Dolphins in a Scaled-Down Mediterranean: The Gulf of Corinth's Odontocetes

The longest recorded movement of an inshore common bottlenose dolphin (Tursiops truncatus)

Information on movements and connectivity among populations of animals is important for the delineation of units to conserve, so that demographic parameters, such as abundance, fecundity and mortality, can be placed in an appropriate population and conservation context. Common bottlenose dolphins (Tursiops truncatus) are often considered relatively ‘resident’ and demonstrating strong site fidelity to specific areas. However, this perception may partly be an artefact of the distribution and ‘habitat use’ of cetacean researchers, rather than animals themselves, and bottlenose dolphins have been shown to be capable of substantial movements, often in relatively short periods of time. Here, we report on two long-distance movements of a common bottlenose dolphin within the Mediterranean Sea, across the Tyrrhenian, Ionian and Adriatic Seas, and subsequently back across all three seas to Ligurian Sea, making these the two longest recorded movement for this species in the Mediterranean Sea to date and some of the longest in the world. We also review published records of long-distance movements in this species worldwide. This study highlights the utility of photo-identification and the importance of regional data sharing. We argue that photo-identification comparisons are always worthwhile and the results are informative regardless of the presence or absence of matches, especially with the ongoing advances in automated matching software.

Density and Abundance of Delphinus delphis in Waters South of Samos Island, Greece (Eastern Mediterranean Sea)

The Mediterranean subpopulation of short-beaked common dolphin Delphinus delphis is ranked as endangered on the IUCN (International Union for Conservation of Nature) Red List because it has sharply declined during the last decades, resulting in sparse and decreasing populations. Monitoring the conservation status of this endangered dolphin species is particularly relevant to fulfil targets under the range of several international agreements. Moreover, estimating the abundance of D. delphis is essential to verify the effectiveness of conservation action to maintain safe population levels in the Mediterranean Sea and to suggest appropriate modifications to limit potential threats. In this regard, a monitoring program of the short-beaked common dolphin in Samos Island (Greece) was carried out from 2016 to 2019, adopting a random line transect sampling method. The overall density and abundance estimates of D. delphis, obtained by applying conventional distance sampling (CDS) on sighting data, were 0.15 individuals/km2 (CV = 13.27%; 95% CI = 0.11–0.19 individuals/km2) and 51 individuals (CV = 13.27%; 95% CI = 40-66 individuals), respectively. Although, a longer time series of sighting data should be collected and a larger area should be investigated to better understand the population trend of D. delphis and its residency pattern, the results contribute to setting up a baseline reference for future assessment of its population in the Eastern Aegean Sea.

Modelling Dolphin Distribution to Inform Future Spatial Conservation Decisions in a Marine Protected Area

As marine predators experience increasing anthropogenic pressures, there is an urgent need to understand their distribution and their drivers to inform spatial conservation planning. We used an ensemble modelling approach to investigate the spatio-temporal distribution of southern Australian bottlenose dolphins (Tursiops cf. australis) in relation to a variety of ecogeographical and anthropogenic variables in Coffin Bay, Thorny Passage Marine Park, South Australia. Further, we evaluated the overlap between current spatial management measures and important dolphin habitat. Dolphins showed no distinct seasonal shifts in distribution patterns. Models of the entire study area indicate that zones of high probability of dolphin occurrence were located mainly within the inner area of Coffin Bay. In the inner area, zones with high probability of dolphin occurrence were associated with shallow waters (2-4 m and 7-10 m) and located within 1,000 m from land and 2,500 m from oyster farms. The multi-modal response curve of depth in the models likely shows how the different dolphin communities in Coffin Bay occupy different embayments characterized by distinct depth patterns. The majority of areas of high (>0.6) probability of dolphin occurrence are outside sanctuary zones where multiple human activities are allowed. The inner area of Coffin Bay is an important area of year-round habitat suitability for dolphins. Our results can inform future spatial conservation decisions and improve protection of important dolphin habitat.

Evidence of introgressive hybridization between Stenella coeruleoalba and Delphinus delphis in the Greek Seas

Natural interspecific hybridization might be more important for the evolutionary history and speciation of animals than previously thought, considering several demographic and life history traits as well as habitat disturbance as factors that promote it. In this aspect, cetaceans comprise an interesting case in which the occurrence of sympatric species in mixed associations provides excellent opportunities for interspecific sexual interaction and the potential for hybridization. Here, we present evidence of natural hybridization for two cetacean species commonly occurring in the Greek Seas (Stenella coeruleoalba and Delphinus delphis), which naturally overlap in the Gulf of Corinth by analyzing highly resolving microsatellite DNA markers and mitochondrial DNA sequences in skin samples from 45 individuals of S. coeruleoalba, 12 D. delphis and three intermediate morphs. Employing several phylogenetic and population genetic approaches, we found 15 individuals that are potential hybrids including the three intermediate morphs, verifying the occurrence of natural hybridization between species of different genera. Their hybrids are fertile and able to reproduce not only with the other hybrids but also with each of the two-parental species. However, current evidence does not allow firm conclusions whether hybridization might constitute a step towards the generation of a new species and/or the swan song of an already existing species (i.e., D. delphis). Given that the focal species form mixed pods in several areas of Mediterranean, this study is an excellent opportunity to understand the mechanisms leading to hybridization in the context of gene flow and urges for the evaluation of the genetic status of common dolphins in the Mediterranean.