THE UNCOMMON COMMON DOLPHIN

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.