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5.4 Fin whale (Balaenoptera physalus) winter feeding ground

The winter feeding ground of the Mediterranean fin whales has been identified by Canese et
al. (2006; Aïssi et al., 2008). Fin whales have been observed to feed mainly on the
euphausid Nyctiphanes couchi swarms formed over the large area within the Lampedusa
plateau (Tunisia shelf), mainly on the south and southeast side of the island (Celona,
personal communication) Even if no further investigations have been carried out in order to
understand the oceanographic processes driving the high concentration of that euphausid
species, it is possible to draft some hypotheses on the light of the bottom topography and
circulation patterns previously described.

The Lampedusa plateau is bounded to the northeast by the Linosa graben, which is
connected to the deeper Ionian plain by its branch Medina–Melita graben, while on the south
Jarrafa graben deeply incises the 200 m bathymetry of the plateau creating the favourable
conditions for the shelf-deep bottom exchanges. According to the general circulation
schemes and models (Astraldi et al., 2001; Sorgente et al., 2003; Ciappa, 2009; Molcard et
al., 2002) it can be roughly stated that the ATC flows following the shelf rims in that area
meandering and generating cyclonic eddy on its left (eastward) over deeper waters and
anticyclonic eddy on its right (westward) over the shelf (Fig. 8).

Cyclonic eddies generate upwelling, while anticyclonic eddies create downwelling and
retention, it is possible to argue that the upwelling transport is also enhanced by the funnel
effect of the above mentioned valleys (grabens), which lies just below, thus nutrients are
easily transported from the deeper bottom to the shelf, where are entrapped by the clockwise
flow close to Lampedusa island.
Moreover this is also consistent with the observations done by Ciappa (2009) about the role
of the Sidra gyre in modifying the ATC flow during the winter. Very likely, the plateau shallow
waters enriched with the nutrients from the closer deep bottoms are a very favourable area
for the grazing and/or herbivorous species, as Nyctiphanes couchi is. Similar topography-
hydrodynamic effect have been observed in the Ligurian sea, where fin whales feed mainly
on Meganycthyphanes norvegica from late spring to early autumn, even if onmuch more
deeper water than in the Sicily Channel (Würtz, 2010).

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