ANNALES • Ser. hist. na t. ■ 11 ■ 2001 • 2 (25) preliminary report UDC 597(262.3) received: 3. 12. 2001 PRELIMINARY OBSERVATIONS ON ABNORMAL ABUNDANCE OF CETORHINUS MAXIMUS (GUNNERUS, 1765) IN THE CENTRAL AND NORTHERN ADRIATIC SEA Marco ZUFFA Museo "L Donini", ¡T-40G64 Ozzano neli'Emiiia, Via Prunaio 1 Alen SOLDO institute of Oceanography and Fisheries, HR-2J000 Split, P.O.BOX 500 E-mail: solclo@izor.hr Tiziano STORAI Museo Cívico di Scienze Natorali della Vaidinievole, tT-510! 7 Pescia, P.zza Leonardo da Vinci 1 E-mai!: tstora@tin.il ABSTRACT During 2000 and in the first months of 2001, numerous records of Cetorhinus max: in us (Cunnerus, 1765) were made in the Central and Northern Adriatic Sea along the Italian, Croatian ana Slovenian coasts. The collected records include 1} sightings of a single specimen and small groups: 2) reports on some accidental captures, the number of records collected in the last two years has highly increased m relation to the records made of basking sharks in previous years, due to which some considerations on the apparent migratory movement in the Adriatic and comparisons with the data concerning the Tyrrhenian Sea are presented Some hypotheses about the c auses of the unusual basking shark abundance in the area are also presented. Key words. Cetorhinus rnaximus, basking shark, Central and Northern Adriatic Sea OSSERVAZiON! PRELIMINARI SULL'ABBONDANZA ANOMALA Dl CETORHINUS MAXIMUS (CUNNERUS, 1765) IN ADRIATICO CENTRALE E SETTENTRIONALE SINTFSI Durante il 2000 e nei primi mesi del 2001, sono state registrate numerose segnalazioni di Cetorhinus rnaximus (Cunnerus, /765; in Adriatic.o centrale e settentrionale, lungo le coste italiane> create e slove.no Le segnalazioni riguardano: 1/ awistamenti di singoli esemplari e di piccoli gruppi, 2) resoconti d> catiure accidentali dl qualche esemplare. Vengono prcsentate alcune considerazioni sull'apparente movimento migratorio in Adriatico ed il raf-fronto con dati provenieriti dal Tirreno. Vengono infine formulate alcune ipotesi suite possibih cause dell'abbon-darua inusuale dello squalo elefante net!'area. Parole chiave: Cetorhinus maximus, squalo elefante, Adriatico centrale e settentrionale 185 ANNALES • Ser. hist. nat. • 11 • 2001 • 2 (2.5} Marco ZUFfA et at.: PRfUMINARY OBSERVATIONS ON ABNORMAi. ABUNDANCE Of CBTORHtNUS MAXIMUS tCUNNERUS, 1V6SI .... 185-192 INTRODUCTION The presence of Cetorhirtus maximus (Gunnerus, 1765) in the Mediterranean basin has been recorded since 1795 (Macri, 1819}. In the past few centuries the basking shark (135 been studied, due to its dimensions and behavioural habits, to a greater degree than any other shark. Accordingly, C maximus is one of the species ort which the modern marine researches have been focused mostly (Harvey-Clark et a!., 1999; Sims, 2000; Sims et a!., 2000; Valeiras et ai., 2001}. Despite this attention, many aspects of the basking shark biology are still unknown. We do know, however, that basking shark is a highly migratory species, noteworthy for its seasonal appearance at different localities of the Pacific, and Atlantic. Oceans and its subsequent disappearance (Compagno, 1984). The same case is in the Mediterranean but, if there is any pattern in migratory movements of basking shark in this area, it stii! needs to be explained. However, periodic or even seasonal occurrences in the Central Mediterranean have been recorded for the Ligurian Sea (Vinciguerra, 1923; Tortonese & Trotti, 1949), Northern and Southern Tyrrhenian Sea (Senna, 1913; Serena & Vacchi, 1996), Sea of Sardinia (Monti, 1910), Sea of Sicily (Monterosso, 1931; La Cascia, 1935) and Tunisian waters (Najai, 1980). In the Adriatic, the presence of C. maximus, has been reported since 1822 (Naccari, 1822), and it. has been considered as occasional (Brusina, 1888; Soldo et ai., 1999; Lipej et ai., 2000). Therefore, the huge increase in the basking shark records, whether captures or sightings, reported in the Adriatic between March 2000 and September 2001 have a notable importance in relation to the actual knowledge on the distribution of the species in the Adriatic. Furthermore, its unusual high abundance in this area has forced researchers of different occupations to carry out more thorough investigations in order to establish the reasons for its unusually large numbers. MATERIALS AND METHODS Being a preliminary study, priority has been given to the collecting of ail available data that can be useful for a general evaluation of the phenomenon. All pieces of information, photographic evidences and videos collected have come from different Marine Institutes, Museums, Harbour Offices, fishermen and other private citizens and articles published In scientific as well as popuiar journals and newspapers, in sortie cases, the main body characteristics, such as iength and weight, have not been measured but merely estimated. 16$. fff' 14 12 V iVi* 18 15 . 10 \21 V 1 Vfi' 11 8 \* 17 \ 7 : i . . CROATIA \ ,6 n V-a . 13? fff. \ ' -.....SU ITALY •x V/ Fig. 1: Distribution of basking shark records in the Adriatic during 2000-2001 according to case numbers, including basking shark recorded on 5'" February 2001 near Callipoli (*)■ SI. 1: Razširjenost morskega psa orjaka v Jadranu v obdobju 2000-2001 glede na poročila iz tega območja, vključno s podatkom o orjaku, ujetem 5. 2. 2001 v bližini Callipolija (*). 186 ANNALES - Ser. Iiist. nat. - 11 • 2001 - 2 (25) Mareo ZU F FA etat.; PKUIMINARY OBSERVATIONS ON ABNORMAL ABUNDANCE OF ClTORHINUS MU0MU5 (GUNNí RUS. I7&S) ..., S85-I92 Tab. 1: Dala on basking shark records in the Adriatic 2000-2001. Tab. 1: Podatki o psih orjakih, opaženih v Jadranu v obdobju 2000-2001. N DATE PLACE LENGTH (in cm) stx NOTES REFERENCES 1 March 2000 1st ran coast near Rovinj (Croatia) 700 Specimen sighted several times, Finally captured by gillnet and released. Soldo & Jardas, 2001 2 22 May 2000 'Piran (Slovenia) 299 male Captured with net some 7 miles off the coast. Weight 120 kg. Lipej el a!., 2000 3 23 May 2000 Blitvenica area (Croatia) 700 Specimen weighing 2000 kg caught by trawl. Soldo & Jardas, 2001 4 5 iune 2000 Blitvenica area (Croatia) 850 Specimen weighing 2500 kg caught by trawl. Soldo & jardas, 2001 5 19 July 2000 Piran (Slovenia] 249 Specimen of 70 kg caught by gillnet 6 4 miles off the coast. Lipej et al., 2000 b November 2000 Pescara (Italy) 500 Specimen caught. Recorded by G. Cu-Rini. De Maddalena, pers. convn. 7 5 February 2001 San Benedetto del Tronto (Italy) 600 male Specimen caught by net. Examined by S. Giacoia and A. Bugari. Anonymus, 2001b 8 5 March 2001 Ancona (Italy) 420 male Specimen of about 300 kg caught by net some 16 miles off the coast. Verified by photographer L. Caretta and local fisherman V. Renzi. Anonymus, 2001c 9 1 5 March 2001 Ancona (Italy) 10001200 Group of some 10 specimens sighted 12! 3 miles off the coast. Harbour Office representatives neared and photographed only 1 individual. Anonymus, 200Id 10 IS March 2001 Cesenatico (Italy) 800 Specimen sighted some 5 miles off the coast and photographed by Harbour Captain. Drudi, 2001 11 20 March 2001 Fano (Italy) 700 Group ol 5 specimens photographed by Harbour Office Captain G. Greco 5 miles off the coast. Anonymus, 2001 e 12 22 March 2001 Umag (Croatia) 600 Sighting. Photographed. Soldo, unpubi. data 13 28 March 2001 Caorle (Italy) 500 Specimen sighted twice on the same day 3 miles off the coast and photographed by Harbour Office Captain CI. Scaltola Caorle. Prevarin, 2001; Anonymus, 2001 f 14 29 March 2001 Caorle (Italy) <500 Specimen sighted several times by the same person as in the previous case is most probably not the same individual, as it may be concluded from the photo. Prevarin, 2001 15 2 April 2001 Ravenna (Italy) 1000 (?) Specimen caught by net 18 miles off the coast and released. Anonymus, 2001g 16 April 2001 Istran coast between Izola and Piran (Slovenia) Croup of around 8 specimens observed several times by fishermen and researchers of the Slovene Marine Biology Station. Lipej, pers. comm., Anonymus, 2001 h 17 25 April 2001 Porto San Giorgio (Italy) 600800 Sighting of 2 specimens. Anonymus, 2001 i 18 7 May 2001 Goro (Italy) 535 female Specimen of 900 kg caught by net 2 miles off she coast and examined by G. Gavanelli and other researchers. Anonymus, 2001 j 19 9 May 2001 Trieste (Italy) 600 Sighting. Anonymus, 2001 k i 20 20 May 2001 Kali / Ugljan Island (Croatia) a 00 Specimen sighted several times. Soldo, unpubi. data 21 12 September 2001 Cattolica (Italy) 430 Capture. Zuffa, unpubi data 187 ANNALES • Ser. hist. nat. • 11 • 2001 • 2 (2.5} Miirco ZUffA el sf.:PK£llMlMARY OBSERVATIONS ON ABNORMAL A8UNOA.NCt Of CHORMNUSMAXIMUS(GUNNiRUS, 17Ml.... 185-192 Fig. 2: Photo relative to ease .?. SI. 2: Fotografija orjaka, ujetega 23. .7. 2000. Aif the data have been carefully examined and cross-compared by eyewitnesses in order to verify every single case. To answer the questions raised by this case, some hypotheses are presumed in the light of the first analysis of the recorded data and the investigations into different elements influencing the migratory behaviour of the species. RESULTS AND DISCUSSION The recent abundance of CetOrhinus maximus in the Adriatic basin {Fig. 1) is indeed anomalous in comparison with the records collected in the last 20 years in the same area. The first examination of 21 cases (Tab. 1) shows the presence of male (cases 2, 7 and 8) and female (case 18) specimens of various dimensions (Figs. 2, 3, 4). This observation underlines the contemporary presence of specimens with varied stages of development. In fact, those stages ranged from young specimens (cases 2 and 5) up to adults of grandiose dimensions (cases 4 and 15). Aggregations of 5-10 sharks have been often observed (cases 9, 11 and 16), which speaks of a true migration than of occasional passage by single specimens. A meaningful sign of the migratory movement from south to north, coming from the Ionian Sea, was the capture and eventual release of a large specimen estimated to be 800-900 cm long (Anonymus, 2001a; A. De Maddatena, pers. comm.). It took place on February 5lh 2001 on the open sea in front of Gallipoli and was the start of a series of recorded sightings and captures during the year of 2001. The case 7 is particularly interesting. Despite the fact that the capture was made during the winter (February 5'"' 2001 off S. Benedetto del Tronto), the specimen was showing gillrakers perfectly developed and apparently functional (S. Glacoia, pers. comm.). This case, together with similar capture of a basking shark with functional gillrakers that was made off Baleares Islands on February 3"J 1985 (Gallego & Alemany, 1985), could suggest that the loss of such organs during the winter season is not obligatory as suggested (Compagno, 1984). This morphological aspect could have a relevant importance on the knowledge of the shark's feeding behaviour, as some authors presumed that during the period without gillrakers, the basking shark could develop a lethargic behaviour (Fllis, 1983; Mojetia, 1997) or change its diet (Lipej eta!., 2000). 188 ANNALES • Ser. hist. nat. • 11 • 2001 • 2 (2.5} m.WCO zu?fa et al.: preliminary observations on' abnormal abundance Of CCTÖRHINUSMAXiMUS (cunnfrus, tï65> .... ibs- Fig. 3: Photo relative to case 13. SI. 3: Fotografija orjaka, opaženega 22. 3. 2001. A preliminary investigation along the Tuscany coasts {about 120 km in length), frequently visited by Cetorhi-nus maximus (Storai & Zuffa, unpubl. data), shows almost (single record in July 2000) total absence of sighting or capture records in this area, while at the same time an increase in the frequency of the basking shark records in the Adriatic Sea has been observed. If this observation could be confirmed by some other cross-comparisons, which would include other factors necessary for a better and true understanding of the basking shark behaviour, this could indicate a true pattern of migration for the basking shark population in the Central Mediterranean. Unfortunately, the collected data do not allow us, at the moment, to make any final conclusions as to the causes of the dealt with phenomenon. Different hypotheses have been taken into consideration, including various elements, as possible causes for the changes in the basking shark behaviour in the Adriatic. The hypotheses, which can throw light on the strange phenomenon presented, are as follows: 1. Climate changes Certain changes in the Adriatic ichthyofauna due to climate changes in the Adriatic have been already observed (DulciC et a!., 1999). Eleven subtropical and tropical fishes have been recorded for the first time, and several species, fairly rare or very rare until now, are more abundant. The main reason for such changes is the warmer seawater that is affecting marine ecosystem. Along with other hydrodynamic factors, such as salinity, this can explain the reasons for the increase of the basking shark records in this area. 2. Changes in zooplankton abundance It is known that basking shark feeds exclusively on small planktonic organisms, such as small copepods, barnacles, decapod larvae and fish eggs (Compagno, Fig. 4: Photo relative to case 18. St. 4: Fotograiija orjaka, ujetega 7. 5. 2001. 1984). Climate changes affect primary and secondary production, so it is possible that, changes in abundance and species composition of zooplankton are causing basking shark to respond and follow these changes in the Adriatic. To prove such hypotheses, we would need new data on the monthly changes of zooplankton in the Adriatic. Currently, some projects regarding this subject are being conducted at the Institute of Oceanography and Fisheries in Split, and as soon as these data and comparison with basking shark data are made, this hypothesis could be proved either right or wrong. 3. Unknown aspects of the basking shark metaboAs there are numerous basking shark biology factors that are still unknown, a more thorough investigation on that subject would be necessary in order to fully understand the pattern of its response to different conditions. At present, the main objectives would be monitoring and a more careful investigation into the absence of basking shark records in the Tyrrhenian Sea and into the 189 ANNA LES • Ser. hist. nat. 11 2001 ■ 2 (25) Marco 2X1 Ff A « a/.; PRELIMINARY OBSERVATIONS ON ABNORMAL ABUNDANCE OF CUTORHINUS MAXtMUS IGUNNESUS. I ?(.S).... 185-1 <12 increase of its records in the Adriatic, which could indicate a possible pattern of migration from the Tyrrhenian to the Adriatic. Furthermore, the increase of records in the Adriatic Sea observed during 2000 and 2001 musl be repeated for a longer period of time, before it could be accepted as a true change in the shark's migratory behaviour However, this abnormal occurrence of the basking shark has attracted the attention of numerous researchers of different occupations to carry out preliminary plans of tagging, photoidentification, genetic analysis, etc. Such plans have been made by a number of international researches joined in the Mediterranean Shark Research Group, whose intention is, among other scopes, to collect and monitor all the basking shark records made in the Mediterranean. Such collaboration and exchange of information between researchers, different groups and institutes studying this subject are certainly most welcome in order to obtain reliable results and extend our knowledge of this giant species. ACKNOWLEDGEMENTS A particular thanks is due to the numerous Harbour Offices that enabled us to collect information and to photograph a number of basking sharks. Another particular thanks go to the Research Institutes that has helped lis to verify certain data. Hence, thanks to Sergio Giacoia, Albano Bugari, l.uigi Carretta, Gianluca Cugini and Boris Suiigoj. Finally, many thanks to Gildo Ga-vaneili, for his precious help. Fig. 5: Photo re lat i ve to čase 16. (Photo: B. Šuligo}) SI. 5: Fotografija orjaka, opaženega aprila 2001 pred Piranom, Slovenija. (Foto: B. Šuligoj) 190 ANNALES • Ser. hist. na t. ■ 11 ■ 2001 • 2 (25) Mart v /..'FFA ci j/ PRI' IMINARV OBSEKVAT ONS ON APsORvA. ABUNOANCf OF CETORMNUS MAXIMUS (OJNNtKUS, 176.7} ... lflS-192 PRVE UGOTOVITVE O NENAVADNO POGOSTEM POJAVLJANJU MORSKEGA PSA ORJAKA CETORH1NUS MAKIMUS (GUNNERUS, 1765) V SREDNJEM IN SEVERNEM JADRANU Marco ZUFFA Museo "L. Oonirii", ¡7-40064 Ozzano neIPEmilia, Via Prunaio 1 Aten SOL DO Inštitut za oceanografijo in rihištvo, HR-21000 Split, P.P. 300 F:-mail: solrlo