Copyright © 2000 Kluwer Academic Publishers. Printed in the Netherlands.Basic biological information for a sublittoral population of the rock goby Gobius paganellus Linnaeus, 1758 is presented based on a 2-year study involving 1680 specimens. The length-weight relationship was given by TW = 0.0089 * TL^3,163 (where TW= total weight in g; TL= total length in mm). Age at length data were inferred by modal analysis of the monthly length-frequency distributions. The parameters of the fitted Von Bertalanffy growth equation (with seasonal component, birth date on the 1st of January) were L1 = 13.8 cm; K = 0.73 yr^-1; to = -0.22 yr; C = 0.95; W = 0.07. This growth rate is much higher than that described for northern Europe populations (where K is about 0.3 yr^-1) and is probably associated with a shorter life span. Macroscopic examination of the gonads, and analysis of the monthly values of the gonadosomatic index, indicated that reproduction occurs in winter and early spring, with a maximum in February and March, when water temperatures are lowest. Individuals become sexually mature around 6-7 cm TL, a size that can be reached in less than 1 year. By contrast, individuals of this goby in the British Isles mature in their second or third year. Stomach contents were mainly small benthic invertebrates, predominantly crustaceans

C Taylor

D Ruple

F Russel

G Fabi

J Dunne

JMN Azevedo

L Arruda

M Costa

MV Lebour

P Sparre

PJ Miller

R Miller

R Patzner

RN Gibson

Azevedo, José M. N.

Simas, Ana M. V.

Repository of the University of the Azores

Age and growth, reproduction and diet of a sublittoral population of the rock goby Gobius paganellus (Teleostei, Gobiidae)

Repositório da Universidade dos Açores

Hydrobiologia 440: 129–135, 2000.M.B. Jones, J.M.N. Azevedo, A.I. Neto, A.C. Costa & A.M. Frias Martins (eds), Island, Ocean and Deep-Sea Biology.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.129Age and growth, reproduction and diet of a sublittoral population of therock goby Gobius paganellus (Teleostei, Gobiidae)Jose´ Manuel N. Azevedo & Ana Maria V. SimasDepartamento de Biologia, Universidade dos Açores, Rua Mãe de Deus, 9500 Ponta Delgada, PortugalTel. + 351 296 653044. E-mail: azevedo@notes.uac.ptKey words: fish biology, length–weight relationship, length–frequency distributions, modal analysis, growth rate,life spanAbstractBasic biological information for a sublittoral population of the rock goby Gobius paganellus Linnaeus, 1758 ispresented based on a 2-year study involving 1680 specimens. The length–weight relationship was given by TW= 0.0089 ∗ TL3.163 (where TW= total weight in g; TL= total length in mm). Age at length data were inferred bymodal analysis of the monthly length–frequency distributions. The parameters of the fitted Von Bertalanffy growthequation (with seasonal component, birth date on the 1st of January) were L∞ = 13.8 cm; K = 0.73 yr−1; to =−0.22 yr; C = 0.95;W = 0.07. This growth rate is much higher than that described for northern Europe populations(where K is about 0.3 yr−1) and is probably associated with a shorter life span. Macroscopic examination of thegonads, and analysis of the monthly values of the gonadosomatic index, indicated that reproduction occurs inwinter and early spring, with a maximum in February and March, when water temperatures are lowest. Individualsbecome sexually mature around 6–7 cm TL, a size that can be reached in less than 1 year. By contrast, individualsof this goby in the British Isles mature in their second or third year. Stomach contents were mainly small benthicinvertebrates, predominantly crustaceans.IntroductionThe rock goby Gobius paganellus Linnaeus, 1758 isa relatively large goby found along the northwest At-lantic coastline, from western Scotland to Senegal, andon the Atlantic islands of the Azores, Madeira andthe Canaries (Miller, 1986). It is the only species ofGobius from the Azores (Miller, 1984) and is con-sidered mainly as an intertidal species (Miller, 1986).The two major studies of its biology (Miller, 1961;Dunne, 1978) used intertidal animals as a matter ofconvenience, however, Dunne (1978) recognized thatthe species is common subtidally “to a depth of ap-proximately five metres”. In the Azores, G. paganellusis found in the rocky subtidal down to 15 m (Patzner etal., 1992), where it is one of the most abundant benthicspecies, together with Tripterygion delaisi, Parablen-nius incognitus and P. ruber (Azevedo, 2000). Thispaper describes various aspects of the biology of asublittoral population of G. paganellus at São Miguel(Azores).Materials and methodsBetween December 1993 and November 1995, 1680specimens were captured at 3–5 m depth on a rockyreef near Ponta Delgada harbour. One to three sampleswere taken each month by SCUBA diving, usingrotenone (3–67 animals per month, average 23, stand-ard deviation 18). Each specimen was measured (totallength, TL, to the nearest mm below), weighed (totalwet weight, TW, in g), fixed in formalin and trans-ferred to 70% ethanol after 2–4 weeks. Later, amonthly sub-sample of up to five specimens for each 1cm length class was selected. Each of these was sexedby external examination of the shape of the genitalpapilla (following Miller, 1961) and by macroscopicobservation of the gonads, which were removed andweighed (gonad weight, GW, in g). Gonad observationand weighing was not generally possible for smallerspecimens (<5 cm TL). A gonosomatic index (GSI)was calculated as GSI=GW/(TW−GW)∗100. Stom-ach contents, from 2 to 6 specimens in each monthly130Table 1. Length-frequency distribution of male and femaleGobius paganellus. Asteriscs indicate the probability of the de-viations from a 1:1 sex ratio, calculated from the binomialdistribution (∗ p<0.05; ∗∗∗ p<0.001)TL (cm) Females Males2 13 8 34 19 10∗5 42 496 20 657 143 99∗∗∗8 134 1219 165 147∗10 99 126∗11 26 92∗∗∗12 1 13∗∗∗13 1 4N 690 726Average 8.0 8.5St. deviation 1.7 1.9t 5.058 p<0.001sample selected to cover the entire size range present,were removed and placed in 70% ethanol. They weresorted under a dissecting microscope and identifiedto the lowest possible taxonomic rank. Results arepresented as percentage occurrence in the stomachs(i.e. number of stomachs with item over total numberof stomach contents examined).Age at length data were deduced from modal ana-lysis of the monthly length–frequency distributionsand used to calculate the parameters of the Von Ber-talanffy seasonalized growth equation, following themethods described in Sparre & Venema (1992) andusing the FiSAT software (Gayanillo et al., 1994). The1st of January was chosen as the date of birth. Age-length data for this species were read from graphsin Miller (1961) and Dunne (1978), and converted tototal length. The age–length points obtained for thesepopulations were used to determine their Von Ber-talanffy parameters in the same way as data from thepresent study.ResultsAge and growthTable 1 shows the length–frequency distributions ofsexed individuals. There is a statistically significantFigure 1. Monthly length–frequency distributions of Gobius pagan-ellus, with superimposed normal curves from modal analysis.difference in average total length between males andfemales due, mainly, to the higher proportion of malesin the largest length classes. The length–weight rela-tionship, calculated separately for each sex, was foundto be statistically identical. The relationship for theentire sample is:TW = 0.0089 ∗ TL3,163 (n = 1152; r = 0.995).The monthly length–frequency distributions, andrespective modal analysis, are given in Figure 1, bothsexes were considered together. Several well-definedmodes can be seen and a displacement to the right canbe followed between successive months. These modeswere assumed to correspond to distinct age groups, orcohorts. The 1994 cohort was the most distinct andcan be traced from its beginning in June 1994 up tothe end of the sampling period. The 1993 cohort wasdeduced by comparison and its displacement couldbe followed until November 1994. Small specimenswere also captured in the summer of 1995 but in muchsmaller numbers than the previous year, making it dif-ficult to follow its modal progression. Based on thesepremises, an age–length key was computed (Table 2).This key contains enough age–length points to allowthe adjustment of a von Bertalanffy growth curve with131Table 2. Age–length key for the Azorean population of Gobius paganellus, based on modal analysis dataDate 1993 1994 1995Age TL (cm) Age TL (cm) Age TL (cm)(years) Average St.dv. n (years) Average St.dv. n (years) Average St.dv. n9/12/93 0.94 7.1 0.5 1810/3/94 1.19 7.7 0.6 1212/4/94 1.28 7.8 0.8 3911/5/94 1.36 8.3 1 508/6/94 1.43 9 1.2 103 0.43 3.4 0.9 2119/7/94 1.55 9.3 0.9 86 0.55 4.7 0.9 11218/8/94 1.63 10 1 171 0.63 6 0.8 11824/9/94 1.73 10.7 0.9 40 0.73 6.7 0.8 229/11/94 1.85 10.4 0.8 46 0.85 7.7 0.7 4818/1/95 1.05 7.7 0.6 5018/2/95 1.13 7.8 0.6 1113/3/95 1.19 8.4 1 595/5/95 1.51 8 0.7 405/6/95 1.42 8.8 0.9 5510/7/95 1.52 9.6 1.2 55 0.52 4.6 1.5 1416/8/95 1.62 10 0.9 6826/9/95 1.73 10.1 0.8 4126/10/95 1.82 10.6 1 51 0.82 7.7 1.4 2618/1/95 1.88 10.6 0.8 39a seasonal component. The parameters obtained were:L∞ = 12.77 cm; K = 0.89 yr−1;to = −0.16 yr; C = 0.99;W = 0.06The L∞ obtained is smaller than the length of thelargest specimen captured (13.8 cm). The latter is alsothe value obtained by the Powell–Wetherall method(see Sparre & Venema, 1992). When this value ofL∞ is made invariable, the new equation parametersbecome:L∞ = 13.8 cm; K = 0.73 yr−1;to = −0.22 yr; C = 0.95;W = 0.07This growth curve, and the points used for itscomputation, are show in Figure 2. This figure alsoshows the growth curves calculated from the data ofMiller (1961) and Dunne (1978), whose parametersare, respectively:L∞ = 12.00 cm; K = 0.31 yr−1;to = −0.32 yr; C = 1; W = 1L∞ = 14.15 cm; K = 0.27 yr−1;to = −0.03 yr; C = 0.86;W = 0.85The Azores population has a much higher growthrate, measured by K . In their first year, Azoreanrock gobies attain an average size double that of theirnorthern conspecifics. Values within 10% of L∞ areattained in just 3 years in the Azores, compared with7–8 years in the British Isles populations.ReproductionMonthly changes in gonadosomatic index for each sexare shown in Figure 3. Minimum values were ob-served between July and November, and maximumvalues between January and May. Females with ripegonads (oocytes extruding under light pressure onthe abdomen) were observed mainly in February andMarch, suggesting that reproduction occurs in winterand early spring.The relationship between length and the gonado-somatic index is given in Figure 4. Well developedgonads were found only in specimens with total lengthequal to or greater than 6 cm (males) or 7 cm (fe-males). These, presumably, represent the size at firstsexual maturation for this population. According to thegrowth equation calculated above, males and femalescan reach sexual maturity in the reproductive seasonimmediately following the one in each they were born.132Figure 2. Growth curves for different populations of Gobius paganellus: Azores (present study), Ireland (Dunne, 1978) and Isle of Man (Miller,1961). Points show age–length values obtained by modal analysis. See text for curve parameters.Figure 3. Monthly values of the gonadosomatic index for each sex of Gobius paganellus. The solid line connects the monthly averages. Notedifferent scales on the GSI axes of each graph.133Figure 4. Length–GSI relationship for both sexes of the studied population of Gobius paganellus. Broken line mark presumptive size at firstsexual maturation. Note different scales on the GSI axes of each graph.FoodTable 3 shows the percentage occurrence of differentfood items in rock goby stomachs. Small benthic in-vertebrates accounted for the bulk of diet. Dominantitems were isopods and amphipods, although gastro-pods, polychaetes and crabs were observed frequently.DiscussionSeparate growth rates for each sex could not be as-sessed by the methods used in the present study, butthe higher proportion of males in the largest lengthclasses suggests that males grow faster than females.For the closely related G. niger from the Adriatic Sea,Fabi & Giannetti (1985) also found a statistically sig-nificant dominance of males in the largest size classes;using otolith data, they showed that males grew fasterthan females. Miller (1961) and Dunne (1978) foundno sexual differences in growth rate for G. paganellus.The former author, however, noted the disappearanceof the larger members of each age group, suggestingthat selective mortality or migration to the subtidalcould account for this. This disappearance could maskthe effects of differences in growth rate between sexes.The main difference between the growth curvededuced in this study from those of more northernlocations (Miller, 1961; Dunne, 1978) is the highervalue of K . This implies a faster growth rate for theAzores population. Arruda et al. (1993) reported age-size values intermediate between these two extremesfor a population of G. paganellus from the Ria deAveiro, near the centre of the west coast of mainlandPortugal. In this latter work, the average length of0+ and 1+ specimens was given as 6.2 and 7.3 cm,respectively. These compare with sizes at the end ofthe first and second years of 3.3–3.7 and 5.9 for theBritish populations, and 7.9 and 10.9 for the Azores.There is an inverse correlation between growth rateand latitude, suggesting that temperature might be the134Table 3. Stomach contents (percentage occurrence) of different populations of Gobius paganellus. Items occurring in more than half thestomachs analysed are shown in boldPresent Gibson Dunne Costastudy (1972) (1978) (1988)Algae 3 16 37Foraminifera 1Cnidaria Hydrozoa 1Annelida Polychaeta 11 7 30 71Mollusca Bivalvia 8Mollusca Gastropoda 28 15 31Arthropoda Chelicerata Pycnogonida 8Acarina 9Crustacea Ostracoda 38 27Copepoda 3 82 34Cirripedia 1Cumacea 5Euphausiacea 3Decapoda 15Decapoda, 3DendrobranchiaDecapoda 14 32 19BrachyuraMysidacea 14Isopoda 69 25 24Amphipoda 100 69 67 33Insecta Coleoptera 1Diptera 8 42 1Echinodermata Ophiuroidea 11 11 1N= 36 170 329 21main factor involved. This was proposed originally byDunne (1978) to explain differences between Carnaand the Isle of Man populations. It must be pointedout, however, that other factors may be involved. Inparticular, the sublittoral habitat of the present popu-lation may be more benign in providing opportunitiesfor feeding, unrestricted by tidal cycles, compared tothe tide-pool habitat studied by the other authors.Most individuals of the Azores population prob-ably reproduce in their first year. This contrasts withthe observations of Miller (1961) and Dunne (1978),who report a size at first maturation which correspondsto an age of 2–3 years. The eggs of G. paganellus,like those of most gobiids (Ruple, 1984), are demersal,with adhesive filaments. They are placed by the femaleunder rocks or empty bivalve shells where they areguarded by the male until hatching (Miller, 1961). Thelarvae remain close to the bottom until metamorphoses(Russel, 1930), after which they assume the crypto-benthic behaviour of the adults. In the Plymouth area,the time span from spawning to absorption of the vi-telline sac by the larvae is 30 days (Lebour, 1919).There is very little information regarding the followingphases of larval development and none about the effectof temperature on development. In the Isle of Man,spawning takes place from mid-April to mid-June.Post-metamorphic juveniles (9.5–10.5 mm in length)are in rock pools by early August (Miller, 1961), al-most 4 months after the beginning of the reproductiveseason. In the present study, the first recruits (TLbetween 2.8 and 4 cm) were captured in May, in bothyears. Even assuming that spawning starts in January,these data are consistent with a faster larval develop-ment in the Azores. This accelerated life history ofAzorean individuals may mean a shorter life span. Theoldest fishes observed by Dunne (1978) belonged tothe 7+ age group, but Miller (1961) reported speci-mens older than 10 years. No direct age determination135was attempted in the present study but, if the longevityestimation measure proposed by Taylor (1962, A0.95,the time an individual takes to reach 95% of L∞) isused, the Azores population scores only 3.9, consider-ably lower than that of the Isle of Man (9.3) or Carna(11.1).In the Azores, the diet of G. paganellus is basedon small benthic invertebrates, generally similar tothat of other populations. Crustaceans were the mainstomach items in Ireland (Dunne, 1978) and in north-ern France (Gibson, 1972). Costa (1988), on theother hand, stated that the most frequent item in thepopulation from the Tagus Estuary was polychaetes.This may be due to the peculiarities of this estuarinehabitat, since the remaining areas were fully marine.Algal fragments have a frequency in the stomach con-tents of between 16% (Costa, 1988) and 37% (Dunne,1978). This latter author suggested that algal materialis ingested accidentally, together with the animal con-stituents of the diet. This view is substantiated by thepresent study, since algal fragments were found onlyin one of the 36 stomach contents analysed.ReferencesArruda, L., J. M. N. Azevedo & A. I. Neto, 1993. Abundance,age-structure and growth, and reproduction of gobies (Pisces;Gobiidae) in the Ria de Aveiro Lagoon (Portugal). Estuar. coast.Shelf Sci. 37: 509–523.Azevedo, J. M. N., 2000. Now you see me, now you don’t: a casestudy of the effect of the sampling method on the perceived struc-ture ichthyological communities. Arquipélago, Life Mar. Sci.Supp. 2(a): 133–143.Costa, M., 1988. Écologie alimentaire des poissons de l’estuarie duTage. Cybium 12 (4): 301–320.Dunne, J., 1978. The biology of the rock goby, Gobius paganellusL., at Carna. Proc. r. Irish Acad. 78B (12): 179–191.Fabi, G. & G. Giannetti, 1985. Growth parameters of the black goby(Gobius niger L.) in the Adriatic Sea, based on otholiths reading.Rapp. Comm. int. Mer Médit. 29(8): 87–90.Gayanillo, F. C. Jr., P. Sparre & D. Pauly, 1994. The FAO-ICLARMstock assessment tools (FiSAT) user’s guide. FAO, Rome: 186pp.Gibson, R. N., 1972. The vertical distribution and feeding relation-ships of intertidal fish on the Atlantic coast of France. J. anim.Ecol. 41: 189–207.Lebour, M. V., 1919. The young of the Gobiidae from the neigh-bourhood of Plymouth. J. mar. biol. Ass. U.K. 12: 48–80.Miller, P., 1984. The gobiid fishes of temperate Macaronesia(Eastern Atlantic). J. Zool., Lond. 204 (3): 363–412.Miller, P., 1986. Gobiidae. In Whitehead, P. J. P., M.-L. Beauchot,J.-C. Hureau, J. Nielsen & E. Tortonese (eds), Fishes of theNortheastern Atlantic and the Mediterranean. UNESCO, Paris:1019–1085.Miller, P. J., 1961. Age, growth, and reproduction of the rock goby,Gobius paganellus L., in the Isle of Man. J. mar. biol. Ass. U.K.41: 737–769.Patzner, R., R. Santos, P. Ré & R. Nash, 1992. Littoral fishes ofthe Azores: an annotated checklist of fishes observed during the‘Expedition Azores 1989’. Arquipélago, Life Earth Sci. 10: 101–111.Ruple, D., 1984. Gobioidei: development. In Moser, H. G. (ed.),Ontogeny and Systematics of Fishes. American Society of Ich-thyologists and Herpetologists, Special Volume nr. 1: 582–587.Russel, F., 1930. The vertical distribution of marine macroplankton.IX. The distribution of the pelagic young of teleostean fishes inthe Plymouth area. J. mar. biol. ass. U. K. 15: 829–850.Sparre, P. & S. Venema, 1992. Introduction to tropical fish stock as-sessment. Part 1. Manual. FAO Fisheries Technical Paper 306/1.FAO, Rome: 376 pp.Taylor, C., 1962. Growth equations with metabolic parameters. J.Con. perm. int. Explor. Mer. 27: 270–286.

José Manuel N. Azevedo

Ana Maria V. Simas

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Age and growth, reproduction and diet of a sublittoral population of the rock goby Gobius paganellus (Teleostei, Gobiidae)

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