NOAA Technical Memorandum NMFS 

AUGUST1983 

PRECISION OF AGE DETERMINATION OF 

NORTHERN OFFSHORE SPOTTED DOLPHINS 

Stephen B. Reilly 

Adeta A. Hohn 

Albert C. Myrick, Jr. 

NOAA-TM-NMFS-SWFC-35 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Center 
NOAA Technical Memorandum NMFS 

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AUGUST 1983 

PRECISION OF AGE DETERMINATION OF 

NORTHERN OFFSHORE SPOTTED DOLPHINS 

Stephen 6. Reilly 

Aleta A. Hohn 

Albert C. Myrick, Jr. 

Southwest Fisheries Center 

National Marine Fisheries Service, NOAA 

La Jolla, California 92038 

NOAA-TM-NMFS-S WFC-35 

U.S. DEPARTMENT OF COMMERCE 

Malcolm Baldrige. Secretary 

National Oceanic and Atmospheric Administration 

John V. Byrne. Administrator 

National Marine Fisheries Service 

William G. Gordon, Assistant Administrator for Fisheries 
CONTENTS 

Page 

ABSTRACT............................................................ 1 

INTRODUCTION...,.................................................... 1 

DATA SOURCES AND SAMPLING STRATEGY.................................. 2 

1973-1978 SAMPLE 

Within-Reader Precision.......................................... 3 

Tests for Trend in Age Estimates................................. 4 

Between-Reader Precision...................... ................... 3 

Age Distribution Comparisons ..................................... 5 

1981 SAMPLE 

Within-Reader Precision.......................................... 6 

Between-Reader Precision......................................... 6 

Age Distributions................................................ 7 

DISCUSSION.......................................................... 7 

LITERATURE CITED.................................................... 

i 

9 
LIST OF TABLES 

Tab1 e Page 

1 Coefficients of variation (CV) for two readers, by 

age groups (3), with analysis of variance testing 

wi thi n-reader di fferences over age groups, and 

t-tests of between-reader differences within age 

groups. Data are from spotted dolphins, both 

sexes, ki 11 ed during 1973-78.. ............................ Indices of precision (D=CV/Jfi) for two 

readers, by age groups (31, with analysis of 

variance testing within-reader differences 

over age groups, and t-tests of between-reader 

differences within age groups. 

spotted dolphins, both sexes, killed in 1973-78........... 

Data are from 

Summary statistics for "agebest," by reader (2) 

age-class (3) and sex. The "agebest" is itself 

a mean of a series of readings per specimen by 

each reader. The within reader precision is 

reported in Tables 3 and 4. Data are from 

offshore spotted dolphins from 1973-78.................... 

Coefficients and tests of significance from 

multiple regressions of last reading age 

estimates on 1) earlier age estimates and, 

2) elapsed time between the estimates. For 

B1, t-tests of the hypothesis last estimate = 

previous estimate (B1=1) are shown. Data 

are from 1973-78 offshore spotted dol phin 

tooth glg readings ........................................ Mu1 tiple analysis of covariance of last reading 

age estimate by reader (21, sex (21, and age 

group (31, with earl ier age estimates and the 

el apsed time between estimates as covari ates.. . 

Frequencies of individuals estimated to be 

within 15 age classes by sex, reader and 

estimate type. Chi-squared tests of 

homogeneity are in Table 9. The full 

distributions from which these were summed 

are reported in Appendices 1-4............................ 

Summary of ~2 statistics from contingency 

table tests of homogeneity of age distributions, across methods (2), readers (2) and 

sexes. 

pooled. 

fetal frequencies are in Table 8. ......................... 

Animals of 15 year or older were 

Each test had 14 degrees of freedom, 

10 

11 

12 

13 

14 

15 

16 

ii 
LIST OF TABLES - Continued 

Tab1 e Page 

8 Coefficients of variation (CV) for two readers, 

broken down by age groups (3), with analysis of 

variance testing wi thi n-reader differences over 

age groups, and t-tests of between-reader 

differences within age groups. Data are from 

female spotted dolphins killed in 1981.. .................. 17 

9 Indices of precision (D=CV/JA) for two readers, 

by age groups (31, with analysis of variance 

testing within-reader differences over age groups, 

and t-tests of between-reader differences within 

age groups. Data are from female spotted dolphins 

killed in 1981............................................ 18 

10 Means and standard deviations of age estimates 

for the 1981 female offshore spotted dolphin 

subsample. Age estimates are broken down by 

age group (61=<4.0; 4.0<62<12.0; 12.0<63), 

reader (2) anhcycle as a repeated measure 

within reader............................................. 19 

11 Analysis of variance for 1981 female spotted 

dol phi n subsampl e. 

agebest. 

size group (3) and cycle as a repeated 

measure within reader..................................... 20 

Dependent vari ab1 e is 

Factors tested were reader (2), 

iii 
LIST OF FIGURES 

Figure Page 

1 Coding format used for dolphin tooth ageing record........ 21 

2 P1 ots of si gned difference between individual readers ' 

mean estimates as a percent of the mean pooled age. The data are from offshore spotted dolphins, 1973-78 

sample: Females in the upper panel, males in the 

lower panel 23 

P1 ots of si gned di ff erence between i ndi vi dual readers ' 

estimates from last readings as a percent of the 

pooled mean age. dolphins, 1973-78 sample: Females in the upper panel, males in the lower panel 

frequencies of female spotted dolphins, as estimated 

by two different readers. 

were means of multiple readings per specimen by each 

reader.................................................... 25 

frequencies of male spotted dolphins, as estimated 

by two different readers. The age statistics compared 

were means of multiple readings per specimen by each 

reader.................................................... 

Plots of signed differences between reader estimates 

as a percent of the pooled mean age. The data are 

from female offshore spotted dolphins, 1981 sample 

............................................... 3 

The data are from offshore spotted 

.................................. 24 

4 Chi-squared values by age-group, from cornparison of 

The age statistics compared 

5 Chi-squared values by age-group, from comparison of 

26 

6 

........ 27 

iv 
PRECISION OF AGE DETERMINATION 

OF NORTHERN OFFSHORE SPOTTED DOLPHINS 

Stephen B. Reilly, Aleta A. Hohn, 

and Albert C. Myrick, Jr. 

Southwest Fisheries Center 

National Marine Fisheries Service, NOAA 

La Jolla, California 92038 

ABSTRACT 

We investigated within- and between-reader precision in estimating age 

for northern offshore spotted dolphins and possible effects on precision from 

the sex and age-class of specimens. Age was estimated from patterns of growth 

layer groups in the dentine and cementum of the dolphins' teeth. Each 

specimen was aged at least three times by each of two persons. Two data 

samples were studied. The first comprised 800 of each sex from animals 

col 1 ected duri ng 1973-78. The second incl uded 45 femal es coll ected during 

1981. There were significant, generally downward trends through time in the 

estimates from multiple readings of the 1973-78 data. These trends were 

slight, and age distributions from last readings and mean estimates per 

specimen appeared to be homogeneous. The largest factor affecting precision 

in the 1973-78 data set was between-reader variation. In light of the 

relatively high wi thin-reader precision (trends considered), the consistent 

between-reader differences suggest a problem of accuracy rather than precision 

for this series. Within-reader coefficients of variation averaged 

approximately 7% and 11%. Pooling the data resulted in an average coefficient 

of variation near 16%. Within- and between-reader preci sion were higher for 

the 1981 sample, and the data homogeneous over both factors. CVs averaged 

near 5% and 6% for the two readers. These results point to further 

refinements in reading the 1981 series. Properties of the 1981 sample may be 

partly responsible for greater precision: by chance there were proportionately 

fewer ol der dol phi ns i ncl uded, and preparation and selection criteria were 

probably more stri ngent. 

INTRODUCTION 

The precision of growth layer group ageing of cetaceans has not been 

previously addressed in the literature. In relation to ageing fish via scale 

pattern readings, Beami sh and Fournier (1981) and Chang (1982) di scussed 

statistics that are generally appl icabl e to measurement of wi thi n-reader 

precision but did not develop quantification of between-reader precision. 

In statistical jar on, accuracy is defined as the closeness of a measured 

or computed value to i 9 s true value; precision is the closeness of repeated 
2 

measurements of the same value (Sokol and Rohlf, 1969). 

refers to lack of bias, while high precision is generally 

vari ance. Thi s termi no1 ogy is fol 1 owed here. 

This paper addresses the precision of estimating 

That is, accuracy 

synonymous with low 

age for northern 

offshore spotted dolphins (Stenell a attenuata). Age is estimated from 

patterns of growth layer groups in the dentine and cementum of the dolphin's 

teeth (Perrin and Myrick 1980). Verification of the accuracy of this 

technique ( for cl osely re1 ated Hawai i an spi nner dol phi ns, Stenel 1 a 

longirostris) is addressed in Myrick et al. (MS) and is not directly 

considered here. Possible confounding of accuracy probl ems with apparent 

precision is discussed, in relation to the slightly different methods used by 

the two readers in this study to estimate age from growth layer group (GLG) 

patterns (Myrick et a1 . 1983). 

We investigated both within- and between-reader precision and possible 

differences due to the sex and age class of specimens. Two data samples were 

selected and analyzed. Analyses for each sample are presented separately, 

then discussed jointly. 

DATA SOURCES AND SAMPLING STRATEGY 

The first data sample was selected from northern offshore spotted 

dolphins killed incidentally in tuna seines during 1973-78 (n=1600; 800 each 

sex, drawn randomly from over 8,000 total ). The original experimental design 

called for each specimen to be read three times by two readers, allowing for 

computation of wi thi n- and between-reader variance. Each reading was made 

"blind," that is, without knowledge of previous readings for the specimen by 

either reader, and with no accessory information on the size or life history 

traits of the specimen. Time interval s between successive readings varied 

from days to months. 

Data from each reading of a tooth preparation were recorded in a common 

format (Figure 1). Additional elements were added to the reading record post 

facto: sex, consecutive reading days and reading number for the specimen by 

the reader. 

During the course of the study, it became apparent that the ageing 

techniques were still under development. Also, through trial and error, the 

tooth preparation techniques improved, and standards for accepting a 

preparation as "readable" became more rigid. Consequently, 1 ater readings 

were not necessarily aged using exactly the same techniques or under the same 

conditions as earlier readings and are therefore not true replicates for 

measuring within-reader precision. 

A second sample was drawn from northern offshore spotted dolphin females 

captured during 1981, to investigate within-reader variance under constant 

conditions. There was some indication of differences in precision for ageing 

"young" and "older" animals in the 1973-78 series (discussed below). The 1981 

sample was drawn in two age groups to address this: below and above 160 cm in 
3 

total length (this was approximately the length at which both between- and 

wi thi n-reader estimates began to diverge: a1 so discussed bel ow). 

Sample size for each stratum was determined with the following criteria: 

95% confidence with a relative bound of 10% (1.5 yrs) on the average age 

estimate (about 15 yrs). Presample estimates of the variance were taken from 

the 1973-78 readings (s2 = approx. 9.5 for <160 cm; 13.5 for >160 cm). For 

each stratum the sample size was estimated as nl = t2s2/b2 (Cochran, 1977). 

The stratum estimates are nl = 4(9.5)/2.25 = 17, and n2 = 24. The sum (41) 

was set as a minimum. A sample of 45 was actually drawn. 

Within-Reader Precision 

Methods 

Acknowledging the possible change over time in methods used for the 1973- 

78 readings, we analyzed the multiple readings per specimen by each reader as 

repeated measures. This was done to compare the magnitude of within-reader 

variance between the 1973-78 and 1981 data, the later having been examined 

with constant methods. Following Chang (1982) the coefficients of variation 

(CV=SDxlOO/mean) and ''indices of precision'' (D=CV/Jii) were calculated. Here 

the sample was divided into three age groups: 0-4 yrs; 4-12; 12+ yrs. Four is 

the approximate age at 160 cm total length (Hohn and Hammond, manuscript: and 

see above), and 12 is the approximate age at sexual maturity (Myrick, 

unpublished). To extend the analysis beyond that defined by Chang (1982), we 

tested for age-group differences in CY and D by analysis of variance 

(ANOVA). We also tested for differences between the readers' precision within 

each age class (and over all ages) by t-tests. 

Results 

Even considering the suggested change in methods over time, the overall 

CVs are not high: 11.2% average for reader 1, 7.7% average for reader 2 

(Table 1). The CVs increase significantly with increasing age for both 

readers. The Ds show a very similar pattern (Table 2). 

CV and D represent relative precision within the readers 

actual mean ages for each reader by age-group cell are in Tab 

Between-Reader Precision 

Methods 

est 

e 3. 

mates. The 

A graphic measure of between-reader differences by age was defined. For 

each specimen, the signed difference between reader 1's mean and reader 2's 

mean as a percentage of the pooled mean age (PM) was plotted against PM. In 

the resulting figures, values greater than zero for a specimen indicate that 

the mean estimate by reader 1 was higher than the mean estimate by reader 2. 
4 

Values less than zero indicate the opposite. Values on the figures are also 

i nformati ve in representing the between-reader di f ferences re1 ati ve to the 

pooled mean value, rather than in absolute time units. 

Between-reader precision was also measured in the CV and D analyses, and 

(as discussed below) in an analysis of covariance. On a per specimen basis, 

significant differences between the readers' means were tested for with ttests. The frequency of significant t values is an indicator of overall 

between-reader preci sion. 

Results 

The reader means appear to diverge with increasing age (Table 3). This 

can be seen in Figures 2 and 3, which show the signed % differences between 

reader means as a function of pooled mean age. Reader 1 estimated higher than 

reader 2 for older animals. For younger animals (~4.0 years pooled mean) 

reader 2's estimates are slightly higher. These differences are apparent in 

the means (Figure 2) and 1 ast readings Figure 3). In all, the mean estimates 

are significantly different for 46.8% of the males, 47.0% of the females (from 

t-tests 1 

The by-reader, within age-class comparisons of mean CV and D show 

significant differences in the magnitude of precision as well, reader 1 

generally having higher CVs and Ds (Tables 1 and 2). 

Tests for Trend in Age Estimates 

Methods 

Multiple regressions of last readings per specimen were made on earlier 

readings (Xl) and elapsed time in days (X2) between the readings. This was 

done for the entire data set, and for six reader (2) by age-group (3) 

categories. In each multiple regression, we tested for departure from unity 

of the coefficient relating last to earlier readings (Bl). Such a departure 

would indicate a trend in ageing method with time. Actual values of the 

coefficients are not easily interpretable, though, due to possible covariance 

with B2, the coefficient for the elapsed time variable. An ANOVA of 

regression coeff i ci ents over groups was a1 so conducted. 

pred 

Resu 

Secondly, we ran a multiple analysis of covariance (MANCOVA) of last 

readings on the same two predictors, with age group (3) sex and reader (2) as 

factors. This method examined the importance of these three factors and their 

i nteracti ons, after adjusti ng for covariance between the 1 ast readi ngs and the 

ctor variables. 

ts 

The multiple regressions show a significant departure from unity in the 
5 

regression coefficients of last on earlier age estimates, in five of six 

reader by age-group cells, and in the combined data (Table 4). The data 

subset with no time trend in age estimates (B1=1) is reader 2, age (4.0. 

Otherwise, the last reading was slightly lower than earlier readings, on 

average (i.e., B1 significantly less than 1.0). The ANOVA indicated that 

regression coefficients were different between the cells. That is, there were 

different rates of change over time, the greatest decrease being in reader 1's 

ageing of "1 arge" animal s. 

The MANCOVA showed that reader and (as expected) age-class were 

significant factors in estimating age from GLGs, as was their interaction 

(Table 7). Sex was not significant in this test nor were any interaction 

terms involving sex. These results can be interpreted as tests of the 

significance of sex, age class and reader on estimated age, after adjustment 

for the covariates, which both relate to a trend with time. 

Age Di stri bution Comparisons 

Methods 

In tests above, reader, age-class and (in some tests) sex have shown 

significant effects on precision and magnitude of estimated ages. Given this, 

it was of interest to see if such differences were translated into the 

resulting age distributions. Possible differences between readers, sexes and 

estimate types (means vs last readings) in the resulting age distributions 

were examined via chi-squared contingency tests. Animals of 15 years or older 

were pooled in these tests. This was done because nearly all 15 year olds are 

sexually mature (Myrick, unpublished data) and because differences in age 

structure are primarily re1 evant to studies of reproductive rates. 

Results 

The age distributions tested (Table 6) are heterogeneous across all 

reader comparisons, within both estimate types (Table 7 1. The distribution of 

chi -squared statistics by age-cl ass for the between-reader comparison of 

females using pooled mean ages (Figure 4) shows that the greatest contribution 

to the difference comes from a few older age groups. Differences in the 15+ 

group contributed heavily in a1 1 between- eader comparisons. The 

distributions for the two estimates (pooled mean and last reading) are not 

different, however. 

For comparison of males using pooled means, the ages contributing to 

between-reader differences were more widely distributed among the 4+ year ol ds 

(Figure 5). Similar patterns were seen in the between-reader comparisons of 

last reading age distributions: significant differences in female- being 

primarily in the 12+ year olds, and in the males being distributed more widely 

in younger age groups. It is not clear why this sex related difference has 

occurred. It is perhaps related to the differing age distributions recorded 

for females and males, the latter having relatively more individuals in the 4 

to 12 year classes. 
6 

1981 SAMPLE 

The overall mean age in this sample of 45 females is 10.912 years 

(SD=8.09), as compared to 13.93 years (SD=8.36) in the larger 1973-78 sample. 

Within-Reader Precision 

Methods 

For the 1981 sample, within-reader precision was determined by three 

methods. The coefficients of variation and indices of precision were computed 

and compared, as with the 1973-78 sample. As a third approach, we conducted 

an ANOVA of agebest, with age-groups (3) and readers (2) as factors, and cycle 

as a repeated measure within reader. While not assigning an explicit 

statistic for precision such as CV or D, this method measures the significance of variance from each effect in determining overall "agebest" and its 

vari ance. 

Results 

For reader 1 the CVs and 'Ds are lower on average in the 1981 than the 

1973-78 sample: CV=4.85 vs 11.28, D=2.79 vs 6.59 for females (Tables 8 and 

9). Reader 2 maintained about the same CV level (6.90 vs 6.12) but declined 

in D (3.53 vs 4.55). For both readers, average coefficients of variation of 

less than 5% indicate high within-reader precision. The change in withinreader precision between the two data sets is discussed further below. 

As in the 1973-78 sample, precision declined with increasing estimated 

specimen age for both readers. For the oldest age-group (over 12 yrs) CVs are 

7.08% and 8.47% for readers 1 and 2, respectively. In the earlier sample, 

these were 13.26% and 8.03%. Cell means and variances for the actual age 

estimates are presented in Table 10. 

Between-Reader Precision 

Methods 

As with the 1973-78 sample, between-reader preci sion was studied using ttests of mean age estimates by specimen, and of mean CV and D within agegroups. The ANOVA described in Wi thi n-Reader Preci si on a1 so tested betweenreader preci si on. 
7 

Results 

The analysis of variance (Table 11) indicates that there is no 

significant difference in cycle within reader, agreeing with the low withinreader CVs. By definition, there is a significant age-group effect, but for 

this data series, there is no reader effect. Nor are the reader by group, or 

group by cycle (within-reader) interactions significant. These results for 

actual age estimates are supported by the lack of significant differences 

between reader mean precision statistics (CVs and Ds) within age classes 

(Tables 8 and 9). A plot of the signed differences between-reader mean ages, 

as percentages of the pooled means, shows a narrow, relatively uniform band 

around zero (Figure 6). In combination, these results indicate that for the 

1981 sample, there was generally high precision both within- and betweenreaders in repeated age estimates. 

The actual percent of cases in which the within-reader mean ages differed 

significantly, by one year or more, was around 20% (9/45). This is down from 

nearly 50% in the 1973-78 sample. In keeping with the generally lower 

precision in ageing older animals, the mean age of the animals for which the 

readers' means were different was 16.7 years, compared to 10.9 for the entire 

sampl e. 

Age Distributions 

Methods 

To compare the age distributions resulting from the 1981 sample 

estimates, we used the same techniques as with the 1973-78 sample: pooling 

animals aged 15 and older and computing chi-squared statistics to test the 

nul 1 hypothesi s of homogenei ty. 

Results 

The age distributions resulting from the 1981 sample readings from the 

two readers are not different, when pooled as described above (X2 = 0.87) 

(Appendix 3). The limited sample of 45 is not, however, really adequate to 

define an age distribution with a maximum age of over 30 years. (The entire 

1981 sample was not prepared for analysis in time for this writing. When it 

i s compl ete , more meani ngful age di stri buti on compari sons i nvol vi ng these data 

can be made.) These preliminary age distribution comparisons are consistent 

with the other between-reader comparisons discussed above in indicating no 

significant between-reader differences. 

DISCUSSION 

There are significant, generally downward trends through time in the 

estimates from multiple readings of the 1973-78 data. These trends are slight 

and do not result in different age distributions from last readings and pooled 
8 

mean estimates (at least on the scale examined here). The largest single 

factor affecting precision in this data series is between-reader variation. 

In light of the relatively high within-reader precision (trends considered), 

the consi stent between-reader di f ferences suggest a problem of accuracy rather 

than precision for the 1973-78 series. That is, the two readers, using 

slightly different methods to determine age from a series of GLGs produced 

significantly different results for this data set. 

Within- and between-reader precision in estimating age is higher for the 

1981 series, and the data are homogeneous over both factors. It may be that 

the readers have refined their respective techniques to the point that they 

produce essentially the same results. No conscious consensus or melding of 

the techniques has taken place. Properties of the 1981 sample and its 

selection may be partly responsible for the greater precision in ageing that 

data set. There were relatively fewer older animals in the 1981 sample 

(Appendix 3): the mean age was significantly younger. In both samples, 

within- and between-reader precision declined with increasing specimen age. 

Also, the quality of tooth preparations was better on average for the 1981 

set, and no "unreadable" specimens were included. However, even for the 

younger animals, precision is higher in the 1981 set. 

Whatever the reasons for change between data sets, the important 

questions remaining relate to interpretation of the 1973-78 data. What is the 

best estimate of age for each specimen? What variance shall be assigned to 

each estimate? 

Given the lack of significant differences between age distributions from 

last readings and pooled means, of those two statistics, it would be 

preferable to use the latter as age estimates, because they allow direct 

estimation of variance (if mu1 tiple readings are regarded as replicates). 

However, because of the large contribution from between-reader differences, 

the overall CVs for the pooled means average near 16%. For the over 12 years 

component, the CVs are nearly 20% (as opposed to 8% and 13% from individual 

reader estimates). This may be too high for some potential uses of age 

data. Consequently, it may be advisable to conduct some subsequent analyses 

with each reader's mean ages separately and examine the sensitivity of results 

to the differences in input. 

The two statistics proposed by Chang (1982) to estimate precision gave 

very similar results here. The coefficient of variation is easier to 

interpret, being in units of percent of actual estimates, and therefore seems 

preferable. 
9 

LITERATURE CITED 

Beamish, R. J., and D. A. Fournier. 1981. A method of comparing the 

precision of a set of age determinations. Can. J. Fish. Aquat. Sci. 

38: 982-983. 

Chang, W. Y. B. 1982. A statistical method for evaluating the 

reproducibility of age determination. Can. J. Fish. Aquat. Sci. 39:1208- 

1210. 

Cochran, W. G. 1977. Sampling techniques. Third edition. J. Wiley and Sons, 

N. Y. 428 p. 

Hohn, A. A. and P. S. Hammond. 1983. Growth in the first year of the 

offshore spotted dolphin, Stenella attenuata, in the eastern tropical 

Pacific. Southwest Fisheries Center, Admin. Rep. No. LJ-83-08, 33 p. 

Myrick, A. C. Jr., E. W. Shallenberger, I. Kang and D. B. MacKay. (In press) 

Calibration of dental layers in seven captive Hawaiian spinner dolphins, 

Stenella longirostris, based on tetracycline labeling, Fish. Bull. (U.S.) 

Myrick, A. C. Jr. , A. A. Hohn, P. A. Sloan, M. Kimura and D. Stanley. 1983. 

Estimating age of spotted and spinner dolphins (Stenel 1 a attenuata and 

Perrin, W. F. and A. C. Myrick, Jr. 1980. Age determination of toothed 

whales and sirenians. Rep. int. Whal. Commn (Special Issue No. 31, 

229 p. 

Sokol, R. R. and F. J. Rohlf. 1969. Biometry. The principles and practice 

of statistics in biological research. W. H. Freeman and Co., San 

Francisco, 776 p. 

Stenel 1 a 1 ongi rostri s) from teeth , NOM-TM-NMFS-SWkC-30, 1/ P* 
10 

Table 1. Coefficients of variation (CV) for two readers, by age groups 

(31, with analysis of variance testing within-reader 

differences over age groups, and t-tests of between reader 

differences within age groups. 

dol phi ns, both sexes, ki 1 led during 1973-78. 

Data are from spotted 

Age Group 

(yrs 1 

(Reader 11 (Reader 2 1 

n mean SD 

cv 

mean SD t (CV 1=CV, ) P(t) 

cv 

n 

*A<4.0 215 8.26 15.56 59 4.39 9.35 2.571 0.009 

4.O<A<12.0 309 8.73 8.21 72 6.39 6.69 3.065 0.005 

12.O<A 724 13.26 10.27 163 8.03 7.32 8.855 <0.001 

a1 1 1248 11.28 11.18 294 6.90 7.74 9.091 <0.001 

Within Sum of Squares 149072.390 

Mean Square 119.737 

D.F. 1245 

Between Sum of Squares 6802.346 

Mean Square 3401.173 

D.F. 2 

16936.677 

58.202 

291 

599.506 

299.7 53 

2 

Equality of Means: 

F 28.405 5.150 

D.F. 2, 1245 2, 291 

P(F1 <0.001 0.006 

*A is the pooled mean age. 
11 

Table 2. Indices of precision (D=CV/Jn') for two readers, by age groups 

(3 1, wi th analysis of variance testing wi thi n-reader 

differences over age groups, and t-tests of between-reader 

differences within age groups. dolphins, both sexes, killed in 1973-78. 

Data are from spotted 

*A<4.0 - 215 5.23 10.40 59 2.85 6.53 2.327 0.020 

4.0 <A< 12.0 309 5.06 4.79 72 4.32 4.64 1.532 0.145 

12.O<A 724 7.64 6.07 163 5.27 4.82 6.482 <0.001 

- 

a1 1 1248 6.59 6.86 294 4.55 5.24 6.620 <0.001 

Within Sum of Squares 56834.582 7 697.605 

Mean Square 45.650 26.728 

D.F. 1245 2 88 

Between Sum of Squares 1925.677 257.050 

Mean Square 962.838 128.525 

D.F. 2 2 

Equality of Means: 

F 21.092 

D.F. 2, 1245 

P(F1 <0.001 

4.809 

2, 288 

0.009 

*A is the pooled mean age. 
12 

Table 3. Summary statistics for "agebest," by reader (2) age-class (3) 

and sex. The "agebest" is itself a mean of a series of 

readings per specimen by each reader. 

precision is reported in Tables 3 and 4. Data are from 

offshore spotted dolphins from 1973-78. 

The within reader 

Age Class (From Pooled Mean) 

Reader Male Female Male Female Male Female Male Female 

1 Mean 2.037 1.965 8.157 8.665 22.823 21.543 14.076 15.112 

SO 1.257 1.176 2.782 2.854 6.165 5.761 10.082 9.397 

n 173 147 217 163 385 464 775 774 

2 Mean 2.189 2.145 7.829 8.186 18.001 17.275 11.278 12.267 

SD 1.210 1.143 2.244 2.358 4.584 4.359 7.467 7.149 

n 175 146 223 163 351 428 749 737 
13 

Table 4. Coefficients and tests of significance from multiple regressions of last reading age estimates on 1) earlier age 

estimates and, 2) elapsed time between the estimates. For B1, 

t-tests of the hypothesis last estimate = previous estimate 

(B1=1) are shown. 

dol phi n tooth gl g readi ngs. 

Data are from 1973-78 offshore spotted 

All n=5867 24943 <0.001 1.032 0.00464 6.80 <O. 001 0.0090 0.0007 

Reader 1, Small 7090 <0.001 0.974 0.00819 3.13 <0.001 0.0002 0.0002 

n=682 

Reader 2, Small 29306 <0.001 1.002 0.00414 0.49 0.688 0.0006 0.0002 

n=409 

Reader 1, Mid 1722 <0.001 0.906 0.01547 6.05 <0.001 0.0007 0.0007 

n=961 

Reader 2, Mid 8815 <0.001 0.974 0.00735 3.52 <0.001 0.0046 0.0006 

n=470 

Reader 1, Large 1837 <0.001 0.876 0.01451 8.58 <O. 001 0.0089 0.0015 

n=2350 

Reader 2, Large 4774 <0.001 0.967 0.00992 3.35 <O. 001 0.0065 0.0014 

n=995 

ANOVA of Regression Coefficients Over Groups: F=38.573, P<O.OOl 
14 

Table 5. Multiple analysis of covariance of last reading age estimate 

by reader (2), sex (21, and age group (31, with earlier age 

estimates and the elapsed time between estimates as 

covari ates. 

Source of 

variation Regression 

ss DF MS F P (F) coefficients 

- Age Class 

- Reader 

3054.369 2 1527.185 149.30 

376.935 1 376.935 36.85 

- Sex 17.714 1 17.714 1.73 

AXR 1349.808 2 674.904 65.98 

AXS 0.579 2 0.289 0.03 

RXS 14.131 1 14.131 1.38 

AXRXS 0.005 2 0.003 0.00 

Previous Estimate 116,889.053 1 116,889.053 11,427.41 

(1st covariate) 

Elapsed Time 609.268 1 609.268 59.56 

(2nd covari ate) 

Both Covariates 117,125.201 2 58,562.601 5725.25 

~~ ~ 

<o. 001 

<o. 001 

0.188 

<0.001 

0.972 

0.239 

0.999 

<o. 001 0.888 

<0.001 0.006 

<o. 001 

Error 59,869.346 5853 10.229 
15 

Table 6. Frequencies of individual s estimated to be within 15 age 

classes by sex, reader and estimate type. Chi-squared tests 

of homogeneity are in Table 9. The full distributions from 

which these were summed are reported in Appendices 1-4. 

Variable: Mean Reading Last Reading 

Reader: 1 2 1 2 1 2 1 2 

Sex : M M F F M M F F 

Age Group 

---------- 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

>15 - 

SUM 

45 40 37 30 

46 44 42 39 

42 41 34 41 

41 59 35 42 

41 23 27 20 

25 42 14 20 

30 19 14 13 

20 33 17 17 

19 28 7 26 

20 34 22 29 

21 28 23 28 

25 29 22 38 

23 35 24 33 

16 25 23 51 

361 269 432 310 

775 749 773 737 

45 40 42 29 

46 44 41 40 

44 40 35 40 

43 60 34 43 

39 22 29 20 

25 43 13 19 

26 23 18 14 

20 30 10 17 

18 29 14 31 

23 33 17 25 

18 27 21 30 

25 30 28 33 

16 33 15 35 

21 26 20 49 

363 258 428 298 

772 738 765 723 
16 

Table 7. Summary of x2 statistics from contingency table tests of 

homogeneity of age distributions, across methods (21, readers 

(2) and sexes. Animals of 15 year or older were pooled. Each 

test had 14 degrees of freedom, fetal frequencies are in Table 

8. 

Data Set 

Females, Mean Readi ngs (Mi 

Males, M1 

Females, Last Reading (M2) 

Males, M2 

Femal es, Rdrl 

Males, Rdrl 

Femal es, Rdr2 

Males, Rdr2 

Rdrl vs Rdr2 

I1 I1 

I1 It 

It I1 

Mi vs M2 

I1 I1 

52.17 

42.74 

61.15 

45.08 

8.83 

2.83 

1.47 

0.87 

<0.001 

<o. 001 

<0.001 

<0.001 

0.842 

0.999 

>o. 999 

>o. 999 
17 

Table 8. Coefficients of variation (CV) for two readers, broken down by 

age groups (31, with analysis of variance testing withinreader differences over age groups, and t-tests of between 

reader differences within age groups. spotted dolphins killed in 1981. 

Data are from female 

Within Sum of Squares 648.536 

Mean Square 15.818 

D.F. 41 

Between Sum of Squares 232.247 

Mean Square 116.124 

D.F. 2 

Equality of Means: 

F 7.341 

D.F. 2, 41 

P(F) 0.0019 

1157.760 

41 

28.238 

205.183 

2 

102.592 

3.633 

2, 41 

0.0353 

*A is the pooled mean age. 
18 

Table 9. Indices of precision (D=CVIJfi) for two readers, by age groups 

(3 1, with analysis of variance testing wi thi n-reader 

differences over age groups, and t-tests of between-reader 

differences within age groups. 

dolphins killed in 1981. 

Data are from female spotted 

*A<4.0 - 12 0.781 1.847 12 1.781 2.459 1.138 0.133 

4.O<A<12.0 - 15 2.948 2.277 15 3.388 3.484 0.418 0.339 

12.0 A 17 4.088 2.576 17 4.889 3.057 0.829 0.206 

a1 1 44 2.798 2.613 44 3.529 3.250 1.169 0.123 

Within Sum of Squares 216.179 

Mean Square 5.273 

D.F. 41 

385.920 

9.413 

41 

Between Sum of Squares 77.416 68.394 

Mean Square 38.708 34.197 

D.F. 2 2 

Equality of Means: 

F 7.341 

D.F. 2, 41 

P(F) 0.002 

3.633 

2, 41 

0.035 

*A is the pooled mean age. 
19 

Table 10. Means and standard deviations of age estimates for the 1981 

female offshore spotted dolphin subsample. Age estimates are 

broken down by age group (G1=<4.0; 4.0<62<12.0; 12.0<63), 

reader (2) and cycle as a repe'ated measure within reader. 

ANALYSIS OF VARIANCE 

Cell Means and Standard Deviations 

Variable . . AGEBEST 

Factor Code Mean Std. Dev. N 95 Percent Conf. Interval 

AGE GROUP 

READER 

CYCLE 

CYCLE 

CYCLE 

READER 

CYCLE 

CYCLE 

CYCLE 

GROUP 

READER 

CYCLE 

CYCLE 

CYCLE 

READER 

CYCLE 

CYCLE 

CYCLE 

GROUP 

READER 

CYCLE 

CYCLE 

CYCLE 

READER 

CYCLE 

CYCLE 

CYCLE 

1 

1 

1 

2 

3 

2 

1 

2 

3 

2 

1 

1 

2 

3 

2 

1 

2 

3 

3 

1 

1 

2 

3 

2 

1 

2 

3 

2.30833 

2.30833 

2.35000 

1.24350 

1.25369 

1.26023 

12 

12 

12 

1.51825 

1.51178 

1.54929 

3.09841 

3.10489 

3.15071 

2.35833 

2.44167 

2.44167 

1.29647 

1.26954 

1.29857 

12 

12 

12 

1.53460 

1.63504 

1.61659 

3.18207 

3.24830 

3.26674 

7.83333 

7.63333 

7.83333 

2.96808 

2.78046 

3.05700 

15 

15 

15 

6.18966 

6.09357 

6.14042 

9.47700 

9.17310 

9.52624 

7.90000 

7.44000 

7.54667 

2.68568 

2.27056 

2.76583 

15 

15 

15 

6.41272 

6.18261 

6.01500 

9.38728 

8.69739 

9.07833 

17.45911 

17.32952 

18.02665 

19.70588 

19.82353 

20.73529 

4.36985 

4.85071 

5.26818 

17 

17 

17 

21.95265 

22.31754 

23.44394 

20.11765 

19.23529 

19.05882 

5.67761 

4.29432 

4.42254 

17 

17 

17 

17.19849 

17.02736 

16.78497 

23.03681 

21.44323 

21.33268 

FOR ENTIRE SAMPLE 10.91212 8.09623 264 9.93098 11.89326 
20 

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1 

1 1 

?? 

11711 1 1 

1 1 21>1 111 ? 

1 2 1!17111 ,> 72 1 1 

5 1 1 11 5246?1? 4 11 

1 1 14 17?475417???11 

1 111 ? 777?3~?27517531?1?1 11 1 1 

11 1 21241272 21131 

>514> 1572711l171 11 11?1 1 

1 5 14 517???2265416U 111??1 1 

?U71 7 16? 177 ?4?15?11?17 l? 2 1 

1 1751 ? 32425q12354271 ?21 17 1 

' 63 7 74?111711 17 1 114 111 1 

7711??21 3 114 1 ll> 2 2 UllO~I?111 2 1 1 11 1 1 

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76. 1 

1' 1 

11 1 

W'511.1 ? 15?U!?!? !???' ?2 

1')hh 7 l? 1 1 

1' 1 

1 '11 1 

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11 

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1 1 

11 2 11 1 

322112 

1 

12 11 

1 121111 1 1 1 

1 1 111 7 2621 12121111 

1 121 11 31 1 2 1 3 1 11 

1 1 1 43 23411524111 11 ' 22 1 1312 1313233133123221 11 2 

1 212 111192 34134254 4 431 12 1 11 

l? 21 35 13333 4331 4 124 1341112 1 1 1 

13 23 222222 4511341 21 121 1 

12182 53223321214331 2 2 1 l? 224 

71825?11?322 2313 1111 13 1 2 

111 1 11 1111131 1 

4864546 1221 2 1 11 1 1 

'?5632 211112 111 11 12 1 

1741215271 1 1 1 

412 31 1 

21 1 

l? 1 1973-1978 

MALE MEAN AGES -0.50 

Figure 2. Plots of signed difference between individual readers' mean 

estimates as a percent of the mean pooled age. The data are from 

female off shore spotted dol phi ns, 1973-78 sampl e: Females in the 

upper panel, males in the lower panel. 
23 

1 

u, 

0.75 1 

0.50 1 

o.oo[ 

1 

1 1 

11 7 

1 11 1 

21 1 1 

1 211 1 11 

11 11 1111 11 

213 1 3 1 21 17 231 

1 1 1 111 7 14 4 

1 

1 1 24 4454 1121 111 1 11 

1 1 1 1 21 24265 323 4 1 11 

1 15 21124 45 45 1 1 11 11 

23 2 123312 22323 21 1 0'25' 1 '1 111 11 3112476541422114 131 

1 79 2 3645131 6714114 7 1 1 1 

24 2 14121 2922335 1 2 111 1 1 

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-1354' 52222 32 1 

937 , I,, r?p, I, I) , , 

43522241 2 1 1 11 2 11 11 

132231 4 1 1 11 1C5D2 431 12 12 

1121 2 1 112 

1111 1 11 

136 1 1 1 

11 

1 1973-1978 

FEMALE LAST RE, 

a 

n 

5i 

w 

a -0.75 

DINGS 

n 

w 

a 

z 1 

1 

1 21 

111 21 112 

11111111 1 1 

1 1 1 1 131 1 11 1 

1 1 2 1 1111 21 1 

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1 1 2 1 21 711 11 213 

1 1 12 11 32 21121121 

2 0.75 

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1 

11 1 1 5 1151424 53 2 111 

21 2 135 111644143313 21 1 2 1 

1 2?2 1?1343 461 7 3121 1111132 2 1 1 

1 14 71 212134 3212111 311 241 1 

12 21 11 4536 121 31 231 11 121 

1 

766'1 113 l? 11 1 11 

7K1 151 7 1 

51 '? 1 

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1973-1978 

-0.50 MALE LAST READINGS 

0 4 8 12 16 20 24 28 32 36 40 

POOLED MEAN AGES 

Figure 3. Plots of signed difference between individual readers' estimates 

from last readings as a percent of the pooled mean age. 

are from offshore spotted dolphins, 1973-78 sample: Females in 

the upper panel, males in the lower panel. 

The data 
24 

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3- L) 
RECENT TECHNICAL MEMORANDUMS 

Copies of thisandother NOAATechnical Memorandums are available from the NationalTechnical 

Information Service, 5285 Port Royal Road, Springfield, VA 221 67 Paper copies vary in price Microfishe copies cost $3 50 Recent issues of NOAA Technical Memorandums from the NMFS Southwest 

Fisheries Center are listed below 

NOAA TM-NMFS SWFC 25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

A preliminary study of dolphin release procedures using model 

D. 6. HOLTS and J. M. COE 

(December 1982) 

"Possible effects of sampling biases on reproduction rate estimates 

for porpoise in the eastern tropical Pacific." 

T. POLACHECK 

(January 1983) 

"Reports of porpoise experiment testing detection of on-track 

R. S. HOLT 

(February 1983) 

"Two computer programs to project populations with time-varying 

T. GERRODETIE, D. GOODMAN & J. BARLOW 

(February 1983) 

Report of eastern tropical Pacific research vessel marine mammal 

R. S. HOLT 

(March 1983) 

Estimating age of spotted and spinner dolphins(Stene1la attenuata 

A. C. MYRICK, JR., A. A. HOHN, P. A. SLOAN, M. KIMURA 

(April 1983) 

Re-estimation of three parameters associated with anchovy 

egg and larval abundance: Temperature dependent incubation 

time, yolk-sac growth rate and egg and larval retention in 

mesh nets. 

N. C. H. LO 

(May 1983) 

"NMFS guidelines on economic valuation of marine recreational 

D. D. HUPPERT 

(June 1983) 

"Summary of environmental and fishing information on Guam and 

the Northern Mariana Islands: A review of the plankton 

communities and fishery resources of Guam and the Comrnonwealth of the Northern Mariana Islands" 

purse seines. 

schools (pet dots), March 7-April 5, 1981 ." 

vital rates." 

survey, May 15-August 3, 1982. 

and Stenella longirostris) from teeth. 

and D. D. STANLEY 

fishing" 

R. N. UCHIDA 

(July 1983) 

"Some data on dolphin mortality in the eastern tropical Pacific 

T. D. SMITH and N. C. H. LO 

(July 1983) 

tuna purse seine fishery prior to 1970."