Comparative Biochemistry and Physiology Part A 120 (1998) 649–653

Orange roughy otolith growth rates: a direct experimental test of theRomanek–Gauldie otolith growth model

R.W. Gauldie a,*, C.R. Romanek b

a 76 Inglis Street, Seatoun Wellington, New Zealandb Sa6annah Ri6er Ecology Laboratory, Uni6ersity of Georgia, Aiken, SC, USA

Received 9 February 1998; received in revised form 11 May 1998; accepted 1 June 1998

Abstract

Endolymph chemistry (Ca2+, pH, Na+) was measured in 17 orange roughy (Hoplostethus atlanticus ; Teleostei; Trachichthyi-dae). Theoretical daily microincrement widths based on the Romanek and Gauldie model of otolith growth were calculated fromthe measure of otolith chemistry. Theoretical daily microincrement widths were well-correlated (r2=59%) with the width of thelast daily microincrement growing at the posterior edge of otoliths taken from the sampled fish. © 1998 Elsevier Science Inc. Allrights reserved.

Keywords: Otolith; Hoplostethus atlanticus ; Endolymph; Microincrement; Growth; Age

1. Introduction

Validation of estimated age in orange roughy (Ho-plostethus atlanticus ; Teleostei; Trachichthyidae) hasemerged as an important management issue in valuablefisheries for H. atlanticus that are as far apart as NewZealand, Australia, South Africa, The Azores and TheFaroes. Estimates of maximum age of H. atlanticusbased on radionuclide ratios are in the range of 150–200 years [2,17], maximum ages based on checks insections of the otolith are 150+ years [17,18], maxi-mum ages based on opaque zones of whole otolithsrange from 16 years [10], through 18 years (personalcommunication, N.-R. Hareide) and 22 years [20], to 29years [11]; maximum ages based on daily microincre-ments are 16+ years [9]; and maximum ages from asmall sample of scales [6] showed similar length-at-agecurves to those based on daily microincrement ages.

Validation of these different age estimation methodshas proved ambiguous. For example, Annala and Sulli-van [1] quote: ‘‘ … age determination … validated by

length-mode analysis of juveniles up to 4 years of age.’’Apart from the extrapolation from 4 to 130+ years,the length-mode analysis does not appear to extend tofurther samples of juveniles of H. atlanticus [4]. Simi-larly, the 210Pb:226Ra ratio radionuclide method of agevalidation [2,17] is confounded by the anomalous distri-bution of 210Pb in otoliths of H. atlanticus [22]; and theloss from otoliths of H. atlanticus of 222Rn an interme-diate daughter product in the decay series from 226Ra to210Pb [7,21].

The Romanek–Gauldie model of validation [16]compares the growth rate of the otolith that is calcu-lated directly from experimental measurements of thechemistry of the endolymph in which the otolith grows,with estimated otolith growth rates from microincre-ment, zone/check, or any other ageing method. Thereare usually three otoliths in each of the pair of en-dolymphatic sacs of the fish inner ear. The largest ofthe three otoliths, the sagitta, is most commonly used infish age estimation, including in H. atlanticus. Thesagitta is a mechano-acoustical transducer that convertssound energy fluctuations into graded action potentialsfrequencies caused by differential bending of the hair* Corresponding author. Tel.: +64 04 3885486.

1095-6433/98/$19.00 © 1998 Elsevier Science Inc. All rights reserved.PII S1095-6433(98)10082-X

R.W. Gauldie, C.R. Romanek / Comparati6e Biochemistry and Physiology, Part A 120 (1998) 649–653650

cells of the inner ear as the sagitta vibrates in responseto different incident sound frequencies. The action po-tentials of the hair cells are dependent on the ratios ofCa2+, Mg2+, Na+ and K+ ions that usually havesimilar concentrations in the endolymph to other bodyfluids that bathe the active membranes of the nervousand muscle systems of vertebrates (see [8,12,13,16] forsummaries of the endolymph physiology literature).The fluid of the endolymphatic sac is bicarbonatebuffered, but teleosts are not good pH regulators (asshown by experimental hypercapnia) so that increasedH+ ion concentrations in the body fluids resulting fromactivity (lactic acid load) are soon transported into theperilymph and endolymph. Temperature also controlsH+ ion availability in bicarbonate buffered systems.The otolith is composed of calcium carbonate whichdissolves below pH 7. The pH of the endolymph istherefore an important control of otolith growth.

The sagitta in most teleosts, including H. atlanticus,is composed of the aragonite morph of calcium carbon-ate. The growth rate of aragonite in solutions of biolog-ically active ions at concentrations and pH typical ofbody fluids has been demonstrated experimentally.Combining observations of endolymph chemistries re-ported in the literature with experimental aragonitegrowth rates allowed calculation of theoretical maxi-mum daily growth of otolith microincrement widths.Comparison of theoretical microincrement widths fromthe model with published observations of daily microin-crement widths from otoliths from fishes from knowntemperature regimes showed a strong relationship be-tween predicted and observed microincrement widths[16]. Thus the Romanek–Gauldie model provides anexperimentally accessible method of validating dailymicroincrement widths.

One approach to validation of the Romanek andGauldie model has been to examine the consequencesof the model itself, i.e. can it be shown that endolymphpH rather than variation in endolymph Ca2+ controlsthe growth rate of the otolith? Laboratory experimentson tank-held Oncorhynchus mykiss by Payan and hisco-workers [11,12] have confirmed that pH, not Ca2+

variation, is indeed the principal physiological controlof otolith growth.

Another direct test the validity of the Romanek andGauldie model is reported in this paper. This was doneby measuring endolymph chemistries, and calculatingtheir instantaneous growth rate from those measure-ments using the model of Romanek and Gauldie [16],from which the theoretical daily growth rate, or dailymicroincrement width, can be extrapolated. The theo-retical daily microincrement width was then comparedwith the widths of the last daily microincrement visiblein the otoliths recovered from the individual H. atlanti-cus from which the theoretical microincrement widthswere calculated.

2. Materials and methods

The direct experimental test of the Romanek–Gauldie method on orange roughy otolith growth ratewas made at sea by measuring the chemistry of theendolymph directly by microelectrodes inserted into theendolymphatic sacs of recently killed H. atlanticus.Ion-specific membranes microelectrodes for Ca2+,Na+ and pH suitable for use with battery-poweredhand-held pH meters were made for this project byMicroelectrodes, Bedford, NH. Microelectrode tipswere less than 0.5 mm in diameter. Standard pH solu-tions and prepared standards for Ca2+ and Na+ wereused to calibrate the electrodes. Errors in measurementstested against pH standards were less than 90.5%, anderrors in measurements tested against standards forCa2+ and Na+ were 910%. The standards used forcalibration during the experiment were 10 mmol l−1

Ca2+, 100 mmol l−1 Na+ and pH 7. Instrument driftwas minimal as shown in Table 1 and was corrected forin calculating theoretical microincrement widths.

Specimens of H. atlanticus were caught in smallnumbers, three to five fish per tow, usually in associa-tion with small (about 20) numbers of cardinal fish,Epigonus telescopus. All tows were made roughly acrossthe same part of the same seamount on the ColvilleRise (about 177°30%E, 36°30%S) at depths ranging from746 to 948 m at bottom temperatures of 8.7°C down to6.4°C measured from the Netsonde. When caught thisway, H. atlanticus were lively on the ship’s deck withlittle scale damage and were sacrificed by spinal andbrain section. The otic capsule of the H. atlanticusexamined (size (FL) range in Table 1) was large, about2×1.5×1 cm, into which the endolymphatic sac con-taining the otoliths (otolith growth radius dimensionsin Table 1) fits loosely, surrounded by the perilymph.Perilymph was removed with Kimwipe tissues and mi-croelectrodes inserted into the sac one by one in theorder pH, Ca2+ and Na+. In some cases two dupli-cates microelectrodes were inserted simultaneously or insuccession to verify measurements. A total of 17 H.atlanticus were processed in this way. The results (in-cluding those of standards) are shown in Table 1.Microelectrodes were cleaned in detergent after eachmeasure and re-equilibrated in the storage solutionbetween measures. The microelectrode drift was moni-tored by measurements made in standard solutions.The entire series of measurements were made over aperiod of 23 h. Only lively fish were used. Bottomtemperatures were used to back-calculate pH and ionicsaturation state typical of the depth of capture fromwhich the width of otolith daily growth increments wascalculated from the Romanek–Gauldie model. Thewidths of orange roughy daily microincrements calcu-lated from the endolymph chemistry are shown inTable 1.

R.W. Gauldie, C.R. Romanek / Comparati6e Biochemistry and Physiology, Part A 120 (1998) 649–653 651

Tab

le1

The

tabl

eco

ntai

nsth

eH

.at

lant

icus

data

For

kle

ngth

Na

Ca

Oto

lith

grow

thra

dius

pHT

heor

etic

alm

icro

incr

emen

tO

bser

ved

mic

roin

crem

ent

Che

ckM

icro

incr

emen

tag

ew

idth

(cm

)ag

ew

idth

(mm

oll−

1)

(mm

oll−

1)

(mm

)

7.0

Stan

dard

9.8

944.

336.

827

0.53

42.5

9.55

7.7

234

30.

538.

24.

555.

822

3.2

220

7.7

39.5

7.9

1.29

2.22

7.8

2037

.53.

110

.922

93.

677.

321

3.25

266

3.9

8.1

38.5

10.1

510

.810

87.

0St

anda

rd8.

33.

383.

567.

329

43.5

10.1

51.

823

47.

91.

54.

916.

624

40.5

9.2

3.6

276

2.14

6.7

25.5

1.45

7.7

41.5

244

6.5

9.25

81.

73.

835.

822

.57.

925

439

.57.

84.

327.

435

6.75

10.3

5.4

45.5

8.2

250

8.8

977.

1St

anda

rd7.

81.

563.

788.

335

45.5

11.6

7.2

244

0.56

10.1

2.56

7.3

276.

823

17.

442

.52.

85.

625

.50.

6841

.57.

87.

623

64.

35.

128.

65.

236.

229

4.1

216

8.2

43.5

8.2

7.62

2.8

6.6

3545

.56.

19.

219

87.

410

4St

anda

rd10

.62.

5310

.33.

797.

427

11.7

206

7.8

42.5

4.18

44.5

6.4

8.85

327.

922

48

3.95

1.95

8.1

322.

9521

97.

944

.511

.37.

1

The

titl

esof

colu

mns

are

self

-exp

lana

tory

.M

icro

incr

emnt

wid

ths

are

inm

m.

Age

sin

year

sar

eca

lcul

ated

asde

scri

bed

inth

ete

xt.

Stan

dard

sin

dica

ted

init

alic

ssh

owth

em

inim

aldr

ift.

The

data

inth

eta

bles

isco

rrec

ted

for

the

inst

rum

ent

drif

t.

R.W. Gauldie, C.R. Romanek / Comparati6e Biochemistry and Physiology, Part A 120 (1998) 649–653652

Measuring the width of the microincrement at thegrowing edge of the otolith requires a specific tech-nique. Microincrement widths of 2–5 mm (typical of H.atlanticus [8]) are easily lost in ground sections of thegrowing edge. Instead, the growing edge of the wholeotolith (the growing edge of the posterior growth radius[8]) was photographed immersed in oil. A similar tech-nique has been used for visualizing microincrements atthe growing edge of the otoliths of Nemadactylusmacropterus [5].

3. Results

The results of the experiment including fish length(FL), endolymph chemistry and observed and theoreti-cal microincrement widths are shown in Table 1. Ob-served microincrement widths at the growing posterioredge of the otoliths of H. atlanticus are plotted againsttheoretical microincrement widths in Fig. 1. The linearregression of observed microincrement width on theo-retical microincrement width was statistically signifi-cant, P=0.0006. However, the slope of the linearregression equation (observed width=2.65+0.4333×theoretical width; r2=59%) indicates that while theRomanek and Gauldie model predicts the trend in theobserved widths, the observed widths were consistentlylarger than theoretically predicted widths. The averageobserved and theoretical microincrement widths were3.81 and 2.67 mm, respectively.

The linear regression of theoretical microincrementwidths on observed pH was statistically significant(P=0.0159) with r2=33%. The linear regression oftheoretical widths on observed Ca2+ was not statisti-cally significant, P=0.52.

Average microincrement width is used to estimateage by dividing the length of the otolith growth axis bythe average daily microincrement width. This method

Fig. 2. Length (FL cm) plotted against age (years) for Microincre-ment ages (open circles) and Check ages (crosses).

has been used for estimating age in H. atlanticus [9] andother species [3,14,15,19]. Dividing the length of thegrowth axes of the otoliths in Table 1 by the averageobserved microincrement width of 3.81 mm yields theages shown in Table 1 as Microincrement Age. Length-at-age curves for H. atlanticus based on the interpreta-tion of 130+ years maximum age, i.e. L�=45 cm,and von Bertalannffy k=0.07 published in [1] yield theages at length listed in Table 1 as Check Age. Lengths-at-age of Colville sample of H. atlanticus based on boththe Microincrement Age and Check Age in Table 1 areshown in Fig. 2.

4. Discussion

Microincrements occur in the otoliths of most speciesof fish. In many of these species experiments haveshown that the microincrements are deposited everyday. Daily microincrements are therefore potentiallyuseful in estimating fish age. However, once past thefirst year of life in which microincrement widths firstincrease and then decrease in width [11,14], the averagemicroincrement widths of many otoliths are fairly simi-lar in width, usually in the range of 1–5 mm. It is thisgeneral similarity that lead to the development of theRomanek–Gauldie model of otolith growth based onthe precipitation rate of the crystalline aragonite (cal-cium carbonate) otolith out of the endolymph. Modelsof aragonite chemistry are well known and were readilyadapted by Romanek and Gauldie allowing the instan-taneous growth rate implied by the chemistry of theendolymph at the time of measurement to be calcu-lated; and theoretical daily microincrement width ex-trapolated from that calculation. We have done this for17 specimens of H. atlanticus. The theoretical microin-crement widths calculated in this way were well corre-lated with observed microincrement widths. Thiscorrelation implies that nearly 60% of the observed

Fig. 1. Observed microincrement widths (mm) plotted against theoret-ical microincrement widths (mm) with fitted linear regression line.

R.W. Gauldie, C.R. Romanek / Comparati6e Biochemistry and Physiology, Part A 120 (1998) 649–653 653

variation on microincrement width can be explained bythe predictions of the theoretical model. Coupled withother confirmation of underlying physiology of themodel, this experiment provides convincing evidence ofthe validity of the Romanek and Gauldie model indescribing otolith growth rates in the orange roughy H.atlanticus. The model implies that growth rates of H.atlanticus are higher than currently believed.

Acknowledgements

We thank Milan Barbarich of Anton Seafoods for hisgenerous offer to let us use the Seamount Enterprise forthis experiment, and the vessel skipper Warwick Smithand his crew for their expertise and kind co-operation.Without the financial support of the Orange RoughyManagement Company this experiment would not havebeen possible.

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