth around 

the otolith edge is not included in the age estimate because it likely occurred 

during the collection year. Viewed with reflected light.
24 Professional Paper NMFS 13

Figure 20

(A) Surface, (B) break-and-burn, and (C) break-and-bake 

patterns from a Bering Sea walleye pollock (Theragra chalcogramma) otolith. Surface pattern is clear with the exception of 

the first annual mark, which appears to be preceded by a faint 

check (indicated by question mark in A). In (B) and (C), the 

first translucent growth zone is more prominent, likely making 

this a 4-year-old with a small first annual mark. Collected from a 

330 mm male on 21 July 2004. Viewed with reflected light.

Figure 21

Distal surface of a walleye pollock (Theragra chalcogramma) 

otolith with moderate crystallization along the ventral edge. 

Viewed with reflected light.
Chapter 4: Groundfish age determination 25

Pacific cod (Gadus macrocephalus)

by Christopher G. Johnston and Delsa M. Anderl

Biology

Pacific cod (Gadus macrocephalus) are found in continental shelf and upper continental slope waters of the 

North Pacific Ocean, from China’s northern Yellow Sea 

along the North Pacific Rim, in the Bering Sea as far 

north as the Chukchi Sea, and south along the Gulf of 

Alaska and the coast of North America to Santa Monica 

Bay, California (Love, 1991; Westrheim, 1996). This 

species is the target of a major multiple-gear fishery in 

Alaskan waters.

Pacific cod are moderately fast-growing and shortlived compared to many other Alaskan groundfish 

species. Spawning generally occurs between January 

and April at depths of 40 to 120 m (Klovach et al., 

1995). Hatching occurs in 8–28 days depending on water temperature and salinity (Alderdice and Forrester, 

1971; Hart, 1973). Larvae are approximately 3–4 mm 

in length at the time of hatching (Palsson, 1990) and 

40 mm at the time of settlement (Laurel et al., 2009). 

Adults inhabit depths ranging from 10 to 875 m, although they are most frequently found between 50 and 

300 m (Allen and Smith, 1988; Love, 1991). Tagging 

studies suggest Pacific cod make seasonal migrations to 

spawn and feed, moving considerable distances within 

the Bering Sea, eastern Aleutian Islands, and Gulf of 

Alaska (Shimada and Kimura, 1994). According to 

AFSC Age and Growth Program data, Pacific cod in 

the Bering Sea tend to grow more slowly and to larger 

sizes than those in the Gulf of Alaska. Von Bertalanffy 

growth parameters were L∞=1220.9 mm, k=0.1353/yr, 

and t0=–0.144 yr (n=1440) and L∞=876.3 mm, k=0.2376/

yr, and t0=–0.118 yr (n=471) for Pacific cod collected 

during trawl surveys in the Bering Sea and Gulf of 

Alaska, respectively, in 2007 (Fig. 22). The maximum 

age reported to date by the Age and Growth Program 

for Pacific cod is 17 years.

Age determination history

Pacific cod is one of the most difficult Alaskan 

groundfish species to age. Various hard parts have 

been tested as aging structures, but no structure consistently shows clear early annual marks (Kennedy, 

1970; Ketchen, 1970). From 1976 to the early 1980s, 

the AFSC Age and Growth Program used scales to 

determine ages of Pacific cod. Thereafter, otoliths 

became the exclusive aging structure for Pacific cod, 

primarily using the break-and-burn or break-and-bake 

methods (Chapter 3).

Figure 22

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for Pacific 

cod (Gadus macrocephalus) collected during trawl surveys in the Bering Sea (BS; 

n=1440) and Gulf of Alaska (GOA; n=471) in 2007.
26 Professional Paper NMFS 13

An unexplained decline in length-at-age observed in 

the early 1990s, combined with the inherent difficulty 

associated with Pacific cod aging, resulted in a decadelong suspension of production age determination of 

Pacific cod at the AFSC. A study to reevaluate Pacific 

cod aging criteria and investigate the length-at-age 

shift was initiated in 1998 (Roberson, 2001; Roberson et al., 2005). Otoliths collected from tagged and 

recaptured Pacific cod were embedded in black resin 

and thin-sectioned to approximately 0.2 mm thick. 

Back-calculation of fish lengths from these otoliths 

showed that criteria-based identification of checks 

and annual marks was correct. Otoliths from this 

study seemed to have better pattern clarity than those 

prepared by the break-and-burn method. However, 

thin-sectioning is a very time-consuming method of 

preparation, and Roberson’s age estimate precision 

was no higher using thin-sections than break-and-burn 

preparations. With regard to the decline in length-atage, it was not possible to identify a single contributing 

factor; Roberson’s results indicated that both a true 

shift in population growth parameters and changes 

in otolith aging criteria may have occurred (Roberson 

et al., 2005).

Since 1990, the AFSC Age and Growth Program has 

aged over 23,000 Pacific cod otoliths from the Gulf of 

Alaska, Bering Sea, and Aleutian Islands. Estimates of 

inter-reader precision reflect the relative difficulty of 

age determination for this species (CV=7.74%; n=7531). 

Pacific cod age estimates have been corroborated by 

Roberson’s work and by tracking the length mode of the 

strong 1977 year class in the eastern Bering Sea (Lai et 

al., 1987; Kimura and Lyons, 1990). Stable isotope and 

bomb radiocarbon studies (Chapter 6) are currently 

underway at the AFSC to provide a stronger form of age 

validation for Pacific cod.

Current age determination methods

Surface examination. Surfaces are viewed using a 

dissecting microscope and reflected light. Pacific cod 

growth patterns range from being barely visible to being 

invisible on the otolith surface. However, surface examination gives age readers a relative idea of the expected 

size and age of the fish. With the exception of very small 

otoliths, determining age from the surface alone is not 

usually an appropriate method for this species.

Break-and-burn/break-and-bake examination. Otoliths are cut transversely through the core using a lowspeed saw (Chapter 3). Care is taken to ensure that 

the saw blade cuts through the otolith cleanly in an 

even plane. Bumps and fractures on the otolith reading surface can produce pattern disruptions, further 

complicating the identification of annual marks. The 

saw cutting speed should be set slower for small otoliths 

than for larger ones. The otolith must be embedded 

firmly in the clay and held in place with either the 

reader’s thumbs or a pair of forceps. This is particularly important at the end of the cutting process when 

loosely-anchored otoliths tend to snap, resulting in an 

uneven reading surface. If necessary, ledges and other 

surface roughness may be sanded using 600-grit sandpaper. Sanding should only be done sparingly because 

it can sometimes blur the exposed pattern.

Before further treatment, the otolith halves are 

placed under water in a Petri dish to examine the cut 

surfaces. Occasionally the pattern appears sharper on 

one of the halves than the other, in which case the clearer of the two should be selected for further treatment. 

If possible, the reader should attempt to determine age 

from the untreated surface, sometimes referred to as 

the unburned face.

Contrast between opaque and translucent growth 

zones is increased by either the break-and-burn method 

or break-and-bake method (Chapter 3). Method choice 

is left to the age reader’s discretion and may be decided 

on a per-otolith basis. Using the break-and-burn method 

for Pacific cod otoliths requires care and attention, but 

a well-controlled burn can result in clear patterns. Burning these otoliths in a very hot flame quickly produces 

unevenly burned or over-burned patterns. A very hot 

flame also can cause fractures in the otolith surface, 

leading to loss of vital material, especially at the dorsal 

and ventral tips. Thus, the break-and-bake method is 

generally the preferred method for age determination 

of Pacific cod at the AFSC. Pacific cod otoliths are usually toasted for a minimum of 4.5 minutes at 390°–460oF, 

but baking time and temperature can be varied to optimize contrast between growth zones.

Oven-baked Pacific cod otoliths typically show more 

uniform contrast and finer growth pattern details compared to burned otoliths. One drawback of baking is 

that it is more likely to expose checks, whereas flame 

burning tends to burn checks away. Unfortunately, it 

is hard to control the flame to avoid burning away the 

annual marks, too. Baked otoliths usually have a fainter 

but more regular pattern of opaque and translucent 

growth zones compared to burned otoliths. Some age 

readers bake the first otolith half, and if they cannot 

decide upon an age using the baked section, they 

will burn the second half. All Pacific cod otolith cross 

sections are viewed with a dissecting microscope and 

reflected light.

Applying aging criteria. A few Pacific cod otoliths have 

distinct, strong annual patterns (Fig. 23), though most 

patterns are faint and thread-like (Fig. 24). Some specimens are easy to prepare for age determination while 

others shatter with minimal heat. Checks are frequently 
Chapter 4: Groundfish age determination 27

Figure 23

Unusually clear Pacific cod (Gadus macrocephalus) otolith collected from an 830 mm female captured 

in March. Age estimate is 7 years. Viewed with reflected light.

Figure 24

A typical Pacific cod (Gadus macrocephalus) otolith, collected in March from a 780 mm female. Identification 

of the first three annual marks (represented by bars) is critical to avoiding over-aging. Age estimate is 7 years. 

Viewed with reflected light.

observed during periods of rapid growth early in life. 

Pacific cod otoliths develop multiple growth checks that 

can be easily mistaken as annual marks during their first 

three years. The difficulty in determining age for this 

species is that the general properties defining checks 

and annual marks do not appear to be applicable to 

Pacific cod. Checks are normally easily identified as 

faint, discontinuous rings between darker, more distinct 

annual marks (Fig. 25). However, difficult early growth 

patterns are fairly common in Pacific cod otoliths (Fig. 

26). Checks in the first two years are sometimes as strong 

as annual marks, and occasionally the first three annual 

marks are as faint and discontinuous as checks.

Specific criteria have been defined to aid age readers in identifying the first three annual marks in 

break-and-burn and break-and-bake preparations. 

From an initial set of clear otoliths, measurements of 

the distance from the dorsal to ventral side of each 

annual mark were used to define expected size ranges 

for the first three annual marks. These are 2.3–3.8 

mm for the first annual mark, 4.6–5.6 mm for the second annual mark, and >6.15 mm for the third annual 

mark. These width ranges are used as supplemental 

information in identifying the first three annual 

marks when the pattern is difficult to discern (Figs. 

24 and 27).

Identifying annual marks after the third year requires close examination of the five reading axes in 

Pacific cod break-and-burn otolith preparations. These 

reading axes are transects between the otolith core and 
28 Professional Paper NMFS 13

Figure 25

Faint Pacific cod (Gadus macrocephalus) otolith with checks (indicated 

with a C) before first and second annual marks. Collected in June 

from a 340 mm male. Age estimate is 2 years. Viewed with reflected 

light.

Figure 26

Faint Pacific cod (Gadus macrocephalus) otolith with multiple checks (indicated with a C). Collected in June from 

a 340 mm male. Age estimate is 3 (or possibly 2) years. Viewed with reflected light.

the following points: the ventral side of the sulcus, the 

dorsal side of the sulcus, the ventral tip, the dorsal tip, 

and the anti-sulcus (Fig. 28). The best reading axes are 

the ventral sulcus and the dorsal sulcus, followed by 

the anti-sulcus and the dorsal tip. Generally, the least 

helpful axis is along the ventral tip. Rarely, the same 

number of annual marks can be observed in more than 

two of the five reading axes (Fig. 29). When possible, 

annual marks should be followed around the otolith 

cross section from one reading axis to another.
Chapter 4: Groundfish age determination 29

Figure 27

Dorsal-ventral measurements of first, second, and third annual marks in a Pacific cod 

(Gadus macrocephalus) otolith cross section. Collected in June from a 320 mm female. 

This fish is small in length for its age, but the diameter of the third (outermost) annual 

mark fits the aging criteria for a 3-year-old. Viewed with reflected light.

Figure 28

Reading axes for Pacific cod (Gadus macrocephalus) otoliths. Viewed with reflected light.
30 Professional Paper NMFS 13

Figure 29

Pacific cod (Gadus macrocephalus) otolith with checks (indicated by C) before first, second, and third 

annual marks. Annual mark counts along the dorsal sulcus, dorsal tip, and anti-sulcus axes are identical, 

but the pattern near the fourth and fifth annual marks is blurry along the ventral sulcus axis. Collected 

in June from a 700 mm female. Bars represent expected diameters of the first, second, and third annual 

marks. Age estimate is 9 years. Viewed with reflected light.
Chapter 4: Groundfish age determination 31

Figure 30

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for Atka 

mackerel (Pleurogrammus monopterygius) collected during trawl surveys in the Gulf 

of Alaska (GOA; n=963) and Aleutian Islands (AI; n=1765) from 2003 through 

2007. One year has been added to all age estimates to account for invisible first 

annual mark (see text).

Atka mackerel 

(Pleurogrammus monopterygius)

by Delsa M. Anderl

Biology

Atka mackerel (Pleurogrammus monopterygius) are common in the northern Pacific Ocean, ranging from the 

Kamchatka Peninsula to southeast Alaska. Atka mackerel abundance is highest around the Aleutian Islands, 

where adults are found in dense and highly localized 

aggregations at depths less than 200 m (Lowe et al., 

2009). Atka mackerel are also the target of a large trawl 

fishery in the Aleutian Islands. They are an important 

food source for Steller sea lions (Eumetopius jubatus) and 

other predators (Sinclair and Zeppelin, 2002; Lowe et 

al., 2009).

The Atka mackerel spawning season lasts from July 

to October (McDermott and Lowe, 1997). Eggs hatch 

44–100 days after spawning, depending on temperature 

(Lauth et al., 2007). Neuston and bongo net surveys by 

the AFSC’s Fisheries-Oceanography Coordinated Investigations (FOCI) program have found Atka mackerel 

larvae in nearshore surface waters from September to 

June. The average standard length (SL) of larvae in 

September is 8.6 mm, increasing to 34.7 mm by the 

following June, and larvae become scarce in nearshore 

collections after April when average standard lengths 

are greater than 19.2 mm (Anderl et al., 1996). Juvenile 

Atka mackerel have been collected in surface tows in 

the central Bering Sea during June, July, and August, 

with average monthly lengths of 49, 54, and 101 mm 

SL, respectively (Anderl et al., 1996). During the spring 

months, young Atka mackerel of about 180 mm fork 

length (FL) start to appear in bottom trawl surveys 

(Anderl et al., 1996).

Female Atka mackerel reach 50% maturity by 3.6 

years (McDermott and Lowe, 1997). According to 

AFSC Age and Growth Program data, Atka mackerel 

from the Gulf of Alaska are larger at age than those 

from the Aleutian Islands (Fig. 30). Von Bertalanffy 

growth parameters were L∞=475.8 mm, k=0.3874/yr, 

and t0=–0.267 yr (n=963) and L∞=426.5 mm, k=0.419/yr, 

and t0=0.044 yr (n=1765) for Atka mackerel collected 

during trawl surveys in the Gulf of Alaska and Aleutian 

Islands, respectively, from 2003 through 2007 (Fig. 30). 

To date, the oldest Atka mackerel aged by the Age and 

Growth Program was 15 years, and the largest recorded 

was 600 mm FL. In the Aleutian Islands, there is strong 

evidence of a longitudinal gradation in average length 

at age; fish in the west are smaller at age than in the 

east (Lowe et al., 1998).

Age determination history

The AFSC Age and Growth Program began age determination of Atka mackerel in 1979 due to increased 
32 Professional Paper NMFS 13

Figure 31

Otolith from a 180 mm Atka mackerel 

(Pleurogrammus monopterygius) collected in May 1985. Only one annual 

mark is visible on the surface, but 

this fish has already lived through two 

winter seasons. Final age estimate is 2 

years (one year added to account for 

invisible first annual mark).

fishery landings. At that time, very few Atka mackerel 

age determination studies were published (Bel’chuk, 

1938; Gorbunova, 1962), and they did not adequately 

describe aging methodology. Though the hypural 

bone was used as an aging structure in one of these 

early studies (Bel’chuk, 1938), the AFSC Age and 

Growth Program selected the sagittal otolith because 

of its relatively clear pattern of alternating translucent 

and opaque growth zones.

Soon after production age determination of Atka 

mackerel began at the AFSC, fishery managers recognized that AFSC otolith-based length-at-age data 

were poorly correlated with published Russian data. 

Atka mackerel aged by the AFSC appeared to be a 

year younger than Russian fish of similar length. 

Adding a year to all Atka mackerel otolith-based 

age estimates resulted in better comparisons and 

revealed a large Atka mackerel year class that coincided with the 1977 phenomenon when large year 

classes were observed in many Alaskan fish species 

(Hollowed and Wooster, 1995). Because of this, U.S. 

fishery managers added a year to otolith-based age 

estimates for stock assessment purposes. Support 

for this decision came later from larval and juvenile 

studies. When otoliths from 180 mm FL spring-captured fish were aged, only one strong annual mark 

was found along the otolith margin (Fig. 31); however, these fish had already lived through two winter 

seasons (Anderl et al., 1996). We determined that if 

an annual mark is deposited during the first year of 

life, its position may be too close to the otolith core 

to be distinguishable. To maintain consistency with 

previous protocols, the AFSC Age and Growth Program continues to report ages to data users without 

a first annual mark, with the understanding that one 

will be added to all Atka mackerel age estimates during stock assessment.

To date, the AFSC Age and Growth Program has 

aged over 18,000 Atka mackerel otoliths. Atka mackerel otoliths are relatively easy to interpret, and 

estimates of precision between age readers tend to 

be very high. Atka mackerel age estimates have been 

corroborated by Anderl’s work described above and 

by edge type analysis (Kimura et al., 2007), described 

in further detail in Chapter 6 of this manual.

Current age determination methods

Surface examination. Atka mackerel otoliths are relatively small and easy to interpret when compared with 

the otoliths of other Alaskan groundfish species. An average 1-year-old otolith is only about 5 mm long. Whole 

otoliths are examined using a dissecting microscope at 

25× magnification with reflected light. Atka mackerel 

otolith surface patterns are sometimes clear enough 

that an experienced age reader can assign age estimates 

up to 6 years without needing to examine the corresponding break-and-burn pattern. The clearest growth 

pattern is typically found on the posterior (rounded) 

side of the otolith, especially along the ventral knob 

(Fig. 32). The first visible annual mark, which is formed 

during the second year, encompasses a wide surface area 

when compared with subsequent annual marks. The 

dorsal-ventral diameter of the first visible annual mark 

measured through the core is typically about 1.4 mm.

In some cases, the anterior otolith half (pointed 

end) is the clearer choice for surface age determination. However, for old or difficult otoliths, examining 

the corresponding break-and-burn pattern (Chapter 3) becomes necessary to determine a final age 

estimate.
Chapter 4: Groundfish age determination 33

Figure 32

Atka mackerel (Pleurogrammus monopterygius) otolith 

with a clear surface pattern. Final age estimate is 7 years 

(one year added to account for invisible first annual 

mark). Collected from a 430 mm female in September 

2007. Viewed with reflected light.

Break-and-burn examination. To produce a breakand-burn pattern, the otolith is gripped firmly in one’s 

fingers and snapped along the transverse axis using 

the thumbnails. The broken end is toasted quickly 

over an alcohol flame until it is golden in color. Atka 

mackerel otoliths quickly char, so only a few passes over 

the flame are necessary. The break-and-burn cross section is then viewed with a dissecting microscope and 

reflected light.

Atka mackerel otoliths produce some of the clearest break-and-burn patterns of all the groundfish 

species aged at the AFSC (Fig. 33). Annual marks on 

a break-and-burn cross section are commonly read 

from the core to the proximal surface of the ventral region. Sometimes the more compressed dorsal 

region is clearer and can help interpret a questionable ventral pattern (Fig. 34). In most cases, annual 

marks are continuous and distinct all the way from 

the ventral edge to the sulcus. Even if the area adjacent to the sulcus is not clear, the banding pattern 

is often strongest near the ventral edge, which may 

be sufficient to determine an otolith age estimate 

(Fig. 35). In the later years, a notch commonly develops on the proximal surface between the outside 

ventral edge and the sulcus, sometimes creating a 

disruption in the continuity of older annual marks 

(Fig. 36).

Recognizing the regular spacing between annual 

marks is integral to Atka mackerel age determination. 

Figure 33

Clear break-and-burn cross sections of Atka mackerel 

(Pleurogrammus monopterygius) otoliths from (A) 430 

mm female with a final age estimate of 7 years and 

(B) 410 mm male with a final age estimate of 8 years. 

(One year was added to each age estimate to account 

for invisible first annual mark.) Opaque growth at edge 

not counted in age estimates because both fish were 

collected in September 2007. Viewed with reflected 

light.
34 Professional Paper NMFS 13

The widest opaque growth zones occur within the first 

2 to 5 years. Faint annual marks are not uncommon 

in Atka mackerel otoliths, and checks can disrupt the 

growth pattern (Fig. 37). However, because the general growth pattern is so regular across specimens, it is 

sometimes obvious where an annual mark should be.

Figure 34

Break-and-burn otolith cross section collected from a 660 mm female Atka mackerel 

(Pleurogrammus monopterygius) on 27 October 2008. Both the dorsal and ventral sides 

were used to arrive at a final age estimate of 8 years (one year added to account for 

invisible first annual mark). Viewed with reflected light.

Figure 35

Break-and-burn cross section of an otolith collected from a 410 mm female 

Atka mackerel (Pleurogrammus monopterygius) on 25 June 2007. Final age 

estimate is 8 years (one year added to account for invisible first annual mark). 

Clearest pattern is along ventral edge. Viewed with reflected light.
Chapter 4: Groundfish age determination 35

Figure 36

Break-and-burn cross section of an Atka mackerel (Pleurogrammus monopterygius) 

otolith, showing proximal surface notch which sometimes disrupts continuity 

of annual marks from the ventral edge to the sulcus in older fish. Final age 

estimate is 8 years (one year added to account for invisible first annual mark). 

Collected from a 420 mm male in July 2007. Viewed with reflected light.

Figure 37

Break-and-burn cross section of an Atka mackerel (Pleurogrammus monopterygius) 

otolith. Growth pattern has multiple checks and is difficult to interpret, 

especially area between proximal surface notch and sulcus. Clearest reading 

axis is ventral edge. Final age estimate is 8 years (one year added to account 

for invisible first annual mark). Collected from a 400 mm male in the Aleutian 

Islands on 15 September 2007. Viewed with reflected light.
36 Professional Paper NMFS 13

Sablefish (Anoplopoma fimbria)

by Delsa M. Anderl

Biology

Sablefish (Anoplopoma fimbria) supports a very important and lucrative fishery in the North Pacific and is 

one of the most studied fish along the North American 

Pacific coast. Sablefish range from Baja California 

through the Bering Sea to Kamchatka and Japan (Hart, 

1973) and are capable of traveling great distances 

(Kimura et al., 1998). Tagging data suggest there may 

be at least two populations of sablefish: an Alaskan 

population and a West Coast population, with some 

mixing occurring from British Columbia to the Oregon 

coast (Kimura et al., 1998). Of the two groups, the 

Alaskan population appears more mobile. In the Gulf 

of Alaska, sablefish perform a two-way migration from 

the continental slope to seamounts located 270–465 km 

offshore (Maloney, 2004). Furthermore, fish tagged at 

ages 0–2 in southeast Alaska have been recaptured in 

British Columbia, the western Aleutian Islands, and the 

eastern Bering Sea (Maloney and Sigler, 2008). Migratory movement and depth preference are dependent 

on age; as a result, sablefish in the age-5 and age-6 

year classes are most vulnerable to the target fishery 

(Maloney and Sigler, 2008).

Sablefish in the northeast Pacific Ocean spawn during winter in offshore waters. Eggs hatch in the water 

column and young larvae then make their way to surface 

waters. Soon afterwards, they migrate to inland waters 

and shallow bays (Kendall and Matarese, 1987). They 

spend about a year in these nursery grounds and then 

return to the open ocean sometime in their second year. 

By age-3 or -4, most fish are in offshore waters (Maloney 

and Sigler, 2008). Young sablefish are fast growers, 

reaching an average length of 600 mm at age-5 before 

growth begins to slow down (Fig. 38). Von Bertalanffy 

growth parameters were L∞=719.8 mm, k=0.2178/yr, 

and t0=–3.633 yr (n=290) and L∞=728.0 mm, k=0.2273/yr, 

and t0=–2.947 yr (n=884) for sablefish collected during 

trawl surveys in the Bering Sea/Aleutian Islands region 

and the Gulf of Alaska, respectively, in 2007 (Fig. 38). 

The maximum age reported to date by the Age and 

Growth Program for sablefish is 94 years.

Age determination history

Of all the groundfish species studied by the AFSC Age 

and Growth Program, sablefish is one of the most difficult species to age. The complex growth patterns 

observed in sablefish otoliths may be related to their 

extensive geographic and depth migrations. Sablefish 

were first aged at the AFSC from whole otolith surface 

patterns. Around 1982, mark-recapture studies demonFigure 38

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for sablefish 

(Anoplopoma fimbria) collected during trawl surveys in the Bering Sea/Aleutian 

Islands (BSAI; n=290) and Gulf of Alaska (GOA; n=884) in 2007.
Chapter 4: Groundfish age determination 37

strated that sablefish is a long-lived species and that the 

break-and-burn method results in more accurate age estimates than using the surface method alone (Beamish 

and Chilton, 1982; Boehlert and Yoklavich, 1985). The 

AFSC later confirmed these findings with a radiometric 

age validation study (Kastelle et al., 1994).

Since 1993, the AFSC Age and Growth Program has 

aged over 24,000 sablefish otoliths. The average interreader CV is 10.8% (n=8052), reflecting the relative 

difficulty of sablefish age determination. The pattern 

complexity of this long-lived species often requires reaging some specimens. Otolith length and fish length 

of sablefish are not always strongly related with age (Fig. 

39); this differs from many of the other groundfish 

studied at the AFSC.

Otoliths collected from sablefish during tagging 

studies (Heifitz et al., 1998; Rutecki and Varosi, 1997; 

Maloney and Sigler, 2008) have provided the Age and 

Growth Program with a rare opportunity to validate 

our sablefish aging criteria. Most of the images in this 

Figure 39

Otoliths collected from two different 600 mm sablefish (Anoplopoma fimbria), both captured in August. 

(A) Otolith surface and (B) break-and-burn preparation from a sablefish aged 4 years. (C) Otolith surface 

and (D) break-and-burn preparation from a sablefish aged 23 years. Otolith thickness and differences between 

otolith surface patterns are good indicators of break-and-burn age estimates. The otolith in (A) is relatively thin 

with distinct, dark annual marks. The otolith in (C) is relatively thick and has a broad, dark area observed on the 

surface following the first several annual marks, indicating the otolith is old. Viewed with reflected light.

chapter are of known-age specimens that were tagged as 

juveniles (ages 0–2) and later recaptured as adults. Ages 

at the time of tagging were determined from non-overlapping length frequencies (Maloney and Sigler, 2008).

Current age determination methods

Sablefish otoliths have several morphologically distinct shapes (Fig. 40). Recognizing each shape and 

its associated pattern is necessary for accurate age 

estimates, because sablefish aging criteria are dependent on both otolith shape and growth pattern. 

Nomenclature helpful in describing sablefish otolith 

reference points is found in Figure 41. At the AFSC, 

sablefish otoliths (surfaces and break-and-burn cross 

sections) are examined using a dissecting microscope 

and reflected light.

Surface examination. Examining the otolith surface 

pattern should always be the first step in sablefish age 
38 Professional Paper NMFS 13

determination. An initial surface age estimate helps 

with interpretation of the break-and-burn pattern, 

especially if the pattern has many checks. An intense 

dark band along the posterior margin, composed of 

multiple compressed annual marks, is usually a sign 

that the otolith is old. An otolith’s thickness relative to 

its dorsal-ventral width is typically an indicator of age. 

However, if an otolith is thick, but the surface pattern 

looks young, the fish could be young but unusually 

fast-growing.

The first several annual marks form distinct grooves 

on the distal surfaces of sablefish otoliths. Break-andburn cross sections should be tipped slightly, as demonstrated in Figures 42 and 43, to better see the groove 

patterns. The grooves are followed from the distal surface through the cut surface of the cross section, making 

it possible to identify the first three to four annual marks 

(Figs. 42 and 43). Checks are weaker in appearance and 

do not coincide with grooves, and thus tipping the cross 

section allows the age reader to discern between checks 

and annual marks. A check is commonly observed between the first two annual marks.

Break-and-burn examination. A few young otoliths 

with clear surface patterns can be assigned age estimates by examining only the whole otolith surface 

pattern. However, the overwhelming majority of sablefish otoliths are aged using the break-and-burn method 

(Chapter 3). The break-and-bake method does not 

produce enough contrast between translucent and 

opaque growth zones, and in some cases it appears to 

accentuate checks. Sablefish otoliths are too small to 

be effectively cut with a low-speed saw, so cross sections 

are made by gripping the otolith in one’s fingers and 

snapping it in half along the transverse axis using the 

thumbnails. An alternative method is to cut through 

the core with a scalpel. The cut face of the cross section is slowly toasted over an alcohol flame. Sablefish 

Figure 40

Four of several morphologically distinct shapes observed in sablefish (Anoplopoma fimbria) otoliths from part of 

the Alaska Fisheries Science Center Age and Growth Program’s known-age collection. The shapes of otoliths in 

(A) (630 mm female) and (C) (670 mm female) are associated with fast-growing fish; the preferred reading axis 

is along the distal margin. The otolith in (B) (590 mm male) has translucent growth zones converging along 

the distal margin. Following each translucent growth zone to the sulcus, when possible, is helpful for identifying 

true annual marks. The otolith in (D) (730 mm male) has the greatest amount of growth along the sulcus. 

Although it can sometimes be difficult to view compressed annual marks in this region of the otolith, the sulcus 

is the best reading axis for this morphological shape.
Chapter 4: Groundfish age determination 39

otoliths are usually aged on the ventral side of breakand-burn cross sections. As described above, surface 

pattern grooves are important in properly aging sablefish otoliths, especially in identifying early annual 

marks. Tipping the break-and-burn cross section to 

observe these grooves aids the age reader in evaluating 

complicated growth patterns (Fig. 43).

Figure 44 is an example of a relatively easy breakand-burn pattern, despite having multiple checks. 

This otolith is from an 8-year-old from the knownage collection. The last five annual marks are clear 

on the surface and correspond to five clear growth 

zones on the break-and-burn cross section. The first 

three annual marks are more problematic, requiring 

tipping the break-and-burn cross section to follow 

individual grooves on the distal surface. The morphological shape of this otolith cross section is relatively 

common.

Another common morphological shape is one in 

which the distal edge abruptly curves towards the sulcus 

(Fig. 45). A transition zone begins after the fourth annual mark, followed by more compressed growth after 

the sixth annual mark. The area of compressed growth 

is usually also evident on the otolith surface as a dark 

band along the margin.

A more extreme case of the pattern described 

above is where the axis of maximum growth is along 

the sulcus and growth at the distal margin is greatly 

reduced (Fig. 46). This pattern is usually reflected 

on the surface by a dark band of compressed annual 

marks. Cross sections from this type of otolith must be 

evenly cut and should be burned slowly and carefully 

to get the best contrast between highly compressed 

annual marks.

Another common but problematic pattern is when 

annual marks converge at a common focal point along 

the distal margin, and identification of individual annual marks is difficult (Fig. 47). In most cases, when 

growth patterns are clear but discrepant on two or 

more reading axes, the area adjacent to the sulcus 

often (though not always) results in a more accurate 

age estimate.

Transition zone patterns can also be difficult. There is 

usually a dark band made up of compressed translucent 

Figure 41

Terminology commonly used to describe sablefish (Anoplopoma fimbria) otoliths. Upper panel: 

whole otolith viewed from the distal surface. Lower panel: break-and-burn cross section. 

Sablefish otoliths are typically aged on the ventral side of break-and-burn cross sections. 

Viewed with reflected light.
40 Professional Paper NMFS 13

Figure 42

Composite image of sablefish (Anoplopoma fimbria) otoliths illustrating clear surface groove patterns that can be 

used to identify early annual marks in break-and-burn cross sections. (A) The translucent growth zone observed 

between the first and second annual marks on the cross section (inset) corresponds to a weak one on the otolith 

surface and therefore is considered a check. (B) Early annual marks appear as strong grooves on the break-andburn distal surface. These are easier to see when the otolith is tipped (left) and correspond to strong marks 

on the whole otolith surface (right). Without first examining the surface pattern for corresponding grooves, 

identifying early annual marks in the two-dimensional cross section is difficult. Viewed with reflected light.

growth zones observed within the transition zone. The 

age reader must decide whether the translucent growth 

zones are components of a single annual mark or if each 

translucent growth zone is an individual annual mark 

(Fig. 48).

Figures 49 and 50 show otoliths with less common 

growth patterns. The oldest sablefish otolith examined 

to date by the Age and Growth Program had an estimated age of 94 years and a surprisingly clear pattern 

considering its age (Fig. 51).
Chapter 4: Groundfish age determination 41

Figure 43

Sablefish (Anoplopoma fimbria) otolith with multiple checks that could easily be over-aged without careful 

examination. Collected from a known-age 5-year-old, 555 mm female recaptured in May. Prominent check 

observed between the second and third annual marks on the otolith surface and break-and-burn cross sections. 

Tipping the otolith cross section as shown (upper right panel) enhances the grooved annual marks and discerns 

between checks and annual marks. Viewed with reflected light.
42 Professional Paper NMFS 13

Figure 44

Known-age 8-year-old sablefish (Anoplopoma fimbria) otolith from a 710 mm 

female recaptured in April. Distinct growth pattern in the last 5 years. First 

3 years are problematic; two translucent growth zones could be mistaken for 

annual marks but are likely checks because they are traced to weak zones on the 

whole otolith surface pattern. Viewed with reflected light.
Chapter 4: Groundfish age determination 43

Figure 45

Known-age 8-year-old sablefish (Anoplopoma fimbria) otolith from a 

590-mm male recaptured in March. Common growth pattern with 

sharp transition from fast to slow growth. Fast growth occurs during 

the first 3–4 years. After deposition of the fourth annual mark, growth 

between annual marks slows down significantly (usually associated 

with a wide dark band along the otolith surface margin). Viewed with 

reflected light.
44 Professional Paper NMFS 13

Figure 46

Otolith from a 670 mm male sablefish (Anoplopoma fimbria). Age estimate is 

21 years. Highly compressed pattern. Axis of maximum growth is along sulcus. 

Distal margin is not useful for age estimation. Viewed with reflected light.

Figure 47

Known-age 8-year-old sablefish (Anoplopoma fimbria) otolith from a 

590 mm male recaptured in July. A common growth pattern, where 

some annual marks merge at a focal point along the distal edge, 

fanning out into clear, regular growth zones toward the sulcus. The 

check between the fifth and sixth annual marks is strong along the 

distal edge but dissipates closer to the sulcus. Viewed with reflected 

light.
Chapter 4: Groundfish age determination 45

Figure 48

Known-age 15-year-old sablefish (Anoplopoma fimbria) otolith from a 700 mm male 

recaptured in April. The transition from fast to slow growth begins at 7 years where 

a thick, dark band made up of three thinner translucent growth zones is observed. 

Spacing between these bands is highly compressed compared to the surrounding 

pattern. True age indicates they should be considered individual annual marks. 

Viewed with reflected light.
46 Professional Paper NMFS 13

Figure 49

Known-age 7-year-old sablefish (Anoplopoma fimbria) otolith from a 670 mm female 

recaptured in June. Otoliths with this morphology always have broad, compressed 

annual marks and multiple checks. Viewed with reflected light.
Chapter 4: Groundfish age determination 47

Figure 50

Known-age 10-year-old sablefish (Anoplopoma fimbria) otolith from a 620 mm male 

recaptured in April. Early years are difficult to interpret. The third annual mark 

is easier to identify after associating the cross-section bands with the surface and 

groove patterns, but the locations of the first and second annual marks on the 

break-and-burn cross section remain questionable. Viewed with reflected light.
48 Professional Paper NMFS 13

Figure 51

Oldest sablefish (Anoplopoma fimbria) otolith aged by the AFSC Age and Growth Program. Collected from an 

Aleutian Islands female sablefish captured and tagged in 1988 and recaptured one year later at 770 mm during 

a NMFS longline survey in the same vicinity where it was tagged. Estimated age is 94 years. Every tenth annual 

mark is indicated by a small black bar. Viewed with reflected light.
Chapter 4: Groundfish age determination 49

Rockfish (Sebastes) species

by B. J. Goetz, Charles E. Piston, 

and Christopher M. Gburski

Introduction

The AFSC Age and Growth Program ages five species 

of rockfish (genus: Sebastes) on a production basis, 

and an additional four species on an occasional basis 

(Table 2). Most rockfish species have similar otolith 

growth pattern characteristics, and aging criteria can 

be applied uniformly with few exceptions. Although 

some annual marks are apparent on otolith surfaces, it 

is typically difficult to estimate age using only surface 

patterns. The standard method for aging all size classes 

of rockfish species is the break-and-burn technique 

(Chapter 3). The otolith break-and-bake method has 

been attempted but was unsuccessful. Thin-sectioning 

is another option if the burning process does not produce satisfactory patterns.

Rockfish age determination methods

Otolith preparation. Most rockfish otoliths are prepared for age determination using the break-and-burn 

method (Chapter 3). Otoliths are cut transversely 

through the core using a low-speed saw, with the sulcus 

facing the saw blade to obtain an even cut. Even when 

using the lightest saw weight, rockfish otoliths may snap 

before the cut has been completed, often resulting in a 

large ledge that must be sanded smooth.

Each rockfish species requires a slightly different 

burning technique which varies by age reader and 

alcohol burner. Age readers should experiment to 

find the best way to burn an otolith. One method that 

works well for most rockfish species is to hold the cut 

surface of the otolith about one inch from an alcohol 

Table 2

Rockfish (Sebastes spp.) species aged by the Alaska 

Fisheries Science Center (AFSC) Age and Growth 

Program, ranked by age determination difficulty.

Very

difficult Difficult Moderate Moderately easy

S. borealis S. aleutianus S. alutus S. ciliatus

S. melanostictus S. polyspinis

 S. proriger*

S. variabilis

S. variegatus*

S. zacentrus*

* limited number aged by AFSC Age and Growth Program

flame, slowly passing it back and forth while it gradually browns. If this does not work well, the otolith can 

be burned with the cut surface facing away from the 

flame. After the entire otolith browns, the otolith half 

is turned so that the cut surface is down and rapidly 

passed through the flame four to five times to darken 

the center.

An optional polishing step either before or after burning can enhance the pattern, especially if foreign matter 

on the otolith has left dark crusts obscuring the pattern. 

Prior to coating the burned surface with mineral oil, 

the surface must be dry, free of any preservative, and 

sufficiently cooled. At the AFSC, all rockfish otoliths are 

examined using a dissecting microscope and reflected 

light.

Readability. Readability varies among rockfish species and is a subjective assessment of pattern clarity, 

ease of burning, and other factors related to the interpretation and identification of annual marks (Table 

2). Pattern difficulties most frequently involve the first 

three annual marks or interpretation of growth at the 

edge.

Reading axis. When looking at break-and-burn cross 

sections, there are a number of landmarks which help 

to orient the age reader (Fig. 52). The sulcus separates 

the dorsal and ventral sides of the otolith. On the ventral 

side, more checks are visible and annual marks have a 

greater tendency to split than on the dorsal side. The 

easiest patterns to interpret are often seen on the dorsal 

side of the sulcus (Fig. 52). Therefore, we tend to concentrate on the dorsal side of break-and-burn cross sections during age determination. The fastest growing axis 

is the dorsal tip. Since splitting is often more prominent 

along the distal surface of the dorsal side, translucent 

growth zones should be followed to the sulcus to identify 

annual marks.

First and second annual marks. Typically, the first and 

second annual marks on rockfish otoliths are difficult to 

interpret. It is quite easy to fail to pass through the core 

when cutting, resulting in a missed or vague first annual 

mark. Often the second annual mark is clearer than the 

first. The expected diameter of the first annual mark 

(measured from clear otoliths) can be used to decide 

whether a first year needs to be added to patterns with 

an obscure or vague center. We have not yet validated 

the aging criteria used to determine the first two years in 

rockfish, but in order to maintain precision we continue 

to apply expected diameter measurements of the first 

annual mark as criteria until more information becomes 

available.

One useful technique to help resolve particularly 

ambiguous early growth patterns is to compare difficult 
50 Professional Paper NMFS 13

otolith patterns to clear otolith patterns. Viewing a 

clearer break-and-burn cross section alongside the problematic pattern may help identify the most probable annual marks. Age readers should note otoliths with clear 

early growth patterns and refer to them when viewing 

otoliths with more difficult patterns.

Growth increment spacing. Rockfish otolith growth 

slows down gradually with age, resulting in relatively 

Figure 52

Break-and-burn preparation showing general terminology used when examining rockfish (Sebastes spp.) otoliths. 

Viewed with reflected light.

regular growth patterns. The expected spacing between 

annual marks helps age readers decide how to interpret 

faintly burned areas. Additionally, vague patterns frequently appear near the edge. Relatively clear patterns 

with wide, dark areas comprised of multiple annual 

marks along the edge are common. By examining the 

spacing between previous annual marks, one can determine (usually within one year) how many annual marks 

fit within the dark area.
Chapter 4: Groundfish age determination 51

Figure 53

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for Pacific 

ocean perch (Sebastes alutus) collected during trawl surveys in the Bering Sea/

Aleutian Islands (BSAI; n=1918) and Gulf of Alaska (GOA; n=2203) from 2004 

through 2007.

Pacific ocean perch (Sebastes alutus)

Biology

Pacific ocean perch (Sebastes alutus) is the most abundant and commercially important rockfish in Alaskan 

waters. This species occurs along the outer continental 

shelf and upper slope regions of the North Pacific 

Ocean and Bering Sea (Quast, 1970; Westrheim, 1970), 

from southern California northward through the Gulf of 

Alaska and the Bering Sea, westward along the Aleutian 

archipelago, Kamchatka Peninsula, and Kuril Islands 

(Phillips, 1957; Moiseev and Paraketsov, 1961), and to 

the east coast of Honshu Island, Japan (Barsukov, 1964).

Pacific ocean perch larvae are pelagic and transform 

to a bottom-dwelling state within the first year (Carlson 

and Haight, 1976). Juveniles predominantly reside in 

benthic habitats with complex vertical structures, such 

as boulders, macrophytes, sponges, and corals (Carlson 

and Haight, 1976; Carlson and Straty, 1981; Love et al., 

1991; Rooper and Boldt, 2005; Rooper et al., 2007). Juveniles migrate from shallower continental shelf waters 

to deeper waters with increasing age, eventually residing 

over the continental slope at adulthood (Carlson and 

Haight, 1976; Carlson and Straty, 1981).

Pacific ocean perch are semi-demersal and spend 

most of their time near the bottom but migrate vertically day or night (Skalkin, 1964; Lyubimova, 1965; Brodeur, 2001) to feed opportunistically on mesopelagic 

zooplankton. Adults undertake a seasonal migration, 

inhabiting shallower depths (150–300 m) in the summer and migrating to deeper water (300–420 m) in the 

fall (Lyubimova, 1963; Paraketsov, 1963; Westrheim, 

1970; Gunderson, 1971; Love et al., 2002). This pattern 

is likely related to summer feeding and winter spawning. Spawning takes place in the fall, and eggs are internally fertilized about two months later (Love et al., 

2002). Larvae are released from March through May 

(Paraketsov, 1963).

Pacific ocean perch are slow-growing and long-lived. 

Age at 50% maturity is 10.5 years in the Gulf of Alaska 

(Hanselman et al., 2007). According to AFSC Age and 

Growth Program data, there are slight differences 

between the Bering Sea and Aleutian Islands (BSAI) 

stock and the Gulf of Alaska (GOA) stock in length 

at age, with those from the BSAI growing more slowly 

and to larger sizes than those from the GOA (Fig. 53). 

Von Bertalanffy growth parameters were L∞=428.3 mm, 

k=0.1259/yr, and t0=–1.243 yr (n=1918) and L∞=412.2 

mm, k=0.1847/yr, and t0=–0.255 yr (n=2203) for Pacific 

ocean perch collected during trawl surveys in the BSAI 

region and the Gulf of Alaska, respectively, from 2004 

through 2007 (Fig. 53). The maximum age reported by 

the AFSC Age and Growth Program for Pacific ocean 

perch to date is 105 years.

Age determination history

The AFSC Age and Growth Program has aged over 

50,000 Pacific ocean perch. This species is moderately 
52 Professional Paper NMFS 13

Figure 54

Break-and-burn otolith cross section from a 370 mm female Pacific ocean perch (Sebastes alutus) caught on 9 

July 2006 in the Bering Sea. The pattern is relatively vague, especially in the first 3 years. Age estimate is 11 

years. Bar represents the expected diameter of the first annual mark (1.65 mm). Viewed with reflected light.

difficult to age, reflected in estimates of precision 

(CV=6.37%; n=8087). Prior to 1982, otolith surface 

examination was the primary method for rockfish age 

determination at the AFSC. Today, all Pacific ocean 

perch otoliths are processed using the break-and-burn 

method (Chapter 3). During the early to mid-1980s, 

periodic Pacific ocean perch otolith exchanges among 

members of the Committee of Age Reading Experts 

(CARE) verified that the general aging criteria used 

by all participating agencies were similar. Pacific ocean 

perch age estimates have been validated using radiometric methods (Kastelle et al., 2000) and bomb 

radiocarbon analysis (Kastelle et al., 2008b); these 

methods are described in further detail in Chapter 6 

of this manual.

Current age determination methods

When compared with other rockfish species aged by 

the AFSC Age and Growth Program, interpretation 

of Pacific ocean perch otoliths is moderately difficult. 

Annual marks are not well defined on Pacific ocean 

perch otolith surfaces. The first four annual marks 

are often vague on very young otoliths and clearer 

on older specimens. Growth patterns are interpreted 

using a dissecting microscope with reflected light at 

magnifications ranging from 6–96×, depending on the 

size of the otolith.

The first annual mark of Pacific ocean perch otoliths is typically larger than those of all other rockfish 

species aged by our program, with the exception of 

shortraker rockfish (S. borealis). The average diameter of the first annual mark is approximately 1.65 

mm, determined from clear Pacific ocean perch 

otoliths.

It is common for age readers to cut otoliths off-center so that the first annual mark’s shape is distorted. If 

the first clear translucent growth zone is much larger 

than the average first annual mark diameter, an extra 

year is added to the age estimate even if it is not clear 

on the break-and-burn preparation. Examples of 

Pacific ocean perch otoliths are shown in Figures 54 

through 59.
Chapter 4: Groundfish age determination 53

Figure 55

Break-and-burn otolith cross section from a 340 mm female Pacific ocean perch (Sebastes alutus) caught on 13 

July 2006 in the Bering Sea. The first annual mark is relatively clear. Age estimate is 11 years. Bar represents the 

expected diameter of the first annual mark (1.65 mm). Viewed with reflected light.

Figure 56

Break-and-burn otolith cross section from a 320 mm female Pacific ocean perch (Sebastes alutus) caught on 6 

July 2006 in the Bering Sea. Pattern is relatively clear. Opaque growth at the dorsal tip not counted because it 

is attributed to the 2006 growth year. Age estimate is 9 years. Bar represents the expected diameter of the first 

annual mark (1.65 mm). Viewed with reflected light.
54 Professional Paper NMFS 13

Figure 58

(A) Vague otolith surface pattern and (B) clear break-and-burn cross section 

from a 150 mm female Pacific ocean perch (Sebastes alutus) caught on 25 

July 2007 in the Gulf of Alaska. Small translucent growth zone inside the first 

annual mark is not counted. Opaque growth on edge is not counted because 

it is attributed to 2007. Age estimate is 2 years. Bar represents the expected 

diameter of the first annual mark (1.65 mm). Viewed with reflected light.

Figure 57

Break-and-burn otolith cross section from a 340 mm female Pacific ocean perch (Sebastes alutus) caught on 16 

May 2006 in the Bering Sea. Relatively clear pattern, but a vague translucent growth zone observed around the 

core was not counted because it is much smaller than the expected first annual mark diameter (white bar, 1.65 

mm). Opaque growth at the dorsal tip was counted since it extends to the sulcus and this otolith was collected in 

the spring. Pattern on the ventral side of the sulcus is relatively clear, corroborating the age estimate of 7 years. 

Viewed with reflected light.
Chapter 4: Groundfish age determination 55

Figure 59

Pacific ocean perch (Sebastes alutus) break-and-burn otolith cross section. Estimated age is 

100 years. Viewed with reflected light.
56 Professional Paper NMFS 13

Figure 60

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for northern 

rockfish (Sebastes polyspinis) collected during trawl surveys in the Gulf of Alaska 

(GOA; n=1035) and Aleutian Islands (AI; n=1050) from 2004 through 2007.

Northern rockfish (Sebastes polyspinis)

Biology

Northern rockfish (Sebastes polyspinis) is a commercially 

important species in Alaskan waters and is the second 

most abundant species caught in the Gulf of Alaska 

(Clausen and Heifetz, 2002). The distribution of northern rockfish encompasses the waters off the extreme 

northern part of British Columbia through Alaska to 

eastern Kamchatka and the northern Kuril Islands 

(Allen and Smith, 1988; Ito et al., 1999). Northern 

rockfish is most abundant in Alaskan waters, from the 

western end of the Aleutian Islands to Portlock Bank in 

the central Gulf of Alaska (Clausen and Heifetz, 2002). 

This species forms large schools over rough, hard bottom and is caught over steep slopes at depths of 75 to 

125 m using bottom trawl gear (Clausen and Heifetz, 

2002). Northern rockfish is a target species in the Gulf 

of Alaska, whereas it is taken primarily as bycatch in the 

Aleutian Islands and Bering Sea.

Little is known about the early life history of northern 

rockfish, particularly the larval and juvenile stages. This 

species is ovoviviparous with internal fertilization. Gulf 

of Alaska research surveys indicate parturition (larval release) occurs in the spring and summer. Older juveniles 

(>200 mm) are found on the continental shelf inshore 

of adult habitat (Heifetz et al., 2007).

Northern rockfish mature around 8 years, or 310 mm 

(Chilton, 2007). According to AFSC Age and Growth 

Program data, northern rockfish from the Gulf of Alaska grow faster and reach a larger maximum length than 

the Aleutian Islands stock (Fig. 60). Von Bertalanffy 

growth parameters were L∞=413.0 mm, k=0.1788/yr, 

and t0=0.108 yr (n=1035) and L∞=351.4 mm, k=0.1602/yr, 

and t0=–0.778 yr (n=1050) for northern rockfish collected during trawl surveys in the Gulf of Alaska and 

Aleutian Islands, respectively, from 2004 through 2007 

(Fig. 60). Despite differences in life history, these stocks 

are genetically homogenous3. The oldest northern rockfish aged to date by the Age and Growth Program was 

88 years and 410 mm.

Age determination history

The AFSC Age and Growth Program began age determination of northern rockfish in 1988. Inter-reader precision is relatively high (CV=4.1%; n=2852) when compared with some of the other rockfish species aged by the 

AFSC. Northern rockfish age estimates have been validated using radiometric methods (Kastelle et al., 2000), 

described in further detail in Chapter 6 of this manual. 

A bomb radiocarbon study is also currently in progress at 

the AFSC to provide further support for our age estimates.

Current age determination methods

Otolith surfaces are not sufficient for accurate age 

determination of northern rockfish; thus, otoliths are 

3 Gharrett, A. J., A. K. Gray, D. Clausen, and J. Heifetz. 2003. Preliminary study of the population structure in Alaska northern 

rockfish, Sebastes polyspinis, based on microsatellite and mtDNA 

variation. Unpubl. contract report. Fisheries Division, School of 

Fisheries and Ocean Sciences, Univ. of Alaska, Fairbanks, Juneau, 

AK 99801,16 p.
Chapter 4: Groundfish age determination 57

Figure 61

(A) Otolith surface and (B) break-and-burn cross section from a 200 mm female 

northern rockfish (Sebastes polyspinis) collected 15 July 2006 in the Aleutian Islands. 

Otolith surface alone is insufficient for age determination. During break-and-burn 

preparation, the otolith was not sectioned directly through the core. Opaque growth 

zone evident on the dorsal tip was not included in the age estimate because it was likely 

deposited during 2006. Age estimate is 9 years. Expected diameter of first annual mark 

(1 mm) indicated by black bar. Viewed with reflected light.

prepared using the break-and-burn method (Fig. 61). 

Growth patterns are interpreted using a dissecting microscope with reflected light at magnifications ranging 

from 6–96×, depending on the size of the otolith. With 

practice it is easier to get a clear break-and-burn pattern in northern rockfish otoliths than in more difficult 

species like Pacific ocean perch. The growth patterns 

of northern rockfish otoliths are very similar to those 

of dusky rockfish (S. variabilis) otoliths.

The first annual mark is typically smaller in northern 

rockfish otoliths than in Pacific ocean perch otoliths. 

The approximate diameter of the first annual mark 

is 1.0 mm. The expected size of the first annual mark 

has been determined from specimens with extremely 

clear growth zones and subsequently verified while 

preparing samples for bomb radiocarbon validation 

experiments. A problem that commonly arises during 

otolith preparation is failure to section the otolith 

directly through the core, which results in distortion 

of the shape of the first annual mark. If the first welldefined translucent growth zone is much larger than 

1 mm, a missed first annual mark is assumed to be 

inside it.

Digital images of northern rockfish otoliths with annotated age estimates are shown in Figures 62 though 

66.
58 Professional Paper NMFS 13

Figure 62

Break-and-burn otolith cross section from a 390 mm female northern rockfish (Sebastes polyspinis) collected on 

10 June 1996 in the Gulf of Alaska. Clear early annual marks. Age estimate is 26 years. Viewed with reflected 

light.

Figure 63

Break-and-burn otolith cross section from a 370 mm female northern rockfish (Sebastes polyspinis) collected on 5 

August 1987 in the Gulf of Alaska. Vague first annual mark. Relatively clear growth pattern after second annual 

mark. Age estimate is 22 years. Expected diameter of first annual mark indicated by black bar (1 mm). Viewed 

with reflected light.
Chapter 4: Groundfish age determination 59

Figure 64

Break-and-burn otolith cross section from a 390 mm female northern rockfish (Sebastes polyspinis) collected on 7 

August 1987 in the Gulf of Alaska. Relatively clear growth pattern. Age estimate is 16 years. Expected diameter of 

first annual mark indicated by black bar (1 mm). Viewed with reflected light.

Figure 65

Clear break-and-burn otolith cross section from a 370 mm female northern rockfish (Sebastes polyspinis) collected 

on 14 July 2006 in the Aleutian Islands. Age estimate is 46 years. Viewed with reflected light.
60 Professional Paper NMFS 13

Figure 66

Break-and-burn otolith cross section from a 250 mm male northern rockfish (Sebastes polyspinis) collected on 15 

July 2006 in the Aleutian Islands. Relatively clear growth pattern. Checks observed near dorsal tip. Vague early 

annual marks. Age estimate is 11 years. Expected diameter of first annual mark indicated by black bar (1 mm). 

Viewed with reflected light.
Chapter 4: Groundfish age determination 61

Rougheye rockfish species complex

Biology

Genetic analysis has shown that the rougheye rockfish 

complex consists of two species: rougheye rockfish 

(Sebastes aleutianus), which ranges from the eastern 

Aleutian Islands off Unalaska Island and the eastern 

Bering Sea at Pribilof Canyon to southern Oregon; 

and blackspotted rockfish (S. melanostictus), which 

ranges from Japan through the Kuril Islands, Aleutian 

Islands, and the Bering Sea south to California (Gharrett et al., 2005; Orr and Hawkins, 2008). At-sea field 

identification of these two species is problematic, and 

misidentification of a third species, shortraker rockfish (S. borealis), can lead to mixed-species samples. 

Although both species have a high market value, they 

are primarily caught as bycatch in trawl and longline 

fisheries targeting other species (Shotwell et al., 2007; 

Spencer et al., 2008).

Rougheye rockfish are typically found at shallower 

depths (45 to 439 m) than blackspotted rockfish (84 

to 490 m). Submersible observations have shown that 

adults have a relatively even distribution over steep, 

rocky areas (Krieger and Wing, 2002). Very little is 

known about the early life history of these species as 

larvae and juveniles must be identified genetically. 

Larval release may take place from December through 

April (McDermott, 1994), and the larvae are thought 

to be pelagic. The length of the larval period is 

unknown.

Age determination history

Rougheye rockfish complex otoliths were first aged by 

the AFSC Age and Growth Program in 2003. While aging these samples, we observed different otolith growth 

patterns; some had very distinct, clear early growth 

patterns whereas others had multiple checks and indistinct patterns that were difficult to interpret. Genetic 

and morphological studies subsequently confirmed 

that the complex consists of two species (Gharrett et 

al., 2005; Orr and Hawkins, 2008). Since 2006, samples 

have been identified at sea as either rougheye rockfish 

or blackspotted rockfish. However, AFSC survey personnel have indicated that accurate field identification of 

these species still lacks precision, and for now we assume 

that collections are comprised of mixed species despite 

at-sea species designation. Otoliths have recently been 

collected together with fin clips for genetic identification and to determine whether otolith morphology can 

be used to distinguish between the two species; this work 

is currently underway.

To date, the program has aged over 6000 rougheye 

rockfish complex otoliths, with a maximum age estimate 

of 132 years and an average CV of 7.8%. Rougheye rockfish age estimates have been validated using radiometric 

methods (Kastelle et al., 2000), described in further 

detail in Chapter 6 of this manual.

Current age determination methods

Rougheye rockfish complex otoliths are typically prepared for age determination using the break-and-burn 

method (Chapter 3). Growth patterns are interpreted 

using a dissecting microscope with reflected light at 

magnifications ranging from 6–96×, depending on the 

size of the otolith.

To date, we have not received any positively identified 

rougheye or blackspotted rockfish otoliths, so we cannot comment on species-specific differences in otolith 

growth. We have noted specimens with particularly 

clear early growth or patterns showing multiple checks. 

Growth patterns are similar to very old Pacific ocean 

perch, and some of the oldest rougheye rockfish complex otoliths have required thin-sectioning (Chapter 3) 

to assign age estimates.

The approximate diameter of the first annual mark is 

slightly over 1.0 mm, roughly the same as northern rockfish. If the first visible annual mark is much larger than 

the expected size of 1.0 mm, an extra year is added to 

the age estimate. Images of rougheye rockfish complex 

otoliths are shown in Figures 67 through 70.
62 Professional Paper NMFS 13

Figure 68

Break-and-burn preparation of a rougheye (Sebastes aleutianus) or blackspotted (S. melanostictus) rockfish otolith. 

Collected from a 660 mm male in the Aleutian Islands on 11 June 2006. A relatively clear growth pattern, 

especially the early annual marks and edge, but the area immediately after the tenth annual mark is vague 

(indicated by question mark). Age estimate is 47 years. Viewed with reflected light.

Figure 67

Break-and-burn preparation of a rougheye (Sebastes aleutianus) or blackspotted (S. melanostictus) rockfish 

otolith. Rare, relatively clear growth pattern. Age estimate is 8 years. Collected from a 330 mm female in the 

eastern Bering Sea in June 2002. Viewed with reflected light.
Chapter 4: Groundfish age determination 63

Figure 69

(A) Otolith surface and (B) break-and-burn patterns from a rougheye (Sebastes aleutianus) 

or blackspotted (S. melanostictus) rockfish. Collected from a 400 mm male in the Aleutian 

Islands on 10 June 2006. Vague growth pattern, especially the second and third annual marks 

(indicated by question marks). The area on the dorsal side adjacent to the sulcus is the best 

reading axis for this otolith. Age estimate is 11 years. Expected diameter of first annual mark 

indicated by white bar (1 mm). Viewed with reflected light.
64 Professional Paper NMFS 13

Figure 70

Break-and-burn preparation of a rougheye (Sebastes aleutianus) or blackspotted (S. 

melanostictus) rockfish otolith. Collected from a 500 mm female in the Aleutian Islands 

on 1 July 2006. Pattern is clear to thirtieth annual mark; edge is vague (indicated 

by question mark). Age estimate is 38 years. Expected diameter of first annual mark 

indicated by white bar (1 mm). Viewed with reflected light.
Chapter 4: Groundfish age determination 65

Figure 71

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for male 

(n=307) and female (n=383) Greenland halibut (Reinhardtius hippoglossoides) 

collected from the Bering Sea from 1990 through 2008.

Greenland halibut 

(Reinhardtius hippoglossoides)

by Delsa M. Anderl

Biology

In the North Pacific, Greenland halibut (Reinhardtius 

hippoglossoides), also known as Greenland turbot, 

is most abundant in the waters of the eastern Bering Sea, followed by the Aleutian Islands (Alton et 

al., 1988). This species is also found in the North 

Atlantic, where it was a target of intensive directed 

fisheries during the latter half of the last century, 

although a decline in spawning stock size led to reduced commercial catches in recent years (Alton et 

al., 1988; Bowering and Nedreaas, 2000). Greenland 

halibut was an important commercial species in the 

eastern Bering Sea until catches declined in the early 

1980s, primarily due to reductions in fishing quotas 

prompted by concerns over poor recruitment (Ianelli 

et al., 2010). The use of two common names dates 

from 1968 when concerns were voiced by the Pacific 

halibut (Hippoglossus stenolepis) fishing industry that 

the name Greenland halibut could mislead consumers to believe they were the same species. The name 

change to Greenland turbot was never successful in 

the Atlantic fisheries, so both common names are 

used interchangeably when referring to this species 

(Scott and Scott, 1988).

Greenland halibut spawning in the Bering Sea and 

Aleutian Islands occurs in winter, most likely from December to January (Sohn, 2009). Larvae appear to have 

a long pelagic duration and may be subject to extensive 

drift pathways prior to juvenile settlement (Sohn, 2009). 

In the Bering Sea, juveniles reside in waters of the 

continental shelf (<200 m) and are thought to move to 

deeper waters over the continental slope (200 to >1000 

m) around age-4 or age-5 (Alton et al., 1988).

This species grows very quickly during the first three 

years of life, reaching lengths up to 160–200 mm after 

the first year and 320–400 mm by the third year (Kodolov and Matveychuk, 1995). Females grow to larger sizes 

than males (Fig. 71). Von Bertalanffy growth parameters 

were L∞=864.88 mm, k=0.10/yr, and t0=–1.00 yr (n=307) 

and L∞=1024.26 mm, k=0.09/yr, and t0=–0.68 yr (n=383), 

respectively, for male and female Greenland halibut collected during trawl surveys in the Bering Sea from 1990 

through 2008 (Fig. 71). Males mature between 5 and 9 

years (500 and 700 mm) and females mature between 5 

and 10 years (500 and 800 mm) (Kodolov and Matveychuk, 1995). The maximum age reported by the AFSC 

Age and Growth Program to date for Greenland halibut 

is 32 years.

Age determination history

Greenland halibut otoliths are morphologically different from other fish otoliths studied by the AFSC 

Age and Growth Program. Greenland halibut otoliths 
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Figure 72

Otoliths from a 460 mm Greenland halibut (Reinhardtius hippoglossoides) captured on 4 July 

1994. (A) Whole right (eyed-side) sagitta. (B) Embedded and sectioned left (blind-side) 

sagitta, cut slightly oblique to the transverse plane. (C) Stained cross-section pattern of the 

anterior half of left sagitta. (D) Stained cross-section pattern of the posterior half of left 

sagitta. Age estimate is 4 years. Note the clarity of the whole otolith surface pattern compared 

to the cross-section pattern which has multiple checks. Viewed with reflected light.

have extended finger-like appendages radiating 

from the core that are easily broken, as well as a 

dome-shaped sulcus bulging proximally from the 

core (Fig. 72). These fragile structures hinder age 

readers from using the standard break-and-burn 

technique (Chapter 3) for Greenland halibut age 

determination.

Our program first attempted to age Greenland 

halibut in the early 1990s by analyzing only the surface pattern, as commonly practiced by Atlantic age 

determination laboratories. Surface patterns typically 

have a myriad of checks, and identifying an annual 

pattern can be difficult, especially near the otolith 

margin. For many years, our program, as well as some 

of the Atlantic laboratories, suspected that the surface 

method of Greenland halibut age determination was 

biased and underestimated true age.

An age validation study in the Northwest Atlantic 

using bomb radiocarbon and oxytetracycline tagging 

confirmed that otolith surface and simple cross-section 

methods underestimate age in larger, older Greenland 

halibut (Treble et al., 2008). Bomb radiocarbon analysis 

estimated longevity to be at least 27 years for this species 

(Treble et al., 2008).

The importance of Greenland halibut in the North 

Pacific and concerns regarding its declining population prompted our laboratory to embark on a study 

to develop viable aging criteria. This study found that 
Chapter 4: Groundfish age determination 67

staining transverse otolith cross sections resulted in 

increased precision for older specimens (Gregg et al., 

2006). Our findings led our laboratory to adopt this 

method as our primary means of Greenland halibut age 

determination.

Greenland halibut otolith growth patterns are difficult to interpret, and inter-reader precision tends to 

be low (CV=11.05%; n=1115). To support the validity of 

our age estimates, a bomb radiocarbon study is in progress at the AFSC. This validation method is described in 

further detail in Chapter 6 of this manual.

Current age determination methods

Our program primarily uses resin-embedded, stained 

otolith cross sections for Greenland halibut age 

determination. However, preparing cross sections is 

time-intensive and may not always be the best method 

for young otoliths with relatively clear surface patterns. 

Age readers should first examine each otolith surface 

pattern and decide whether the surface pattern is sufficiently clear to estimate age. Some otoliths have very 

clear surface patterns but checks within their crosssection patterns (Fig. 72). Correlating surface and 

cross-section patterns is a valuable method to prevent 

over-aging the cross-section pattern (Fig. 73). In general, otoliths from fish 300 mm or smaller are better 

aged from surface patterns because they are generally 

no older than 3 years. Furthermore, making precise 

cuts through the core of very small embedded otoliths 

is very difficult and likely to result in poor cross-section 

patterns.

The orientation and nomenclature used to describe 

Greenland halibut otoliths are shown in Figures 72 

and 74. Both the blind-side and eyed-side otoliths are 

relatively flat with a dome-shaped bulge at the core. In 

older fish, this dome-shaped core is a dominant characteristic of the blind-side otolith’s proximal surface. 

The centric orientation of the core and the prominent 

domed sulcus found in the blind-side otolith makes it 

the otolith of choice for cross-sectioning. The recommended cutting plane is transversely through the core 

and sulcus (Fig. 72).

Surface examination. In general, the first annual 

mark, with a diameter of about 2 mm, is easy to identify 

on the otolith surface, as is the second annual mark. 

Typically the second and third annual marks are around 

3 and 4 mm in diameter, respectively. The surface pattern contains checks thereafter, and successive annual 

marks can be difficult to identify.

If the otolith is older than 4–5 years and does not 

have an exceptionally clear pattern, it should be crosssectioned. However, prior to embedding, the surface 

pattern of the intact otolith should be examined and 

used to develop an age determination strategy for the 

cross section. Greenland halibut is not an easy species 

to age and requires more time investment in pattern 

analysis. As illustrated in Figure 72, viewing the cross 

section without the surface pattern may be ambiguous 

and misleading. Otoliths with widely spaced translucent 

growth zones are generally from younger, fast-growing 

fish, while otoliths with tightly spaced translucent 

growth zones, especially those that are dense along the 

margin, are usually from older, slow-growing fish.

Cross-section examination. To prepare otolith cross 

sections for age determination, the left (blind-side) otolith is embedded in polyester resin and cut in half using 

a low-speed saw. The cut is made through the transverse 

plane and adjusted to ensure that the saw blade bisects 

the core and passes through the thickest part of the 

domed sulcus (Fig. 72). The cut face is polished with 

800 grit wet-dry sand paper to remove saw marks.

Staining techniques have been adapted from Richter 

and McDermott (1990). Polyester resin blocks containing cut otoliths are submerged in a solution of 1% 

Aniline Blue WS in 1% acetic acid. The stain solution 

should be stored at 20–23°C. Staining times vary but 

average around 13 minutes, after which the otoliths are 

rinsed with fresh water and wiped clean to ensure that 

residual acid and stain are removed. Both halves of the 

stained cross section are examined under a dissecting 

microscope at 12–50× magnification, using reflected 

light. Mineral oil is applied to the polished surfaces to 

eliminate surface glare.

To the trained eye, the first and second annual marks 

are readily identifiable if the otolith was sectioned 

through the core. Typically observed inside the first annual mark is a well-defined, mushroom-shaped check 

(Fig. 75). The first annual mark sometimes appears to 

be merged with or just slightly bordering this check. 

If the early growth pattern is difficult to interpret, the 

first three annual marks are identified using average 

measurements from clear specimens. The expected diameters of the first three annual marks are 1.5–2.5 mm, 

~3 mm, and ~4 mm, respectively. Occasionally, the first 

and second annual marks are difficult to separate from 

surrounding checks, but the third annual mark may be 

clearly identified using expected measurements (Fig. 

76). If the core is not visible due to a bad cut, extra care 

must be taken in identifying early annual marks.

The remaining annual marks are identified by examining both sides of the sulcus. If the cross section 

includes the domed sulcus, annual marks may be seen 

crossing the sulcus connecting the dorsal and ventral 

patterns (Fig. 77). Age estimates are typically made using the domed sulcus as a reading axis. The dorsal and 

ventral tips often have too many checks to be useful for 

determining a precise age estimate, although they may 
68 Professional Paper NMFS 13

provide supplementary information helpful in interpretation of the later annual marks. 

A recurring pattern in many Greenland halibut 

otoliths is a transition zone around the sixth and seventh annual marks (Fig. 77). It is not currently known 

Figure 73

Composite image of the left sagitta in Figure 72. Stained cross-section patterns of each 

half are superimposed onto the corresponding surface image to elucidate the annual 

pattern. Together, the cross sections and surface patterns yield an age estimate of 4 years. 

Viewed with reflected light.

why this transition zone occurs in some otoliths, what 

percentage of otoliths it is clearly identified in, and 

whether it is associated with biological changes. In 

difficult otoliths, the transition zone can be a useful 

reference point.
Chapter 4: Groundfish age determination 69

Figure 74

Orientation of whole Greenland halibut (Reinhardtius hippoglossoides) sagittal otoliths. Viewed with reflected 

light.
70 Professional Paper NMFS 13

Figure 75

Stained otolith cross sections from two different Greenland halibut (Reinhardtius hippoglossoides). (A) Cut is not 

directly through the core, resulting in problems identifying the early annual marks. Age estimate is 4 years. 

(B) Cut is made directly through the core, and early annual marks are easier to identify. Mushroom-shaped 

check inside first annual mark is outlined by white dots. Age estimate is 12 years. Viewed with reflected light.
Chapter 4: Groundfish age determination 71

Figure 76

Measurements help identify early annual marks in otoliths from two different Greenland halibut (Reinhardtius 

hippoglossoides). (A) Otolith with a clear surface pattern. (B) Stained cross section where measurements were 

used to reduce ambiguity in identification of the first, second, and third annual marks. Viewed with reflected 

light.
72 Professional Paper NMFS 13

Figure 77

Stained otolith cross section from a 590 mm male Greenland halibut (Reinhardtius hippoglossoides) collected 

in July 1994. Annual marks can be followed from the dorsal side to the ventral side of the otolith through the 

sulcus. Checks are observed after the seventh annual mark. The annual pattern is clearer within the domed 

sulcus. A transition zone commonly observed in Greenland halibut otoliths is visible after the seventh annual 

mark (indicated by arrows). Age estimate is 16 years. Viewed with reflected light.
Chapter 4: Groundfish age determination 73

Figure 78

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for 

arrowtooth flounder (Atheresthes stomias) collected during trawl surveys in the 

Bering Sea (BS; n=593) in 2004 and Gulf of Alaska (GOA; n=729) in 2005.

Arrowtooth flounder (Atheresthes stomias)

by John D. Brogan

Biology

Arrowtooth flounder (Atheresthes stomias) is a large, 

piscivorous flatfish species ranging from central California to the eastern Bering Sea (Hart, 1973). Arrowtooth 

flounder is one of the most abundant fish species in the 

Gulf of Alaska (von Szalay et al., 2010).

Research conducted along the Washington coast has 

demonstrated that arrowtooth flounder is a groupsynchronous batch spawner (Rickey, 1995). Although 

there are differences in specific spawning times for 

different areas along the Pacific coast, spawning in Alaskan waters generally occurs from December through 

February (Witherell, 2000; Blood et al., 2007).

In the Gulf of Alaska, female arrowtooth flounder 

attain 50% maturity at 7 years or 460 mm (Stark, 

2008). Males reach 50% maturity at a length of 420 

mm (Zimmerman, 1997). According to AFSC survey 

data, females grow more slowly and much larger than 

males, and arrowtooth flounder from the Bering Sea 

exhibit slightly faster growth rates than those from 

the Gulf of Alaska (Fig. 78). Von Bertalanffy growth 

parameters were L∞=816.2 mm, k=0.1253/yr, and 

t0=–0.262 yr (n=593), and L∞=836.3 mm, k=0.0931/yr, 

and t0=–0.843 yr (n=729), for arrowtooth flounder 

collected during trawl surveys in the Bering Sea in 

2004 and Gulf of Alaska in 2005, respectively (Fig. 

78). The maximum age reported to date by the AFSC 

Age and Growth Program for arrowtooth flounder is 

32 years.

Age determination history

The AFSC Age and Growth Program began age determination of arrowtooth flounder in 1975. In 1991, we discovered that specimens collected as arrowtooth flounder (Atheresthes stomias) included a different species, 

Kamchatka flounder (A. evermanni). Over 9000 positively identified arrowtooth flounder have been aged 

since the early 1990s. The majority of specimens have 

been aged using the break-and-burn method; however, 

surface examination has also been used for young specimens with clear growth patterns. Arrowtooth flounder 

otoliths are generally more difficult to interpret than 

some of the other flatfish species aged at the AFSC, reflected by somewhat lower estimates of inter-reader precision (CV=8.4%; n=2487). To date, there are no published studies on arrowtooth flounder age validation.

Current age determination methods

Surface examination. Whole arrowtooth flounder 

otoliths are examined at 6× magnification using a dissecting microscope and reflected light. The average 

distance from the anterior tip to the posterior tip of 

a 1-year-old blind-side otolith is 4.0 mm. The clearest 
74 Professional Paper NMFS 13

Figure 79

Distal surfaces of blind-side (left) and eyed-side (right) arrowtooth flounder (Atheresthes 

stomias) otoliths.

Figure 80

Arrowtooth flounder (Atheresthes stomias) 

otolith. Faint second annual mark close to 

the first. Age estimate is 7 years.

growth patterns are typically found on the anterior 

side of either the distal or proximal surface (Fig. 79). 

Arrowtooth flounder otoliths can be difficult to age on 

the surface; however, an experienced age reader can 

assign age estimates up to 7 years for otoliths with clear 

surface patterns (Fig. 80).

Using expected measurements can be helpful in 

identifying the early annual marks. The first annual 

mark encompasses a wide surface area and has an average dorsal-ventral diameter of about 1.5 mm. The first 

and second annual marks can be very close together, 

and it can be easy to mistake the second annual mark 

as a check. Figure 80 shows this type of pattern, with a 

faint second annual translucent growth zone very close 

to the first. Because it forms a complete ring around 

the otolith, it should be counted as an annual mark.

When the surface pattern is difficult to interpret or 

there are questions about the surface age estimate, the 

specimen should be aged using the break-and-burn 

method or the break-and-bake method (Chapter 3). 

The criteria applied for age determination are the same 

for both methods.

Break-and-burn examination. As with surface examination, 

the blind-side otolith is preferred for the break-and-burn 

method. The first three translucent growth zones should 

be traced on the surface of the otolith with a pencil prior 
Chapter 4: Groundfish age determination 75

to cutting and burning because the early years of the 

break-and-burn pattern often contain too many checks 

to definitively identify annual marks. Also, because the 

otoliths are small, precise cutting through the core is 

sometimes difficult, and a traced first year will help determine whether the core was missed during sectioning.

Typically, the posterior otolith half is burned, although sometimes the anterior half is also burned to 

clarify questionable annual marks. If the cut surface 

is too rough, the otolith is sanded using 600 grit sandpaper. The otolith can be burned by quickly passing it 

over an alcohol flame a few times; however, it is easy to 

over-burn arrowtooth flounder otoliths, causing them to 

crumble. An easy way to prevent over-burning is to use 

Figure 81

Clear break-and-burn cross section of an arrowtooth flounder (Atheresthes stomias) otolith. Dots indicate 

annual marks and preferred reading axes for arrowtooth flounder otoliths. Age estimate is 7 years.

the break-and-bake method, which results in a uniform 

pattern. It is best to bake arrowtooth flounder otoliths 

for 7–8 minutes at 500°F.

Cross sections are examined at 25× magnification 

using a dissecting microscope and reflected light. Unlike most other flatfish otoliths, arrowtooth flounder 

otoliths do not typically have distinct annual marks 

near the sulcus. Arrowtooth flounder otoliths are usually read from the core to the dorsal and ventral tips as 

illustrated in Figure 81. When possible, annual marks 

should be followed from the distal edge toward the 

sulcus. If spacing is irregular in the first few years, the 

pencil marks on the otolith surface will help to determine annual marks. For example, Figure 82 shows a 

Figure 82

Arrowtooth flounder (Atheresthes stomias) otolith break-and-burn cross section with some irregular 

spacing. Tracing the translucent growth zones on the distal surface with a pencil (indicated by black 

ticks) helped identify the first three annual marks. Age estimate is 9 years.
76 Professional Paper NMFS 13

ure 83 shows a 15-year-old otolith with irregular banding patterns. The tip of the cross section is a clearer 

reading axis than the sulcus; however, there can still be 

trouble interpreting the edge of this specimen. When 

reading the tip, the tenth through twelfth translucent 

growth zones appear to be a single annual mark with 

some splitting. However, closer examination reveals 

that these translucent growth zones are likely three separate annual marks observed throughout the otolith.

Figure 83

Arrowtooth flounder (Atheresthes stomias) otolith break-and-burn cross section with some ambiguity 

in the growth pattern near the edge. Age estimate is 15 years.

9-year-old otolith with a questionable second annual 

mark. The second annual mark looks discontinuous, 

but it can be followed to the sulcus, and this fact, combined with the pencil marks, clearly shows that it is an 

annual mark and not a check.

In older arrowtooth flounder specimens, the annual 

marks are narrowly spaced after the third or fourth 

year. In some of these cases, it can be helpful to use the 

sulcus and dorsal and ventral tips as reading axes. Fig-
Chapter 4: Groundfish age determination 77

Figure 84

Length-at-age data fit with von Bertalanffy growth functions (VBGF) for male 

(n=883) and female (n=1149) yellowfin sole (Limanda aspera) collected during 

trawl surveys in the Bering Sea from 2006 through 2008.

Yellowfin sole (Limanda aspera)

by Wes Shockley and Mary Elizabeth Matta

Biology

Yellowfin sole are found in continental shelf waters of 

the North Pacific Ocean, ranging from the Chukchi and 

Bering Seas south along the continental shelves to the 

Sea of Japan and Vancouver Island, British Columbia 

(Bakkala, 1981; Wilderbuer et al., 1992). This species 

supports the largest commercial flatfish fishery in the 

United States, with 121,029 metric tons caught in the 

eastern Bering Sea in 2007 (Wilderbuer et al., 2008). 

Yellowfin sole experienced a dramatic decline in stock 

size due to overfishing in the late 1950s and early 1960s, 

but with reduced fishing pressure abundance increased 

in the 1980s and has since stabilized (Wilderbuer et al., 

2008).

Yellowfin sole spawn between May and August 

(Fadeev, 1970; Wilderbuer et al., 1992). Juveniles reside 

in shallow bays and gradually move to deeper waters 

as they age (National Research Council, 1996). Adults 

overwinter along the continental shelf-slope break in 

waters 100–270 m deep and migrate to the inner shelf 

(<100 m) during the summer months for the purposes 

of spawning and feeding (Bakkala, 1993; Wilderbuer 

et al., 1992). In the eastern Bering Sea, size at 50% 

maturity is 203 mm for males and 288 mm for females, 

corresponding roughly to the ages of 7 and 11 years, 

respectively (Wilderbuer et al., 1992). According to 

AFSC Age and Growth Program data, females are larger 

at age than males after reaching maturity (Fig. 84). Von 

Bertalanffy growth parameters were L∞=352.46 mm, 

k=0.16/yr, and t0=0.68 yr (n=883) and L∞=398.27 mm, 

k=0.13/yr, and t0=0.37 yr (n=1149), respectively, for male 

and female yellowfin sole collected during trawl surveys 

in the Bering Sea from 2006 through 2008 (Fig. 84). 

To date, the oldest specimen aged by the AFSC was a 

39-year-old female collected in the Bering Sea. A strong, 

positive correlation (r=0.90) between bottom temperature and the width of yellowfin sole otolith growth zones 

has been observed (Matta et al., 2010).

Age determination history

Of the flatfish species routinely aged by the AFSC Age 

and Growth Program, the growth patterns of yellowfin 

sole otoliths are typically the easiest to interpret, with 

a high level of agreement between readers (CV=2.9%; 

n=5643). Since 1982, over 22,000 yellowfin sole have 

been aged by the Age and Growth Program. In some 

cases otolith surface patterns have been used to estimate age for this species, but the majority of yellowfin 

sole otoliths have been aged using the break-and-burn 

method. Yellowfin sole age estimates have been corroborated by edge type analysis (Kimura et al., 2007) 

and synchronous growth patterns observed among 

individuals (Matta et al., 2010). A bomb radiocarbon 

study is currently in progress at the AFSC to provide a 
78 Professional Paper NMFS 13

stronger method of validation for this species. Validation methods are described in further detail in Chapter 

6 of this manual.

Current age determination methods

Surface examination. Yellowfin sole otoliths usually 

possess readily identifiable first annual marks, distinct 

translucent growth zones, and fairly regular spacing. 

Depending on the specimen, surface age determination (Fig. 85) is generally reliable for otoliths up to 12 

years. Yellowfin sole otolith surfaces are examined using 

a dissecting microscope at 6–16× magnification with 

reflected light.

Break-and-burn examination. When the otolith 

growth pattern is difficult to interpret, the break-andburn method is used (Chapter 3). Yellowfin sole otoliths 

are usually cut transversely through the core using a 

scalpel. Yellowfin sole otoliths should be burned longer than otoliths of most other flatfish species, as this 

clarifies the growth patterns for easier interpretation. 

Break-and-burn cross sections are examined at 32–60× 

magnification using a dissecting microscope with reflected light.

Age is determined by counting annual marks from 

the core to the otolith edge (Fig. 86). In yellowfin 

sole otoliths, the first annual mark is relatively small 

in diameter and is normally clear. However, the first 

Figure 85

Clear proximal surface pattern of a yellowfin sole (Limanda aspera) otolith. Age 

estimate is 9 years. Viewed with reflected light.

annual mark can be difficult to detect in some breakand-burn patterns, as it sometimes can be diminished 

by the burning process (Fig. 87). Furthermore, the 

heat from the flame may cause the accreted otolith 

layers to physically separate, such that the material 

corresponding to the first year of growth may break 

away from the rest of the otolith (Fig. 88). The surface 

pattern should always be compared with the breakand-burn pattern to ensure that the first annual mark 

is not overlooked.

Age determination problems. While yellowfin sole 

age determination is typically easy relative to other 

groundfish species, they occasionally exhibit some of 

the problems of interpretation common to all otoliths. 

Crystallization is probably the most frequently encountered impediment to age determination of yellowfin sole otoliths. Crystallization can render a specimen difficult or impossible to age because it often 

obscures the edges and the most recently deposited 

annual marks.

Other problems one encounters in yellowfin sole 

otoliths are irregular spacing, splitting, and pre-annular 

checks. Particularly on older specimens, irregular spacing can lead to doubt as to whether the presumed annual marks are true or are the result of splitting, where 

two translucent growth zones are deposited within the 

same year (Fig. 89). Because spacing between annual 

marks may be a result of interannual differences in 
Chapter 4: Groundfish age determination 79

Figure 86

Clear yellowfin sole (Limanda aspera) otolith (A) surface pattern and (B) break-and-burn 

pattern. Collected from a 310 mm male on 24 February 2007. Age estimate is 11 years. 

Viewed with reflected light.

water temperature (Matta et al., 2010), age readers 

should approach otoliths displaying irregular spacing 

with caution. When close spacing of annual marks occurs, the areas adjacent to the sulcus are typically the 

best reading axes (Fig. 90). Pre-annular checks are a 

fairly common problem in yellowfin sole (Figs. 90 and 

91). Pre-annular checks tend to be pronounced near 

the distal surface but insignificant or absent closer to 

the sulcus; they are therefore seldom a problem for age 

determination.
80 Professional Paper NMFS 13

Figure 87

(A) Otolith surface and (B) break-and-burn growth patterns from a 

yellowfin sole (Limanda aspera) collected on 4 March 2007. Faint first annual 

mark, especially on the break-and-burn face. Only 14 annual marks can be 

viewed on the surface pattern, illustrating the importance of comparing 

preparation methods. Age estimate is 18 years. Viewed with reflected light.
Chapter 4: Groundfish age determination 81

Figure 88

(A) Otolith surface and (B) break-and-burn patterns from a yellowfin sole 

(Limanda aspera) collected 9 March 2007. In the upper cross section, layers of 

material including the first annual mark broke away from the otolith during 

the burning process, leaving a pit behind. Age estimate is 12 years. Viewed 

with reflected light.
82 Professional Paper NMFS 13

Figure 89

(A) Otolith surface and (B) break-and-burn patterns from a yellowfin sole 

(Limanda aspera) collected in April 2007 with irregularly spaced annual 

marks. Faint first annual mark and narrow spacing between the third and 

fourth annual marks compared to the wider spacing of the fifth, sixth, 

and seventh annual marks. Questionable sixth annual mark (indicated 

by question mark in (A) noted on the surface pattern is most likely a 

check, evident from the break-and-burn pattern (indicated by arrow). Age 

estimate is 25 years. Viewed with reflected light.
Chapter 4: Groundfish age determination 83

Figure 90

(A) Otolith surface and (B) break-and-burn patterns from a yellowfin 

sole (Limanda aspera) collected on 19 February 2007. Pre-annular checks 

(indicated by “C”) and closely spaced annual marks (indicated by “S”) are 

visible in the break-and-burn pattern. The clearest reading axes are along 

either side of the sulcus. Age estimate is 19 years. Viewed with reflected light.
84 Professional Paper NMFS 13

Figure 91

(A) Otolith surface and (B) break-and-burn patterns from a yellowfin sole 

(Limanda aspera). Pre-annular checks (indicated by “C”) can be distinguished 

from annual marks by their faintness and lack of continuity. Collected from a 320 

mm female on 17 February 2007. Age estimate is 10 years. Viewed with reflected 

light.
85

Fish age determination typically relies on the interpretation of perceived annual marks in hard structures. 

While age readers are expertly trained in pattern recognition and the use of species-specific aging criteria, 

some degree of subjectivity is typically involved. In age 

determination science, the term “accuracy” describes 

how well estimates of age (such as from counting annual marks) compare with true ages. “Precision” refers 

to the degree to which an age estimate is reproducible, 

either by the same age reader or among different age 

readers. For age data to be useful, high levels of both 

accuracy and precision are required. However, without 

known-age materials, accuracy is difficult to evaluate. 

Precision statistics do not necessarily reflect accuracy 

but may provide useful measures of the relative difficulty and repeatability of age determinations (Campana, 

2001).

This chapter briefly outlines the procedures used 

by the AFSC Age and Growth Program to quantify 

age determination precision and relative bias. These 

procedures are more thoroughly described in Kimura 

and Anderl (2005). Campana (2001) also provides a 

thoughtful review of methods used to measure precision 

and accuracy in fish age determination. Methods used 

to assess age estimate accuracy are discussed in Chapter 

6 of this manual.

Precision testing

Age determination at the AFSC is based on the discipline of precision testing. A random subsample of at 

least 20% of all specimens is tested by a second age 

reader (Kimura and Anderl, 2005). To generate the 

subsample for a given collection of specimens, a number from one to five is randomly chosen and every fifth 

specimen thereafter is selected for testing. The second 

reader (hereafter referred to as the tester) ages the subsample without any knowledge of the first reader’s age 

estimates. The reader and tester generally use the same 

preparation method to age a given specimen, but occasionally the tester may prefer a different method based 

on the relative clarity of the preparation. To ensure 

that repeated readings are statistically independent, 

age readers do not have knowledge of any other ages 

estimated from the structures. This means that if the 

same age reader makes repeated readings, the readings 

are separated in time so that age determinations are not 

done from memory.

Chapter 5: Precision and bias in age determination

by Daniel K. Kimura, Delsa M. Anderl, and Mary Elizabeth Matta

Specimens without agreement between age estimates 

(Kimura and Anderl, 2005) are re-examined, and any 

remaining discrepancies are resolved between reader 

and tester. A second independent test may be required 

for samples which fail to meet an acceptable level of 

precision, and in some cases the primary age reader 

must re-age the sample. The 20% test sample has been 

used by our program for over 20 years to calculate 

precision statistics and is the standard that maintains 

our aging criteria over time and among age readers.

We typically calculate percent agreement (PA), average percent error (APE; Beamish and Fournier, 1981) 

and the coefficient of variation (CV; Chang, 1982) to 

evaluate inter-reader precision for a given sample. High 

PA values and low CV and APE values indicate good 

precision between age estimates. All precision statistics 

are relayed to data end users at the AFSC.

Because of tradition and simplicity, no statistic will 

likely ever replace PA as the fundamental statistic in 

age precision studies. It is simple to calculate and is usually used to make gross comparisons between samples. 

However, it does not consider specimen age, a problem 

given that exact agreement generally decreases with increasing age for most species (Kimura and Lyons, 1991). 

Both APE and CV account for age, allowing easier 

comparisons among species or samples with dissimilar 

age ranges. These measures can give a general idea of 

the relative difficulty associated with estimating age in 

different species (Kimura and Lyons, 1991; Kimura and 

Anderl, 2005). Precision statistics for all species aged at 

the AFSC are available on our website (Alaska Fisheries Science Center, http://www.afsc.noaa.gov/REFM/

Age/Default.htm).

Measures of bias

At the AFSC, all samples are scrutinized for relative bias 

between reader and tester age estimates. The single 

most important tool used by our laboratory for comparing age readers is a simple cross-tabulation of age 

estimates, so that the x-axis is reader age and the y-axis is 

tester age (Kimura and Anderl, 2005). These tables are 

used to see precisely where the reader and tester differ 

in their age estimates. For example, the tester may generate consistently older age estimates than the reader 

only for fish older than 20 years. Thus, a cross-tabulation 

is an easy way to identify potential systematic biases that 

may occur disparately among age classes.
86 Professional Paper NMFS 13

Bias is statistically assessed using Bowker’s test for 

symmetry (Bishop et al., 1975). A significant test statistic indicates that the reader and tester are interpreting 

aging structures differently. Bowker’s test is a convenient index of overall similarity between reader and 

tester but may not be adequate for determining differences when sample sizes are small or when age ranges 

are broad.

Age bias plots are also used to graphically show bias 

(Campana et al., 1995), either on a relative basis (such 

as describing the difference between two age readers) or 

on an absolute basis (such as describing the difference 

between true age and estimates of age). Typically, the 

true (or most credible) age is plotted on the x-axis. The 

mean test age and error bars corresponding to each of 

the true age categories are plotted on the y-axis. Deviance from a 1:1 equivalence line indicates the presence 

of aging bias.

When bias occurs at the AFSC, readers and testers 

typically examine discrepant specimens together to 

pinpoint potential systematic differences in aging criteria. Sometimes a third age reader is asked to read the 

specimens independently. Conversely, the tester may be 

required to age another 20% subsample, or the primary 

age reader may have to re-age the sample, depending on 

the situation. It is a standard practice of the AFSC Age 

and Growth Program to resolve any bias issues prior to 

releasing age data to end users.
87

Introduction

Validation of fish age estimates and aging methods has 

been recognized as a critical element of the Age and 

Growth Program at the AFSC. Growth zones can often 

be enumerated in hard structures like otoliths, but it is 

necessary to validate the annual nature of translucent 

and opaque growth zone deposition (Beamish and 

McFarlane, 1983). In recent history our program has 

undertaken a number of special studies using different age validation methods to confirm the accuracy of 

routinely estimated fish ages (Table 3). The validation 

method chosen depends upon the specific research 

question being addressed, the species’ estimated age 

range, the availability of suitable specimens and ancillary biological data, and the resolution of the validation 

methods available. The goals in age validation studies 

can be as broad as confirming a general age range or as 

specific as an aging error of one year.

In this chapter, age validation methods are broadly categorized as “direct” or “indirect.” However, the quality of 

an age validation study realistically exists on a continuum 

that depends not only on the method used but also on 

the quality of data employed (Kimura et al., 2006).

Direct methods

Known-age studies

The age validation studies that are considered to be the 

most reliable and direct are those that utilize knownage specimens released into the wild. In this method, 

juvenile fish of known age are marked and released into 

their natural environment with the expectation that 

some percentage of them will be caught in several years 

as adults. The number of growth zones seen in the aging structure can be compared to the known age of the 

fish to reveal aging error. This age validation method 

assumes that the act of initial tagging and releasing 

does not affect the fish’s future growth rate or the aging 

structure’s appearance. However, the stress of tagging 

or the long-term presence of an external tag could 

change the fish’s ability to survive or grow in the wild.

Sablefish (Anoplopoma fimbria) is the only groundfish 

species studied at the AFSC for which known-age studies have been undertaken. From 1985 to 2005, juvenile 

fish were caught as confirmed age-0 to age-2 specimens, 

tagged, and released (Heifetz et al., 1998; Maloney and 

Chapter 6: Age validation methods used at the Alaska Fisheries Science Center

by Craig R. Kastelle, Daniel K. Kimura, and Charles E. Hutchinson

Sigler, 2008). Recaptured sablefish have been used to 

validate age estimates and aging criteria up to 18 years.

Bomb-derived radiocarbon validation

Bomb-derived radiocarbon (14C) has the ability to validate fish age estimates directly with much more resolution than most other methods (Campana, 2001). Bombderived radiocarbon was first used to validate fish age 

estimates by Kalish (1993) and since has seen extensive 

use across many species worldwide (Kalish, 1995; Campana, 1997; Kerr et al., 2004; Piner and Wischniowski, 

2004; Treble et al., 2008).

Above-ground testing of atomic bombs during the 

1950s and 1960s produced an increase of environmental 14C above the natural levels (Nydal, 1993). This 14C became part of the biota, and the increase can be 

measured in calcified marine biological structures and 

used as a time signal (Kalish, 1993). To apply this validation method, bomb radiocarbon is first measured in 

otoliths from fish with known birth years to establish a 

“reference chronology” of 14C (Kerr et al., 2004; Piner 

and Wischniowski, 2004). When otolith-derived age 

estimates require validation, 14C is measured in otolith 

cores and posited birth years are determined from the 

growth zone-based age estimates. The timeframe of the 

14C increase in the reference chronology is then compared to that of the species being validated (i.e., the 

“test species”). If the increases in 14C of the test species 

and reference chronology are in phase, the test species’ 

age estimates are considered validated and accurate.

Radiometric validation

The radiometric age validation method was pioneered 

by Bennett et al. (1982) in a study on splitnose rockfish 

(Sebastes diploproa) and has since proven very useful at 

the AFSC. In early studies, general age ranges were 

validated, such as in sablefish (Kastelle et al., 1994). In 

more recent studies, the process has been refined to 

be more specific, such as in the case of walleye pollock 

(Theragra chalcogramma), for which age estimates of 

young fish were validated (Kastelle and Kimura, 2006). 

Typically, this method excels at answering general questions such as, “What is the maximum age range of this 

species – is it long-lived or short-lived?”

The radiometric age validation method relies on the 

disequilibrium between a daughter-parent pair of radionuclides, 226Ra and one of its progeny, 210Pb, both part 
88 Professional Paper NMFS 13 Table 3 Age validation studies for species aged at the Alaska Fisheries Science Center (AFSC). This list is not exhaustive for all species aged at the AFSC. Studies in process at the AFSC as of 2010 are listed as “In process.” Direct validation methods Indirect validation methods Species Known age Radiometric Bomb radiocarbon Edge type analysis Other methods Alaska skate Matta and Gunderson (2007) (Bathyraja parmifera) Atka mackerel Anderl et al. (1996) Anderl et al. (1996) (Pleurogrammus monopterygius) Kimura et al. (2007) Dover sole Kastelle et al. (2008a) (Microstomus pacificus) Giant grenadier Burton (1999) In process (Albatrossia pectoralis) Greenland halibut Treble et al. (2008) (Reinhardtius hippoglossoides In process Northern rock sole Kimura et al. (2007) (Lepidopsetta polyxystra) Northern rockfish Kastelle et al. (2000) In process Kimura et al. (2007) (Sebastes polyspinis) Pacific cod Roberson et al. (2005) (Gadus macrocephalus) Pacific ocean perch Kastelle et al. (2000) Kastelle et al. (2008b) Kimura et al. (2007) (Sebastes alutus) Rougheye rockfish Kastelle et al. (2000) (Sebastes aleutianus) Sablefish Heifetz et al. (1998) Kastelle et al. (1994) Kimura et al. (2007) Beamish and Chilton (1982) (Anoplopoma fimbria) Maloney and Sigler (2008) Beamish and McFarlane (2000) Rutecki and Varosi (1997) Beamish et al. (1983) In process McFarlane and Beamish (1995) Pearson and Shaw (2004) Shortraker rockfish Hutchinson et al. (2007) In process (Sebastes borealis) Kastelle et al. (2000) Shortspine thornyhead Kastelle et al. (2000) (Sebastolobus alascanus) Kline (1996) Walleye pollock pollock Kastelle and Kimura (2006) Kimura et al. (2006) Kimura et al. (2006) (Theragra chalcogramma) Kimura et al. (2007) Yellowfin sole In process Kimura et al. (2007) (Limanda aspera)
Chapter 6: Age validation methods used at the Alaska Fisheries Science Center 89

of the naturally occurring decay chain of 238U. During 

metabolism, 226Ra is deposited along with calcium in 

calcified structures such as otoliths (Swanson, 1985; 

Porntepkasemsan and Nevissi, 1990). When 226Ra is deposited in otoliths and remains immobile in the closed 

calcium carbonate structure, a disequilibrium is created 

between 226Ra and all of its progeny. Over time, the activity (decays/min) of shorter-lived daughter products 

increases due to radioactive decay of the parent. The difference between the half-lives of 226Ra and 210Pb is great, 

so the activity ratio of 210Pb/226Ra is a function of the 

time elapsed since deposition. In a time span of about 

100 years, the activity of both 226Ra and 210Pb will become 

equal as secular equilibrium is reached asymptotically. 

Therefore, this method is only effective for validating 

age estimates of specimens under 100 years old.

In the mid-1990s, the AFSC developed a small radiochemical facility geared specifically toward analyzing 

210Pb and 226Ra. This pair of radionuclides has since been 

used to age species younger than 5 years (Kastelle and 

Kimura, 2006) and as old as about 75 years (Kastelle et 

al., 2000). Other daughter-parent pairs, such as 228Th 

and 228Ra, have been used to validate fish age estimates 

ranging from 1 to 10 years (Smith et al., 1991), but the 

capability does not exist at the AFSC. The 210Pb and 226Ra 

radionuclides are comparatively easy to extract from 

otoliths and analyze, and the applicable age range is 

appropriate for most species aged at the AFSC.

Indirect methods

Several indirect age validation methods have been used 

on species routinely aged at the AFSC. These are considered to be weaker forms of validation and as such are 

usually considered along with other evidence. Many of 

these methods may be more appropriately called “corroboration” and not validation (Kimura et al., 2006). 

However, when several lines of corroborating evidence 

all lead to the same conclusion, strong confidence can 

be placed in the probable accuracy of the age estimates.

Mark-recapture studies

Oxytetracycline (OTC) mark-recapture has been used 

successfully to indirectly validate sablefish aging criteria 

(Beamish and Chilton, 1982; Beamish et al., 1983; McFarlane and Beamish, 1995; Beamish and McFarlane, 

2000). Fish, often as adults, are caught, tagged, injected 

with OTC, and released. When a fish is recaptured some 

years later, the otolith displays an OTC mark and the 

apparent number of otolith growth zones after initial tagging can be compared to the number of years at liberty. 

One caveat associated with this method is that age is 

usually unknown upon tagging fish with OTC. Thus, 

upon recapture, age must be estimated from counts of 

unvalidated growth zones observed prior to the OTC 

mark. In sablefish, as many as the first 50 years were left 

unvalidated and were only presumed accurate based on 

the similar appearance of growth zones before and after 

the OTC mark (Beamish and McFarlane, 2000).

Other indirect age validation methods using tagrecapture data have proven useful at the AFSC. Pacific 

cod age estimates were validated two ways with otoliths 

from spaghetti-tagged and recaptured fish (Roberson et 

al., 2005). First, the estimated length of each fish at time 

of release was back-calculated from measurements of 

otolith annual marks. The estimated length agreed with 

observed fish length at time of release, supporting the 

assumption of annual growth zone deposition. Second, 

otolith age and von Bertalanffy parameters, estimated 

from tagging data, were used to predict how much each 

fish grew in length during its time at liberty. Otolith aging criteria applied to the tagged and recaptured fish 

accurately predicted the observed growth increments. 

Thus, multiple lines of evidence led to the conclusion 

that Pacific cod age estimates are accurate.

Edge type analysis

In edge type analyses, estimated fish ages are validated 

indirectly by determining whether an annual pattern is 

present through evaluation of the edge growth of aging structures. These types of studies can range from a 

qualitative assessment of edge type (i.e., translucent or 

opaque) to a quantitative marginal increment analysis, 

in which new growth is measured and assessed as a proportion of the previous year’s growth. This method has 

been used to verify the existence of an annual growth 

pattern for eight species of groundfish aged by the AFSC 

Age and Growth Program (Table 3).

Other validation methods

Following a strong year class over time, where it annually 

advances by one year, is another way to indirectly validate age estimates. This method was used with walleye 

pollock, where a strong 1978 year class was confirmed 

by the age-independent method of length mode analysis 

and subsequently tracked for 10 years in age frequency 

distributions (Kimura et al., 2006).

Timing of first translucent growth zone deposition 

in Atka mackerel (Pleurogrammus monopterygius) otoliths was validated using three methods: larval seasonal 

length data, counts of daily growth zones in larval otoliths, and edge type analysis (Anderl et al., 1996). Atka 

mackerel spawn and hatch in the fall, and this research 

confirmed that the first translucent growth zone was 

formed at an age of 1.5 years during the second winter 

or spring.
90 Professional Paper NMFS 13

Acknowledgments

We are grateful to the many people who have contributed to the general knowledge and methodology 

of the AFSC Age and Growth Program over the past 

several decades of its existence, particularly J. Gregg, 

G. Hirschhorn, S. Jay, and J. Lyons. Our work has been 

further enhanced by collaborations with other age 

determination laboratories such as the Alaska Department of Fish and Game; the Department of Fisheries 

and Oceans, Canada; and the International Pacific 

Halibut Commission. J. Short (AFSC) provided von 

Bertalanffy growth function figures. Earlier drafts of 

this manuscript were improved thanks to the thoughtful 

comments of the Scientific Editor, the AFSC Publications Unit, T. Helser (AFSC), and several anonymous 

reviewers. Cover image by D. Anderl (AFSC).

Literature cited

Alderdice, D. F., and C. R. Forrester.

1971. Effects of salinity, temperature, and dissolved oxygen on 

the early development of Pacific cod (Gadus macrocephalus). J. 

Fish. Res. Board Can. 28:883–902.

Allen, M. J., and G. B. Smith.

1988. Atlas and zoogeography of common fishes in the Bering Sea and northeastern Pacific. NOAA Tech. Rep. NMFS 66, 

151 p.

Alton, M. S., R. G. Bakkala, G. E. Walters, and P. T. Munro.

1988. Greenland turbot Reinhardtius hippoglossoides of the eastern Bering Sea and Aleutian Islands region. NOAA Tech. Rep. 

NMFS 71, 31 p.

Anderl, D. A., A. Nishimura, and S. A. Lowe.

1996. Is the first annulus on the otolith of the Atka mackerel, 

Pleurogrammus monopterygius, missing? Fish. Bull. 94:163–169.

Aydin, K., and F. Mueter.

2007. The Bering Sea – A dynamic food web perspective. Deep 

Sea Res. II 54:2501–2525.

Bailey, K. M., T. J. Quinn, P. Bentzen, and W. S. Grant.

1999. Population structure and dynamics of walleye pollock,

Theragra chalcogramma. Adv. Mar. Biol. 37:180–255.

Bakkala, R. G.

1981. Population characteristics and ecology of yellowfin sole. 

In The eastern Bering Sea shelf: oceanography and resources, 

Vol. 1 (D. W. Hood and J. A. Calder, eds.), p. 553–574. U.S. 

Dep. Commer., NOAA, Off. Mar. Poll. Assess., U.S. Gov. Printing Office, Washington, D.C.

1993. Structure and historical changes in the groundfish complex of the eastern Bering Sea. NOAA Tech. Rep. NMFS 114, 

91 p.

Bakkala, R., T. Maeda, and G. McFarlane.

1986. Distribution and stock structure of pollock (Theragra 

chalcogramma) in the North Pacific Ocean. Int. North Pac. 

Fish. Comm. Bull. 45:3–20.

Barsukov, V. V.

1964. Intraspecific variability in the Pacific ocean perch (Sebastodes alutus Gilbert). In Soviet Fisheries Investigations in 

the Northeast Pacific (P. A. Moiseev, ed.), p. 231–252. Tr. 

Vses. Naucho-Issled. Morsk. Rybn. Khoz. Okeanogr., Part 

II. (Transl. in 1968 from Russian by Israel Program for Sci. 

Transl., Jerusalem.) 

Barto, T.

1999. Comparison of break-and-burn and break-and-bake ageing techniques on halibut otoliths. Int. Pac. Halibut Comm. 

Rep. Assess. Res. Activities 1999:281–290.

Beamish, R. J.

1979. New information on the longevity of Pacific ocean perch 

(Sebastes alutus). J. Fish. Res. Board Can. 36(11):1395–1400.

Beamish, R. J., and D. E. Chilton.

1982. Preliminary evaluation of a method to determine the 

age of sablefish (Anoplopoma fimbria). Can. J. Fish. Aquat. Sci. 

39:277–287.

Beamish, R. J., and D. A. Fournier.

1981. A method for comparing the precision of a set of age 

determinations. Can. J. Fish. Aquat. Sci. 38:982–983.

Beamish, R. J., and G. A. McFarlane.

1983. The forgotten requirement for age validation in fisheries 

biology. Trans. Am. Fish. Soc. 112:735–743.

2000. Reevaluation of the interpretation of annuli from otoliths 

of a long-lived fish, Anoplopoma fimbria. Fish. Res. 46:105–111.

Beamish, R. J., G. A. McFarlane, and D. E. Chilton.

1983. Use of oxytetracycline and other methods to validate a 

method of age determination for sablefish. In Proceedings of 

the International Sablefish Symposium, p. 95–116. Alaska Sea 

Grant Rep. 83-8, Univ. Alaska, Fairbanks.

Bel’chuk, L. A.

1938. K voprosu ob opredelenii vozrasta odnoperogo terpuga 

(Pleurogrammus azonus). [Determining the age of Pleurogrammus azonus.] Izvestiya TINRO, Vol.14.

Bennett, J. T., G. W. Boehlert, and K. K. Turekian.

1982. Confirmation of longevity in Sebastes diploproa (Pisces, 

Scorpaenidae) from Pb-210/Ra-226 measurements in otoliths. Mar. Biol. 71:209–215.

Bishop, Y. M. M., S. E. Fienberg, and P. W. Holland.

1975. Discrete multivariate analysis, 557 p. MIT Press, Cambridge, MA.

Blood, D. M., A. C. Matarese, and M. S. Busby.

2007. Spawning, egg development, and early life history dynamics of arrowtooth flounder (Atheresthes stomias) in the Gulf 

of Alaska. NOAA Prof. Paper NMFS 7, 28 p.

Boehlert, G. W., and M. M. Yoklavich.

1985. Larval and juvenile growth of sablefish, Anoplopoma 

fimbria, as determined from otolith increments. Fish. Bull. 

83:475–481.

Bowering, W. R., and K. H. Nedreaas.

2000. A comparison of Greenland halibut (Reinhardtius hippoglossoides (Walbaum)) fisheries and distribution in the Northwest and Northeast Atlantic. Sarsia 85:61–76.

Brodeur, R. D.

2001. Habitat-specific distribution of Pacific ocean perch (Sebastes alutus) in Pribilof Canyon, Bering Sea. Cont. Shelf Res. 

21:207–224.

Burton, E. J.

1999. Radiometric age determination of the giant grenadier 

(Albatrossia pectoralis) using 210Pb:226Ra disequilibria. M.S. thesis, 91 p. San Francisco State Univ., San Francisco, CA.

Butler, J. L.

1992. Collection and preservation of material for otolith analysis. In Otolith microstructure examination and analysis (D. 

K. Stevenson and S. E. Campana, eds.), p. 13–17. Can. Spec. 

Publ. Fish. Aquat. Sci. 117.

Cailliet, G. M., W. D. Smith, H. F. Mollet, and K. J. Goldman.

2006. Age and growth studies of chondrichthyan fishes: the 

need for consistency in terminology, verification, validation, 

and growth function fitting. Environ. Biol. Fish. 77:211–228.

Campana, S. E.

1997. Use of radiocarbon from nuclear fallout as a dated mark-
91

er in the otoliths of haddock Melanogrammus aeglefinus. Mar. 

Ecol. Prog. Ser. 150(1–3):49–56.

1999. Chemistry and composition of fish otoliths: pathways, 

mechanisms and applications. Mar. Ecol. Prog. Ser. 188:263–

297.

2001. Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods. J. Fish Biol. 59:197–242.

Campana, S. E., M. C. Annand, and J. I. McMillan.

1995. Graphical and statistical methods for determining the 

consistency of age determinations. Trans. Am. Fish. Soc. 

124:131–138.

Campana, S. E., and S. R. Thorrold.

2001. Otoliths, increments, and elements: Keys to a comprehensive understanding of fish populations? Can. J. Fish. 

Aquat. Sci. 58:30–38.

Carlson, H. R., and R. E. Haight.

1976. Juvenile life of Pacific ocean perch, Sebastes alutus, in 

coastal fiords of southeastern Alaska: Their environment, 

growth, food habits, and schooling behavior. Trans. Am. Fish. 

Soc. 105:191–201.

Carlson, H. R., and R. R. Straty.

1981. Habitat and nursery grounds of Pacific rockfish, Sebastes

spp., in rocky, coastal areas of southeastern Alaska. Mar. Fish. 

Rev. 43(7):13–19.

Chang, Y. B.

1982. A statistical method for evaluating the reproducibility of 

age determination. Can. J. Fish. Aquat. Sci. 39:1208–1210.

Chilton, E.

2007. Maturity of female northern rockfish Sebastes polyspinis in 

the central Gulf of Alaska. Alaska Fish. Res. Bull. 12:264–269.

Chilton, D. E., and R. J. Beamish.

1982. Age determination methods for fishes studied by the 

Groundfish Program at the Pacific Biological Station. Can. 

Spec. Publ. Fish. Aquat. Sci. 60, 102 p.

Clausen, D. M. and J. Heifetz.

2002. The northern rockfish, Sebastes polyspinis, in Alaska: 

Commercial fishery, distribution, and biology. Mar. Fish. Rev. 

64(4):1–28.

CARE (Committee of Age Reading Experts).

2006. Manual on generalized age determination procedures 

for groundfish, 52 p. [Available online at http://care.psmfc.

org/docs/CareManual2006.pdf, accessed March 2011.]

Degens, E. T., W. G. Deuser, and R. L. Haedrich.

1969. Molecular structure and composition of fish otoliths. Mar. 

Biol. 2:105–113.

Dorn, M., K. Aydin, S. Barbeaux, M. Guttormsen, K. Spalinger, and 

M. Wilkins.

2010. Assessment of the walleye pollock stock in the Gulf of Alaska. In Stock assessment and fishery evaluation report for the 

groundfish resources of the Gulf of Alaska, p. 53–156. North 

Pac. Fish. Mgmt. Council, Anchorage, AK.

Fadeev, N. W.

1970. The fishery and biological characteristics of yellowfin 

soles in the eastern part of the Bering Sea. [In Russ.] (Translated by Isr. Prog. Sci. Transl., 1972.) In Soviet fisheries investigations in the northeastern Pacific, Part V (P. A. Moiseev, ed.), 

p. 332–396. [Avail. From U.S. Dep. Commer., Natl. Tech. Inf. 

Serv., Springfield, VA as TT 71-50127.]

Forsberg, J. E.

2001. Ageing manual for Pacific halibut: Procedures and methods used at the International Pacific Halibut Commission. Int. 

Pac. Halibut Comm. Tech. Rep. 46, 56 p.

Gharrett, A. J., A. P. Matala, E. L. Peterson, A. K. Gray, and Z. Li.

2005. Two genetically distinct forms of rougheye rockfish are 

different species. Trans. Am. Fish. Soc. 134:242–260.

Gorbunova, N. N.

1962. Spawning and development of greenlings (family Hexagrammidae). In Greenlings: Taxonomy, biology, inter-oceanic 

transplantation (T.S. Rass, ed.). Tr. Inst. Okeanol. Akad. Nauk 

SSSR 59:121–185. [Engl. Transl. avail. U.S. Dep. Commer., 

Natl. Tech. Inf. Serv., Springfield, VA 22161, as TT69-55097.]

Gove, P. B. (ed.).

1961. Webster’s third new international dictionary of the English language, unabridged. Merriam-Webster, Inc., Springfield, MA.

Gregg, J. L., D. M. Anderl, and D. K. Kimura.

2006. Improving the precision of otolith-based age estimates 

for Greenland halibut (Reinhardtius hippoglossoides) with 

preparation methods adapted for fragile sagittae. Fish. Bull. 

104:643–648.

Gunderson, D. R.

1971. Reproductive patterns of Pacific ocean perch (Sebastes 

alutus) off Washington and British Columbia and their relation to bathymetric distribution and seasonal abundance. J. 

Fish. Res. Board Can. 28:417–425.

Hanselman, D. H., J. Heifetz, J. Fujioka, S. K. Shotwell, and J. N. 

Ianelli.

2007. Gulf of Alaska Pacific ocean perch. In Stock assessment 

and fishery evaluation report for the groundfish resources of 

the Gulf of Alaska, Section 9:563–622. North Pac. Fish. Mgmt. 

Council, Anchorage, AK.

Hart, J. L.

1973. Pacific fishes of Canada. Fish. Res. Board Can., Bull. 180, 

740 p.

Heifetz, J., D. Anderl, N. E. Maloney, and T. L. Rutecki.

1998. Age validation and analysis of ageing error from marked 

and recaptured sablefish, Anoplopoma fimbria. Fish. Bull. 

97:256–263.

Heifetz, J., D. Hanselmann, D. Courtney, and J. Ianelli.

2007. Gulf of Alaska northern rockfish. In Stock assessment 

and fishery evaluation report for the groundfish resources 

of the Gulf of Alaska, Section 10:623–648. North Pac. Fish. 

Mgmt. Council, Anchorage, AK.

Hollowed, A. B., and W. S. Wooster.

1995. Decadal-scale variations in the eastern subarctic Pacific: II. Response of Northeast Pacific fish stocks. In Climate 

change and northern fish populations (R. J. Beamish, ed.), p. 

373–385. Can. J. Fish. Aquat. Sci. 121.

Hughes, S. E., and G. Hirschhorn.

1979. Biology of walleye pollock, Theragra chalcogramma, in the 

western Gulf of Alaska, 1973–75. Fish. Bull. 77:263–274.

Hutchinson, C. E., C. R. Kastelle, D. K. Kimura, and D. R. Gunderson.

2007. Using radiometric ages to develop conventional ageing 

methods for shortraker rockfish (Sebastes borealis). In Biology, assessment, and management of North Pacific rockfishes (J. Heifetz, J. DiCosimo, A. J. Gharrett, M. S. Love, V. M. 

O’Connell, and R. D. Stanley, eds.), p. 237–249. Alaska Sea 

Grant Program Rep. No. AK-SG-07-01, Univ. Alaska, Fairbanks.

Ianelli, J. N., S. Barbeaux, T. Honkalehto, S. Kotwicki, K. Aydin, and 

N. Williamson.

2007. Eastern Bering Sea walleye pollock. In Stock assessment 

and fishery evaluation report for the groundfish resources 

of the Bering Sea/Aleutian Islands regions. North Pac. Fish. 

Mgmt. Council, Anchorage, AK, Section 1:41–138.

Ianelli, J. N., T. K. Wilderbuer, and D. Nichol.

2010. Assessment of Greenland turbot in the eastern Bering 

Sea and Aleutian Islands. In Stock assessment and fishery evaluation report for the groundfish resources of the Bering Sea/

Aleutian Islands regions, Section 5:645–696. North Pac. Fish. 

Mgmt. Council, Anchorage, AK.
92 Professional Paper NMFS 13

Ito, D. H., P. D. Spencer, and J. N. Ianelli.

1999. Pacific ocean perch. In Stock assessment and fishery evaluation report for the groundfish resources of the Bering Sea/

Aleutian Islands regions, Section 10:519–557. North Pac. Fish. 

Mgmt. Council, Anchorage, AK.

Kalish, J. M.

1993. Pre-bomb and post-bomb radiocarbon in fish otoliths. 

Earth Planet Sci. Lett. 114:549–554.

1995. Application of the bomb radiocarbon chronometer to 

the validation of redfish Centroberyx affinis age. Can. J. Fish. 

Aquat. Sci. 52:1399–1405.

Kalish, J. M., R. J. Beamish, E. B. Brothers, J. M. Casselman, R. I C. C. 

Francis, H. Mosegaard, J. Panfili, E. D. Prince, R. E. Thresher, C. A. 

Wilson, and P. J. Wright.

1995. Glossary for otolith studies. In Recent developments in 

fish otolith research (D. H. Secor, J. M. Dean, and S. E. Campana, eds.), p.723–729. Belle W. Baruch Library in Marine Science No. 19. Univ. South Carolina Press, Columbia.

Kastelle, C. R., and D. K. Kimura.

2006. Age validation of walleye pollock (Theragra chalcogramma) from the Gulf of Alaska using the disequilibrium of Pb210 and Ra-226. ICES J. Mar. Sci. 63:1520–1529.

Kastelle, C. R., D. K. Kimura, A. E. Nevissi, and D. R. Gunderson.

1994. Using Pb-210/Ra-226 disequilibria for sablefish, Anoplopoma fimbria, age validation. Fish. Bull. 92:292–301.

Kastelle, C. R., D. K. Kimura, and S. R. Jay.

2000. Using Pb-210/Ra-226 disequilibrium to validate conventional ages in Scorpaenids (genera Sebastes and Sebastolobus). 

Fish. Res. 46:299–312.

Kastelle, C. R., D. M. Anderl, D. K. Kimura, and C. G. Johnson.

2008a. Age validation of Dover sole (Microstomus pacificus) by 

means of bomb radiocarbon. Fish. Bull. 106:375–385.

Kastelle, C. R., D. K. Kimura, and B. J. Goetz.

2008b. Bomb radiocarbon age validation of Pacific ocean 

perch (Sebastes alutus) using new statistical methods. Can. J. 

Fish. Aquat. Sci. 65:1101–1112.

Kendall, A. W., Jr., and A. C. Matarese.

1987. Biology of eggs, larvae, and epipelagic juveniles of sablefish, Anoplopoma fimbria, in relation to their potential use in 

management. Mar. Fish. Rev. 49(1):1–13.

Kennedy, W. A.

1970. Reading scales to age Pacific cod (Gadus macrocephalus) 

from Hecate Strait. J. Fish. Res. Board Can. 27:915–922.

Kerr, L. A., A. H. Andrews, B. R. Frantz, K. H. Coale, T. A. Brown, and 

G. M. Cailliet.

2004. Radiocarbon in otoliths of yelloweye rockfish (Sebastes 

ruberrimus): A reference time series for the coastal waters of 

southeast Alaska. Can. J. Fish. Aquat. Sci. 61:443–451.

Ketchen, K. S.

1970. An examination of criteria for determining the age of 

Pacific cod (Gadus macrocephalus) from otoliths. Fish. Res. 

Board Can. Tech. Rep. 171, 42 p.

Kimura, D. K., and D. M. Anderl.

2005. Quality control of age data at the Alaska Fisheries Science Center. Mar. Freshwat. Res. 56:783–789.

Kimura, D. K., and J. J. Lyons.

1990. Choosing a structure for the production ageing of Pacific cod (Gadus macrocephalus). Int. N. Pac. Fish. Comm. Bull. 

50:9–23.

1991. Between-reader bias and variability in the age determination process. Fish. Bull. 89:53–60.

Kimura, D. K., A. M. Shimada, and F. R. Shaw.

1998. Stock structure and movement of tagged sablefish, Anoplopoma fimbria, in offshore northeast Pacific waters and the 

effects of El Niño Southern Oscillation on migration and 

growth. Fish. Bull. 96:462–481.

Kimura, D. K., C. R. Kastelle, B. J. Goetz, C. M. Gburski, and A. V. 

Buslov.

2006. Corroborating the ages of walleye pollock (Theragra chalcogramma). Aust. J. Mar. Freshwat. Res. 57:323–332.

Kimura, D. K., D. M. Anderl, and B. J. Goetz.

2007. Seasonal marginal growth on otoliths of seven Alaska 

groundfish species support the existence of annual patterns. 

Alaska Fish. Res. Bull. 12:243–251.

Kline, D. E.

1996. Radiochemical age verification for two deep-sea rockfishes. M.S. thesis, 124 p. San Jose State Univ., San Jose, CA.

Klovach, N. V., O. A. Rovnia, and D. V. Kol’stov.

1995. Biology and exploitation of Pacific cod, (Gadus macrocephalus), in the Anadyr Navarin region of the Bering Sea. J. 

Ichthyol. 35:9–17.

Kodolov, L. S., and S. P. Matveychuk.

1995. Stock condition of Greenland turbot (Reinhardtius hippoglossoides matsuurae Jordan et Snyder) in the northwestern 

Bering Sea. In Proceedings of the International Symposium 

on North Pacific Flatfish, p. 451–465. Alaska Sea Grant College Program, Univ. of Alaska, Fairbanks.

Krieger, K. J., and B. L. Wing.

2002. Megafauna associations with deepwater corals (Primnoa

spp.) in the Gulf of Alaska. Hydrobiologia 471:83–90.

Lai, H. L., D. R. Gunderson, and L. L. Loh.

1987 Age determination of Pacific cod, Gadus macrocephalus, 

using five ageing methods. Fish. Bull. 85:713–723.

LaLanne, J. J.

1979. The validity and consistency of age determinations from 

otoliths of Pacific pollock (Theragra chalcogramma). Int. North 

Pac. Fish. Comm., Annual Report 1977:99–107.

Laurel, B. J., C. H. Ryer, B. Knoth, and A. W. Stoner.

2009. Temporal and ontogenetic shifts in habitat use of juvenile Pacific cod (Gadus macrocephalus). J. Exp. Mar. Biol. Ecol. 

377:28–35.

Lauth, R. R., J. Guthridge, D. Nichol, S. W. McEntire, and N. 

Hillgruber.

2007. Timing and duration of mating and brooding periods 

of Atka mackerel (Pleurogrammus monopterygius) in the North 

Pacific Ocean. Fish. Bull. 105:560–570.

Love, M. S.

1991. Probably more than you want to know about the fishes 

of the Pacific coast, 215 p. Really Big Press, Santa Barbara, CA.

Love, M. S., M. H. Carr, and L. J. Haldorson.

1991. The ecology of substrate-associated juveniles of the genus Sebastes. Environ. Biol. Fish. 30:225–243.

Love, M. S, M. Yoklavich, and L. Thorsteinson.

2002. The rockfishes of the Northeast Pacific, 405 p. Univ. California Press, Los Angeles, CA.

Lowe, S. A., D. M. Van Doornik, and G. A. Winans.

1998. Geographic variation in genetic and growth patterns of 

Atka mackerel, Pleurogrammus monopterygius (Hexagrammidae), in the Aleutian archipelago. Fish. Bull. 96:502–515.

Lowe, S., J. Ianelli, M. Wilkins, K. Aydin, R. Lauth, and I. Spies.

2009. Stock assessment of Bering Sea/Aleutian Islands Atka 

mackerel. In Stock assessment and evaluation report for the 

groundfish resources of the Bering Sea/Aleutian Islands regions, Section 16:979–1054. North Pac. Fish. Mgmt. Council, 

Anchorage, AK.

Lyubimova, T. G.

1963. Basic aspects of the biology and distribution of Pacific 

rockfish (Sebastodes alutus Gilbert) in the Gulf of Alaska. In

Soviet Fisheries Investigations in the Northeast Pacific (P. A. 

Moiseev, ed.), p. 308–318. Tr. Vses. Naucho-Issled. Morsk. 

Rybn. Khoz. Okeanogr., Part I. (Transl. in 1964 from Russian 

by Israel Program for Sci. Transl., Jerusalem.)
93

1965. The main stages of the life cycle of the rockfish Sebastes 

alutus Gilbert in the Gulf of Alaska. In Soviet Fisheries Investigations in the Northeast Pacific (P. A. Moiseev, ed.), p. 

85–111. Tr. 58 Vses. Nauchno-Issled. Inst. Morsk. Rybn. Khoz. 

Okeanogr., Part IV (Available from U.S. Dep. Commer. Clearinghouse Fed. Sci. Tech. Inform. TT 67-51206. Springfield, 

Virginia 22151.)

MacLellan, S. E.

1997. How to age rockfish (Sebastes) using S. alutus as an example – the otolith burnt section technique. Can. Tech. Rep. 

Fish. Aquat. Sci. 2146, 39 p.

Maloney, N. E.

2004. Sablefish, Anoplopoma fimbria, populations on Gulf of 

Alaska seamounts. Mar. Fish. Rev. 66(3):1–12.

Maloney, N. E., and M. F. Sigler.

2008. Age-specific movement patterns of sablefish (Anoplopoma 

fimbria) in Alaska. Fish. Bull. 106:305–316.

Matta, M. E., and D. R. Gunderson.

2007. Age, growth, maturity, and mortality of the Alaska skate, 

Bathyraja parmifera, in the eastern Bering Sea. Environ. Biol. 

Fish. 80:309–323.

Matta, M. E., B. A. Black, and T. K. Wilderbuer.

2010. Climate-driven synchrony in otolith growth-increment 

chronologies for three Bering Sea flatfish species. Mar. Ecol. 

Prog. Ser. 413:137–145.

McDermott, S. F.

1994. Reproductive biology of rougheye and shortraker rockfish, Sebastes aleutianus and Sebastes borealis. M.S. thesis, 76 p. 

Univ. Washington, Seattle, WA.

McDermott, S. F., and S. A. Lowe.

1997. The reproductive cycle and sexual maturity of Atka mackerel, Pleurogrammus monopterygius, in Alaska waters. Fish. Bull. 

95:321–333.

McFarlane, G. A., and R. J. Beamish.

1995. Validation of the otolith cross-section method of age determination for sablefish (Anoplopoma fimbria) using oxytetracycline. In Recent developments in fish otolith research (D. H. 

Secor, J. M. Dean, and S. E. Campana, eds.), p. 319–329. Belle 

W. Baruch Library in Marine Science No. 19. Univ. South Carolina Press, Columbia.

Moiseev, P. A., and I. A. Paraketsov.

1961. Nekotorye dannye ob ekologii morskikh ershei (sem. 

Scorpaenidae) severnoi chasti Tikhogo okeana [Information of the ecology of rockfishes (family Scorpaenidae) of the 

northern part of the Pacific Ocean]. Vop. Ikhtiol. Vol. 1, No. 

1(18):39–45. (Preliminary trans. Fish. Res. Bd. Can., Biol. Sta., 

Nanaimo, B.C., FRB Transl. Ser. 358.)

Morales-Nin, B.

1992. Determination of growth in bony fishes from otolith 

microstructure. FAO Fish. Tech. Paper 322, 52 p. Food Agr. 

Organ. U.N., Rome.

National Research Council.

1996. The Bering Sea Ecosystem, 307 p. National Academy Press, 

Washington, DC.

Nishimura, A.

1993. Occurrence of a check in otoliths of reared and seacaught larval walleye pollock Theragra chalcogramma (Pallas) 

and its relationship to events in early-life history. J. Exp. Mar. 

Biol. Ecol. 166:175–183.

Nishimura, A., and J. Yamada.

1988. Geographical differences in early growth of walleye pollock Theragra chalcogramma, estimated by back-calculation of 

otolith daily growth increments. Mar. Biol. 97:459–465.

Nydal, R.

1993. Application of bomb 14C as a tracer in the global carbon 

cycle. Trends Geophys. Res. 2:355–364.

Orr, J. W., and S. Hawkins.

2008. Species of the rougheye rockfish complex: Resurrection 

of Sebastes melanostictus (Matsubara, 1934) and a redescription 

of Sebastes aleutianus (Jordan and Evermann, 1898) (Teleostei: 

Scorpaeniformes). Fish. Bull. 106:111–134.

Palsson, W. A.

1990. Pacific cod (Gadus macrocephalus) in Puget Sound and 

adjacent waters: Biology and stock assessment. Wash. Dep. 

Fish. Tech. Rep. 112, 137 p, Olympia, WA.

Panfili, J., H. de Pontual, H. Troadec, and P. J. Wright (eds.).

2002. Manual of fish sclerochronology, 464 p. Ifremer-IRD coedition, Brest, France.

Paraketsov, I. A.

1963. On the biology of Sebastodes alutus of the Bering Sea. In

Soviet Fisheries Investigations in the Northeastern Pacific, 

Part I (P. A. Moiseev, ed.), p. 319–327. [Available from U.S. 

Dep. Commer., Natl. Tech. Inf. Serv, Springfield, VA. As TT67-

51203.]

Pearson, D. E., and F. R. Shaw.

2004. Sources of age determination errors for sablefish (Anoplopoma fimbria). Fish. Bull. 102:127–141.

Penttila, J., and L. M. Dery (eds.).

1988. Age determination methods for northwest Atlantic species. NOAA Tech. Rep. NMFS 72, 135 p.

Phillips, J. B.

1957. A review of the rockfishes of California (Family Scorpaenidae). California Fish Bull. 104, 158 p. Ca. Dep. Fish Game, 

Mar. Fish. Branch. Los Angeles.

Piner, K. R., and S. G. Wischniowski.

2004. Pacific halibut chronology of bomb radiocarbon in otoliths from 1944 to 1981 and a validation of ageing methods. J. 

Fish Biol. 64(4):1060–1071.

Popper, A. N., J. Ramcharitar, and S. E. Campana.

2005. Why otoliths? Insights from inner ear physiology and 

fisheries biology. Mar. Freshwat. Res. 56(5):497–504.

Porntepkasemsan, B., and A. E. Nevissi.

1990. Mechanism of radium-226 transfer from sediments and 

water to marine fishes. Geochem. J. 24:223–228.

Quast, J. C.

1970. Reduction in stocks of the Pacific ocean perch, an 

important demersal fish off Alaska. Trans. Am. Fish. Soc. 

101(1):64–74.

Richter, H., and J. G. McDermott.

1990. The staining of fish otoliths for age determination. J. 

Fish Biol. 36:773–779.

Rickey, M. H.

1995. Maturity, spawning, and seasonal movements of arrowtooth flounder, Atheresthes stomias, off Washington. Fish. Bull. 

93:127–138.

Roberson, N. E.

2001. Age determination of Alaska Pacific cod (Gadus macrocephalus). M.S. thesis, 44 p. Univ. Washington, Seattle.

Roberson, N. E., D. K. Kimura, D. R. Gunderson, and A. M. Shimada.

2005. Indirect validation of the age-reading method for Pacific 

cod (Gadus macrocephalus) using otoliths from marked and recaptured fish. Fish. Bull. 103:153–160.

Rooper, C. N., and J. L. Boldt.

2005. Distribution of juvenile Pacific ocean perch Sebastes alutus in the Aleutian Islands in relation to benthic habitat. Alaska Fish. Res. Bull. 11:102–112.

Rooper, C. N., J. L. Boldt, and M. Zimmerman.

2007. An assessment of juvenile Pacific ocean perch (Sebastes 

alutus) habitat use in a deepwater nursery. Est. Coast. Shelf 

Sci. 75:371–380.

Rutecki, T. L., and E. R. Varosi.

1997. Distribution, age, and growth of juvenile sablefish, 
94 Professional Paper NMFS 13

Anoplopoma fimbria, in southeast Alaska. In Biology and management of sablefish, Anoplopoma fimbria (M. E. Wilkins and 

M. W. Saunders, eds.), p. 45–54. NOAA Tech. Rep. NMFS 130.

Scott, W. B., and M. G. Scott.

1988. Atlantic fishes of Canada. Can. Bull. Fish. Aquat. Sci. 

219, 731 p.

Shimada, A. M., and D. K. Kimura.

1994. Seasonal movements of Pacific cod, Gadus macrocephalus, 

in the eastern Bering Sea and adjacent waters based on tagrecapture data. Fish. Bull. 92:800–816.

Shotwell, S. K., D. Hanselman, and D.M. Clausen.

2007. Gulf of Alaska rougheye rockfish. In Stock assessment 

and fishery evaluation report for the groundfish resources 

of the Gulf of Alaska, Section 11:675–716. North Pac. Fish. 

Mgmt. Council, Anchorage, AK.

Sinclair, E. H., and T. K. Zeppelin.

2002. Seasonal and spatial differences in diet in the western 

stock of Steller sea lions (Eumetopias jubatus). J. Mammal. 

83:973–990.

Skalkin, V. A.

1964. Pitanie morskikh okunei v Beringovom more (The feeding of ocean perch in the Bering Sea). In Soviet fisheries investigations in the northeastern Pacific Ocean, Part 2 (Sovetskie 

rybokhozyaistvennye issledovaniya v severo-vostochnoi chasti 

Tikhogo okeana, vyp. 2). Tr. Vses. Nauch.-issled. Inst. Morsk. 

Ryb. Khoz. Okeanogr. 51):151–166. (Trans. P. 159–174 in complete col. Transl. avail. Nat. Tech. Inform. Service, Springfield, 

VA, as TT 67-51204.)

Smith, D. C., G. E. Fenton, S. G. Roberston, and S. A. Short.

1995. Age determination and growth of orange roughy, Hoplostethus atlanticus: A comparison of annulus counts with radiometric ageing. Can. J. Fish. Aquat. Sci. 52:391–401.

Smith, J. M., R. Nelson, and S. E. Campana.

1991. The use of Pb-210/Ra-226 and Th-228/Ra-228 dis-equilibria in the ageing of otoliths of marine fish. In Radionuclides 

in the study of marine processes (P. J. Kershaw and D. S. Woodhead, eds.), p. 350–359. Elsevier Applied Science, London.

Sohn, D.

2009. Ecology of Greenland halibut (Reinhardtius hippoglossoides) during the early life stages in the eastern Bering Sea and 

Aleutian Islands. M.S. thesis, 68 p. Oregon State Univ., Corvallis, OR.

Spencer, P. D., R. R. Reuter, and C. N. Rooper.

2008. Blackspotted and rougheye rockfish. In Stock assessment 

and fishery evaluation report for the groundfish resources of 

the Bering Sea/Aleutian Islands regions, Section 13B:1041–

1094. North Pac. Fish. Mgmt. Council, Anchorage, AK.

Stark, J. W.

2008. Age- and length-at-maturity of female arrowtooth flounder (Atheresthes stomias) in the Gulf of Alaska. Fish. Bull. 

106:328–333.

Stepanenko, M. A., and A. V. Nikolaev.

2004. Interannual variability of spatial distribution and abundance of the eastern Bering Sea pollock and environment 

conditions in summer of 1999–2003. Izv. TINRO 136:148–161.

Summerfelt, R. C., and G. E. Hall (eds).

1987. Age and growth of fish, 544 p. Iowa State Univ. Press, 

Ames, IA.

Swanson, S. M.

1985. Food-chain transfer of U-series radionuclides in a northern Saskatchewan aquatic system. Health Phys. 49:747–770.

Treble, M. A., S. E. Campana, R. J. Wastle, C. M. Jones, and J. Boje.

2008. Growth analysis and age validation of a deepwater Arctic 

fish, the Greenland halibut (Reinhardtius hippoglossoides). Can. 

J. Fish. Aquat. Sci. 65:1047–1059.

VanderKooy, S. (ed.).

2009. A practical handbook for determining the ages of Gulf 

of Mexico fishes. Gulf States Mar. Fish. Comm. Publ. No. 167, 

155 p.

von Szalay, P. G., N. W. Raring, F. R. Shaw, M. E. Wilkins, and M. H. 

Martin.

2010. Data report: 2009 Gulf of Alaska bottom trawl survey. 

NOAA Tech. Memo. NMFS-AFSC-208, 245 p.

Westrheim, S. J.

1970. Survey of rockfishes, especially Pacific ocean perch, in 

the northeast Pacific Ocean, 1963–1966. J. Fish. Res. Board 

Can. 27:1781–1809.

1996. On the Pacific cod (Gadus macrocephalus) in British Columbia waters, and a comparison with Pacific cod elsewhere, 

and Atlantic cod (G. morhua). Can. Tech. Rep. Fish. Aquat. 

Sci. 2092, 390 p.

Wilderbuer, T. K., G. E. Walters, and R. G. Bakkala.

1992. Yellowfin sole, Pleuronectes asper, of the eastern Bering 

Sea: Biological characteristics, history of exploitation, and 

management. Mar. Fish. Rev. 54(4):1–18.

Wilderbuer, T. K., D. G. Nichol, and J. Ianelli.

2008. Yellowfin sole. In Stock assessment and fishery evaluation report for the groundfish resources of the Bering Sea/

Aleutian Islands regions, Section 4:521–592. North Pac. Fish. 

Mgmt. Council, Anchorage, AK.

Witherell, D.

2000. Groundfish of the Bering Sea and Aleutian Islands area: 

Species Profiles 2001. North Pac. Fish. Mgmt. Council, Anchorage, AK, 15 p.

Wright, P. J. J. Panfili, B. Morales-Nin, and A. J. Greffen.

2002. Otoliths. In Manual of fish sclerochronology (J. Panfili, 

H. de Pontual, H. Troadec, and P. J. Wright, eds.), p. 31–57. 

Ifremer-IRD coedition, Brest, France.

Zimmerman, M.

1997. Maturity and fecundity of arrowtooth flounder, Atheresthes stomias, from the Gulf of Alaska. Fish. Bull. 95:598–611.
95

Accuracy: the closeness of an age estimate to the true 

age; contrast with precision

Aging criteria: standard rules that an age reader uses to 

recognize regular growth patterns and estimate age 

in a given species

Age determination (also age estimation): the process of 

assigning age estimates to fish

Age estimate: an age assigned to a fish in an effort to approximate its true age; this is typically accomplished 

by counting annual marks on an aging structure and 

applying an edge interpretation consistent with the 

international birth date convention; see also age determination

Aging structure: any calcified body part that grows consistently throughout the life of an organism and forms 

annual growth patterns that can be used to estimate 

age; in fish, includes otoliths, fin rays, spines, thorns, 

scales, opercles, cleithra, and vertebrae; the structure 

most appropriate for age determination depends 

upon the species

Age reader: within the context of age and growth studies, a person who generates age estimates

Age reading: the process of examining aging structures

to determine age estimates

Annual mark: within an aging structure, a growth zone 

that forms yearly and can be used to estimate the age 

of fish; in general, consists of one opaque growth zone

and one translucent growth zone; age readers at the 

AFSC often use this term to refer solely to the narrow 

translucent growth zone

Annulus: a concentric growth zone in an aging structure; by convention, age readers often assume this 

term is equivalent to “annual mark” even though 

“annulus” is derived from the Latin “anus,” meaning 

ring, not from “annus,” meaning year

Anterior: towards the head of an animal’s body; in fish, 

synonymous with the term “cranial”

Anti-sulcus: the area on the distal side of an otolith opposite the sulcus

Appendix. Glossary 

by Mary Elizabeth Matta, Daniel K. Kimura, Delsa M. Anderl, and Craig R. Kastelle

Asterisci: (pl.) one of the three pairs of otoliths found 

in the internal ear of teleosts

Average percent error (APE): a statistic used to measure 

precision within or between age readers or aging 

structures; see also coefficient of variation

Back-calculation: a method used to estimate fish lengths 

at previous ages by assuming a relationship between 

the size of an aging structure and fish length

Bias: a systematic error where an age reader consistently 

estimates older or younger ages than the true age; see 

also relative bias

Bomb radiocarbon (14C) age validation: a method used 

to directly validate age estimates by tracking the increase in otolith 14C corresponding to atomic bomb 

testing in the 1950s and 1960s; one of the strongest 

methods of age validation

Bowker’s test of symmetry: a statistical test used to detect bias between two age readers’ age estimates using 

paired age readings from the same fish

Break-and-bake method: a method used to clarify otolith

growth patterns, where the otolith is sectioned in half 

transversely and baked in an oven for several minutes; 

sometimes referred to as “toasting”

Break-and-burn method: a method used to clarify otolith growth patterns, where the otolith is sectioned in 

half transversely and the exposed surface is passed 

over an alcohol flame

Check: a non-annual translucent growth zone observed 

in the growth pattern of an aging structure; can be 

distinguished from annual marks by faint appearance 

and inconsistency throughout an aging structure; 

should not be counted in age estimates; may form due 

to physiological or environmental stress

Clearing: a technique of improving the visibility of 

growth zones, usually by immersion in a solution that 

reduces opacity such as water, ethanol, oil, or glycerin; 

see also over-clearing

Coefficient of variation (CV): a statistic used to measure 

precision within or between age readers or aging 

structures; see also average percent error

Cohort: within a population, a group of fish born within 

the same time period; see also year class

Core: within an otolith, the primordium and surrounding areas; in conventional age determination, generally refers to the part of the otolith deposited early 

within the first year of life, but in radiometric age 

validation studies, may refer to the part of the otolith 

deposited within the first several years of life; see also 

nucleus

This glossary provides definitions for terms commonly 

used by the AFSC Age and Growth Program. Several 

sources were used to compile and verify accuracy of 

these terms, including Chilton and Beamish (1982), 

Summerfelt and Hall (1987), Penttila and Dery (1988), 

Kalish et al. (1995), CARE (2006), and VanderKooy 

(2009). In composing the definitions below, all attempts were made to maintain consistency with previous publications.
96 Professional Paper NMFS 13

Corroboration: a comparison of age data with other 

data sources to infer the accuracy of fish age estimates; the consistency or repeatability of age estimates; the strongest methods of age corroboration 

are sometimes referred to as age validation

Crystallization: abnormal biomineralization in fish otoliths where part or all of the otolith has a glass-like or 

crystalline appearance, often resulting in obscured 

growth patterns that cannot be aged; in sagittae, typically caused by replacement of the normal form of 

calcium carbonate (aragonite) with the less common 

form (vaterite)

Daily rings: growth increments that form on a daily basis 

within an aging structure

Distal surface: in situ, the part of an otolith facing away 

from the midline of the body; the surface opposite 

the sulcus; sometimes referred to as the “external 

surface;” see also anti-sulcus

Dorsal tip: in situ, the part of an otolith corresponding 

to the dorsal (back) side of the fish; best seen in otolith transverse sections

Edge: the outermost part of an aging structure corresponding to the most recent period of growth; an 

edge interpretation is required to arrive at an age 

estimate; synonymous with margin

Edge interpretation: a judgment concerning growth observed on the edge of an otolith made according to the 

international birth date convention; the date of capture and the amount and type of growth along the otolith edge are used to decide whether the edge should 

be included in the final age estimate (i.e., whether an 

additional year should be added to the annual mark

count), thus assigning fish to the proper year class

Edge type: the characterization of the extent of opaque

or translucent growth present on the edge of an aging 

structure; see marginal increment analysis

Fishery observer: a scientifically-trained person who 

collects data and biological samples from commercial 

fishing vessels and transmits the information to the 

National Marine Fisheries Service for the purpose of 

monitoring and managing fisheries

Flatfishes: any of the teleost species which are laterally 

compressed and have both eyes on the same side of 

the body, including halibuts, flounders, and soles

Focus: the center (origin) of a vertebral centrum; sometimes refers to the point of origin of an otolith or scale

Growth function: any parametric model that describes 

length or weight at age; the most commonly used 

form in fish biology is the von Bertalanffy growth 

function (VBGF) 

Hyaline growth zone: synonymous with translucent 

growth zone; within an aging structure, a zone of low 

calcification that allows the passage of light; may be 

an annual mark or a check; the term “translucent” is 

preferred

International birth date convention: an international 

convention adopted by many age determination agencies, where all fish in the northern hemisphere are 

assumed to have a birth date of 1 January regardless 

of spawning season

Lapilli: (pl.) one of the three pairs of otoliths found in 

the internal ear of teleosts

Longitudinal section: any section along the long axis 

of an organism (or aging structure), perpendicular 

to the transverse plane; sagittal sections are a type of 

longitudinal section

Margin: the outermost part of an aging structure; synonymous with edge

Marginal increment analysis: an indirect method for 

validation of age estimates; the relative amount of 

new growth on the edge of aging structures is compared throughout the year to determine whether 

annual marks are laid down once per year; see edge 

type

Mark-recapture: a field method where fish that have 

been marked (tagged) and re-captured can be used 

to estimate abundance, analyze movements, and estimate growth parameters; when used in conjunction 

with a chemical marker such as oxytetracycline, can 

provide a form of age estimate validation

Nucleus: an ambiguous term usually used to refer to the 

otolith primordium or core

Opaque growth zone: within an aging structure, a zone 

of high calcification that restricts the passage of light; 

appears light in color when viewed with reflected light

against a dark background; contrast with translucent 

growth zone

Otoliths: three pairs of calcified structures (sagittae, 

lapilli, and asterisci) that are located in the heads of 

teleost fishes; age readers often use this as a general 

term for the sagittae, which form annual growth patterns that can be used for age determination

Over-clearing: during the process of clearing, the point 

at which the otoliths have spent too much time in a 

clearing solution and the growth zones are no longer 

visible

Oxytetracycline (OTC): a chemical marker that binds 

to calcified tissues upon injection or immersion; used 

to validate age estimates from otoliths or other aging 

structures in mark-recapture studies

Percent agreement: a measure of precision used to compare age estimates within or between age readers or 

aging structures

Posterior: towards the tail of an animal’s body; in fish, 

synonymous with the term “caudal”

Pre-annular check (growth pattern): a phenomenon 

in otoliths where, prior to deposition of an annual 

mark, there is a broad, indistinct, and poorly defined 

translucent growth zone or check; sometimes referred 

to as “shadowing”
97

Precision: the degree to which an age estimate is reproducible, either by the same age reader or between age 

readers; contrast with accuracy

Primordium: the initial structure of an otolith; see also 

core

Production aging: routine age determination carried 

out over time in order to develop a database of ages 

that can be used for stock assessment modeling

Proximal surface: in situ, the part of the otolith facing 

towards the midline of the body; the surface containing the sulcus; sometimes referred to as the “internal 

surface”

Radiometric age validation: a method used in age validation studies to determine age in fish by measuring 

the ratio of 210Pb/226Ra or of 228Th/228Ra in otolith 

cores

Readability: the relative ease with which an age reader

can assign an age to a given aging structure; can be affected by the method of preparation used to estimate 

age of the structure as well as physical characteristics 

within the structure such as crystallization

Reading axis: within an aging structure, the preferred 

path of reading annual marks from the core or focus 

to the edge

Reflected light: light that is shone down onto an aging 

structure from above; the use of reflected or transmitted light affects the appearance of opaque and 

translucent growth zones

Relative bias: a systematic error between age estimates; 

can occur between age readers, when one person consistently attains either older or younger age estimates

than another person; can also occur between aging 

structures or aging methods

Ring: a general term describing concentric growth 

zones; is not necessarily synonymous with “annual 

mark”

Sagittae: (pl.) for nearly all species, the largest of the 

three pairs of otoliths found in the internal ear of 

teleosts; the otolith pair most frequently used in age 

determination studies

Sagittal section: a section dividing the right and left 

sides; see also transverse section

Splitting (growth pattern): a type of check commonly 

observed in otoliths from certain species, where two 

or more closely-spaced translucent growth zones were 

deposited within the same year of growth and are 

equivalent to a single annual mark

Sulcus: a deep groove on the proximal side of an otolith

containing a thickened part of the otolithic membrane; the areas adjacent to the sulcus are often used 

as reading axes in transverse sections of otoliths from 

various species

Surface examination: the practice of using the intact 

surface of an aging structure to determine age; sometimes adequate for generating age estimates but is 

generally less desirable for old specimens or difficult 

species; in otoliths, either the distal or proximal surface can be used for surface examination

Teleost: any of the higher bony fishes (Teleostei); 

excludes more ancestral groups such as bichirs and 

sturgeons

Test sample: a subset of specimens aged by a second age 

reader; at the AFSC, production species are typically 

tested at a rate of 20%

Tester: a second, usually more experienced age reader, 

who ages a subset of specimens for comparison with 

the primary age reader to estimate between-reader 

precision and to detect potential relative bias as a 

means of quality control

Thin section: an aging structure preparation method 

typically used at the AFSC for difficult or long-lived 

species such as certain rockfish species and skates; 

time-intensive but generally produces even reading 

surfaces with relatively high resolution compared to 

the break-and-burn method

Translucent growth zone: within an aging structure, a 

zone of low calcification that allows the passage of 

light; appears dark in color when viewed with reflected light against a dark background; sometimes 

referred to as a hyaline growth zone, although “translucent” is the preferred term; may be an annual mark

or a check

Transmitted light: light that is projected through an aging structure from beneath; the use of transmitted or 

reflected light affects the appearance of opaque and 

translucent growth zones

Transition zone: within an otolith, an area of significant 

change in the growth pattern; usually corresponds to 

significant events during the life of the fish such as the 

process of sexual maturation; older otoliths are often 

observed with one or more transition zones

Transverse section: a section dividing the anterior and 

posterior ends of an aging structure; generally the 

preferred method of sectioning otoliths; see also 

sagittal section

True age: the actual age of an organism, a very difficult 

quantity to know with certainty; most commonly, age 

estimates are used to approximate true age

Unburned face: a term used by AFSC age readers to 

describe the untreated cut surface of a transversely 

sectioned otolith; for some otoliths, it can occasionally be used to determine age

Validation: any of a variety of strong methods used to 

confirm the accuracy of age estimates; see also corroboration

Ventral tip: in situ, the part of an otolith corresponding 

to the ventral (belly) side of a fish; best seen in otolith 

transverse sections

Year class: a group of fish of the same stock born within 

the same year; see also cohort
1. Atlas of abundance and distribution patterns 

of ichthyoplankton from the Northeast Pacific 

Ocean and Bering Sea ecosystems based on research conducted by the Alaska Fisheries Science 

Center (1972-1996), by Ann C. Matarese, Deborah 

M. Blood, Susan J. Picquelle, and Jan L. Benson. 

September 2003, 281 p.

2. The economic importance of marine angler expenditures in the United States, by Scott 

Steinback, Brad Gentner, and Jeremy Castle. 

January 2004, 169 p.

3. Development of kelp rockfish, Sebastes atrovirens (Jordan and Gilbert 1880) and brown rockfish, S. auriculatus (Girard 1854), from birth to 

pelagic juvenile stage, with notes on early larval 

development of black-and-yellow rockfish, S. chrysomelas (Jordan and Gilbert 1880), reared in the 

laboratory (Pisces: Sebastidae), by William Watson 

and Larry L. Robertson. June 2004, 30 p.

4. A taxonomic guide to the mysids of the South 

Atlantic Bight, by Richard W. Heard, W. Wayne 

Price, David M. Knott, Rachael A. King, and 

Dennis M. Allen. June 2006, 37 p.

5. Emerging technologies for reef fisheries research and management, by J. C. Taylor (editor). 

December 2006, 116 p.

6. Juvenile and small resident fushes of Florida 

Bay, a critical habitat in the Everglades National 

Park, Florida, by Allyn B. Powell, Gordon Thayer, 

Michael Lacroix, and Robin Cheshire. March 

2007, 210 p. 

NOAA Professional Papers NMFS

All publications are available online at http://spo.nmfs.noaa.gov/trlist.htm

7. Spawning, egg development, and early life 

history dynamics of arrowtooth flounder (Atheresthes stomias) in the Gulf of Alaska, by Deborah M. 

Blood, Ann C. Matarese, and Morgan S. Busby. 

May 2007, 28 p.

8. A guide to the Thalassinidea (Crustacea: Malacostraca: Decapoda) of the South Atlantic Bight, 

by Richard W. Heard, Rachael A. King, David M. 

Knott, Brent P. Thoma, and Susan ThorntonDeVictor. October 2007, 30 p.

9. Genetic and morphological identification 

of pelagic juvenile rockfish collected from the 

Gulf of Alaska, by Arthur W. Kendall Jr., Christine 

Kondzela, Zhuozhuo Li, David Clausen, and 

Anthony J. Gharrett. December 2007, 26 p.

10. Larval development and identification of 

the genus Triglops (Scorpaeniformes: Cottidae), 

by Deborah M. Blood and Ann C. Matarese. May 

2010, 43 p.

11. Bathyraja panthera, a new species of skate 

(Rajidae: Arhynchobatinae) from the western 

Aleutian Islands, and resurrection of the subgenus Arctoraja Ishiyama, by James W. Orr, Duane 

E. Stevenson, Gerald R. Hoff, Ingrid Spies, and 

John D. McEachran. March 2011, 50 p.

12. A guide to the deepwater sponges of the 

Aleutian Island Archipelago, by Robert P. Stone, 

Helmut Lehnert, and Henry Reiswig. September 

2011, 187 p.
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