Central-southern Mediterranean submarine canyons and steep slopes : role played in the distribution of cetaceans, bluefin tunas and elasmobranchs

Abstract

Morphologically, the Strait of Sicily comprises the continental shelf and slope. However, in the area there are also banks, seamounts, irregular incised slopes forming troughs and canyons of V, U and flat-bottomed types. The latter referred to as grabens due to their origin, considered to have been produced by subsidence of the earth's crust between two faults. These canyon-like grabens are governed by a fault system that extends throughout the Sicily Channel from Southern Sicily to Tunisia and which has also been responsible for the major tectonic and geomorphological development of the Maltese islands (Illies, 1981; Galea, 2007). The Maltese islands lie in the Sicily Channel on a relatively stable plateau of the African foreland, the Pelagian Platform, about 200 km south of the convergent segment of the Europe-Africa plate boundary that runs through Sicily (Galea, 2007). In the extension of the Sicilian continental shelf toward the Pantelleria Rift (Adventure Bank and Graham Bank plateaus), five volcanic seamounts have been recognised (Tetide, Anfitrite, Galatea, Cimotoe and Graham). Two other much larger seamounts, Bannock and Nameless Bank (Banco Senza Nome), are located between the Malta and Pantelleria basins and close to the eastern border of the Nameless Bank respectively (EC, 2011). A strip of deeper water approximately 96 km wide (200–1700m deep, GEBCO 08, 2010) stretches in the middle of contrasting shallow Tunisian waters on one side and the banks of Sicily and Malta on the other side, in a manner that links the deeper waters of the two main western and eastern Mediterranean basins through this relatively restricted deep water passage. In turn this passage has narrower, deeper troughs and canyon-like grabens that may channel the deepest waters down to a 600-1700m depth. The deepest water passages in the Sicilian Channel appear to link to the Bizerte Canyon and the Ustica trough in the Western basin, and to the Heron Valley in the Eastern basin. The area of particular interest for this paper includes the deep Malta, Linosa and Pantelleria troughs or canyon-like grabens, that run parallel to Malta and Sicily in a north west direction and up to the volcanic islands of Linosa and Pantelleria. Also of interest are regions where steep slopes lead to deeper waters. Submarine canyons are defined as deeply incised, steep-sided underwater valleys winding along or across the continental shelf or continental slope. Their cross sections tend to be V-shaped along the upper course and U-shaped in the lower course (Danovaro et al., 2010). As submarine canyons may also be described in relation to submarine troughs, thus indicating a relationship that may be found in the Straits of Sicily, the submarine canyon-shaped parts of the troughs found in the region need to be considered as contributing to the deep canyon and steep slope formations in this Central-Southern Mediterranean region. The contrast between the shallow and deep waters found may have encouraged a greater diversity of species adapting to the variable conditions or isolating them in patchy distributions. The review on global distribution of large submarine canyons by Harris and Whiteway (2011) is an important basis on which to build further awareness and pursue research on these marine features, especially highlighting the fact that for the Mediterranean it is valuable to consider not only the large submarine canyons but also smaller canyons which may still contribute unique habitat and conditions for unique biodiversity. Therefore, as indicated by various researchers (Harris and Baker, 2012), different seafloor geomorphologies need to be studied to provide important data and possible relationships between various seafloor features, including submarine canyons, steep slopes and regions close to seamounts, and the conditions which provide specific benthic habitats and affect the distribution of micro to mega species distribution. In general, Mediterranean canyons are found to be more closely spaced than in other areas of the world (Harris and Whiteway, 2011) thus constituting a probable hotspot for submarine canyon species. The deep-sea submarine canyons of the Mediterranean vary in size, linking the coastal areas to the deep sea. These canyons may act as conduits of coastal detritus (organic and inorganic) to the deep-sea grounds (Gardner, 1989). Submarine troughs, canyons and valleys can also connect various deep water areas allowing for exchange of materials and deep water species horizontally as well as vertically. The seafloor geomorphology may also affect pelagic species distribution in both direct and indirect ways. Most prominently through the availability of refugia and prey in these deep water formations.