Age determination and growth of turbot and brill in the Adriatic Sea: reversalof the seasonal pattern of otolith zone formation

By E. Arneri, S. Colella and G. Giannetti

Istituto di Ricerche sulla Pesca Marittima (IRPEM), Consiglio Nazionale delle Ricerche, Largo Fiera della Pesca, Ancona, Italy

Summary

The growth of two commercially important ¯at®sh, turbot

(Psetta maxima) (L.) and brill (Scophthalmus rhombus) (L.),was investigated in the Adriatic using whole otoliths (sagittae)and stained otolith sections. At variance with the pattern

usually observed in temperate seas, the opaque zone was foundto be laid down in autumn and winter, and the translucentzone in spring and summer. Growth rates di�ered according to

sex, with the females attaining greater body lengths. Thevon Bertalan�y growth parameters were: L¥� 66.2 cm,K� 0.31 years±1, and t0� ±0.14 years for turbot males,L¥� 81.5 cm, K� 0.21 years±1, and t0� ±0.48 years for turbot

females; L¥� 40.2 cm, K� 0.49 years±1, and t0� ±1.03 yearsfor brill males; L¥� 50.1 cm, K� 0.27 years±1, and t0� ±1.75years for brill females. Growth rates and maximum age

recorded for turbot were comparable to those reported in theNorth Sea.

Introduction

Turbot, Psetta maxima (L.), and brill, Scophthalmus rhombus(L.), are widely distributed from Norway to the Mediterranean

and the Black Sea (Nielsen 1986). They inhabit sandy andmuddy bottoms in the shallower part of the continental shelf toa depth of about 70±80 m. In the Adriatic, where they have

always been highly prized commercial ®sh (Faber 1883), theyare caught mainly by bottom otter and beam trawls, as well asby ®xed gear such as trammel nets and bottom long lines. In the

North Sea (Rae and Devlin 1972; Jones 1974), their contribu-tion to the ®shing industry lies not in the absolute weight oflandings but rather in their high market value. Since in o�cial

catch statistics the two species are often pooled together, andbecause small individuals tend to be confused with other¯at®shes, such data are unreliable. In one study, the annualcatch of turbot and brill in the Adriatic was tentatively

estimated at 400±500 tonnes (Cingolani et al. 1986).Otoliths have long been used to estimate age of turbot in the

North Sea (Mengi 1963; Rae and Devlin 1972; Jones 1974),

and turbot and brill in the north-western Mediterranean(Robert and Vianet 1988) by employing various preparationtechniques (Hassager 1991) based on the observation that

opaque and translucent zones are alternately formed each yearon the otolith, with a roughly seasonal pattern. In temperatewaters the opaque zone is usually laid down in the warm

season, which corresponds to a period of fast ®sh growth; thetranslucent zone is usually laid down in winter, when growth isslow (Pannella 1974; Williams and Bedford 1974). This patternis reversed in sole (Solea vulgaris Quensel) in the Adriatic

(Froglia and Giannetti 1986), and in sole (S. vulgaris), plaice

(Platichthys ¯esus) (L.), brill (S. rhombus) and turbot

(P. maxima) in the Gulf of Lions (Golfe du Lion) (NWMediterranean) (Robert and Vianet 1988; Vianet et al. 1989).The present study provides evidence of the reversal of the

seasonal pattern of zone formation on the otoliths of turbotand brill in the Adriatic and describes for the ®rst time thegrowth rates of these species, no other studies being available

in this area.

Materials and methods

The material for this study has two sources: one is from a seriesof samples collected by IRPEM (Istituto di Ricerche sulla PescaMarittima) in the Adriatic from 1974 to 1996 on the occasion of

various scienti®c activities at sea, and the second is a series ofsamples obtained from the landings of commercial ®shingvessels at the port of Ancona (central Adriatic) fromNovember

1994 to July 1996 in the framework of a European Union-funded research project (Study Contract MED/93/017). Be-cause of their high market value, pooling the samples fromdi�erent years and sampling activities was necessary to obtain a

su�cient number of specimens. The ®shing gear used by theresearch vessel and the commercial ¯eet were in most casesItalian bottom otter and beam trawls with a mesh size of 40 mm

(stretched) or less at the cod end. Bias caused by gear selectivityin sampling young age-classes was thus negligible. Occasionallylarge individuals were obtained from bottom long lines. For

each ®sh total length (TL), weight and sex were recorded beforeremoving and dry-storing the otoliths. Overall, the otoliths of414 brill, S. rhombus, ranging from 11 to 50.5 cm TL, and 227

turbot, P. maxima, ranging from 4 to 79 cm TL, were studied.The otoliths were placed in ethanol and observed whole

using a stereomicroscope under re¯ected light against a darkbackground. The annual periodicity of zone formation was

validated for both species by marginal-increment analysis(Morales-Nin 1992), i.e. by classifying the margins as opaqueor translucent on otoliths collected in di�erent months of the

year. One otolith (usually dorsal) was then embedded in epoxyresin and thin-sectioned (75±80 lm) using a low-speed saw(Remet Micromet). The sections were stained with acidi®ed

neutral red (Richter and McDermott 1990; R. Millner,CEFAS, Lowestoft, UK, personal communication) andobserved with a stereomicroscope under re¯ected light. Theconventional birth date for both species was considered to be 1

January, and the ®sh were aged by counting the number oftranslucent zones on the whole otolith and on the stainedsection. In ®sh caught from July to December, the translucent

zones on the otolith margin were considered as recently laiddown and therefore excluded from the calculation. Fish were

J. Appl. Ichthyol. 17 (2001), 256±261Ó 2001 Blackwell Wissenschafts-Verlag, BerlinISSN 0175-8659

Received: May 2, 2000Accepted: September 1, 2000

U.S. Copyright Clearance Centre Code Statement: 0175-8659/2001/1706±0256$15.00/0 www.blackwell.de/synergy

aged separately by sex; 22 turbot and 71 brill belonging to age-classes 0 and 1 and indeterminate were added to both sex

groups because no evidence of growth di�erences between thesexes was observed in the ®rst 2 years of life. A conventionalage in months was then attributed, based on the conventional

birth date and date of capture. Only individuals of age-class 1and older were considered as fully recruited to the sample andused in the computation of the growth curves. A vonBertalan�y growth function was ®tted to all length-at-age

data in each age class for each species, separately for males andfemales, using nonlinear regression analysis (Vaughan andKanciruk 1982) to obtain estimates of asymptotic length (L¥),

growth coe�cient (K), theoretical age at length zero (t0), andtheir 95% con®dence intervals.Length±weight relationships were computed in the form

W� a ´ Lb, where W is the weight in grams without guts andgonads, L is total length in centimetres and a and b areconstants. Data were ®rst linearized by log-transformation,and an analysis of covariance (F-test, Snedecor and Cochran

1967) was performed to test for di�erences in the length±weight relationships between the sexes. Student's t-test

(Snedecor and Cochran 1967) was used to check for isometry(b� 3) or allometry (b ¹ 3) in the length±weight relationship.

Results

Whole otoliths of both species viewed under re¯ected lightgenerally presented a small central opaque zone presumably

laid down in the ®rst weeks of life, then a wide, translucentzone laid down during the ®rst summer followed by an opaquezone deposited in winter (Figs 1 and 2). The opaque zone was

formed approximately from November to April and thetranslucent zone from May to October (Fig. 3). The patternwas clearly evident in the ®rst 2±3 years of life, but also could

be seen in most older ®sh. One opaque and one translucentzone were laid down each year in both species, although brillwere in general about 1 month in advance in the process ofzone formation.

Fig. 2. Otoliths of brill viewed underre¯ected light against a dark back-ground. Left: brill caught in Augustwith translucent margin laid down insummer; right: brill caught in Januarywith opaque margin laid down inwinter. Scale bar 1 mm

Fig. 1. Otoliths of turbot viewedunder re¯ected light against a darkbackground. Left: turbot caught inApril with opaque margin laid down inwinter; right: turbot caught in Octoberwith translucent margin laid down insummer. Scale bar 1 mm

Age determination of turbot and brill in the Adriatic Sea 257

In specimens of both species older than 4±5 years, thealternate opaque and translucent zones became increasingly

di�cult to read owing to increasing otolith thickness. Theobservation of stained sections yielded clearer results (Figs 4and 5) because the sliced surface exposed the entire internal

structure of the otolith and the staining enhanced the contrastbetween the translucent and opaque zones. As the number oftranslucent zones on the whole otolith and on the stainedsections closely corresponded up to 5 years of age, the

validation of the annual periodicity of translucent-zoneformation obtained using the whole otolith was also assumedto apply to the sections. In older individuals, the number of

translucent zones in the section was sometimes much higherthan the number observed on the unsectioned otolith; thisdiscrepancy was ascribed to the increasing thickness and

diminishing transparency of the otolith. The length-at-age dataused to compute the von Bertalan�y growth curves (Table 1;

Fig. 4. Stained thin section of turbototolith (total length 77 cm): estimatedage 15 years. Scale bar 1 mm

Fig. 5. Stained thin section of brillotolith (total length 49 cm): estimatedage 12 years. Scale bar 1 mm

Fig. 3. Monthly percentage of turbot (black bars, n� 164) and brill(white bars, n� 160) with opaque zone on the otolith margin versusmean temperature in the central Adriatic in the 50±80-m depthrange

258 E. Arneri et al.

Figs 6 and 7) were thus based on the age obtained from thesections. While in some cases only a study of the sectionsallowed the ®sh to be aged, about 5% from both species wereunreadable even when applying this method and were thus

discarded.The F-test revealed no signi®cant di�erences (P < 0.05)

between the sexes in the allometric parameter b of the length±

weight relationship in either turbot (F� 0.0002 df 1, 151) orbrill (F� 1.58 df 1, 145). Parameters of the length±weightrelationships for both sexes combined were a� 0.011,

b� 3.104 (n� 155, r2� 0.99, range 25±79 cm) for turbot anda� 0.016, b� 2.928 (n� 149, r2� 0.98, range 15±49 cm) forbrill. Student's t-test detected signi®cant positive allometry

(b > 3; P < 0.05) in both sexes of turbot, but not in brill.

Discussion

According to Artegiani et al. (1997), two extreme seasons areclearly distinguishable in the Adriatic: winter from January to

April, and summer from July to October. At 50±80-m depths,where turbot and brill spend most of their life, the average

monthly temperature in the Adriatic shows a minimum ofaround 11°C from February to April, an increase from May toNovember up to about 16°C, with a sharp decrease in

December and January (Fig. 3; IRPEM ATOS data, Artegianiet al. 1997). The period of opaque-zone formation reasonablycorresponds to the low-temperature winter months, whilesummer is de®nitely a period of translucent-zone formation.

Thus, if a link between sea temperature and type of zoneformation does exist, it is reversed in these ®sh. In theintermediate seasons this correspondence is less clear: in

November and December the temperature is still high, butbrill are already forming an opaque zone; in May turbot arestill laying down an opaque zone as the temperature increases.

The factors that control zone formation are probably notonly exogenous, such as temperature, but also endogenous(Beckman and Wilson 1995): both species spawn in winter orat the end of it, both in the Mediterranean (Tortonese 1975)

and in the Adriatic (V. Caputo, Istituto di Biologia e Genetica,UniversitaÁ di Ancona, Italy, personal communication) andopaque-zone deposition corresponds broadly to the period of

gonad maturation. It has been observed that in the Adriaticthe spawning period of brill precedes that of turbot by a coupleof months (Tortonese 1975; V. Caputo, Istituto di Biologia e

Genetica, UniversitaÁ di Ancona, Italy, personal communica-tion); this is re¯ected by the advance of approximately1 month in the beginning of the opaque-zone deposition

observed in the present study.In cold and temperate waters, the pattern of an otolith

usually shows an opaque zone laid down in the warm season,which broadly corresponds to a period of intense ®sh growth.

The deposition of the translucent zone takes place in the coldermonths of the year, when ®sh growth is slower (Pannella 1974;Williams and Bedford 1974). Our data do not allow us to state

whether opaque-zone deposition in winter is associated withfaster ®sh growth or, alternatively, that the translucent zonecorresponds to faster growth in summer. We suspect that in

the Adriatic the growth of turbot and brill is fairly continuousthroughout the year, independently of the zone being laiddown on the otolith. In our 0-group samples, turbot wereusually about 5 cm long in June±July and around 19 cm in

January, while age-class 1 showed an average length of around25 cm. Brill belonging to the 0 group (winter-born) averagedabout 13 cm in July, 20 cm in October and 27 cm at 1 year.

Our data for young brill suggest that summer is a period ofintense growth associated with the deposition of the translu-cent zone. Further circumstantial evidence for intense growth

in summer is provided by the width of the ®rst translucent zone(Figs 1 and 2). High growth rates and translucent-zonedeposition were also observed in summer by Vianet et al.

(1989) in 0-group plaice (P. ¯esus) in the Gulf of Lions. Highgrowth rates in summer associated with translucent-zonedeposition are likely to be a common feature of ¯at®shes(sole, plaice, turbot and brill) in the northern Mediterranean

(Adriatic and Gulf of Lions).

Fig. 6. Von Bertalan�y growth curves and experimental data for male(dashed line, black dots) and female (full line, white dots) turbot

Table 1Von Bertalan�y growth parametersfor male and female turbot(P. maxima) and brill (S. rhombus)with 95% con®dence intervals;n number of specimens

L¥ (cm) K (years)1) t0 (years) n

P. maxima males 66.2 (� 3.7) 0.31 (� 0.07) ) 0.14 (� 0.34) 112P. maxima females 81.5 (� 8.4) 0.21 (� 0.06) ) 0.48 (� 0.43) 120S. rhombus males 40.2 (� 4.2) 0.49 (� 0.28) ) 1.03 (� 0.86) 104S. rhombus females 50.1 (� 3.4) 0.27 (� 0.08) ) 1.75 (� 0.64) 114

Fig. 7. Von Bertalan�y growth curves and experimental data for male(dashed line, black dots) and female (full line, white dots) brill

Age determination of turbot and brill in the Adriatic Sea 259

The growth rate of turbot in the Adriatic is relatively fast inthe ®rst 5±6 years, slowing down after the 8±9th year. Femalesshow a greater average size±at±age from 3 years onwards(Fig. 6) and larger asymptotic size (Table 1). Growth curves

obtained by di�erent authors using di�erent methodologies aredi�cult to compare; only preliminary conclusions can bedrawn. For instance, the length-at-age data are broadly

comparable with those reported for the Scottish area of theNorth Sea by Rae and Devlin (1972) and those reported byIvanov and Beverton (1985) for the Black Sea (Table 2). The

growth coe�cient (K), indicating the speed at which asymp-totic size (L¥) is reached, does not vary too much in thevarious areas (Table 2), therefore di�erences in asymptotic

sizes might indicate di�erences in growth rates. In the Adriatic,the growth rate is slightly higher than in the North Sea (Mengi1963; Jones 1974) and considerably higher than in the BalticSea (Szlakowski 1990) and the north-western Mediterranean

(Robert and Vianet 1988). Although turbot is geneticallyhomogeneous throughout its geographical range (Blanqueret al. 1992), its growth rate shows a high geographical

variability; Adriatic turbot appear to belong to the populationgroup with the fastest growth rates. A possible reason for thehigh growth rate of turbot in the Adriatic could be that the

temperature never falls below levels inhibiting growth in thisspecies (Jones 1973; Iglesias et al. 1987; Burel et al. 1996;Imsland et al. 1996), although food availability throughout theyear must also play an important, if unknown, role.

Growth of brill in the Adriatic is rapid during the ®rst2 years of life, then drops very quickly and further slows afterthe 5±6th year. For this reason the von Bertalan�y growth

curve gave high negative values of theoretical age at lengthzero (t0) and a high value of the growth coe�cient (K). As inturbot, females showed greater average size±at±age from 3

years onwards (Fig. 7) and larger asymptotic sizes (Table 1).Brill growth was studied in the Gulf of Lions by Robert andVianet (1988), who reported similar fast growth rates (Table 2)

in the ®rst few years as well as asymptotic lengths slightlylower than those recorded in this study.Since turbot older than 10 years and brill older than 6 years

were few in our samples, our estimates of the asymptotic

lengths must be considered with some caution. Such a lownumber of older individuals is not surprising in light of theheavy exploitation of demersal resources in the Adriatic

(Arneri 1996). Indeed, an even smaller age range could havebeen expected since Robert and Vianet (1988), for instance,found no individuals of either species older than 6 + years in

the Gulf of Lions, another intensely exploited area of theMediterranean. The maximum age recorded for turbot in oursample, 18 years, could thus indicate that these ®sh have agreater potential longevity in the Adriatic, comparable to that

reported for the North Sea.

Acknowledgements

This work is part of a study of the growth, feeding habitsand reproductive biology of turbot and brill funded by the

European Union (Contract MED/93/017). We are indebtedto Drs Maria Emilia Gramitto and Carlo Froglia ofIRPEM, Ancona, Italy and to Dr Vincenzo Caputo of the

University of Ancona, Ancona, Italy who participated in thestudy and shared with us their ®ndings on feeding habits andreproductive biology. We are also grateful to Dr AnielloRusso of IRPEM for the oceanographic data, and to Dr

Silvia Modena who reviewed the English version of themanuscript.

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Author's address: Enrico Arneri, Istituto di Recerche sulla PescaMarittima, Consiglio Nazionale delle Ricerche,Largo Fiera della Pesca, I-60125 Ancona, Italy.E-mail: arneri@irpem.an.cnr.it

Age determination of turbot and brill in the Adriatic Sea 261