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of the basin. It seems that movement of bluefin tuna tagged in the central and western
Mediterranean Sea are more pronounced than in the east. Seasonal prey abundance (e.g. E.
encrasicolus, S. pilchardus, M. norvegica, S. scombrus, A. rochei, etc.) drives the
concentration of both young and adult specimens in Mediterranean areas not used for
reproduction.

Surface fronts can affect the spatial aggregation of bluefin tuna schools, at least at certain
scales. Shelf-break fronts can be seen all around the basin and very close to the shore,
where the continental shelf is narrow. Coastal fronts are also visibile along the southern
Sicilian coast.

The Gulf of Gabes in Tunisia shows a decline in frontal density during September, possibly
linked to tidal phenomena. The association between the distribution of juvenile bluefin tuna
schools and thermal fronts was found valid only over a limited range of spatial scales (10-40
km). This indicates that other processes occur on small scales (over-aggregation due to
unseen prey clusters or other behavioural processes) and larger scales (in and out
movement at the border of the studied area).

The relationship between tuna aggregates and frontal meanders is most probably indirect
and trophic-related. Advected material at fronts can provide favourable feeding grounds for
small clupeids, which are in turn sought by bluefin tuna. Interannual and seasonal variations
in frontal density in the Mediterranean Sea may not have a direct influence on the basin’s
global carrying capacity. However, it may have an important impact on the local aggregation
of nutrients, phytoplankton and zooplankton species, and eventually on fish schools, thus
leading to possible changes in density-dependent responses in marine populations.
Transient surface fronts are particularly difficult to observe and assess (Royer et al., 2005)
(Fig. 54).

Koched et al. (2012) have studied the spatial distribution and ecology of the larvae of three
tuna species (Thunnus thynnus, Auxis rochei and Euthynnus alletteratus) in the Gulf of
Gabes. The A. rochei (bullet tuna) larvae showed a more widespread distribution than the
other species, being found at both inshore and offshore stations. E. alletteratus (Atlantic
black skipjack) larvae distribution covers a wide area over the continental shelf of this region.
The larvae of the large migratory tuna T. Thynnus (Atlantic bluefin tuna) were mainly
sampled from the offshore stations, suggesting that spawning possibly takes place mainly
near the shelf break. Tuna larvae were mainly collected in oligotrophic and mixed waters
resulting from the confluence of surface water of recent Atlantic origin (ATC) and resident
surface Mediterranean waters, as shown by their preference for lower chlorophyll a
concentrations (from 1.4 to 2.5 mg m-3) and moderate salinity values (between 37.35 and
37.75). Significantly, tuna larvae seemed to avoid the more eutrophic and saltier waters of
the gulf situated very close to the coast and around Kerkennah and Djerba islands.

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