Abundance, distribution and diet of the common dolphin, Delphinus delphis, in the northern Aegean Sea (Greece)

Abstract

1. The abundance, distribution and diet of the short-beaked common dolphin were investigated as part of the first detailed study on cetacean populations in the North Aegean Sea. Since 2004, the area has been proposed by national and international entities as a marine protected area for common dolphins and other cetacean species owing to its high biodiversity.2. Abundance and distribution were investigated between 2005 and 2013 through dedicated scientific marine transect surveys, covering 14,701 km, in sea conditions of Beaufort sea state 3 and below. The line-transect sampling method was used to estimate relative abundance, using Distance 6.0 software. Encounter rate for this species in the study area was estimated to be 0.24 groups/100 km (1.5 dolphins/100 km), with a mean group size of 6.88 (SE = 1.90).3. Common dolphin sightings were analysed for correlations with eight environmental variables (distance from the coast, depth, slope, median salinity, gradient of salinity, median temperature, gradient of temperature and mean current) using generalized additive modelling. Common dolphin sightings were significantly correlated to depth, temperature and salinity. 4. Stomach-content analyses were performed on eight suitable samples from common dolphins stranded in the study area. The trophic level of the species was calculated, indicating that this species is a top predator.5. This research provides the first estimates for abundance and distribution and the first diet analysis for this Mediterranean Endangered dolphin species in the study area based on a year-round long-term study.

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

“…The estimated ER values could be used as an effective baseline for future assessment of the common dolphin conservation status in the area, where an increasing trend in small-scale fishery exploitation and tourist flow have been recorded in the last few years, representing potential harmful threats for the species. The mean group size estimated for the waters south of Samos Island confirmed the pattern observed for the species in the Inner Ionian Sea Archipelago [6,32] and the Thracian Sea [35]. The small-sized aggregation of common dolphin groups is probably due to a behavioral feeding strategy caused by the distribution of prey (small pelagic fish) over many small patches in the study area [5,28,54], which leads dolphins to split into smaller groups for more efficient predation [55].…”

“…Estimations of encounter rate, group size, density, and abundance values of D. delphis were made by the application of CDS analysis to sighting data collected during standardized surveys carried out near Samos Island, Greece (Eastern Aegean Sea), providing a comparison with analogous values estimated in similar studies carried out in other Mediterranean areas ( Table 3). The encounter rates estimated in the study area were higher than values reported for the Alborán Sea [4], in waters off Ischia Island [14], in the Inner Ionian Sea Archipelago [6,32], and in the Thracian Sea [35], indicating that the waters off the south of Samos Island are an eligible habitat for D. delphis. The estimated ER values could be used as an effective baseline for future assessment of the common dolphin conservation status in the area, where an increasing trend in small-scale fishery exploitation and tourist flow have been recorded in the last few years, representing potential harmful threats for the species.…”

“…The overall density estimate of the common dolphin in the study area is generally in line with the corresponding values reported in both the western and central-eastern Mediterranean regions ( Table 3). The overall abundance estimated in the waters south of Samos Island is lower than the values reported for the Thracian Sea [35] and for the Eastern Aegean Sea [36] as well as for the Alborán Sea [4,11]. In contrast, abundance is higher than the latest estimates reported for the Inner Ionian Sea Archipelago [33] and the Gulf of Corinth [17,34].…”

“…However, both the presence and distribution of these small pelagic fish can fluctuate with the seasons depending on environmental factors such as sea surface temperature and chlorophyll concentration, causing a variation in the presence of the common dolphin, both in its group size and spatial distribution. In effect, during warmer months, the common dolphin shows a preference for coastal waters due to the movement to epipelagic areas by small pelagic fish both in the western [3,11] and eastern Mediterranean areas [35,56]. During the colder season, the species seems to move in offshore waters in the western Mediterranean areas, whereas in the Greek seas it seems to remain relatively close to the coast (not exceeding 400 m) also thanks to the presence of Levantine Intermediate Water.…”

“…Currently, records of this species are documented in the Pelagos Sanctuary, near Lampedusa Island and in the Messina Strait [13], off western Sardinia [24], in the Cap Bon area (northern Tunisia) [25], in the eastern Ionian Sea [15], in the Aegean Sea [26][27][28], and in the Levantine Sea [29,30]. Local abundance estimates of this species have only been provided for long-term monitored areas such as the Alborán Sea [4,11], the waters off Malta [31], the Inner Ionian Sea Archipelago [6,32,33], the Gulf of Corinth [17,18,34], and the Thracian Sea [35]. Recently, an attempt to estimate abundance of a local population of common dolphin inhabiting waters near Lipsi and Samos islands, in the Eastern Aegean Sea, was carried out by the authors of [36].…”