The Strait of Gibraltar is known to be a barrier for certain species and a transition
zone for many others. The Strait acts as a biogeographical frontier particularly for
deep marine fishes, as the depth of the Gibraltar Strait, which is only 320 m, has
made it an effective barrier to them (Fredj and Maurin 1987). This border sometimes
functions as a semipermeable frontier, working more effectively at hindering the
pass from east to west than vice versa, especially for species that make use of the
superficial water, where the current is important and always flows eastwards. This is
the case for the Loggerhead turtles, as turtle youngsters enter the Mediterranean but
are unable to cross back the Strait, since they lack the strength to overcome the
incoming current of Atlantic water until they reach a carapace size of at least 55 cm
(Bellido et al. 2007, 2010, 2010; Revelles et al. 2007; Eckert et al. 2008).
In any case, this frontier is permeable, and the Strait of Gibraltar and the Alboran
Sea constitute an important transition zone, particularly for highly migratory fish
species and air-breathing vertebrates. The highly migratory species (HMS) have
been listed in Annex 1 of UNCLOS (United Nations Convention of the Law of the
Sea) and include the tunas, billfishes, oceanic sharks, and other migratory fishes
(dolphin fishes, pomfrets, and sauries), in addition to cetaceans. On the other hand,
cetaceans, sea turtles, and marine birds conform the group of marine air-breathing
vertebrates, which are also highly migratory marine species. Currently, the Alboran
Sea represents an important area for the distribution of many HMS and air-breathing
vertebrate species, since it acts as the entry point to the Mediterranean from the
Atlantic for migrating sharks (Megalofonou et al. 2005; Meléndez et al. 2015),
cetaceans (de Stephanis et al. 2008), and sea turtles (Camiñas et al. 1992; Camiñas
1997). This affects vagrant or casual species such as, for example the pelagic sharks
Galeocerdo cuvier and Sphyrna mokarran, which have been reported in the Alboran
Sea (Pinto de la Rosa 1994; Denham et al. 2007), and groups of species that are
known to migrate periodically eastward or westward through the Alboran Sea, which
include large pelagic vertebrate fishes (tuna and others tuna-like species),
chondrichthyans (sharks and batoidea), mammals, sea turtles (loggerhead and leatherback) and different species of sea birds.
This periodic and systematic transition through a strait and a narrow sea pathway,
as is that represented by the Alboran Sea, imply predictable concentrations of
individuals and densifications of passing populations that have been exploited by
man since ancient times. The fishing gears mainly used to this aim were driftnets and
trap nets, designed for improving the fishing yield mostly of tunas and swordfish
(Silvani et al. 1999; Tudela et al. 2005). Nevertheless, driftnets gears have been
proven to concentrate a high bycatch rate of other HMS species of conservation
concern and, for this reason, they were recently banned (UNEP-MAP-RAC/SPA.
2015).
The highly mobile behaviour of these species, which is typical of large pelagic
fishes, implies a link between the fishing stocks at both sides of the Strait of
Gibraltar. However, recent studies revealed significant genetic differentiation
between Mediterranean and Atlantic populations of certain fish species. As a consequence, the population of species such as, for instance, Coryphaena hippurus,
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R. Real et al.
zone for many others. The Strait acts as a biogeographical frontier particularly for
deep marine fishes, as the depth of the Gibraltar Strait, which is only 320 m, has
made it an effective barrier to them (Fredj and Maurin 1987). This border sometimes
functions as a semipermeable frontier, working more effectively at hindering the
pass from east to west than vice versa, especially for species that make use of the
superficial water, where the current is important and always flows eastwards. This is
the case for the Loggerhead turtles, as turtle youngsters enter the Mediterranean but
are unable to cross back the Strait, since they lack the strength to overcome the
incoming current of Atlantic water until they reach a carapace size of at least 55 cm
(Bellido et al. 2007, 2010, 2010; Revelles et al. 2007; Eckert et al. 2008).
In any case, this frontier is permeable, and the Strait of Gibraltar and the Alboran
Sea constitute an important transition zone, particularly for highly migratory fish
species and air-breathing vertebrates. The highly migratory species (HMS) have
been listed in Annex 1 of UNCLOS (United Nations Convention of the Law of the
Sea) and include the tunas, billfishes, oceanic sharks, and other migratory fishes
(dolphin fishes, pomfrets, and sauries), in addition to cetaceans. On the other hand,
cetaceans, sea turtles, and marine birds conform the group of marine air-breathing
vertebrates, which are also highly migratory marine species. Currently, the Alboran
Sea represents an important area for the distribution of many HMS and air-breathing
vertebrate species, since it acts as the entry point to the Mediterranean from the
Atlantic for migrating sharks (Megalofonou et al. 2005; Meléndez et al. 2015),
cetaceans (de Stephanis et al. 2008), and sea turtles (Camiñas et al. 1992; Camiñas
1997). This affects vagrant or casual species such as, for example the pelagic sharks
Galeocerdo cuvier and Sphyrna mokarran, which have been reported in the Alboran
Sea (Pinto de la Rosa 1994; Denham et al. 2007), and groups of species that are
known to migrate periodically eastward or westward through the Alboran Sea, which
include large pelagic vertebrate fishes (tuna and others tuna-like species),
chondrichthyans (sharks and batoidea), mammals, sea turtles (loggerhead and leatherback) and different species of sea birds.
This periodic and systematic transition through a strait and a narrow sea pathway,
as is that represented by the Alboran Sea, imply predictable concentrations of
individuals and densifications of passing populations that have been exploited by
man since ancient times. The fishing gears mainly used to this aim were driftnets and
trap nets, designed for improving the fishing yield mostly of tunas and swordfish
(Silvani et al. 1999; Tudela et al. 2005). Nevertheless, driftnets gears have been
proven to concentrate a high bycatch rate of other HMS species of conservation
concern and, for this reason, they were recently banned (UNEP-MAP-RAC/SPA.
2015).
The highly mobile behaviour of these species, which is typical of large pelagic
fishes, implies a link between the fishing stocks at both sides of the Strait of
Gibraltar. However, recent studies revealed significant genetic differentiation
between Mediterranean and Atlantic populations of certain fish species. As a consequence, the population of species such as, for instance, Coryphaena hippurus,
438
R. Real et al.