17 Aug 2020

NENAVADNI NAVADNI DELFIN I THE UNCOMMON COMMON DOLPHIN

 

 

Društvo Morigenos je v uveljavljeni mednarodni znanstveni reviji Aquatic Conservation: Marine and Freshwater Ecosystems objavilo pregledno raziskavo o pojavljanju navadnega delfina v Tržaškem zalivu in severnem Jadranu.


Navadni delfin (Delphinus delphis) je bil nekoč v Jadranskem morju in celotnem Sredozemlju nekaj povsem navadnega. Toda od 1970-ih let naprej je ta vrsta v Jadranu in drugih delih Sredozemlja postala tako redka, da je sredozemska populacija na Rdečem seznamu Mednarodne zveze za varstvo narave (IUCN) opredeljena kot Ogrožena. Vrsta delfinov, ki sicer stalno živi ob slovenski obali in v severnem Jadranu ter jo v društvu Morigenos redno preučujemo, ni navadni delfin, temveč velika pliskavka (Tursiops truncatus). Nekoč je vode Jadrana naseljeval tudi navadni delfin, a je s tega območja praktično povsem izginil. Zadnjih trideset let ga v Jadranu obravnavamo kot regionalno izumrlo vrsto, k čemer je najverjetneje prispevalo predvsem namerno in sistematično pobijanje v sredini 20. stoletja. Takrat sta Italija in nekdanja Jugoslavija izplačevali denarne nagrade za vsakega ubitega delfina, saj se jih je obravnavalo kot škodljivce, ki z ribištvom tekmujejo za ribolovne vire. Poleg omenjenega ubijanja sta verjetna vzroka za upad populacije tudi pomanjkanje hrane zaradi prekomernega ribolova ter splošna degradacija morskega okolja.

 

Nekoč stalna, danes izjemno redka vrsta

Viri iz 1970-ih let navajajo, da so bile zadnje večje skupine navadnih delfinov na območju Tržaškega zaliva opažene v 1940-ih letih. Od takrat pa vse do danes nimamo niti enega samega zanesljivega zapisa te vrste v celotnem Tržaškem zalivu, vse do primerov dokumentiranih v tej raziskavi. Tudi naše 18-letne sistematične raziskave potrjujejo, da je na tem območju stalno prisotna le velika pliskavka. Zanimivo je, da je v slovenski strokovno-poljudni literaturi navadni delfin kljub temu naveden kot slovenska avtohtona vrsta, kljub temu, da pravzaprav ni niti enega samega dokumentiranega primera opažanja, najdbe ali pojava te vrste na območju Slovenije.  Spiridon Brusina, hrvaški naravoslovec, je v zapisih iz leta 1888 poročal o enem poginulem primerku iz Žavelj pri Trstu, ki naj bi ga nekoč hranili v Tržaškem prirodoslovnem muzeju. Omenjenega primerka Tržaški prirodoslovni muzej že dolgo ne hrani več, tudi če ga kdaj je. Na podlagi tega zapisa, ter glede na to, da je razdalja med Žavljami in slovensko-italijansko mejo le nekaj kilometrov, je bil navadni delfin uvrščen na seznam sesalcev Slovenije, pod predpostavko, da je v nekem trenutku moral prečkati tudi slovenske vode. Poleg tega je vrsta uvrščena tudi v slovenski Rdeči seznam ogroženih vrst sesalcev kot Ogrožena. Vendar pa navadni delfin v slovenskih vodah nikoli zares ni bil dokumentiran, vse do primerov, ki jih omenjamo v nadaljevanju.

 

Prvi potrjeni primeri v Tržaškem zalivu in Sloveniji

Prav zaradi redkosti so vsa opažanja te vrste v Jadranu in Sredozemlju danes nekaj posebnega in pomembnega, zato jih je pomembno dokumentirati. V društvu Morigenos smo navadnega delfina med leti 2009 in 2012 večkrat dokumentirali na območju Tržaškega zaliva, tako na podlagi neposrednih opažanj na morju, kot tudi na podlagi najdenih poginulih živali. Naravne oznake na hrbtnih plavutih so nam omogočile foto-identifikacijo (prepoznavanje osebkov po naravnih oznakah s pomočjo fotografij) nekaterih od teh, s čemer smo ugotovili, da so se v tem obdobju tukaj pojavili vsaj štirje različni osebki.

 

Prvi potrjeni primer te vrste v Sloveniji in Tržaškem zalivu smo dokumentirali leta 2009, ko so potapljači pred Izolo s plovila posneli delfina in fotografije posredovali društvu Morigenos. V društvu smo na podlagi fotografij nemudoma ugotovili, da gre prav za redkega navadnega delfina. Leto zatem smo v pristanišču Monfalcone (Tržič) dokumentirali samico z mladičem, ki se je tam zadrževala več mesecev. Na podlagi foto-identifikacije smo s pomočjo kolegov iz Grčije ugotovili, da je ta samica leta 2008 že bila opažena v Jonskem morju v Grčiji, več kot 1000 km stran. Njena pot do Tržaškega zaliva predstavlja doslej najdaljše dokumentirano potovanje pri tej vrsti na svetu, o čemer smo že poročali pred leti. Njen mladič je v začetku leta 2011 žal izginil, kar glede na njegovo starost najverjetneje pomeni, da je poginil. Njegovo mamo smo na območju Tržaškega zaliva videvali še nekaj mesecev, nakar je tudi ona izginila. Istega leta smo pred Izolo našli zelo razpadlo truplo delfinjega mladiča. Zaradi razpadlosti ni bilo možno ugotoviti za katero vrsto gre, vendar smo na podlagi natančnega pregleda očiščenih lobanjskih kosti ugotovili, da gre za navadnega delfina. Navadni delfini se namreč od vseh drugih vrst delfinov razlikujejo prav po zgradbi lobanje, saj imajo nebnico (kost ustnega neba) posebej oblikovano v dva žleba. Čeprav je nemogoče ugotoviti ali gre za istega mladiča, ki smo ga pred tem videvali s svojo mamo, je zelo velika verjetnost, da gre resnično za isto žival. Leta 2012 pa smo pred Piranom fotografirali še enega navadnega delfina, na podlagi naravnih oznak pa potrdili, da gre za novi osebek, ki ga doslej še nismo zabeležili.

 

Zanimivo je, da je bilo na tem območju v razmeroma kratkem obdobju opaženih razmeroma veliko primerkov te vrste, sploh glede na njeno redkost in skupno število vseh dokumentiranih primerov v celotnem Jadranu. V okviru raziskave smo namreč opravili tudi temeljit pregled literature in dosedanjih zapisov o pojavljanju te vrste v celotnem Jadranu. A vendar navadni delfin v severnem Jadranu zaenkrat ostaja redka vrsta. Če se bo sčasoma v večjem številu vrnila v Jadran, zaenkrat težko rečemo. Na žalost so možnosti za to precej majhne, saj nikjer v Sredozemlju ne beležimo porasta številčnosti ali opažanj. Upamo pa, da bo ta raziskava služila kot izhodišče in spodbudila poročanje o morebitnih prihodnjih primerih, za boljši vpogled v pojavljanje navadnih delfinov v Jadranu. Raziskava je prosto dostopna na: https://onlinelibrary.wiley.com/doi/full/10.1002/aqc.3407

 

V kolikor na morju opazite kite ali delfine, vas prosimo, da nam to sporočite na 031 77 10 77, saj nam s tem pomagate zbirati pomembne in koristne informacije o teh živalih.


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Morigenos has published a new review study on the occurrence of common dolphins in the Gulf of Trieste and the northern Adriatic Sea, published this week in the renowned scientific journal Aquatic Conservation: Marine and Freshwater Ecosystems.

 

The common dolphin (Delphinus delphis) used to be very common in the Adriatic Sea and other parts of the Mediterranean Sea. However, from the 1970s onwards it had become so rare that the Mediterranean population is now listed as Endangered on the Red List of the International Union for the Conservation of Nature (IUCN). The dolphin species which is otherwise regularly present in the Gulf of Trieste and the northern Adriatic Sea, and regularly studied by Morigenos, is the bottlenose dolphin (Tursiops truncatus). The waters of the Adriatic Sea were once also inhabited by the common dolphin, but is not generally absent from this region. During the last 30 years, this species is considered as regionally extinct in the Adriatic Sea, likely due to intentional and systematic killing during mid 20th century. Back then, both Italy and the former Yugoslavia used to pay monetary rewards for every dolphin killed, because dolphins were considered pest that compete with the fisheries. Apart from direct kills, the population declines are likely also due to overfishing and the general degradation of the marine environment.

 

Once common, today extremely rare

Sources from the 1970s report that the last large groups of common dolphins in the Gulf of Trieste were seen in the 1940s. Since then, there have been no records of this species from the Gulf of Trieste, nor from nearby areas, until the records presented here. Our own 18-year systematic research also shows that the bottlenose dolphin is the only regular dolphin species in these waters. Interestingly, despite this, in Slovenian popular science literature, the common dolphin is nevertheless listed as a native Slovenian species, even though there are no actual documented cases of this species in Slovenia. A report from 1888 refers to a dead specimen from Zaule near Trieste, supposedly preserved in the Trieste Museum of Natural History. The museum no longer holds that specimen, even if it did so in the past. Based on this record, and given that the distance between Zaule and the Slovenian–Italian border is only a few kilometres, the common dolphin was included in the list of mammals of Slovenia, on the premise that it must have crossed Slovenian waters at some point. The species is also listed in the Slovenian Red List of Mammals as Endangered. However, the common dolphin has never actually been documented in Slovenian waters at any point in history, prior to records reported here.

 

First confirmed records in the Gulf of Trieste and Slovenia

Due to their rarity, all records of common dolphins in the Adriatic and many other Mediterranean areas today are very special and important, so it is vital to document them. We documented several records of common dolphins in the Gulf of Trieste between 2009 and 2012, through sightings of live animals or recovery of dead stranded animals. Dorsal fin markings allowed the photo‐identification of some of these, suggesting that at least four different live individuals occurred here in recent times.

 

We documented the first confirmed case of the common dolphin in Slovenia and the Gulf of Trieste in 2009, when divers photographed a dolphin from a boat, off Izola, and sent photographs to Morigenos. Based on these photographs we immediately determined that this was the rare common dolphin. A year later we documented a female with a calf in the port of Monfalcone, where they stayed for several months. Based on photo-identification and with the help of our colleagues working in Greece, we discovered that the female had already been sighted in the Ionian Sea in Greece, more than 1000 km away. Her trip to the Gulf of Trieste represents the longest documented movement in this species worldwide, which we reported some years ago. Unfortunately, her calf disappeared in early 2011, which likely meant it had died. We kept seeing the female for a few more months, after she left the area. The same year we recovered a highly decomposed carcass of a dolphin calf near Izola. Due to advanced carcass decomposition, we could not immediately determine the species. However, after carefully examining the cleaned skull bones, we were able to identify the animal as the common dolphin. Common dolphins are distinguishable from all other dolphin species by the anatomy of their skull, which features special grooves in their palatal bone. Even though it is impossible to determine whether this was the same calf we had observed before, it is highly likely to be the same animal. Finally, in 2012 we photographed another common dolphin off Piran. Photo-identification showed that this was a new individual, not seen in previous years.

 

In our study, we also reviewed all published records of this species in the Adriatic to date. It is interesting that our new records are relatively numerous for such a short time period, especially considering the rarity of the species and the total number of documented cases in the entire Adriatic Sea to date. Unfortunately, the species continues to be rare in the region. It is difficult to say if the species is likely to make a comeback to the Adriatic Sea. The chance for that are likely slim, as there is currently no evidence of any increase in common dolphin abundance or sightings anywhere in the Mediterranean Sea. But hopefully this contribution can serve as a baseline and encourage potential future cases to be reported, in order to provide further insights into the occurrence of common dolphins in the region. The study is freely available at: https://onlinelibrary.wiley.com/doi/full/10.1002/aqc.3407

 

If you see whales or dolphins in the Gulf of Trieste, please report your sightings to +38631771077 and help us collect important information about these animals.

 

 

  

 

 

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