M. Iglesias á E. B. Brothers á B. Morales-Nin

Validation of daily increment deposition in otoliths.Age and growth determination of Aphia minuta(Pisces: Gobiidae) from the northwest Mediterranean

Received: 30 December 1996 /Accepted: 16 April 1997

Abstract The transparent goby Aphia minuta (Risso,1810) is one of the main target species of the small-scale®shery o� the Island of Majorca. Otolith microstructureand length-frequency analysis were used to study the ageand growth of this species during the 1982/1983 and1992/1993 ®shing seasons. Daily periodicity of incre-ment formation was determined by experiments withmarked otoliths in individuals maintained in captivity.The length range of the catches during the 11 yr periodwas between 12 and 49 mm, with a main distribution(89%) between 24 and 40 mm. Otolith age-readings in-dicate that the population exploited in the commercial®shery consists of seven age-groups (2 to 8 mo old), witha very high proportion of individuals (95%) between 3and 6 mo old. Population growth-curves revealed nodi�erences between males and females. The growth pa-rameters for the whole population are: asymptotic length,L1 � 53:69mm; growth coefficient; K � 2:23 yrÿ1; the-oretical age at length zero, t0 � ÿ0:005 yr. Those indi-viduals of A. minuta caught in Majorca during the winterperiod reached a maximum age of 7 or 8 mo.

Introduction

The transparent goby Aphia minuta is a pelagic neriticgoby that belongs to a monotypic genus. The species is

common in the European Atlantic from Gibraltar to thecoasts of Norway and the Baltic Sea, and in the Medi-terranean, including the Black Sea and the Azov Sea(Tortonese 1975). This goby is the object of an impor-tant small-scale ®shery during the winter months (De-cember to March/May) o� the island of Majorca(Balearic islands) (arrowed in Fig. 1), on sand and mudbottoms between 15 and 40 m in depth (Iglesias andMartorell 1992; Iglesias et al. 1994). Annually, the catchcomprises '4:5% of the commercialised landings in the®sh market of Palma (sale centre for catches from thewhole island), and is normally caught in company with asmall number of two related goby species, Pseudaphiaferreri (De Buen and Fage, 1908) and Cristallogobiuslinearis (von DuÈ ben, 1845).

There are ®sheries similar to that of Majorca in otherareas of the Mediterranean (Fig. 1), such as the south-eastern Spanish coast (Martinez-BanÄ o et al. 1990, 1993),northern Tyrrhenian (Serena et al. 1990), centralAdriatic (Froglia and Gramitto 1989) and southern Ad-riatic (Rizzoli 1983; Ungaro et al. 1994).

Aphia minuta is a small species (not >60mm in totallength in the Mediterranean Sea) that gathers in shoalsclose to the bottom to spawn, after which it dies. Itprobably never lives longer than one year (De Buen1931). Little additional data are available on its lifehistory, and the biology of A. minuta is not very wellknown. Reproduction is from December to March in thewestern Mediterranean, in May in the Adriatic, fromMay to July in the Black Sea, and from July to August inthe Atlantic (Norway) (Fischer et al. 1987). It feeds onzooplankton, principally copepods and cirripede andmysid larvae (Miller 1986). The only available data onage and growth come from a preliminary study whichanalysed size frequencies and growth increments in theotoliths (Iglesias and Morales-Nin 1992).

In recent years, numerous studies have demonstratedthat the examination of otolith microstructure andenumeration of daily growth increments (DGI) allow theage of individual ®sh to be determined with great pre-cision. This technique is of greatest use for obtaining

Marine Biology (1997) 129: 279±287 Ó Springer-Verlag 1997

Communicated by A. RodrõÂ guez, Puerto Real

M. Iglesias (&)Instituto Espanol de Oceanografõ a,Centro Oceanogra ®co de Baleares, Apartado 291,E-07080 Palma de Mallorca, Spain

E.B. BrothersEFS Consultants, 3 Sunset West, Ithaca,New York 14850, USA

B. Morales-NinInstitut Mediterrani d'Estudis AvancË ats (IMEDEA),CSIC/UIB, Campus Universitario,E-07071, Palma de Mallorca, Spain

information on the growth of larvae and juvenile ®shesduring their ®rst year of life (Campana and Neilson1985; Jones 1992). The short life of Aphia minuta makesDGI counts the ideal approach for obtaining informa-tion on its age and growth. However, in order to de-termine age based on growth-increment counts, it isnecessary to validate the periodicity of deposition(Beamish and McFarlane 1983; Ge�en 1992).

The present study attempts to determine the size/agerelationship and the growth rate of Aphia minuta bymeans of otolith microstructure-analysis and length-frequency data. The periodicity of increment formationwas determined in experiments with marked otoliths ofindividuals maintained in captivity.

Materials and methods

Validation of daily increment formation in otoliths

In order to con®rm that the microstructural increments detected inthe otoliths of Aphia minuta (Risso, 1810) are formed daily, avalidation test was carried out in captivity by marking the otolithswith strontium chloride (SrCl2).

In March 1994, '100 individuals of Aphia minuta (between 20and 40 mm in length) were captured at a depth of 30 m and a watertemperature of 13.5 °C. On board ship, they were kept in a 50-litrecontainer without water renewal or aeration for 4 h. Upon landing,they were transported to an aquaculture station, where they wereplaced in a 250-litre tank, with ¯ow-through seawater, constantaeration and an initial in situ temperature of 14.5 °C. On the fol-lowing day, feeding with Artemia spp. (enriched with fatty acidsand vitamins) was commenced. After one week, the ®sh were ac-climatised to the environment and were responding favourably tothe diet. They were then marked by immersion in SrCl2 (a methodthat had been used with success for larvae, juveniles and adults ofother gobies: Brothers 1990). The procedure involved placing the®sh in a static aquarium with a solution of 1.25 g SrCl2 per litre

seawater for 24 h. After this time, the water was completely re-newed and the ®sh were returned to the initial holding conditions.This treatment produced bands of strontium-enriched material inthe otoliths that was detectable with a scanning electronic micro-scope (SEM), back-scattered electron image (BEI) and an energy-dispersive X-ray spectrometer (EDS) (Brothers 1990).

Fifteen days after being marked, 15 individuals were sacri®ced,and a similar number were subsequently sampled every 7 d. Thetotal number of marked and sacri®ced individuals was 61; the studyended in mid-May. Between March and the end of the experimentin May, the water temperature increased from 14.5 to 20 °C.

In both the light microscopy (LM) and SEM preparations, aclear mark was observed at a distance from the otolith (sagitta)edge. Once SEM had con®rmed that this mark corresponded withthe strontium mark in the otoliths of some individuals and that thethickness of presumptive DGIs was within the detection limit of theLM, the ®nal counting of the DGIs from the Sr mark to the otolithborder was carried out with LM. The resolution of the mark byLM is due to the contrasting optical properties (greater refractiveindex) of strontium carbonate compared to the calcium carbonatepredominating in the otolith.

Sampling

From the 1982/1983 to the 1992/1993 ®shing seasons (December toMarch/May), weekly and fortnightly (®shing season 1992/1993)length-frequency measurements were made on samples of Aphiaminuta obtained from landings of the commercial small-scale ®sh-ery at the central ®sh-auction wharf in Palma. Total ®sh length(TL) was measured to the nearest lowest millimetre (n � 40681).

Growth analysis was based on samples from the commercialcatches of Aphia minuta during the 1992/1993 ®shing season (De-cember to March). During this period, eight fortnightly sampleswere taken. Total length was measured to the nearest millimetre for1007 individuals whose sizes ranged between 14 and 44 mm. Withineach sample, three individuals representative of each size intervalwere selected for biological measurements (n � 394): wet weightwas measured to the nearest 0.01 g, sex was determined, and thesagittae were extracted.

The otoliths were cleaned with a solution of KOH (5%), driedfor 24 h, mounted on slides for microscopy with Petropoxy, andpolished with a polishing rotating wheel and 0.3 lm alumina. The

Fig. 1 Commercial ®shingareas for Aphia minuta in Medi-terranean Sea (d), showingstudy site (arrowed )

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sagittal sections thus obtained displayed a complete increment-counting path from the primordium to otolith margin. The sectionsfor SEM observation were decalci®ed to reveal the increment se-quence using 0.1 M EDTA (ethylenediamine tetraacetic acid)(Brothers 1990; Morales-Nin 1992).

Age interpretation

Increment reading was carried out under LM. Pending validation,growth increments were presumed to have been deposited daily andtotal counts could be equated to age in days. The microscope had ahigh-resolution television camera attached, and the image of theotolith was displayed on a closed-circuit television monitor. Theincrements were read on the monitor along the longest radius of theotolith, from the primordium to the margin. Two readings weremade for each otolith. The magni®cations used for reading theDGIs were 630 and 1000´, with oil immersion and plane-polarisedlight.

The mean of the two readings (when counts did not di�er by >5increments) was considered as the age in days for each individual.As it was not known when the formation of the increments began,the age obtained was approximate, and lacked a correction factorcorresponding to the period between hatching and initiation ofincrement deposition.

Otolith dimensions (minimum and maximum diameters) weremeasured using an analysis-image system (MIP, Microm EspanÄ a)with an iterative measurement application.

Growth calculation

The length±weight relationship was calculated for the 1992/1993®shing season both for males, females and indeterminates sepa-rately and for the population as a whole. For the entire 1985/1986to 1992/1993 study period, only the population relationship wasdetermined. The applied power regression equation was:

Total weight (mg) � a� total lengthb (mm) : �1�The allometric indexes (b) for the males, females and indeter-

minates, and the possibility of isometry were examined by a Stu-dent's t-test (Sokal and Rohlf 1981), using the expression:

t � b1 ÿ b2=Sb1ÿb2 ; �2�where Sb1ÿb2 � standard error of the di�erence between the re-gression coe�cients.

The growth parameters of the von Bertalan�y (1938) equa-tion were calculated for males and females and for the popula-tion as a whole by means of a statistical programme, Fishparm(Prager et al. 1987), based on the size±age pairs of values de-termined by the DGI count in the otoliths and using the Mar-quadt algorithm for the iterative calculation of the parameters.Growth parameters based on length-frequency were also esti-mated using the Complete Elefan software package (Gayaniloet al. 1988) for the 1992/1993 ®shing season and for all ®shingseasons (1982/1983 to 1992/1993) combined.

As L1 and K are correlated, the growth performance indexU � 2 log L1 � log K (Munro and Pauly 1983) was used to com-pare the growth parameters estimated by these two methods withthe growth parameters of other gobies recorded in the literature.

Weight, W1 was calculated using the length±weight relationshipparameters for the entire population and the L1 calculated bymeans of the DGI count:

W1 � a� Lb1 : �3�

Results

Length±weight relationship

Table 1 gives the parameters of the length±weight rela-tionship for male, female and indeterminate Aphia mi-nuta alone, and for the whole population. Positiveallometric growth was observed in both sexes and for thewhole population. A Student's t-test did not reveal anysigni®cant di�erences between the allometric coe�cientin any of the size-ranges considered ( p < 0:05).

Otolith characteristics

The sagittae of Aphia minuta were of a concave±convexform, disc-shaped for small-sized individuals and be-coming slightly oval in form with increasing size. In oursamples, minimum diameter (Dmin) ranged between 0.26and 0.82 mm, and maximum diameter (Dmax) was be-tween 0.31 and 1.16 mm. Relationship between thesediameters and total ®sh length was linear for each otolithdiameter, con®rming a direct relation between oto-lith diameter and ®sh growth:

Dmin � ÿ0:08806� 0:02032 TL r2 � 0:89 ; �4a�Dmax � ÿ0:15738� 0:02725 TL r2 � 0:87 : �4b�

LM observation revealed an elongated primordium(Fig. 2a), occasionally slightly expanded at the extrem-ities and surrounded by increments composed of cleardark (D) and light (L) zones similar to deposition layersof other ®sh species. The ®rst increments formed aroundthe primordium were very ®ne and, in some cases, dif-®cult to resolve (arrowed in Fig. 2a). The thickness ofthe increments increased gradually with otolith growthbut with no sign of secondary nuclei or transition zones,

Table 1 Aphia minuta. Esti-mated parameters a and b ofrelationship y � a� xb betweentotal length and total weight (r2

determination coe�cient; nnumber of specimens)

a b r2 (n) Range(mm)

1992/1993Males (M) 0.000492 3.64 0.95 (182) 23±44Females (F) 0.000529 3.62 0.96 (168) 22±44M+F 0.000512 3.63 0.95 (350) 22±44Indeterminates (I) 0.000633 3.54 0.92 (117) 16±38Total (M+F+I) 0.000407 3.69 0.96 (467) 16±44

1985/1986 to 1993/1994Total 0.00071 3.55 0.93 (1965) 16±45

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and then diminished in the marginal zone of the largersized specimens (Fig. 2b).

Those sections etched with EDTA and observed bySEM showed the microarchitecture of Aphia minutaotoliths to be characterised by small needle-shapedcrystals. This particular microstructure and the contin-uous addition of crystalline prisms perpendicular to theotolith surface made it di�cult to obtain sections withde®ned DGIs observable by SEM. The LM preparationsallowed a greater depth of ®eld, and revealed the in-crements with greater clarity (Fig. 2b).

DGI validation

SEM observations demonstrated that we could clearlydistinguish the strontium-enriched band as a brighterconcentric mark in the sectioned otoliths of labelled ®sh.At greater magni®cation (LM: Fig. 2c), it was apparentthat the number of rings between the strontium markand the edge of the otolith coincided with the dayselapsed between labelling and subsequent sacri®ce of theindividuals (Otolith F17, Table 2).

A total of 18 otoliths (30% of those labelled) dis-played a clear strontium mark with well-de®ned incre-ments, allowing a DGI count between the mark and the

otolith margin (Table 2). The relationship between thenumber of DGIs and the days elapsed (DE ) from la-belling was not statistically di�erent from 1:1.

DGI � DE ÿ 0:13723 r2 � 0:99 : �5�Thus, the increments formed in the otoliths of Aphia

minuta were deposited daily. In the micrographs(Fig. 2d) a zone of slow otolith growth (®ne increments)appears just before the strontium mark in the otolith.This zone consisted of approximately seven increments��7 d�, a period that corresponds to the interval be-tween the capture of the individuals and their subse-quent marking.

Otolith reading and age determination

The size range of those individuals whose otoliths werereadable �n � 241� was 14 to 44 mm, which covers thewhole length range of this species that is present inMajorca waters during the ®shing season. The DGIcount, corresponding to days of life, ranged between 63and 250.

The age±length relationship (in months) obtainedwas used to ®t the von Bertalan�y growth-curve formales and females (Fig. 3). The ages of both sexes and

Fig. 2 Aphia minuta. Micrographs (LM) of sagittal otolith showingelongated primordium (arrowed ) typical of Gobiidae and narrow ®rstincrements (a), thickness of increments (b), increments laid downbetween strontium-rich band produced by marking (arrow) and

sacri®ce date (c), and changes in thickness of daily growth increments(arrowheads) 7 d before marking resulting from capture stress (d) (Allscale bars � 10 lm)

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indeterminates ranged from 2 mo (63 d) to 8 mo (250 d)(Tables 3, 4 and 5). The majority of individuals (95%)were 2 to 6 mo old (Table 5). Representatives of theyoungest ages (0 to 1 mo), corresponding to the earlydevelopment, were not found. The older age groups(7 and 8 mo old) were poorly represented in the popu-lation (3.7 and 1.2%, respectively) (Table 5).

These data con®rm that the individuals of Aphiaminuta caught in Majorca during the winter period didnot exceed 1 yr of age, and indicate that this speciesattains a maximum age of 7 or 8 mo.

Growth parameters

The parameters of the von Bertalan�y growth equationestimated by the DGI count in the otoliths (Fishparm)are summarised in Table 6. Females had a higher growthcoe�cient and a lower theoretical asymptotic lengththan males, although the growth performance index, U,was very similar.

The growth coe�cient of this species (K � 2:23) ishigh, in accordance with its short life-span. The highestlength observed in the catches was 49 mm; thus, thecalculated length, L1, for males (61.01 mm: Table 6) issigni®cantly higher than the observed values.

Length frequency

The monthly length-frequency distribution in thecatches of Aphia minuta ranged between 12 and 49 mm,with mean size increasing throughout the ®shing season

Table 2 Aphia minuta. Marking experiment (1994). Number ofdays between marking and sacri®ce and number of daily growthincrements (DGI ) in otoliths (numbered A1 to F17 ) with clearmarks (M males; F females; TL total length)

Days after marking(sacri®ce date)

TL(mm)

DGIafter Sr band

Sex

15 d (27 Mar.)A1 41 15 MA3 42 15 FA8 40 15 FA11 40 15 FA12 41 15 F

22 d (3 Apr.)B1 40 22 MB4 44 22 MB5 38 22 MB10 43 21 FB11 42 22 F

29 d (10 Apr.)C10 41 29 F

37 d (18 Apr.)D1 45 37 MD2 39 36 FD8 44 37 FD9 47 37 M

43 d (24 Apr.)E1 38 44 F

53 d (4 May)F8 40 52 FF17 44 52 M

Fig. 3 Aphia minuta. Growth curves for males (n � 118), for females(n � 123), and for whole population (n � 241)

Table 3 Aphia minuta. Age±length relationship of males. Data areage in months of various age classes at given sizes (TL total length;n number of specimens; x mean length)

TL(mm)

Age classes

II III IV V VI VII VIII

16 120 221 3 222 2 123 1 2 124 3 125 2 1 126 5 327 2 528 2 4 129 1 5 130 1 2 231 4 332 2 433 1 3 134 1 635 5 236 2 4 237 1 4 2 138 1 139 1 2 240 141 1 142 144 1

(n) (10) (19) (41) (34) (8) (5) (1)x 21.8 25.32 30.17 33.68 37.25 40.2 41SD 2.27 2.7 4.41 3.2 2.28 2.48% 8 16 35 29 7 4 0.8

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from December (when the ®rst shoals appeared) untilApril (the end of the ®shing season). The larger sizesdisappeared in April, with a sharp decline of the abun-dance of ®sh >41mm (Fig. 4).

Examination of the distribution of mean length dur-ing the 1982/1983 to 1992/1993 ®shing season (Fig. 5)revealed monthly variations in the growth rate. Meanlength increased rapidly from December to February(3.16 mm increase from December to January; 2.22 mmincrease from January to February), and then decreasedby 1.32 mm between February and March and increasedagain by 2.32 mm between March and April. The de-crease in May may have been due to the disappearanceof the larger individuals due to migration or mortality.

Based on the length frequencies, the parameters ofthe von Bertalan�y equation were calculated using theElefan method (Gayanilo et al. 1988), ignoring di�er-ences between the sexes (Table 7). The data supportedthe otolith results.

Discussion and conclusions

The sagittae of Aphia minuta are of similar morphologyand structure to those of other gobies, with the charac-teristic primordium ± elongated and with a centralconstriction ± described by Brothers (1984) for 15 genera

of gobies and related families (Gobiidae, Microdes-midae, Eleotridae, Gobioididae), and not observed inany other group (see also Johnson and Brothers 1994).The otoliths of A. minuta di�er from the sagittae ofother gobies in not bearing a ``settling mark'' (transitionmark). Its absence is probably related to the pelagiccharacter of the transparent goby adults. A. minuta,together with Pseudaphia ferreri and Cristallogobiuslinearis, constitute the only group of gobies in theMediterranean that, apart from the mating season, havea pelagic mode of life.

Table 4 Aphia minuta. Age±length relationship of females. Furtherdetails as in legend to Table 3

TL(mm)

Age classes

II III IV V VI VII VIII

14 117 120 221 1 2 122 3 223 2 324 1 225 4 126 1 2 227 3 328 2 3 229 2 2 230 2 4 331 5 132 1 3 333 2 334 4 3 135 3 3 236 2 1 137 2 238 1 2 3 139 2 1 140 1 3 1 141 1 242 1 1

(n) (12) (25) (39) (28) (13) (4) (2)x 21.17 25.68 31.36 33.64 38.08 39.75 41SD 3.05 2.94 3.97 3.52 2.3 1.48 1% 9.7 20 32 23 11 3 1.6

Table 5 Aphia minuta. Age±length relationship for whole popula-tion (i.e. males, females, and indeterminates). Further details as inlegend to Table 3

TL(mm)

Age classes

II III IV V VI VII VIII

14 116 117 120 2 221 4 2 322 5 323 3 5 124 4 325 6 2 126 1 7 527 5 828 4 7 329 3 7 330 3 6 531 9 432 1 5 733 3 6 134 5 9 135 8 5 236 4 5 337 3 6 2 138 2 3 3 139 3 3 340 2 1 3 1 141 1 3 142 3 144 1

(n) (22) (44) (80) (62) (21) (9) (3)x 21.45 25.52 30.75 33.66 37.76 40 41SD 2.74 2.84 4.24 3.35 2.33 2 0.82% 9.1 18.3 33.2 25.7 8.7 3.7 1.2

Table 6 Aphia minuta. Growth parameters determined for age±length relationship obtained by means of DGI counts (Fishparm:Prager et al. 1987) (L1 asymptotic length; K growth coe�cient; t0theoretical age at length zero; U growth-performance index; ASEasymptotic standard error)

Males (ASE) Females (ASE) Total,males+females (ASE)

L1(mm) 61.01 (17.59) 49.58 (5.84) 53.69 (6.63)K yr)1 1.68 (0.98) 2.76 (0.87) 2.23 (0.65)t0 (yr) )0.036 (0.068) 0.019 (0.039) )0.005 (0.04)U 3.79 3.84 3.81

284

The present study has con®rmed the periodicity ofincrement deposition to be daily, at least in the sizeclasses studied. However, without direct knowledge ofthe age of an individual at initiation of incremental

growth, the age obtained from counts can only beapproximate; it lacks a correction factor correspondingto the period between fertilisation or hatching andthe formation of the ®rst counted increment. Thisvalue is expected to be small, amounting to but a fewdays.

The percentage of otoliths with clear strontium marksand de®ned DGIs after the mark was not very high(30%). This may have been the result of stress inducedby capture, handling, con®nement, and subsequent la-belling, and the maintenance conditions in captivity ±which were not optimal for growth. No previous studiesare available which document experiments of this typewith Aphia minuta. The diet, based on Artemia spp.,although adequate for larvae and juvenile ®sh, may nothave satis®ed the metabolic requirements of adult Aphiaminuta that feed principally on zooplankton (copepods,cirripede larvae and mysids). It was not practicallypossible to reproduce the composition of the naturaladult diet. Another important factor that could havea�ected the irregular laboratory growth was the gradualincrease in the water temperature of the holding tanksthroughout the experiment (14.5 °C in March to 20 °Cin May), due partly to their limited capacity (250 litres)and their location inside a greenhouse. In the bays whereA. minuta are ®shed, during the months in which thespecies is caught the water temperature is only 13 to16 °C. Various authors have demonstrated that elevatedtemperatures can provoke premature ageing of ®sh (Liuand Walford 1970; Craig 1979; Craig and Fletcher1984). However, the experimental conditions can nothave been excessively inadequate, since some individualsspawned in empty mussel shells on the bottom of thetank, although the eggs were not fertile. This spawningactivity may also have a�ected the growth rate. In themajority of otoliths of freshly captured adult specimens,increments deposited close to the edge were very narrow.Narrow increments and sexual maturity may be related.The study period corresponded to the spawning season,with a progressive increase in the percentage of maturespecimens in the catches (Iglesias unpublished data).

Some otoliths exhibited a decrease in incrementthickness corresponding to capture and acclimatisationto maintenance conditions (Fig. 2d). As somatic growthand otolith growth are generally closely related, incre-ment thickness can be expected to vary as a function ofgrowth rate (Gutierrez and Morales-Nin 1986; Campanaand Jones 1992), at least over periods of several days orlonger. Narrower increment marks probably result fromstress produced by capture and coincident environmen-

Fig. 4 Aphia minuta. Monthly length-frequency distribution (1982/1983 to 1992/1993) (n number of individuals sampled)

Fig. 5 Aphia minuta. Mean monthly growth rates. Line connectsmeans

Table 7 Aphia minuta. Growth parameters determined by means oflength-frequency analysis (Elefan: Gayanilo et al. 1988) (L1 asymp-totic length; K growth coe�cient; U growth-performance index)

Period L1(mm) K yrÿ1 U

1982/1983 to 1993/1994 53 2.13 3.781992/1993 47 2.36 3.72

285

tal changes that caused a decrease in otolith growth-rateand probably somatic growth. Stress associated withcapture appears to have a�ected the deposition rate ofboth the D- and L-zones of the increments.

The growth parameters determined by means ofotolith interpretation indicated a rapid growth rate.Similar data were obtained when we used a performanceindex (U) to compare growth model parameters (L1 andK ) with those of other gobies ± Pomatoschistus minutusfrom the Wadden Sea (Fonds 1973) and Gobius nigerfrom the Adriatic Sea (Fabi and Giannetti 1985) (seeTable 8 of present study).

The length range sampled during the 11 yr studyperiod was 12 to 49 mm. The net used retains 100% of®sh from �10:5mm (Serena et al. 1990). Thus, the ab-sence of small ®sh and the scarcity of individuals from12 to 24 mm could be related to the reproductive natureof the schools. The youngest ®sh sampled were 2 mo old;this age corresponds with the onset of maturation. Atthe end of the ®shery in April, the mean ®sh age was6 mo, with very few 7 and 8 mo-old ®sh present. Thelow abundance of the older ®sh could be related tochanges in the species' depth distribution after spawningor to a high mortality rate. However, the observed re-cuperation of the growth rate both as mean length andin the DGI during the postspawning period in Aprilsuggests that the ®sh were recovering. Thus, some ofthem may survive to be captured occasionally in summer(40 to 50 mm TL) (personal communications of local®shermen). Also, blue®n-tuna with stomachs full ofAphia minuta are reported to have been caught in Au-gust 1994 (personal communication of local ®shermen).In occasional catches o� the coast of Malaga (SE Spain)mature A. minuta (mean length 39 mm) were found at adepth of 80 m during October (Cruise Mersel 10/1994,Instituto EspanÄ ol de OceanografõÂ a).

The length range recorded for the catches suggeststhat the Mediterranean Aphia minuta corresponds to theproposed dwar®sh race postulated by De Buen (1931).The Atlantic race is considerably larger, with a maxi-mum length of 72 mm in the Ria de Arosa (NW Spain)(Iglesias 1981) and 79 mm in the Ria de Aveiro Lagoon(Portugal) (Arruda et al. 1993). In other areas of theMediterranean (Adriatic Sea) A. minuta is caught duringits reproductive aggregations in spring and summer, andhere a maximum total length of 60 mm was reported byMancini and Cavinato (1969) and of 56 mm by Frogliaand Gramitto (1989).

The present study has documented the presence ofAphia minuta in Majorca waters in the period December

to March/May, with the ®sh aggregating within bays ata depth of 5 to 40 m for reproductive activities. The onlyavailable data were from catches of the commercial®shing ¯eet. When the ®shing season for this speciesends, the ¯eet concentrates on other species (Iglesiaset al. 1994). A. minuta probably occurs all-year-roundclose to the coast of Majorca, although when the meansurface-water temperature increases in summer (14 °C inwinter, 23 °C in summer), the species may leave theshallow areas of the bays for greater depths.

Our results demonstrate that the ®sh captured by the®shery were spawned the previous autumn (Septemberto October), suggesting that there is more than one co-hort per year.

Acknowledgements We are grateful to all persons from the aqua-culture station ``Es Murtera '' (Alcudia) and especially toS. Massutõ .

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Table 8 Von Bertalan�ygrowth parameters (L1 and K )and growth-performance index(U) for various gobids

Species L1(mm) K yrÿ1 F Method (source)

Aphia minuta 53.69 2.23 3.81DGI counts (Fishparm)Aphia minuta (males) 61.01 1.68 3.79

Aphia minuta (females) 49.58 2.78 3.84Pomatoschistus minutus 89 0.92 3.86 Length-frequency analysis (Fonds 1973)Gobius niger (males) 185 0.30 4.00 Otolith reading (Fabi and Giannetti 1985)Gobius niger (females) 169 0.19 3.74

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