NOTES ON THE FOOD HABITS AND PREDATORY BEHAVIOUR OF THE DUSKY GROUPER, EPINEPHELUS MARGINATUS (LOWE, 1 834) (PISCES : SERRANIDAE) IN THE AZORES
JOAO P. BARREIROS & RICARDO S. SANTOS
BARREIROS,JOAO P. & RICARDOS. SANTOS1998. Notes on the food habits and predatory behaviour of the dusky grouper, Epinephelus marginatus (Lowe, 1834) (Pisces: Serranidae) in the Azores. Arquipklago. Life and Marine Sciences 16A: 29-35. Ponta Delgada. ISSN 0873-4704.
Between 1992 and 1995, 57 individuals of E. marginatus were collected near Terceira Island. Their sizes ranged from 60 to 138 crn, total length, with two size categories. Stomach contents were analysed. Vacuity coefficient was 440% for medium size specimens and 31,3% for large specimens. Octopus and fishes were the dominant prey. Two different hunting postures were also observed and described.
BARREIROS,JoAo P. & RICARDOS. SANTOS.1998. Notas sobre os hibitos alimentares e comportamento predatdrio do mero, Epinephelus marginatus (Lowe, 1834) (Pisces: Serranidae) nos Aqores. Arquipdago. CiCncias Biol6gicas e Marinhas 16A: 29-35. Ponta Delgada. ISSN 0873-4704.
Entre 1992 e 1995, forarn colhidos e examinados 57 exernplares de E. marginatus da ilha Terceira. 0s seus cornprirnentos totais variararn entre 60 e 138 crn corn duas categorias de tarnanho. 0 coeficiente de vacuidade observado foi de 44,0% para exernplares de rnMio porte e 31,3% para grandes exernplares. As presas predominantes forarn o polvo comurn e peixes. Duas posturas de caGa diferentes foram obsewadas e s5io descritas.
Joiio P. Barreiros, Universidade dos Agores, Departamento de Ci2ncias Agrrfrias, TeFra Chti. PT- 9700 Angra do Herotsmo, Azores, Portugal. E-mail: [email protected] - Ricardo S. Santos, Universidade dos Agores, Departamento de Oceanografia e Pescas, PT-9900 Horta, Azores, Portugal.
INTRODUCTION Studies on E. marginatus have been carried out, mainly in the Mediterranean, where it became The dusky grouper, Epinephelus marginatus a symbol of marine life in the early 40's when (Lowe, 1834) is a well known teleost fish of the scuba diving began to develop and expand as a rocky shores of the Azores. The species has a leisure/sport activity. Several aspects of the wide geographical range and has been recorded biology of the species were studied by BOU-Am from the British Isles to South Africa, southern et al. (1983), BRUSL~(1985) and GHAFIR& Mozambique, Mediterranean, Azores, Madeira, GUERRAB (1992) while aspects of the Canaries and Cape Verde (HEEMSTRA199 1). It is reproductive biology have been reported by also recorded from Brazil to Uruguay (RIGUELET BRUSL~& BRUSL~(1976), BOU-AIN & SLAU & ARAMBURU1960), India (REDDY1984) and (1983) and SKARAMUCAet al. (1989). Growth the island of RCunion (TAQUET& DIRINGER studies (BRUSL~& PRUNUS 1980; CHAUVET 1992). 1988) have shown relevant aspects of the species lengtwweight relationship. Demography and
29 general behaviour were reported by NEILL(1967) and weighed, with a dynamometer, with a and CHAWET& FRANCOUR(1988) and feeding precision of 0.01 g. by CASTELLO-ORVAYet al. (1992) and DERBAL& KARA(1996). FOOD HABITS In the Azores, only some aspects of the Stomachs were removed and preserved in the biology of the dusky grouper were studied. freezer. In the laboratory, food items were Juvenile ontogeny was described by PATZNERet thoroughly examined and identified to the lowest al. (1992) and AZEVEDo et al. (1995). possible taxonomic level. After drying with BARREIROS(1995), BARREIROS& SANTOS absorbing paper, food items were weighed in an (1996) and PORTEIROet al. (1996) published analytical balance to the hundredth of the gram. preliminary results on its reproduction and The vacuity coefficient (VC) (HUREAU1970) feeding behaviour. was determined, in which: Groupers are a diverse taxon of predators with a wide distribution, thus resulting in a wide prey VC = No of empty stomachs I Total stomachs choice. The genus Epinephelus comprises more observed x 100 than 100 species distributed worldwide in tropical and temperate waters (NELSON1994). The dusky Importance of prey items on fish diet was grouper is the only Epinephelus species occurring determined based on feeding coefficient (Q) of in the Azores (SANTOSet al. 1997) and is HUREAU(1970) and GEISTDOERFER(1975) index probably the most common large (> 100 cm) (F), in which: benthopelagic predator from the surface down to 100 meters. Q = (%Cng x % Cp& being Knowledge of its food habits and thus its ecological role as a top predator is an important %Cna = no of g preys 1 Total no of preys x 100 step towards the understanding marine foodwebs and % Cpg = weight of all g preys / weight of all of the Azores. stomach contents x 100 Besides its ecological importance, the dusky grouper has a special protection status in the GEISTDOERFER(1975) proposed the index "F" Azores (SANTOSet al. 1995), where spedishing for different prey items, in which: for this spec& is prohibited. F% = No of stomachs with the item a I Total of full stomachs x 100 MATERIAL AND METHODS The use of index Q gives an idea of the role of All specimens sampled were collected near a determined item of prey in the whole of the diet Terceira Island by spearfishing under permit. This while F expresses the frequency in which a prey is method prevents regurgitation (e.g. BOWEN1983) captured. and makes selection easier (DERBAL& KARA Results are presented for the whole sample 1996). and for two size categories: A. medium sized Material was collected and ethological (Total Length - TL < 90 cm) and B. large sized observations made throughout the year during (TL > 90 cm). BARREIROS(1995) indicated that daytime. The specimens were sexed, measured to sex change in the population of E. marginatus the nearest millimetre with a measuring board, studied in Terceira Island, Azores, occurs when fishes attain a TL of ca. 90 cm. PREDATORY BEHAVIOUR Table 1 presents the results for the whole sample and Tables 2 and 3 show the differences found in During the sampling fieldwork we made several the two size categories considered. observations on predatory behaviour (around 100 hours) on E. marginatus. These observations were PREDATORY BEHAVIOUR made by carefully approaching (at the closest possible distance, without disturbing the fishes) Similar results for both Q and F for Octopus some individuals that displayed hunting postures, vulgaris and whole of fishes show these to be while hiding ourselves as much as possible. The preferred prey (Table 1). Crustaceans are bulk of these observations were made by accessory prey (Q) or frequent secondary prey snorkelling (87 hours) and the remaining with (F). Centrolabrus trutta was the most common scuba gear (22 hours). fish in the diet. Underwater sketches of the postures observed were made for behavioural Table 1 illustrations. Values of Q and F found in the whole sample of E. marginatus (n = 35) SPECIES Cn% Cp% Q F Octopus vulgaris 47.5 70.4 3345.9 54.3 RESULTS Total molluscs 47.5 70.4 3345.9 54.3 Scyllarides latus 5 6.1 30.3 5.7 FOOD HABITS Scyllarus arctus 7.5 1.4 10.4 8.6 Total crustaceans 12.5 7.4 92.9 14.3 A total of 57 stomachs were analysed. Centrolabms trutta 17.5 9.2 160.1 11:4 The vacuity coefficient (VC) for the Ophioblennius atlanticus 5 0.5 2.5 5.7 whole sample was of 38,6%. Larger Synodus saurus . 2.5 1.3 3.3 2.9 specimens (> 90 cm TL) had a lower VC Mullus surmuletus 2.5 1.8 4.5 2.9 (3 1,2%) than medium sized specimens Sarpa salpa 2.5 2.5 6.2 2.9 (44,00%). Forty prey species were found Unid. Teleostei 10 3.0 29.8 5.7 with a total weight of 23,324 kg. Among Total fishes 40 18.3 730.4 31.5 the prey, five fish (Centrolabrus trutta, Ophioblennius atlanticus atlanticus, Synodus In size A class (Table 2), 0. vulgaris, crustaceans saurus, Mullus surmuletus and Sarpa salpa), two and fishes are main preys (Q). Fish constitute the crustaceans (Scyllarus arctus and Scyllarides main preferred prey while Octopuses and the latus) and two molluscs (Octopus vulgaris and crustaceans are occasional main preys. When Haliotis coccinea (Tables 1-3) could be considered individually, Scyllarus arctus, identified to species level. Three teleost fishes Centrolabrus trutta and Ophioblennius atlanticus were too digested for an accurate identification atlanticus are frequent preys and all other items and bony structures (scales, otoliths, and are secondary preys. vertebrae) could not be recognised. In the case of For fish larger than 90 cm TL 0. vulgaris is the gastropod Haliotis coccinea we did not the main preferred prey (Table 3), the fish, include it in Q and F calculations because only Centrohbrus trutta, and crustaceans were of three empty worn shells were found. Since these lesser importance. The hunting posture illustrated were in stomachs that also contained octopus in Fig. 1 was observed 23 times. Of these, 87.0% remains, we considered that the shells could be (20 individuals) were smaller than 90 cm. Figures attached to the octopuses, thus not being a true 2 and 3 illustrate another hunting posture, prey item (see Discussion). observed on 21 occasions. Fig. 1. Hunting posture performed by small to medium-sized (<90 cm TL) individuals when attacking fishes. (Drawings by Jo2o Pedro Barreiros).
Fig. 2. Hunting posture performed by some large (>I00 cm 'l'L) individuals when stallang and attacking octopus.
Fig. 3. Hunting posture observed in some large (>I00 cm TL) individuals when stalking and attacking octopus. Table 2 2. What can be stated so far is that Food parameters found in size class A (c 90 cm TL) of the sample of fishes in these two postures actually E. marginatus (n = 13) performed them while hunting. SPECIES Cn% Cp% Q F Octopus vulgaris 15.4 50.9 782.4 15.4 Total molluscs 15.4 50.9 782.4 15.4 DISCUSSION Scyllarides latus 7.7 12.4 95.4 7.7 Scyllarus arctus The vacuity coefficient of 38,6% is Total crustaceans lower than those reported by DERBAL Centrolabrus trutta & KARA (1996), of 46,3% and Ophioblennius atlanticus GHAFIR& GHERAB(1992), of 42,1%, Synodus saurus for E. marginatus. However, in our Mullus sunnuletus samples, large specimens had a lower Sarpa salpa VC (31,3%) than medium ones Unid. Teleostei (44,0%) which is closer to the values Total fishes above mentioned. This could mean that large fish may have improved hunting strategies, probably due to their Table 3 older age and, thus, experience. Also the Food parameters found in size class B (> 90 cm TL) of the sample of E. marginatus (n = 22) fact that large fishes ciin eat larger prey could imply that these would remain for longer periods and in idehtifiable condition SPECIES Cn% Cp% Q F in their stomachs. Octopus vulgaris 63.0 77.7 4890.1 77.3 Other works that focus on food habits of Total molluscs E. marginatus (e.g. NEILL 1967; BRUSL~ Scyllarides latus 3.7 3.7 13.7 4.5 1985; SMALE1986) largely consider fish Scyllarus arctus 3.7 0.7 2.6 4.5 and crustaceans as main items of prey. Total crustaceans 7.4 4.4 32.6 9.1 Octopuses and other molluscs (Haliotis Centrolabrus trutta 18.5 9.6 178.0 9.1 sp. by NELL 1967 and small bivalves by Unid. Teleostei 11.1 3.0 33.4 4.5 &EVEDO et al. 1995) are also considered Total fishes 29.6 12.6 373.9 13.6 and is according to CHAUVET(1991), the preferred prey of E. marginatus. All individuals performing this posture were SMALE (1986) working in South Africa larger than 100 cm. observed that larger individuals fed mainly on These ambushindhunting postures were and reef-associated fishes. However, in the Azores, it are consistently observed in groupers of all sizes, seems that as E. marginatus grows larger it alters both in captivity and in natural conditions. From the diet towards a higher percentage of Octopus our observations all individuals performing the vulgaris (AZEVEDOet al. 1995). hunting posture illustrated in Fig. 1 were seen attacking fishes while from the ones performing This fact is confirmed in our results. For large the hunting posture illustrated in Fig. 2, 18 (86%) individuals, octopus represents clearly the were seen carefully approaching octopuses with 7 preferred prey. Fish constitute the preferred prey attacks actually occurring. of medium sized fish. Here octopus and However, one must state that these postures crustaceans appear only occasionally as main may not be exclusively performed during hunting prey. However, the small size of our sample and moments. The posture in Fig. 1 may also be the high VC for medium sized individuals must displayed as a territorialitylresidential stance lead us to consider this results cautiously. while resting fishes also performs the one in fig. As we did not study fishes with TL lower than REFERENCES 60 cm, there is a lack of data for small individuals and juveniles. However, AZEVEDO et al. (1995) AZEVEDO,J.M.N., J.B. RODRIGUES,M. MENDIZABAL& found crabs, fish (including other E. marginatus) L.M. ARRUDA.1995. 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