Cetaceans in the Mediterranean Sea: Encounter Rate, Dominant Species, and Diversity Hotspots

Abstract

We investigated the presence and diversity of cetaceans in the Mediterranean Sea, analysing the data collected by 32 different research units, over a period of 15 years (2004–2018), and shared on the common web-GIS platform named Intercet. We used the encounter rate, the species prevalence, and the Shannon diversity index as parameters for data analysis. The results show that cetacean diversity, in the context of the Mediterranean basin, is generally quite low when compared with the eastern Atlantic, as few species, namely the striped dolphin, the bottlenose dolphin, the fin whale, and the sperm whale, dominate over all the others. However, some areas, such as the Alboran Sea or the north-western Mediterranean Sea, which includes the Pelagos Sanctuary (the Specially Protected Area of Mediterranean Interest located in the northern portion of the western basin), show higher levels of diversity and should be considered hotspots to be preserved. Primary production and seabed profile seem to be the two main drivers influencing the presence and distribution of cetaceans, with the highest levels of diversity observed in areas characterized by high levels of primary production and high bathymetric variability and gradient. This collective work underlines the importance of data sharing to deepen our knowledge on marine fauna at the scale of the whole Mediterranean Sea and encourages greater efforts in the networking process, also to accomplish the requirements of the Marine Strategy Framework Directive, with particular reference to Descriptor 1: biological diversity is maintained.

Surface-sourced eDNA to track deep-diver cetaceans: the study case of Cuvier’s beaked whale (Ziphius cavirostris) at Caprera Canyon and surrounding areas (Western Mediterranean Sea)

“…Preliminary results based on visual surveys carried out in 2011-2013 have speculated this area as a favorable habitat for the Cuvier’s beaked whale (e.g. Bittau and Manconi, 2016; Gnone et al, 2023), and the results presented here, with data collected in recent year and with a continuous monitoring, can only support this hypothesis. In addition, the control stations used in this study (Figure 3) confirmed this result as well: the Cuvier’s beaked whale was detected only in stations B and C, which are located in proximity to the Canyon, but not in station A, located approximately 12 km north of it.…”

“…Although DNA metabarcoding is more useful and cost-efficient when detecting greatest numbers of species at the same time (Gargan et al, 2022), cryptic species needs highly specific and sensitive approaches to be identified (Vörös et al, 2017). Compared to other cetaceans, the Cuvier’s beaked whale is one of the most difficult to study: these animals are able to dive at depth greater than 1 km (Schorr et al, 2014; Gnone et al, 2023) and for extended time period (1 hour, or more), and they also spend short periods at the surface (typically between 2 and 8 minutes) (Baird et al, 2006; Tyack et al, 2006; see also Figure 1). These traits make standard monitoring methods reasonably ineffective since for cetaceans that spend much of their life submerged, abundance estimates based on visual surveys become challenging and prone to underestimates.…”

“…As a matter of fact, it has been assessed that only 40% of cetacean groups that are directly on the transect line would be detected in outstanding conditions during visual sighting surveys conducted from a large vessel (Barlow, 2015). Even so, our current knowledge on this and other cetacean species largely rely on the visual approach, which remains the most informative to date, especially if based on long-term data (Cañadas et al 2018, Gnone et al 2023). Moreover, when dealing with Cuvier’s beaked whales, traditional surveys that incorporate boat- and aerial-based surveys, as well as satellite-tracked tags, and offshore passive acoustic monitoring may result in very expensive monitoring programs.…”

“…This species is found both in the western and eastern basins of the Mediterranean Sea (Podestà et al, 2016). However, although the analysis of a massive long-term sightings dataset has recently better delineated the Mediterranean areas usually frequented by this species (Arcangeli et al, 2023; Gnone et al, 2023), information on the spatial ecology of the Cuvier’s beaked whale is still limited and mainly restricted to certain areas (such as the Pelagos Sanctuary for Mediterranean Marine Mammals and Alboran Sea, e.g. Moulins et al, 2007; Tenan et al, 2023; Tepsich et al, 2014; Torreblanca et al, 2022), leaving knowledge gaps where the study effort is non-existent (Cañadas and Notarbartolo di Sciara, 2018).…”

Targeting fin whale conservation in the North-Western Mediterranean Sea: insights on movements and behaviour from biologging and habitat modelling

“…In general, both home and core ranges encompass critical habitats for fin whales in the North-Western Mediterranean Sea and agree well with prior knowledge on the presence and behaviour of fin whales in this portion of the Basin. Recent summer wide-scale aerial surveys [78][79][80], as well as studies based on photographic mark recapture [40,81] and visual [82] and acoustic surveys [27,83], 0q 3q E 6 q E 9 q E 1 2 q E 0 q 3q E 6 q E 9 q E 1 2 q E 0q 3q E 0 preferred habitat (% of daily occurrence) (for frequency estimate >1% of total days) 20 40 60 80 100 show a consistent seasonal use of the Balearic Sea, the slope, and offshore waters of the Gulf of Lion and the Corso-Provençal-Ligurian Basin for feeding purposes. In this context, our study demonstrates that animal-borne tracking technology can be used to support and consolidate other approaches.…”