Intercalibration of age readings of deepwater black scabbardfish, Aphanopus carbo (Lowe, 1839

ICES Journal of Marine Science, 59: 352–364. 2002doi:10.1006/jmsc.2001.1154, available online at http://www.idealibrary.com on

Intercalibration of age readings of deepwater black scabbardfish,Aphanopus carbo (Lowe, 1839)

B. Morales-Nin, A| . Canha, M. Casas, I. Figuereido,L. S. Gordo, J. D. M. Gordon, E. Gouveia,C. G. Pineiro, S. Reis, A. Reis, and S. C. Swan

Morales-Nin, B., Canha, A| ., Casas, M., Figuereido, I., Gordo, L. S., Gordon,J. D. M., Gouveia, E., Pineiro, C. G., Reis, S., Reis, A., and Swan, S. C. 2002.Intercalibration of age readings of deepwater black scabbardfish, Aphanopus carbo(Lowe, 1839). – ICES Journal of Marine Science, 59: 352–364.

To facilitate intercalibration of the age readings of black scabbardfish, Aphanopuscarbo, a collection of otoliths was exchanged among the participating laboratories inPortugal, Scotland, and Spain and a workshop was held to discuss the results. The ageprecision was significantly improved by the intercalibration exercises but remainedrelatively low. The indirect validation methods applied showed that the growthincrements used for age determination were consistent. However, some discrepancieswere found between otoliths from different areas, probably due to the complex spatialdistribution of the species and the differing length structure, depending on the area andthe method of fishing. Interpretation criteria were defined pending more definitivevalidation studies.

� 2002 Published by Elsevier Science Ltd. on behalf of International Council for the Explorationof the Sea

Keywords: age determination, Aphanopus carbo, black scabbardfish, incrementalgrowth, otoliths, microstructure.

Received 13 June 2001; accepted 5 November 2001.

B. Morales-Nin: CSIC/UIB, Institut Mediterrani d’Estudis Avancats, Miguel Marques21, 07190 Esporles, Spain. A| . Canha and A. Reis: DOP, Faial, Azores, Portugal.M. Casas and C. G. Pineiro: Instituto Espanol de Oceanografıa, 36280 Vigo, Spain.I. Figuereido: Instituto de Investigacao das Pescas e do Mar, Av. Brasilia, 1449-006,Lisboa, Portugal. L. S. Gordo: Faculdade de Ciencias de Lisboa, Dep. Zoologıa eAntropologıa, Bloc C2, Campo Grande, 1749-016 Lisboa, Portugal. J. D. M. Gordon andS. C. Swan: Scottish Association for Marine Science, Dunstaffnage Marine Laboratory,Oban, PA34 4AD, UK. E. Gouveia and S. Reis: DRP-DSIP, Estrada da Pontinha,9000-017 Funchal, Madeira, Portugal. Correspondence to B. Morales-Nin: tel:34971611721; fax: 34971611761; e-mail: [email protected]

Introduction

Black scabbardfish, Aphanopus carbo (Lowe, 1839)Family Trichuridae, is a benthopelagic species inhabit-ing the continental shelf and slope between 200 and1600-m depth (Nakamura and Parin, 1993). There issome evidence that it makes excursions into midwater,with these migrations taking place mostly at night, forfeeding. There are often statements in the literature thatjuveniles are pelagic, although there is a lack of evi-dence to support this. The smallest specimens known,10–15 cm, were taken from the stomachs of a midwaterfish, Alepisaurus ferox, off Madeira (Maul, 1948). Blackscabbardfish are widely distributed in the northeastern

1054–3139/02/040352+13 $35.00/0 � 2002 Published by Elsevier S

Atlantic along the European continental slope and alsoaround isolated island groups and seamounts. Severalcountries, namely Portugal, the UK, France, andIreland, already commercially exploit black scabbard-fish. In Portugal, black scabbardfish is the target speciesof the longline fishery off Sesimbra (Portuguese main-land) and off Madeira (Martins and Ferreira, 1995;Martins et al., 1989, 1994). In more northern waters, tothe west of the British Isles and in the internationalwaters of the Rockall and Hatton Banks, the blackscabbardfish is mainly a by-catch of the unregulateddeepwater trawl fishery (Gordon, 2001).

Biological information on this species is scarce and
most of the publications deal with its distribution (Maul,
cience Ltd. on behalf of International Council for the Exploration of the Sea

353Intercalibration of age readings of deepwater black scabbardfish

Figure 1. Photomicrographs of Aphanophus carbo sagittal otoliths. (a) Whole otolith from Madeira corresponding to a fish of125 TL. (b) Transverse section of an otolith from Madeira corresponding to a fish of 130 cm TL. R, rostrum; D, dorsal side;V, ventral side; N, nucleus; SC, sulcus acusticus.

354 B. Morales-Nin et al.

Table 1. Results of the age interpretation for set 1 (whole otoliths from Madeira).

Fishno.

Fishlength(cm)

Catchmonth

IMEDEAReader

1

IEO-1Reader

2

IEO-2Reader

3

DSIPReader

4

SAMS-1Reader

5

SAMS-2Reader

6

IPIMARReader

7

FCReader

8

DOP-1Reader

9

DOP-2Reader

10Modal

ageAge

difference

1 125 6 7 6 8 6 7 8 8 8 8 7 8 22 144 6 4 9 8 8 12 6 10 10 8 8 8 83 135 6 9 9 8 8 8 6 9 10 8 7 8 44 130 6 10 9 8 10 6 12 12 12 12 12 65 77 6 6 7 6 3 6 6 5 5 6 8 6 56 80 6 4 6 6 4 6 7 6 6 7 7 6 37 56 6 7 4 2 3 6 5 5 6 6 6 58 64 6 6 4 3 3 4 4 4 8 7 4 59 103 6 2 8 7 3 5 4 4 8 7 6 8 6

10 79 6 1 7 5 4 5 8 6 7 6 7 711 84 6 4 5 3 5 6 5 5 6 6 5 313 87 6 6 6 7 5 6 6 7 7 6 5 6 214 138 6 7 8 7 11 6 9 9 7 7 7 515 121 6 8 6 8 6 9 6 8 8 8 7 8 316 123 6 7 7 7 7 10 7 10 10 7 8 7 317 108 6 8 ? 8 9 7 9 9 8 10 8 318 141 6 7 8 7 12 ? 7 6 7 619 120 6 9 6 7 6 7 7 8 7 7 320 131 6 7 9 10 8 10 7 11 8 10 7 7 421 115 6 8 9 7 10 9 11 11 8 8 8 422 118 6 7 9 6 7 9 6 7 9 7 9 7 323 124 6 8 7 7 8 7 8 11 11 7 8 8 424 122 6 9 7 6 6 9 2 10 10 8 7 9 825 112 6 12 9 8 10 6 11 11 11 8 11 626 114 6 6 ? 8 6 10 6 10 10 8 7 6 427 126 6 7 10 8 8 11 10 11 11 8 8 428 117 6 7 7 9 8 11 7 10 10 7 7 7 429 119 6 8 8 a 9 7 9 9 8 7 8 230 111 6 6 10 7 7 9 6 10 10 10 7 10 431 113 7 7 8 8 6 9 3 632 71 7 2 7 5 3 5 4 6 6 6 7 6 533 79 7 5 8 6 4 4 3 6 6 7 6 6 534 81 7 3 7 6 4 4 2 6 6 7 7 7 535 74 7 3 6 5 3 4 2 9 8 3 736 68 7 1 7 4 2 2 2 4 4 8 5 4 737 63 7 1 8 8 2 2 1 4 4 8 7 8 738 75 7 4 6 4 3 3 1 539 83 7 6 5 5 4 4 2 7 7 6 7 7 540 65 7 2 5 4 2 3 2 4 4 7 6 2 541 82 7 4 7 5 4 7 4 6 6 6 6 6 342 67 7 1 5 4 3 5 2 5 5 6 5 5 543 56 7 2 7 4 2 3 1 4 4 5 5 4 646 60 7 1 7 6 2 2 1 5 5 7 7 7 647 86 7 2 7 6 4 7 2 7 7 6 7 7 548 69 7 3 7 5 3 449 90 7 3 8 6 4 9 3 7 7 10 8 3 750 70 7 4 6 5 3 6 1 5 5 7 7 5 6

1948; Tucker and Palmer, 1949; Templeman andSquires, 1963; Bone, 1971; Fitch and Gotshall, 1972;Piotronski, 1981), anatomy (Holl and Meinel, 1968;Blaxter et al., 1971; Parin and Becker, 1972; Howe et al.,1980), and general biology (Tucker, 1950; Zilanov andShepel, 1975). In northern European waters, apart fromsome work on feeding (Mauchline and Gordon, 1984),

there is no information on the biology of this species.Age and growth data are available from studies onwhole otoliths of fish caught off Madeira, and they showa relatively fast growth rate, with maximum ages of8 years (Morales-Nin and Sena-Carvalho, 1996). Apreliminary study with sectioned otoliths showed agesup to 32 years in Irish waters (Kelly et al., 1998). There


are no data available on growth rates in other areas andthere is no established methodology for age reading andinterpretation.

In 1997, mainland Portugal, Madeira, the Azores,Spain, and Scotland initiated a project (EuropeanCommission Study Project 97/0084) to improve theunderstanding of the biology of and fishery of thisspecies, focusing specifically on age determination inorder to form a basis for future stock assessments andbiological studies. Similar to many other deepwaterspecies (Bergstad, 1995), both validation of age readingsand intercalibration among readers was needed. Thispaper summarises the efforts carried out, first to inter-calibrate the age determinations and establish commonotolith reading methodology, and second to attempt tovalidate the age readings exploring the feasibility ofsemi-direct methods.

Material and methods

Table 2. Results of the age interpretation for set 2 (sectioned otoliths from Madeira).

Fishno.

Fishlength(cm)

Catchmonth

IEO-2Reader 3

DSIPReader 4

SAMS-1Reader 5

IPIMARReader 7

FCReader 8

DOP-1Reader 9

DOP-2Reader 10

Modalage

Agedifference

1 125 6 12 8 8 12 8 8 42 144 6 14 11 12 10 46 80 6 11 6 6 7 8 6 57 56 6 5 6 4 28 64 6 9 4 4 7 6 4 59 103 6 8 11 14 8 8 9 7 8 7

10 79 6 9 5 8 9 7 9 211 84 6 10 6 6 7 9 7 7 412 76 6 9 14 5 6 9 6 9 813 87 6 8 13 7 7 5 7 815 121 6 18 11 8 8 7 1116 123 6 15 20 16 13 11 10 1017 108 6 19 13 12 11 10 919 120 6 13 8 9 7 520 131 6 20 24 17 12 8 9 1622 118 6 13 20 16 12 9 7 1323 124 6 17 13 10 10 326 114 6 10 8 8 8 227 126 6 13 13 11 10 13 328 117 6 13 19 12 10 11 930 111 6 10 11 131 113 7 10 22 11 11 8 8 11 1432 71 7 7 8 7 7 133 79 7 7 12 7 6 7 7 7 634 81 7 7 12 6 8 6 6 635 74 7 9 8 13 7 5 8 8 8 836 68 7 9 7 9 6 5 6 5 9 437 63 7 6 8 5 4 7 7 7 438 75 7 7 14 7 7 7 7 7 739 83 7 4 7 10 6 440 65 7 7 9 8 5 5 6 5 5 441 82 7 6 8 14 7 6 7 7 7 842 67 7 7 5 5 6 6 6 243 58 7 6 8 7 6 4 6 5 6 448 69 7 9 11 11 6 5 7 5 11 649 90 7 9 9 14 7 7 10 6 9 850 70 7 10 8 10 7 6 7 6 10 4

Otolith interchange

Four sets of otoliths were used in this intercalibrationexchange. The first set was composed of 50 whole leftotoliths from fish of 56–144 cm total length (TL), caughtoff Madeira in June–July 1998. These otoliths wereslightly burned and read whole, following the proceduredescribed by Morales-Nin and Sena-Carvalho (1996).The second set consisted of a thin transverse section(McCurdy, 1985) of the other otolith from each pair,mounted on slides with Eukitt and moistened withimmersion oil before observation. Twenty otolith pairsfrom fish 82–112 cm TL caught in the Rockall Trough inJune 1998 were split into two sets, the left otoliths wereinterpreted whole (otolith set 3) and the right otoliths


were embedded in black resin and cut into thin trans-verse sections (otolith set 4) following standard pro-cedures (Pineiro et al., 1996). The preliminary criterionfor age determination was that one opaque and onetranslucent growth increment were laid down annuallyand comprised an annulus. Since the spawning season ofblack scabbardfish is not well known, the age class wasnot determined and the age was considered to bethe number of growth increments. Participants wererequested to comment on their results in relation to anyproblems encountered, the nature of the otolith age, andage reliability. The otolith terminology used was basedon Kalish et al. (1995).

The results were analysed for each set separately usinga specially designed spreadsheet developed by Eltink(1994), following the recommendations of the Workshopon Sampling Strategies for Age and Maturity (ICES,1994). For each set of otoliths the modal age werecalculated, as well as the highest and lowest ages (Eltink,1997).

In addition, the age and length data obtained forMadeira set 1 were used to parameterise the von

Bertalanffy growth model. Moreover, for the Madeiraotoliths (sets 1 and 2), a comparative analysis of theresults from whole and sectioned otoliths was carriedout by subtracting the age obtained from the wholeotolith from the age obtained by reading the section, andplotting the results.

Table 3. Results of the age interpretation for set 3 (whole otoliths from Rockall Trough).

Fishno.

Fishlength(cm)

Catchmonth

IMEDEAReader

1

IEO-1Reader

2

IEO-2Reader

3

DSIPReader

4

SAMS-1Reader

5

SAMS-2Reader

6

IPIMARReader

7

FCReader

8

DOP-1Reader

9

DOP-2Reader

10Modal

ageAge

difference

1 92 6 6 7 10 6+ 6 4 7 7 9+ — 7 62 100 6 7 7 9 7 ? 3 7 7 10 — 7 73 93 6 7? 8? 9 8 6 5 7 7 9/10 — 7 44 97 6 8 7? 10 7 7 6 7 7 8+ — 7 45 87 6 ? 6? 7–8 6 6 4 8 7 8 — 6 46 95 6 6 6 8 7 6 5 8 8 8 — 8 37 107 6 9 7 7? 7+ 11 ? 10 10 7/8 — 10 48 97 6 7* 7 7 7+ 7 5 9 8 7/8? — 7 49 104 1 10? 9? 9? 11 6 9 9 8/9 — 11 5

10 112 6 10 8? 6+ 9 4 9 9 8 — 9 611 85 6 6* 6 5? 6? 5 3 7 7 6 — 6 4

Table 4. Results of the age interpretation for set 4 (sectioned otoliths from Rockall Trough).

Fishno.

Fishlength(cm)

Catchmonth

IPIMARReader 7

FCReader 8

DOP-1Reader 9

DOP-2Reader 10

Modalage

Agedifference

1 92 6 11 7 9 9+ 9 42 100 6 11 10 6/7? 10 10 13 93 6 9 8 7/9 9/10 9 14 97 6 9 7 7/8 8+ 7 25 87 6 9 8 9 8 8 16 95 6 8 6 8 8 8 07 107 6 12 10 8/9 7/8 8 28 97 6 10 8 7 7/8? 7 29 104 6 10 9 8 8/9 1

10 112 6 11 9 7/8 8 8 311 85 6 6 7 4/5 6 6 1

Sagittal otolith growth increment analysis

The results obtained from the otolith interchange werediscussed at a Workshop held during 1999, where thebest methodology was determined, the interpretationdiscrepancies were reduced, and interpretation guide-lines were established. Also an additional set of otolithswas studied during the workshop to investigate whetherimmersing unburned otoliths in water for 24 h enhancedotolith clarity. There was no improvement in the resultsand the readers decided that using burned otoliths readin 50% glycerol-alcohol was the best method. It wasagreed that the outer edge was clearer in burnt otoliths,although the centre was probably clearer in theuntreated otoliths. It was agreed that a second step in


the intercalibration exercise was to explore the agevalidation methods. To assess if the growth incrementswere annual, the following indirect validation methodswere applied: (1) The growth increment distances to thecore were analysed in order to determine the incrementformation pattern and homogeneity in the population,using 20 sections of otoliths embedded in black resinfrom Madeira, 100 whole otoliths from Madeira, and100 whole otoliths from Sesimbra, mainland Portugal.(2) The nature of the otolith edge was examined for 124otoliths from Madeira. These otoliths corresponded tofish between 56 and 128 cm TL. (3) The otolith micro-structure was examined on sectioned otoliths from

Madeira, which were washed with distilled water usingultrasound and etched with 0.1 M HCl for one minute.Finally, they were washed with distilled water and driedat 60�C. The otolith slides were glued to scanningelectron microscope (SEM) stubs using colloidalsilver and sputtered with gold palladium for SEMobservation.

Results

20406080

100120140160

Reader 1

20406080

100120140160

Reader 2

20

Len

gth

(cm

)

406080

100120140160

Reader 3

20406080

100120140160

Reader 4

2015

Age0 5 10

406080

100120140160

Reader 5

20406080

100120140160

Reader 6

20406080

100120140160

Reader 7

20406080

100120140160

Reader 8

20406080

100120140160

Reader 9

2015

Age0 5 10

406080

100120140160

Reader 10

Figure 2. Age (years)–length relationships determined by ten readers on the whole otoliths from Madeira (set 1).

Otolith interchange

Black scabbard fish otoliths are oblong and show trans-
lucent and opaque growth increments around a central


0

20

Difference>5 5 4 3 2 1 0 –1 –2 –3 –4 –5 >–5

5

10

15

0

20

Difference>5 5 4 3 2 1 0 –1 –2 –3 –4 –5 >–5

Reader 10

5

10

15

20

Nu

mbe

r

Reader 5

5

10

15

20Reader 9

5

10

15

20Reader 3

5

10

15

20Reader 8

5

10

15

Reader 7

Figure 3. Frequency distribution of differences (years) between the readings of the sectioned and whole otoliths from Madeira.

opaque elongate area or otolith nucleus (Figure 1a). Thegrowth increments could not be followed all around theotolith and could only be located in the dorsal andventral otolith zones, whereas in the rostrum andantirostrum the increments were unclear and split inmultiple rings. Tables 1–4 show discrepancies notonly between readers but also between otolith prep-aration procedures. The maximum age determined fromwhole otoliths was 12 and 24 from sections. The main

difficultes were in locating the first translucent growthincrement and interpreting the marginal growthincrements. This resulted in greater discrepancies inreadings from the youngest and oldest specimens.Depending on the reader, the age–length relation-ships obtained for the whole otoliths showed differenttrends (Figure 2). Some readers obtained ages rangingfrom 1 to 12 years, while others obtained a narrowerage-range.


In the otolith sections, these problems were com-pounded by the presence of multiple false rings (Figure1b) making interpretation more difficult. All the readersconsidered the sectioned otoliths from Madeira to beunclear, due in part to the slide preparation method,thus a second small set of otoliths from the RockallTrough were sectioned (set 4). However, these sectionswere also indistinct and proved difficult to interpret.

While the ages obtained from whole and sectionedotoliths for Madeira were compared, it was clear thathigher age estimates were obtained from the sections,although there were some important differences in theage range depending on the reader (Figure 3).

Indirect validation by otolith growth pattern

The distance between growth increments, measured asincrement diameter for the Madeira whole otoliths andas increment radius for the Madeira sectioned otoliths,showed the steadily decreasing growth pattern charac-teristic of otoliths (Figure 4). Taking the standarddeviation as a measure of variability, the measurementof the radius (Figure 4b) was less variable than that ofthe diameter (Figure 4a), probably due to the difficultyof locating the increment in the otolith contour. Thescatter of the widths increased in otoliths with olderestimated ages (Figure 4a). This growth pattern is con-sidered to be an indicator of the annual nature of theincrements (Morales-Nin et al., 1998).

A plot of the increment diameter distances of the firstseven increments from the 100 Madeira otoliths (Figure5a) and for the radius measured from the core to

increment distances for the 100 Sesimbra otoliths(Figure 5b) showed low overlap between contiguousincrements, indicating a high otolith growth rate. Theincrement frequencies showed two peaks in most of theincrements, suggesting that two modes were present.This might be due to differential growth between sexes,as reported for hake, Merluccius merluccius (Morales-Nin et al., 1998), or to the presence of more than onestock in the samples.

The percentage of opaque edges at the otolith marginwas relatively high all year around with a peak duringOctober, but with a decrease in February, Septemberand November–December (Figure 6). In a study basedon a larger data set (n=686), the highest percentage ofopaque margins occurred during October and decreasedfrom November to December (Morales-Nin and Sena-Carvalho, 1996). This study showed a similar trend,although the low number of otoliths examined (n=124)resulted in high variability. The formation of translucentincrements might be related to the reproductive periodwhich, in Madeira, is from November–December(Morales-Nin and Sena-Carvalho, 1996).

014

3

Age (years)

Incr

emen

t (m

m)

1 1312111098765432

1

2

(b)

09

4

Age (years)

Incr

emen

t (m

m)

1 8765432

1

2

(a)

3

Figure 4. Increment distances (mm) from the otolith core byage for (a) Madeira whole otoliths and (b) Madeira transversesections.

Otolith microstructure

Otolith sections from Madeira were examined under theSEM and 22% of the samples did not show any growthstructure. The remainder had different growth structuresdepending on the otolith zone examined. Two SEMemission modes were tested, using either primary orsecondary back-scattered electrons, without anyimprovement in the increment resolution (Figure 7).Although the periodicity of the growth increments hasnot been tested, we presume them to be daily growthincrements (DGI) due to the omnipresence of thisrhythm in marine fishes (Morales-Nin, 2000). Radialaragonite microcrystals were clearly visible with thindiscontinuities that corresponded to the discontinuousunit of the DGI. The discontinuous units of the DGIwere faint, with their appearance depending on theetching agent and time.

The core, laid down during the embryonic period, wasoblong and had a mean size of 58.4 �m�41.58 �m. Thefirst DGI laid down around the core were relativelythick (2.14 �m) and at around 0.2 mm from the core,several accessory nuclei initiate the formation of theprismatic growth described in several Gadidae (Morales-Nin and Aldebert, 1997). The first DGI formed in theprisms were also comparatively thick, measuring inaverage 5.54 �m (s.d.=1.41) (Figure 7a–c). The DGIthickness decreased progressively along the prisms, withan average width of 2.11 �m (s.d.=0.57) (Figure 7d).Several growth discontinuities or checks preceded theformation of thinner DGI (mean=1.017 �m, s.d.=0.39).These thinner DGI probably comprise the macrostruc-tural translucent increment. The discontinuities were


rhythmic in several regions of the otolith and wereprobably related to growth changes. When the trans-lation was smooth and without a check, this mightcorrespond to one of the translucent rings observed atmacroscopic level inside the opaque increments, whichwere considered to be false rings. However, in somecases the growth discontinuity or check between thinand thick increments occurred only in part of the otolith.As the discontinuities are rich in organic matter, thesemight correspond to structures that control otolithshape (Gauldie, 1990).

The SEM preparations with a complete sequence ofDGI, or with a radius with more than 95% of thesequence (n=6), were used to determine the age lengthrelationship. If in some parts of the otolith the DGI wereunclear, their number was extrapolated from the widthof the DGI laid down before and after the unclear area.The numbers of DGI were converted into years and theages at length were plotted together with those obtainedfrom readings of whole otolith (Figure 8).

21

1

41

Radius

(b)

Per

cen

tage

61116

263136

1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.6 3.8 4.0

20

0

35

Diameter

(a)

Per

cen

tage

5

10

15

25

30

3.2 3.7 4.1 4.5 4.9 5.3 5.7 6.1 6.5 6.9 7.3 8.5

1.3

8.17.7

Figure 5. Distribution of the first seven increments distances (mm) for (a) increment diameter for whole Madeira otoliths and (b)increment radius for whole Sesimbra otoliths.

012

80

Month

Per

cen

tage

opa

que

edge

1

20

40

60

2 3 4 5 6 7 8 9 10 11

Figure 6. Percentage of otoliths with an opaque edge in eachmonth for whole Madeira otoliths.

Discussion

The aims were to standardise the otolith preparationtechnique as well as the interpretation criteria in order totest and improve the precision of age readings. The twoprevious studies on A. carbo age used different method-ologies and gave age ranges up to 8 years using wholeotoliths (Morales-Nin and Sena-Carvalho, 1996) and upto 32 years of age using thin otolith sections (Kelly et al.,1998). Although the age range differences should bepartly caused by the study area (Madeira and theRockall Trough, respectively), the main discrepanciesshould be due to the preparation and interpretationbecause the fish length range is similar in both studies.The main difficulty in black scabbardfish otolith


Figure 7. Scanning electron photomicrographs showing Aphanophus carbo otolith microstructure. (a) Examples of emission mode,lower panel: back-scattered primary electrons, upper panel: secondary electrons. Otolith from a fish of 92 cm TL. (b) and (c)Accessory primorida (arrows) in otoliths from fish of 87.1 cm TL and 94 cm TL, respectively. (d) Rhythmical growth patterns inan otolith from a fish of 94 cm TL.

interpretation is the lack of well-defined growth incre-ments that could be followed all around the otolith.Morales-Nin and Sena-Carvalho (1996) reading wholeotoliths and Kelly et al. (1998) using thin sections(0.25–0.35 mm) reported that the otolith interpretationis difficult due to split zones. Having in mind theseresults, both otolith preparation methods were tested

and some preliminary conclusions were reached,although further validation studies should be carried outbefore black scabbard fish age determination is resolved.

The relatively simple preparation technique ofburning whole otoliths proved successful, improvingthe growth increment individualisation and providedreasonable age readings. The preparation of thin otolith


20

140

Age (years)

Len

gth

(cm

)

0

40

60

80

100

120

2 4 6 8 10 12

Figure 8. Age–length relationships determined from whole Madeira (black dots) and Sesimbra (circles) otolith interpretation usingannual increments and from Madeira otolith microstructure examination (triangles). The number of presumed daily growthincrements (DGI) has been transformed to years.

sections was not advantageous because it resulted in theadded difficulty in locating the true increments, becausethe false rings became more evident. However, someinterpretation problems remain unsolved. The interpret-ation of the first increment is one of the main problemsdue to the variability in the morphology of the nucleus.Frequently, two to three faint rings are visible in thecentral area followed by a more marked one, consideredto be the first increment. Moreover, there is considerableotolith variability both between geographic areas andwithin the same sampling locality. Some of the discrep-ancies found in the age interpretation might be due tothe different geographical origin of the samples. Thecauses of this variability are not clear. Black scabbard-fish in the North Atlantic might be composed of morethan one stock and have complex migratory behaviourwith a complex spatial distribution, with both small andlarge fish present at Madeira and intermediate sizes inSesimbra. Spawning tends to occur over a restrictedperiod off Madeira. No spawning has been observed inother areas. There appears to be no seasonality in the

occurrence of small fish, which are found in smallnumbers throughout the year off Madeira (Morales-Ninand Sena-Carvalho, 1996). This possible presence ofmore than one stock might be also the cause of thepresence of two peaks in the increment measurementsand in the variability percentage of opaque increments inthe otolith edge.

The general interpretation criteria of following anincrement all around the otolith to determine if it is anannulus or a false ring could not be applied to blackscabbardfish, since it was often impossible to follow anincrement through the entire otolith perimeter. Also,there was considerable variability in the numbers ofrings in different zones (rostral, dorsal, ventral, andantirostrum) of the same otolith due to split rings.

Once a general methodology was defined some vali-dation was attempted. The indirect validation methodsused appear to support the view that the growth incre-ments are consistently interpreted. The decreasing inter-val between increments and the consistent incrementsizes are considered as indicators that the increments are


annual. However, the evolution of the otolith edge wasunclear and did not allow the period of incrementformation to be determined.

The daily nature of the presumed DGI has not beenproved, but if they are formed on a daily basis, then theage determined from their thickness and the otolithradius, shows a good agreement with the ages deter-mined from growth increments interpretation. Theotolith microstructure appears to support the interpret-ation criteria. The microstructure has revealed differ-ences between the false rings and growth increments,suggesting that the latter correspond to major periodicevents.

Similar accessory primordia to those found in blackscabbardfish have been described in flatfish and Gadidae(Morales-Nin, 2000) and are attributed to changes inotolith shape related to the transition of juvenile fishfrom the pelagic to the demersal habitat. Nothing isknown about the early life stages of this species, but theotolith structure suggests that they might have tran-sitional changes similar to the juvenile habitat migrationof flatfish and Gadidae, probably passing from beingpelagic larvae to benthopelagic juvenile and adults.

The question arose as to whether such large fish couldhave such low ages (12 years maximum age from wholeotoliths) and whether there were any comparable deep-water fish. In the Mediterranean Lepidopus caudatusbelonging to the same family, grows rapidly (Demestreet al., 1993). However, it was considered that it might bemore appropriate to think in terms of fish weight ratherthan fish length in a fish with such an elongated laterallycompressed body. The otolith microstructure appears tosupport the view that the fish live in quite good con-ditions that allow them to have good growth rates, asindicated by the width of what are considered to be dailyincrements (Morales-Nin, 2000).

References

Bergstad, O. A. 1995. Age determination of deep-water fishes:experiences, status and challenges for the future. In Deep-water fisheries of the North Atlantic Oceanic Slope,pp. 267–283. Ed. by A. G. Hopper. Kluwer AcademicPublishers, The Netherlands.

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Intercalibration of age readings of deepwater black scabbardfish, Aphanopus carbo (Lowe, 1839

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