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This article was downloaded by: [Pennsylvania State University] On: 15 March 2013, At: 05:15 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Transactions of the American Fisheries Society Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/utaf20 Otolith Age Validation and Growth Estimation from Oxytetracycline-Marked and Recaptured American Shad William J. Duffy a b , Richard S. McBride a , Michael L. Hendricks c & Kenneth Oliveira b a National Oceanic and Atmospheric Administration, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts, 02543, USA b Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport Road North Dartmouth, Massachusetts, 02747, USA c Pennsylvania Fish and Boat Commission, Benner Spring Fish Research Station, 1735 Shiloh Road, State College, Pennsylvania, 16801, USA Version of record first published: 31 Oct 2012. To cite this article: William J. Duffy , Richard S. McBride , Michael L. Hendricks & Kenneth Oliveira (2012): Otolith Age Validation and Growth Estimation from Oxytetracycline-Marked and Recaptured American Shad, Transactions of the American Fisheries Society, 141:6, 1664-1671 To link to this article: http://dx.doi.org/10.1080/00028487.2012.720631 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Otolith Age Validation and Growth Estimation from Oxytetracycline-Marked and Recaptured American Shad

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This article was downloaded by: [Pennsylvania State University]On: 15 March 2013, At: 05:15Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Transactions of the American Fisheries SocietyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/utaf20

Otolith Age Validation and Growth Estimation fromOxytetracycline-Marked and Recaptured American ShadWilliam J. Duffy a b , Richard S. McBride a , Michael L. Hendricks c & Kenneth Oliveira ba National Oceanic and Atmospheric Administration, Northeast Fisheries Science Center, 166Water Street, Woods Hole, Massachusetts, 02543, USAb Department of Biology, University of Massachusetts Dartmouth, 285 Old Westport RoadNorth Dartmouth, Massachusetts, 02747, USAc Pennsylvania Fish and Boat Commission, Benner Spring Fish Research Station, 1735 ShilohRoad, State College, Pennsylvania, 16801, USAVersion of record first published: 31 Oct 2012.

To cite this article: William J. Duffy , Richard S. McBride , Michael L. Hendricks & Kenneth Oliveira (2012): Otolith AgeValidation and Growth Estimation from Oxytetracycline-Marked and Recaptured American Shad, Transactions of the AmericanFisheries Society, 141:6, 1664-1671

To link to this article: http://dx.doi.org/10.1080/00028487.2012.720631

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.

Transactions of the American Fisheries Society 141:1664–1671, 2012C© American Fisheries Society 2012ISSN: 0002-8487 print / 1548-8659 onlineDOI: 10.1080/00028487.2012.720631

ARTICLE

Otolith Age Validation and Growth Estimation fromOxytetracycline-Marked and Recaptured American Shad

William J. Duffy*National Oceanic and Atmospheric Administration, Northeast Fisheries Science Center,166 Water Street, Woods Hole, Massachusetts 02543, USA; and Department of Biology,University of Massachusetts Dartmouth, 285 Old Westport Road North Dartmouth,Massachusetts 02747, USA

Richard S. McBrideNational Oceanic and Atmospheric Administration, Northeast Fisheries Science Center,166 Water Street, Woods Hole, Massachusetts 02543, USA

Michael L. HendricksPennsylvania Fish and Boat Commission, Benner Spring Fish Research Station, 1735 Shiloh Road,State College, Pennsylvania 16801, USA

Kenneth OliveiraDepartment of Biology, University of Massachusetts Dartmouth,285 Old Westport Road North Dartmouth, Massachusetts 02747, USA

AbstractThis study validated a whole-otolith aging method using known-age American shad Alosa sapidissima from the

Delaware River system. Although scale ages are commonly used in autecological and assessment studies of Americanshad, scale ages from the same fish could not be validated. New data reported here used known-aged otoliths andscales available from shad marked by oxytetracycline as larvae in a hatchery and recaptured as adults on or near theirspawning ground. A subset of whole otoliths were examined and annuli—defined as a pair of translucent and opaquebands—were counted using males and females ranging in age from 3 to 9 years. The reading and interpretationof annuli by the more experienced reader were accurate with respect to the known age, whether measured as thepercent agreement (PA = 80%) or Chang’s coefficient of variation (CV = 3.11). The use of otoliths provided moreaccurate results than scales obtained from the same fish (PA = 46%; CV = 7.84). A second reader, who had noprevious experience with this species, had lower performance scores but also performed better with otoliths; thisdemonstrated the need for training and testing when using the whole-otolith aging method. Growth modeling usingages of known-age juveniles and adults confirmed dimorphic growth. Females grew larger than males (von BertalanffyL∞ = 552 and 495-mm FL, respectively). The maximum age observed for females was only slightly older than males(9 versus 8 years). The superiority of the otolith-based age method makes it difficult to compare our results witholder, scale-based demographic studies, but it represents an improved method for generating ages for future stockassessments.

American shad Alosa sapidissima are an economically andculturally valuable herring (Clupeidae) native to the east coast ofNorth America (Munroe 2002). Dramatic declines have been

*Corresponding author: [email protected] February 15, 2012; accepted July 31, 2012Published online October 31, 2012

documented for nearly two centuries, and age-based assess-ments have been used to evaluate certain stocks that range fromCanada to Florida (Limburg et al. 2003; ASMFC 2007). Scale

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OTOLITH AGE VALIDATION AND GROWTH ESTIMATION 1665

aging is the only validated method (Cating 1953; Judy 1960),although this stands in contrast to many other species for whichotolith aging methods replaced scale aging methods decadesago (Summerfelt and Hall 1987). Scales of American shad are,in fact, difficult to read; their accuracy has been challenged formore than a century (LaPointe 1958; ASMFC 2007). Early scaleaging methods were repeatedly discredited (Borodin 1924; Leim1925) until Cating (1953) developed a scale aging method thatwas validated by Judy (1960) in the Connecticut River. Nonethe-less, a recent blind study using scales of known-age fish froma Pennsylvania river system failed to accurately age Americanshad, once again calling into question broad-scale use of Cat-ing’s scale-aging method (McBride et al. 2005). Most recently,Duffy et al. (2011) concluded that the application of Cating’smethod to age American shad scales is inappropriate across itsentire native range.

The choice of aging method is not merely a matter of conve-nience or tradition since using inaccurate age and growth datacan have negative effects on biomass estimates and manage-ment efforts (Beamish and McFarlane 1983; 1987). One of themost cited case studies on aging error occurred on the westerncoast of Canada with the sablefish Anoplopoma fimbria. Usingan unvalidated scale aging method, the commercial fishery wasthought to comprise age-3 to age-8 fish; once a validated otolithmethod was developed, it was found that the commercial fisheryconsisted of fish ages 4 to 40 (Beamish and McFarlane 1987).Such aging errors lead to inaccurate estimates of mortality thatresult in annual catch recommendations too high for sustainabil-ity or too low for industry efficiency (Beamish and McFarlane1983; 1987).

Validation studies have increased over the last two decadessince Beamish and McFarlane’s (1983) classic “The ForgottenRequirement for Age Validation in Fisheries Biology” was pub-lished. Since then, more emphasis has been put on validatingaging methods. There are many types of validation studies, butvery few of them validate all age-classes of a fish species, whichis the primary goal of any aging study (Beamish and McFarlane1983; Campana 2001). Most studies use marginal incrementanalysis, but for only one or a few of the youngest age-classes;however, validating the formation of one scale annulus per yearin this manner is usually insufficient for even fish with mediumlongevity because scale annuli are difficult to discern after a fewyears of growth (Beamish and McFarlane 1987). Other studiesuse tagging and recapturing of wild-caught fish to validate theformation of the annulus. This is accomplished by either usingan external tag or internally injecting the fish with a chem-ical marker (Beamish and McFarlane 1983; Campana 2001;DeCicco and Brown 2006). This method can confirm the fre-quency of annulus deposition, but it does not validate the abso-lute age of the fish because the age of the tagged fish is rarelyknown (Beamish and McFarlane 1983; Campana 2001).

In this study, we used otoliths of known-aged American shadcollected from the Delaware Bay ecosystem as an alternate agingstructure to scales. Aging American shad using otoliths has been

attempted, but the method has not been validated (Limburg2001; Aschenbach et al. 2006). We validate an otolith-basedaging method across all typical ages found on the spawninggrounds and simultaneously compare the results with a scale-based aging method. Finally, given the uncertainty of the agedata obtained from scales used in the previous stock assessmentfor Delaware River American shad (ASMFC 2007), we fit sizeand age of known-aged fish to the von Bertalanffy growth modelto update growth parameter estimates.

METHODSSample collection.—We used a collection of known-

age American shad females and males available from thePennsylvania Fish and Boat Commission (PFBC). The PFBChas cultured and marked shad as larvae using oxytetracycline(OTC) since the mid-1980s (Hendricks et al. 1991). More than1,100 marked, known-age juveniles and adults were recapturedand sampled between 1995 and 2007 from the Delaware Riverecosystem, primarily from two of its tributaries: the LehighRiver and Schuylkill River.

Hatchery-raised shad larvae were repeatedly immersed inOTC to produce multiple marks on their otoliths (Hendrickset al. 1991). Most annual cohorts were given a unique seriesof OTC marks to identify the river of origin and cohort. Thesemarked fish were released as larvae into the Delaware Riverecosystem, where they spend a few months in freshwater beforemigrating out to sea as part of their anadromous life history.When juveniles (age 0) or adults were recaptured, fish size wasmeasured as fork length to the nearest millimeter. Also, foradults, 10–20 scales were removed from the area just under thedorsal fin, and both sagittal otoliths were excised from the head.

Aging.—One otolith was used to determine if the shad waswild or hatchery raised based on OTC markings, and the otherwas used to examine for annuli (Hendricks et al. 1991). Mostotoliths had been encased in epoxy resin, and the majority ofthese were difficult to read because of discoloration and crack-ing of the epoxy. A few dozen otoliths were stored in vialsand submerged in mineral oil, and were very suitable for aging.All otoliths stored in vials were used for the validation (if notbroken). Only some of those encased in epoxy could be used,specifically when the epoxy did not restrict visibility or distortthe appearance of the otolith (Table 1). Otoliths were not sec-tioned, sanded, or polished, so we refer to this as a whole-otolithaging method.

Matching scale samples were prepared when the otolith forthat fish was used, unless the scales were missing or regener-ated. Scales were prepared and scale annuli were recognizedfollowing the methods outlined by Duffy et al. (2011).

Shad otoliths were viewed under a dissecting microscopebetween 15× and 20× using reflected light. The otoliths wereaged by counting alternating translucent and opaque bandsfrom the otolith core to the margin of the postrostrum. Winter(translucent) bands appeared dark, and summer (opaque) bands

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1666 DUFFY ET AL.

TABLE 1. Sample size, by age, for both the validation and the growth mod-eling aspects of this study. Otoliths and scales were from known-age fish andwere aged by both readers. The numbers in parentheses are otoliths encased inepoxy. The ages used in the growth model were calculated by time differencefrom hatching to recapture.

Age validation

Otoliths Scales Growth modelAge N N N

0 0 0 6301 0 0 02 0 0 03 2 (2) 2 184 11 (10) 8 1105 35 (9) 25 2006 29 (12) 24 1347 9 (6) 6 228 1 (1) 1 89 1 (0) 1 1Total 88 (40) 67 1,123

appeared white (Penttila et al. 1988). Although we assumed thatthe band pairs were formed annually and we tested this as partof this study, the appearance of these band pairs changed fromthe core to the edge of the otolith. The first annulus boundeda broad translucent band extending from an opaque band justafter the nucleus of the otolith, which was dark under reflectedlight (Figure 1a). Between the first and second annulus, a diffusetranslucent band was often evident, but this did not represent afull year of growth (i.e., a false annulus; Figure 1b). Summerfeltand Hall (1987) define a false annulus as an abrupt discontinuityin a band with irregular spacing. The distance between the firstand second annulus also tended to be greater than the distancesbetween subsequent annuli. After the third year, the spacing be-tween translucent zones was fairly consistent in distance and theannuli were more coherent (Figure 1b). The edge of the otolithwas considered the final annulus because shad were caught ontheir spawning grounds.

Age validation.—Two readers conducted blind studies of ac-curacy and precision using the otoliths and scales from the samefish. As in Summerfelt and Hall (1987), we define accuracy asthe proximity of an age estimate to the actual value, and preci-sion as the measure of consistency in age estimates when count-ing annuli more than once. The simplest performance measurewas to calculate percent agreement (PA) as the percentage ofages that agreed with either the known-age fish (i.e., accuracy)or within or between readers (i.e., precision), expressed as

PA = 100 · A

N,

where A is the number of correct replicate ages (i.e., to the exactyear), and N is the total number of fish aged. A PA greater than80% was considered acceptable (Campana 2001).

Chang’s coefficient of variation (CV; Chang 1982) was alsoused as a second performance measure of precision and accu-racy. Chang’s CV first calculates the standard deviation of multi-ple age readings from a single fish, divides this by the mean ageof that fish, and, in the form we use, sums these values amongall fish aged as

CV = 100 · 1

N

N∑j=1

√∑Ri=1

(Xij −Xj )2

R−1

Xj

,

where N is the total number of fish aged, R is the number oftimes each fish is aged, Xij is the ith age determination of thejth fish, and Xj is the mean age estimate of the jth fish (Campanaet al. 1995). A CV < 5 was considered acceptable (Campana2001).

When PA was below 80% or Chang’s CV was greater than5, Bowker’s (1948) test of symmetry was used to determine ifdeviations from the diagonal on an age frequency table (firstread versus second read) show systematic differences in pairedages (i.e., between scale and otoliths, by reader, by multiplereads):

X2 =m−1∑i=1

m∑j=i+1

(nij − nji)2

nij + nji

,

where nij is the observed frequency in the ith row and the jthcolumn, and nji is the expected frequency in the jth row andthe ith column (Hoenig et al. 1995). If p > 0.05, based on thechi-square distribution, we concluded there was no bias in thepaired ages.

In addition, age bias plots were used to illustrate the precisionand accuracy between methods, readers, and multiple reads ofthe same reader (Campana et al. 1995). We also tested the biasbetween scale age and otolith age from the same fish using aregression analysis. Specifically, to accept that the scales andotoliths from the same fish produce the same age we tested anull hypothesis that slope (β) = 1.

Growth modeling.—Analyses of growth included allmarked–recaptured juveniles and adults (Table 1), where agewas assigned as the time difference between hatching and re-capture. Size-at-known-age data were fit to the three-parametervon Bertalanffy growth model:

Lt = L∞�1 − e−k(t−t0)�,

where Lt is the predicted length at age, L∞ is the length atwhich the American shad reaches a mean asymptotic size, K isthe Brody growth coefficient which describes the growth ratetowards the maximum size, and t0 is the estimated age when thefish length equals zero. Because the sexes were not determinedfor the juveniles, when fitting data to the model we randomlyassigned half of them to be males and the other half to be female,

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OTOLITH AGE VALIDATION AND GROWTH ESTIMATION 1667

FIGURE 1. Otoliths from known-age American shad from the Delaware River: (a) otolith from a 525-mm, 5-year-old female viewed under mineral oil;(b) otolith from a 509-mm, 8-year-old female viewed mounted in epoxy; (c) otolith from a 594-mm, 7-year-old female viewed mounted in epoxy resin; (d) otolithfrom a 508-mm, 5-year-old female viewed mounted in epoxy resin. ∗ = annulus and � = a false annulus.

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1668 DUFFY ET AL.

Reader 2 Otolith Age (years)

0 1 2 3 4 5 6 7 8 9 10 11

Rea

der

2 S

cale

Age

(ye

ars)

0

1

2

3

4

5

6

7

8

9

10

11

R2 = 0.15

B

(2)

(12)

(14)

(4)

A

Reader 1 Otolith Age (years)

0 1 2 3 4 5 6 7 8 9 10

Rea

der

1 S

cale

Age

(ye

ars)

0

1

2

3

4

5

6

7

8

9

10

(1)

(5)

(3)

(3) (1)

(1)

(1)(3)(2)

(7)(2)

(2)

(1) (3)

R2 = 0.26

(4)

(14)

(12)

(2)

(1)

(2) (1)

(3)(5)(2)

(3) (6) (6) (2) (1)

(1)(8)(8)(3)

(3)(7)(2)

(1)(1)(1)

A

FIGURE 2. Age bias plots showing both readers otolith versus scale age forthe same fish (N = 67). (A) Reader 1 otolith age versus scale age, and (B)reader 2 otolith age versus scale age. The number in parentheses represents thenumber of observations. The 45◦ line represents 100% agreement. The boldsolid line represents the best linear fit (i.e., Y = a + b·X) between the scale andotolith age.

assuming a 50:50 sex ratio. The juvenile ages were recorded indays and then converted into decimal years. As the modelingresult demonstrate, juvenile lengths at age were important forobtaining a biologically realistic estimate of t0. A maximumlikelihood ratio was performed to determine if using separatemodels for each sex was appropriate (Haddon 2001).

RESULTSAging scales and otoliths from the same fish produced differ-

ent results. A t-test of the regression slope showed a significantdifference between the interpreted otolith and scale ages (β �= 1,p < 0.05; Figure 2). Accuracy and precision measures for eachaging structure confirmed that otoliths were a more suitablestructure for aging American shad than scales.

We explore this first with reader 1, the more experienced ager.Otoliths were aged accurately (PA = 80%, CV = 3.11, N = 88;

B

Scale Known Age (years)

2 3 4 5 6 7 8 9 10

Rea

der

1 (y

ears

)

2

3

4

5

6

7

8

9

10

B

CV = 7.84PA = 46%

Otolith Known Age (years)

2 3 4 5 6 7 8 9 10

Rea

der

1 (y

ears

)

2

3

4

5

6

7

8

9

10

A

PA = 80%

CV = 3.11

FIGURE 3. Age bias plots showing reader 1’s accuracy results. (A) Accuracytest of otolith age versus known age, and (B) accuracy test of scale age versusknown age. The 45◦ line represents 100% agreement. Error bars represent 95%confidence limits.

Figure 3A) and with good precision (PA = 78%, CV = 3.11,N = 88). Technically, the PA for precision (first versus sec-ond read by reader 1) was just below the accepted measure of80%, but Bowker’s test (p = 0.54) confirmed that there wereno systematic differences between repeated reads. When scalesfrom the same fish were aged, the percent agreement and CVscores were much lower and did not pass the accepted thresholds(PA = 46%, CV = 7.84, N = 67; Figure 3B). When the scaleswere read a second time, the PA and CV were still below the ac-ceptable measures (PA = 62%; CV = 5.79; N = 67). A Bowker’stest (p = 0.48) showed no systematic biases each time the scalewas aged.

Reader 2’s accuracy scores with otoliths did not pass theacceptable threshold for PA and CV (PA = 65%, CV = 6.11,N = 88; Figure 4A), but they were much better than for scales(PA = 40%, CV = 9.53, N = 67; Figure 4B). Reader 2’s

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OTOLITH AGE VALIDATION AND GROWTH ESTIMATION 1669

B

A

Scale Known Age (years)

2 3 4 5 6 7 8 9 10

Rea

der

2 (y

ears

)

2

3

4

5

6

7

8

9

10

B

PA = 40%

Otolith Known Age (years)

2 3 4 5 6 7 8 9 10

Rea

der

2 (Y

ears

)

2

3

4

5

6

7

8

9

10

PA = 65%

A

CV = 6.11

CV = 9.53

FIGURE 4. Age bias plots showing reader 2’s accuracy results. (A) Accuracytest of otolith age versus known age, and (B) accuracy test of scale age versusknown age. The 45◦ line represents 100% agreement. Error bars represent 95%confidence limits.

ages also had improved precision scores with otoliths (PA 63%;CV = 5.8; N = 88) than with scales (PA = 61%; CV = 6.29;N = 67).

Finally, using the same otolith ages, the precision betweenreaders was calculated (PA = 69%; CV = 4.70; N = 88). Whilethe PA was below the acceptable measure the CV was not, anda Bowker’s test of symmetry (p = 0.39) showed no significantbias between the two readers.

Growth parameters were poorly estimated by the von Berta-lanffy growth model using adult data only (Table 2, group A).Adding the juvenile data made the parameter estimates biolog-ically realistic and improved their precision (Table 2, group B).The full von Bertalanffy growth model, with sex-specific pa-rameters, produced the best fit (maximum likelihood ratio: p <

0.001; Table 2, group C; Figure 5). In the full model, femalesgrew to a larger maximum size.

TABLE 2. von Bertalanffy parameters for Delaware River American shad.Group A = sexes are combined and no juvenile data are included (N = 493),group B = sexes are combined and juvenile data are included (N = 1,123), andgroup C = sexes are separate and juvenile data are included (N = 1,123). SeeMethods for explanation of the growth model.

95% confidencelimits

Group Parameter Estimate SE Lower Upper

A L∞ 815 245.8 332 1,298K 0.09 0.07 –0.03 0.23t0 –3.26 1.85 –6.90 0.38

B L∞ 563 8.0 547 579K 0.33 0.01 0.30 0.35t0 –0.14 0.01 –0.16 –0.12

C L∞ 553 7.8 537 568Females K 0.41 0.02 0.37 0.46

t0 –0.07 0.01 –0.10 –0.04L∞ 496 6.6 482 508

Males K 0.44 0.02 0.40 0.48t0 –0.09 0.01 –0.12 –0.06

DISCUSSIONThe whole-otolith aging method was validated successfully

for American shad from the Delaware River system, but the scaleaging method was not. Although we did not find a strong bias inscale ages, aging shad using scales was much less precise and

Age (years)

0 1 2 3 4 5 6 7 8 9 10

Leng

th (

mm

)

0

100

200

300

400

500

600

700

Predicted Female Predicted MaleObserved FemaleObserved Male Observed Juvenile (unsexed)

FIGURE 5. Observed size at age for individual juvenile and adult Americanshad. Predicted lines are fitted to the von Bertalanffy growth equation (see Table2 for parameter estimates). Observed male and female sizes at age are plottedon either side of the whole-age value for graphical clarity.

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1670 DUFFY ET AL.

less accurate than aging using otoliths. We conclude that whenotoliths can be collected from American shad, whole otolithaging is more reliable than scale aging.

The samples from this study were from known-age, mark–recaptured fish, which allowed us to pursue a direct validationof two aging methods. Because American shad spend their firstfew years at sea, not all age-classes could be obtained from thespawning grounds, but this did not confound our results. Themost common age-classes (3 through 9) observed in this riversystem were included. Age-1 and age-2 fish are unavailablesimply because they are not spawning in the river. Evidently, acouple of 3-year-old males matured early, but the majority ofmales start spawning between ages 4 and 5 (ASMFC 2007).There were only a few 4-year-old fish because most of themwere encased in epoxy, so they were unsuitable for aging. Themajority of the fish in the study were age 5 and 6, which istypical of the Delaware River (Chittenden 1975). Few fish olderthan age 6 were available, but this is also consistent with historicdata (Chittenden 1975).

Interreader differences in accuracy and precision were mostlikely due to differing experience. Reader 1 had prior experienceaging American shad scales and was familiar with their otoliths.Reader 2 had extensive experience with both scales and otolithsbut had never aged American shad. Although the whole-otolithaging method is relatively easy to learn, training and testing isrequired to perform successfully.

The results were also affected by the quality of the otoliths,many of which were encased in epoxy and therefore of marginalquality for such a test. In this case, we used the available materialto its best advantage, but we do not recommend encasing inepoxy as was done here. As is typically done, otoliths shouldbe stored in vials, dry or in mineral oil, which allows the readerto manipulate the otolith for viewing. The value of sectioningotoliths to improve annulus recognition was not investigated inthis study. Although future studies may find that sectioning mayimprove precision further, such an extra effort is not necessaryfor routine aging of large numbers of samples (i.e., productionaging) that would be typical for stock assessments.

Consistent results were also affected by the axis chosen toread annuli. When aging American shad, the clearest axis wasfrom the base of the first annulus to the dorsal edge of thepararostrum (Figure 1c). A second plane used to age was theaxis from the base of the first annulus to the ventral side of thepostrostrum (Figure 1d). Age readers can internally check theirresults by using both axes. For many clupeids the rostrum isused as an axis for aging (L. M. Dery-Wells, National MarineFisheries Service, personal communication); for American shad,however, growth anomalies can be overly pronounced on therostrum, making it difficult to assess annular zones (Figure 1b).

Otolith aging produces more reliable results but does notappear to radically alter our general perceptions of Americanshad life history in the Delaware River system. We simply con-firm that this population exhibits sexual dimorphism, wherefemales grow to a larger maximum size. Males mature earlier

than females and return to their spawning grounds at a youngerage. Both males and females tend to grow rapidly during theirfirst few years, until they reach maturity. Given that each riveris a separate spawning stock, our life history characterizationapplies, in the strictest sense, only to the Delaware River popu-lation. Leggett and Carscadden (1978) reported a positive trendin American shad size and age with respect to latitude, and ap-plication of the whole-otolith aging method is warranted acrossa latitudinal range to confirm if these or other age-based traitsstill hold up to previous reports. A new, more reliable agingmethod should also be of use to address the causes and poten-tial remedies for declines in stock size and extensive changesin demographics that have been noted for many American shadpopulations over the last few decades (e.g., Tuckey and Olney2010; Latour et al. 2012).

Accepting the whole-otolith aging method is likely to havean impact on options for and quality of future stock assessments.Previous assessments conducted for American shad elected notto use age data obtained from many managed river systems,including the Delaware River system, because of the uncertaintyand unreliability of aging American shad using scales (ASMFC2007). In the most recent assessment, concern was expressedthat not enough research was conducted on otoliths at the timeto use this method as an alternative (ASMFC 2007). Broaderuse of the otolith method should now lead to more accurateestimates of abundance and mortality for American shad thanwas possible with scale-based ages.

Aging American shad using otoliths provides an acceptableand preferable alternative to using scales, but there are still ad-vantages and disadvantages for both methods. Spawning markscan be recognized in scales, leading to important data on thenumber of repeat spawners in a population. Optical patterns inotoliths do not provide spawning histories, although explorationof microchemistry signals may be useful in this regard in thefuture. Collecting the otolith is lethal, which may be unaccept-able in stocks at very low abundance. Sampling mortality aside,scales are simply much easier to collect in the field and pro-cess in the laboratory. However, even with the technologicalaid of digital imaging, American shad scales are very difficultto read: many can be regenerated, many more can be of verypoor optical quality, and significant erosion of the scale margincan occur in fish experiencing long spawning runs (McBrideet al. 2005; Duffy et al. 2011). Cating’s (1953) method was de-veloped to address these issues of poor quality, particularly inthe central portion of the scale, but Duffy et al. (2011) showedthat the key assumption of the Cating method—that the num-ber of transverse grooves are a function of age—was not trueacross the latitudinal range of American shad. When precisionand accuracy are the priority, the whole-otolith aging method isrecommended.

In a strict sense, our conclusions only apply to the DelawareRiver system, just as Cating’s validation of a scale aging methodonly applies to the Connecticut River system. The requirementsfor a direct validation, such as done here, are difficult to achieve,

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OTOLITH AGE VALIDATION AND GROWTH ESTIMATION 1671

and unfortunately we are not aware of any other hatchery thatis proactively releasing American shad that can be assigned tospecific year-classes. The anadromous life history of Americanshad also makes them difficult to use for other types of agevalidation (McBride et al. 2005). Advancing technology, partic-ularly otolith microchemistry, may be useful in this regard buthas yet to be applied. Even if such approaches can be conceivedor funded, they are not likely to be applied to more than a fewriver systems, but they do not need to be. Strategically, if it canbe demonstrated that an aging method works across the majorbiogeographic zones of the North American east coast (e.g.,Acadian, Virginian, Carolinian), then that should be sufficient.In that regard, we recommend researchers consider how to ex-pand such validation tests of American shad scales and otolithsto a river system south of Cape Hatteras, another in the Gulf ofMaine, and a third in the Canadian provinces.

ACKNOWLEDGMENTSWe thank those at the PFBC for their assistance in the field,

hatchery, and laboratory. We would also like to thank S. Cadrin,K. Limburg, J. Burnett, E. Robillard, and L. M. Dery-Wells fortheir assistance. Funding was provided by the National Oceanicand Atmospheric Administration’s Advanced Study Program asa part of a Master’s thesis from the University of Massachusetts–Dartmouth. Reference to trade names does not imply endorse-ment by the U.S. Government.

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