Mediterranean submarine canyons as stepping stones for pelagic top predators: The case of sperm whale (Physeter catodon)

Submarine canyons have long been considered important areas for marine life supporting abundant and unique communities of megafauna, including sometimes species not observed in other environments. These emerged valleys are becoming increasingly recognised as recurrent sources of enhanced productivity (Allen et al., 2001; Genin, 2004) and habitat heterogeneity. Indeed, they provide important habitat for various life stages of benthic and demersal fishes and invertebrates along continental margins (Demestre & Martìn, 1993; Stefanescu et al., 1994). These physiographic features may also serve as ‘keystone structures’ and aggregation area of top predators like tuna, sharks and marine mammals. The bottom topography of the north-western Mediterranean Sea is not a uniform flat environment. It’s characterized by the presence of some geomorphologic features as seamounts, and is incised for the most part by several submarine canyons. These deep incised valleys occupy nearly 50% of the continental slope (Gili et al., 2000). This area constitutes one of the pools of greatest diversity and highest production (Franqueville, 1971; Andersen et al., 2001), stimulating the regular presence of eight cetacean species (Notabartolo di Sciara, 2002). The aggregation of various teutophageous, planktophageous and ichtyophageous cetaceans may reveal the high productivity of this area. Several studies attempted to correlate cetacean distribution to geomorphologic features inside the Mediterranean Sea (Gannier, 1998; Cañadas et al., 2002; Azzellino et al., 2008; Aïssi et al., 2008; Moulins et al., 2008). It seems that as well as depth and slope, submarine canyons play an important influence on cetacean distribution, even if modalities and intensities depend on hydrological, topographical, and biological contexts. It was confirmed that habitat selection for Cuvier’s beaked whale is associated particularly to steep slopes and submarine canyons (Moulins et al., 2007). For example, the Genoa canyon was described as specific attractive area and overlaps hotspot ecosystems for striped dolphin, Risso’s dolphin and sperm whales (Moulins et al., 2008). Recent studies have been conducted in the Pelagos sanctuary using a wide range of spatial and temporal scales to try to elucidate the relationship between preferred habitat of sperm whale and specific physiographic features. This area encompasses an aggregation of seamounts located in the south-eastern part and successive submarine canyons. This part of the Mediterranean is considered as the most densely canyoned margin segment in the entire basin. Of the most important and prominent canyons considered we refer to the Genoa and the Imperia canyons in the northern part of the Ligurian Sea. The others were relatively smaller and located north and west of the Corsica Island as Saint Florent, Porto, Sagone and Ajaccio canyons (Fig. 1). Visual and acoustic surveys have been carried out over three consecutive years (2009-2011) focusing this largest toothed whale in the Pelagos Sanctuary. Observations have been reported throughout this area. However, the target species does not appear to be homogeneously distributed. Sightings occurred over a large bottom depth range, with a trend to increase on the continental slope and the abyssal plain. The distribution of the depth data appeared to be bimodal, with one marked peak of sightings in waters around 1400 m deep, and a majority of sightings in waters around 2000 m deep (Fig. 2). The spatial distribution of sperm whales was reflected in three dissimilar morphological regions: submarine canyons (region 1), inter-canyons (region 2) and seamounts areas (region 3). Among the survey effort spread along the whole study area, the survey division within the three regions was relatively homogeneous. A relevant distinction of encounter rate has been registered between these three zones characterized by different topography. Sightings were greater in region 1 (n=63/113), to a lesser extent in region 2 (n=41/113), and even lower in region 3 (n=9/113). Thus, a marked preference for the canyon area has been evidenced by the sighting frequency (56%). Sperm whales seemed to favour in particular submarine canyon habitat, which appeared in ranked order as the major influent, than inter-canyons (36%) to seamounts (8%) (Fig. 3). This tendency was supported by several dedicated observations and the frequent use of submarine canyons has been previously reported in the Mediterranean Sea (Gannier & Parca, 2007; David, 2000). Preferential use of submarine canyons has also been suggested in other regions of the globe: in the Gully region off Nova Scotia shelf (Whitehead et al., 1992), in the Mississippi Canyon in the Gulf of Mexico (Davis et al., 1998), the Kaikoura Canyon on the east coast of New Zealand (Larivière, 2001) and in the northeast United States (Waring et al., 2001). The canyons generally open on the continental shelf, at around 100 to 200m depth and extend down to the abyssal plain, over 2000m deep. The head of the canyon was delimited by the 200m contour and the bottom by the 2000m isobaths. In order to evaluate the variability of distribution of sperm whales and to investigate their affinity within canyon structure, this area was split initially into three classes: head (200-500m deep), middle (500-1000m deep) and mouth (more than 1000m deep). Sightings were located for the most part at the opening of submarine canyons and their opposite extremity. The encounter rate has been assessed in each class, and appeared to increase particularly in the outer canyon. Thus, the spatial distribution of sperm whales highlights habitat suitability in the deep area of submarine canyons. To delineate the characteristics and spatial distributions in relation to canyon morphology, a new split related to the middle axe of canyon has been applied leading to segregation into eastern and western canyon. The spatial distribution of sperm whales appears to concentrate exclusively in the western part of canyons. In particular, the pattern of distribution of this species is associated with the steep and complex topography of these emerged valleys. Indeed, submarine canyons studied in this area are characterised by a relatively genteel slope (mean slope 80 m/Km) in their upper heads located close to the shore. However, a steeply slope (mean slope 180 m/Km) extends far over the abyssal plain of these valleys on the sea floor. Sperm whale distribution in the Pelagos Sanctuary seems to be influenced mainly by the structure of submarine canyons which affects hydrological and biological phenomena. Certainly, the steep topography of submarine canyons induces upwelling that supports elevated cetacean diversity and abundance compared to surrounding waters (e.g. The Gully in eastern Canada; Hooker et al. 1999). The bottom relief modifies currents, leading to concentration of organisms. Hydrological features, including eddies and topographically-induced upwellings generate fronts and bring nutrients, which in turn increase primary productivity, and the aggregation of zooplankton from enhanced secondary production. Internal waves, which are produced by complex and steep topography, can also lead to the concentration of prey species. Submarine canyons can strongly modify flow, shelf-slope exchanges of water and material (Hickey, 1995; Perenne et al., 2001) and this coupling can aid the transport of particulate organic matter that influences productivity. Submarine canyons can also act as funnels for water upwelling from deeper oceanic levels to shallower shelf regions, providing nutrient inputs to the marine ecosystem (Flaherty, 1999) and enhancing productivity. Sperm whale diet includes large variety of food items, but consists primarily of mesopelagic and bathypelagic cephalopods (Clarke, 1980; Kawakami, 1980). It has therefore been impossible to directly relate sperm whale distribution to the distribution of their prey because methods of effectively sampling these deep-living squid have not yet been developed (Clarke, 1987).Many papers report evidence that cetaceans occupy the continental slope, especially the part of submarine canyons that cut into the slope. Steep slopes and submarine canyons play an important role in influencing the water patterns in and around the surrounding area because of their size and the area they occupy (Hickey, 1995). Physical processes in submarine canyons have received much attention (Shepard et al., 1974; Freeland & Denman, 1982; Noble & Butman, 1989; Breaker & Broenkow, 1994; Alvarez & Tintore, 1996), but studies on the ecological processes of canyons are still limited. The refuge role of submarine canyons may be especially important for protecting in general top predators, especially cetacean populations from extirpation in ecological cul-de-sacs.